Structural Timber Design To Eurocode 5

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Timber frame housing: UK Structural recommendations

(TRADA TECHN0L0G

Third edition 2006

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First published in Great Britain by TRADA Technology Ltd. 2006.

Copyright of the contents of this document is owned by TRADA Technology. All rights are reserved. No copying or reproduction of the contents of the printed book is permitted without the consent of the copyright holder, TRADA Technology, application for which should be addressed to the publisher.

© TRADA Technology 2006

Whilst every effort is made to ensure the accuracy of the advice given, the company cannot accept liability for loss or damage arising from the use of the information supplied.

ISBN (10 digit): 1-900510-50-2 ISBN (13 digit): 978-1-900510-50-9

Cover illustration courtesy CCB Evolution

Printed in England on paper with a high recycled content

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TRADA Technology Timber frame housing: UK Structural recommendations

Contents

Page

Introduction

5

I Loading on structural elements

6

1.1

Structural duties of timber frame elements Timber frame wall panels Horizontal diaphragms

1.1.1

1.1.2

1.2

Determination of loads acting on each element Calculation of vertical load Calculation of wind loads Calculation of roof loads Checking strength and stability

1.2.1

1.2.2 1.2.3 1.2.4

6 6 7

9 10 11

19

20

1.3

Weights of materials

21

1.4

Weights of typical constructions

23

Roofs Floors

1.4.1

1.4.2 1.4.3

1.5

Walls

General requirements of prefabricated wall, floor and roof elements

2 Walls 2.1

24 25

Wall studs Minimum desirable stud sizes Multiple studs Lateral support Load capacities Design procedure Drilling of studs

2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 2.1.6

23 23 24

25 25 25 26 26 29 30

2.2

Framing around openings

30

2.2.1

Lintel capacities

31

2.3

Racking resistance of timber frame walls Basic design and materials Adhesively bonded panels Overturning effects Deflection

2.3.1 2.3.2 2.3.3 2.3.4

3 Floors 3.1 3.1.1

3.1.2 3.1.3 3.1.4 3.1.5

41 41

42 43 46

48

Joists and beams Reactions, moments and deflections in a single span, simply supported beam Solid timber beams Structural timber composites Prefabricated engineered timber joists Steel fitch beams

3.2 Floor decking

48 49 50 52 52 53

63

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TRADA Technology Timber frame housing: UK Structural recommendations

4 Roofs

64

4.1

Trussed rafter roofs

64

4.1.1

Bracing

64

Connections

67

4.2

5 Foundations

68

5.1

68

Loading

6 Multi-storey buildings

69

6.1

General design considerations

69

6.2

Construction

69

6.3

Disproportionate collapse

69

7 Example of calculations for a complete dwelling

70

7.1

Vertical loads

72

7.2

Horizontal loads

73

7.3

Overall stability calculations

75

7.3.1 7.3.2 7.3.3 7.3.4

7.4 7.4.1

7.4.2 7.4.3 7.4.4

7.5 7.5.1 7.5.2 7.5.3

Wind pressures and self-weight Overturning Sliding Resistance to wind uplift

Racking calculations Wind loads Racking forces Design method for racking Recommended design procedure

Wall panel studs Ground floor rear wall — very short term Ground floor rear wall — medium term and long term Ground floor gable wall — very short term

75 76 77 78

78 78 79 79 91

92 92 94 95

7.6

First floor platform

96

7.7

Wall panel lintels

98

7.7.1

7.7.2

7.8 7.8.1 7.8.2 7.8.3 7.8.4

2.4 clear span opening at eaves level 1.2 m clear span opening at first floor level

Cripple studs First floor rear wall — very short term Check bearing on bottom rail of panel — medium term load Ground floor front wall — very short term Check bearing on bottom rail of panel — medium term load

8 References and further reading

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99 99 101 101

102

103

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TRADA Technology Timber frame housing: UK Structural recommendations

Introduction Timber Frame Housing - Structural Recommendations covers well established principles and methods for the structural design and strength and stability checking of timber frame buildings. The guidance is based on the recommendations in BS 5268-2 Code of practice for the structural use of timber. Permissible stress design, materials and workmanship, BS 52686.1:1996 Structural use of timber. Code of practice for timber frame walls. Dwellings not exceeding four storeys but it also includes procedures now widely used in the design of timber frame houses but which are not specifically covered in these Codes. Worked examples, including calculations for a complete house, are included. The guidance does not exclude other ways of showing that a design satisfies the requirements of Building Regulations and Standards regarding strength and stability. The design of timber frame buildings, for example, can be undertaken using the guidance given in the Eurocodes suite, BS EN 1990:2002 Eurocode. Basis of structural design, BS EN 1991 Eurocode 1. Actions on structures (in 5 parts) and BS EN 1995-1-1:2004 Eurocode 5. Design of timber structures. General. Common rules and rules for buildings. However, these do not include specific guidance for timber frame structures and currently most buildings are designed or checked using the relevant parts of the British Code, BS 5268.

Materials and components are covered by other standards as listed in the References. Span tables for floor joists, rafters and purlins for roofs, and joists for flat roofs are available in a separate TRADA Technology document Span tables for solid timber members in floors, ceilings and roofs (excluding trussed rafter roofs) for dwellings. This publication deals solely with the engineering aspects of timber frame design; it does not address issues such as fire safety and performance, thermal and acoustic performance, weatherproofing and durability and design detailing. These are covered in a companion publication Timber Frame Construction, last updated in 2001. Timber frame: Standard details for houses and flats, published in 2006, provides typical details for 'open' panel timber frame wall panels for three storey houses requiring 30 minutes fire resistance and for flats requiring 60 minutes fire resistance. The details show floors with solid timber joists which can readily be used for timber frame buildings up to four storeys in height. This is the third edition of Timber Frame Housing - Structural Recommendations. The first edition, published in 1979, was itself an updated version of Section 9 Structural Recommendations of the TRADA Design Guide for Timber Frame Housing first issued in 1967.

TRADA Technology TRADA Technology is the leading independent timber research, consultancy and information provider for the construction industry. Technical expertise is at the heart of our business, working with clients to get the most from their timber products. Specialist services include frameCHECK (timber frame quality assessment), structural and condition surveys on site, design assistance, materials and product evaluation. In the event of a dispute, expert witness services are also available. Clients cover the whole construction delivery chain, including architects, engineers, designers, specifiers, contractors and builders, in the UK and overseas.

Acknowledgements TRADA Technology wishes to acknowledge the assistance of CCB Evolution in the preparation of this publication. CCB Evolution, Consultants in prefabrication and modular design, is a specialist engineering consultancy with expertise in timber frame structures and cold rolled steel frames. More information is available from www.ccbevolution.co.uk.

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TRADA Technology Timber frame housing: UK Structural recommendations

I Loading on structural elements 1.1 Structural duties of timber frame elements 1.1.1 Timber frame wall panels Timber frame panels used in house construction have three major structural duties to perform: support of vertical loading, resistance to deformations caused by horizontal loading in their plane and resistance to wind loading perpendicular to their plane.

Resistance to vertical loads is checked according to normal engineering principles, bearing in mind that in timber design the duration of each load (long-, medium-, short- or very short-term) has to be considered because the strength of the timber members depends on the duration of the loads. The racking or shear resistance of wall panels to horizontal load is calculated according to procedures set out in BS 5268-6.1 Structural use of timber— Code of practice for timber frame walls — Dwellings not exceeding four storeys, which are based on data from tests on typical timber frame wall panels.

1ICHN0

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TRADA Technology Timber frame housing: UK Structural recommendations

1.1.2 Horizontal diaphragms The horizontal diaphragms formed by floor, ceiling and roof systems are usually required to take loads in their own plane.

The illustration shows a floor and roof diaphragm resisting wind load on a gable wall. The ends of the gable wall are supported directly by the front and rear walls of the building, and the bottom edge by the foundation. The load on the rest of the wall is transferred via the horizontal diaphragms into the front and rear walls where their shear resistance can transfer it to the foundations. The first floor diaphragm takes half the neff load on the ground floor walls plus half the nett load on the first floor walls. The roof diaphragm takes half the nett load on the first floor walls plus load from the roof. For wind parallel to the ridge it is usual to assume that half the nett horizontal wind load on the spandrels or roof is transferred to the roof diaphragm; for wind perpendicular to the ridge all the nett horizontal roof load is transferred to it. Where adjacent panel edges are fastened with nails or screws to the same timber members such as joists or blocking, the resulting connection between the panels enables shear forces to be transferred from one panel to the next. This is an essential part of the diaphragm action. It may be assumed that conventional floors and flat roofs, in which a woodbased panel product is fastened to timber joists, have adequate strength and stiffness as horizontal diaphragms, provided that:

• • •

•

the diaphragm span : depth ratio does not exceed 2:1 in either wind direction (BS 5268-6-2 Clause 6.5) the span does not exceed 12 rn between supporting walls (BS 5268-6-2 Clause 6.5) the fixing around the edges of the panels complies with standard recommendations (eg 3.00 mm diameter ringed shank nails at 150 mm centres for plywood or 3.35 mm ringed shank nails at 300 mm for wood particleboard and OSB, with a length equal to 2.5 times the board thickness) the perimeter of the diaphragm is attached to the walls with fastenings of equivalent strength.

Plasterboard ceilings in roofs that comply with BS 5268-3 Annex A may also be assumed to provide adequate diaphragm action provided that truss clips are used to secure every truss to a head binder and the fixing around the edges complies with standard recommendations (3.5 mm diameter plasterboard nails or screws 40 mm long at 150 centres). It is recommended that in areas of high wind load (eg with a dynamic wind pressure> 1500 N/rn2), and always for horizontal diaphragms outside the range given above, the required fastener spacings should be calculated by ensuring that

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TRADA Technology Timber frame housing: UK Structural recommendations

Va

h

adm,1 Slkedge,1

+ dm,2 S2kedge,2

where the suffices 1 and 2 refer to the decking and plasterboard respectively (if present) and Va

h fadm

k

= maximum shear force (normally half the total wind load applied to the diaphragm) (N) = horizontal depth of diaphragm measured in direction of wind (mm) = permissible load per fastener for very short-term loading (N) = a value shown in the following table k

Blocked floors and flat roofs, in which all four edges of the panels are joined to adjacent panels by fastening them to joists or blockings Floors and flat roofs in which adjoining tongued and grooved edges are not fastened to the same timber member but are glued together Unblocked floors and flat roofs in which adjoining tongued and grooved edges are not fastened to the same timber member and are not glued together Plasterboard ceilings in which all four edges of the panels are joined to adjacent panels by fastening them to joists or blockings

1.0

1.5

1.4

For a plywood listed in BS 5268-2 the code provides instructions for calculating fadm for specified types of hand driven nail. A machine driven nail made from steel with a tensile strength of 600 N/mm2 or more has a loadcarrying capacity at least equal to that of a standard hand-driven nail of the same length and diameter.

For wood chipboard and OSB, there is no method in BS 5268-2: 2002 for determining fadm. The value of fadm for Group I plywoods of a similar thickness, reduced by a factor of 0.9 for OSB or 0.8 for wood chipboard, should be safe to use with suitable structural grade materials. (Group 1 plywoods are listed in a footnote to BS 5268-2 Table 63.) Alternatively a value for fadm in OSB can be calculated using Eurocode 5 (BS EN 1995-1-1) for wind loading in service class 1 and dividing the design value by 1.5. For plasterboard fastened to trussed rafters with plasterboard nails or screws of at least 3.5 mm diameter, the Trussed Rafter Association recommends a value for fadm of 190 N for 12.5mm thick board and 213 N for 15mm thick board.

For unconventional building forms or unconventional types of floor or roof, the structural adequacy of the diaphragm action should be checked, not only for the fasteners but also for shear, bending and deflection. Where necessary, specialist advice should be sought.

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TRADA Technology Timber frame housing: UK Structural recommendations

1.2 Determination of loads acting on each element Loads in general:

The following figures show various different combinations of loading that occur in two-storey terraced housing, and how the forces and loads acting on the floor, wall and roof systems may be calculated. In each example, a load bearing internal wall is used on the ground floor to bring about economy in design. If a more flexible plan is required the joists may often be designed to span the whole ground floor without interior support. Prefabricated timber I-joists are particularly common for longer spans, while timber-flanged joists with open metal webs, glulam, laminated veneered lumber (LVL) and other structural timber composites can all span significantly further than solid timber joists of the same depth.

Dead load:

The dead load is the weight of the construction and should include water tanks plus their contents, and services such as pipes, ducts, heating units and similar appliances that are classed as fixtures. Water tanks and heating units can be positioned over load bearing walls to avoid subjecting beams, trusses and joists to heavy dead loads. Where tanks are supported by roof trusses the additional dead load should be considered in the truss design.

The dead loads should be calculated from the unit weights given in BS 648 Schedule of weights of building materials or from the actual known weights of the materials used. Weights of materials commonly employed in house construction are given in Section 1 .3.

Imposed floor load:

The imposed floor load specified by BS 6399-1 Loading for buildings. Code of practice for dead and imposed loads for a self-contained unit designed for occupation by a single family is a long-term distributed load of 1.5 kNlm2 or a point load of 1.4 kN, whichever is more severe. However, for solid timber floor joists BS 5268-7.1 recommends instead a long-term distributed load of 1.5 kN/m2 for joists with an effective span of 2400 mm or more, or for shorter joists a load of 3.6 kN per metre width of floor (measured perpendicular to the span) uniformly distributed over the entire span. All these loads may also be used in self-contained dwellings within multi-storey buildings.

Imposed ceiling load:

The imposed ceiling load specified by BS 6399-1 for dwellings is a long-term distributed load of 0.25 kN/m2, assuming that the space above the ceiling is used for storage rather than a living room.

Imposed roof load:

The exact value of the imposed roof load depends on the location and size of the building, and BS 6399-3 Loading for buildings. Code of practice for imposed roof loads should be consulted. In general for roofs up to and including 30° pitch where access is limited to maintenance and repair, the imposed load is a medium-term distributed load of 0.75 kN/m2 measured on plan, or a short-term point load of 0.9 kN, whichever is more severe. For roof slopes between 30° and 60° the distributed load may be obtained by linear interpolation between the value for a 30° pitch and nil. For roof slopes of 60° or more the distributed load is nil. The point load may be ignored for slopes of 30° or more.

Wind loads:

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Wind loads are calculated using BS 6399-2 Loading for buildings. Code of practice for wind loads.

TRADA Technology Timber frame housing: UK Structural recommendations

1.2.1 Calculation of vertical load 1.2.1.1 Roof and floor spanning front to rear (load bearing internal wall on ground floor) Stud and joist spacing = S

Roof and ceiling load Wç kN/rn2 total

Shaded areas carry roof or floor loads

,

Load at each stud point

L 2

= WrXSX_ Load in each stud

4\Nr xsxJ+(Ww xsxh) Load at each interior joist support

=(Wf xsx.J+(Ww xsxh) For beam designs see Section 3.1

Floor load W k N/rn2

Load at each stud point

1\NrXSxJ +(wwXsXh)+\NfXsX_J Load in each stud

Stud supporting bearn, designed as for lintel support

1\NrXSxJ +(wWxsxh)+[wfxs4j +(W xsxh) Self weight of wall W kN/m2

Typical loading For weights of materials and construction, see Section 1.3 Wr = 1.97 kN/m2

W = 0.86 kN/m2 for tile hanging (upper storey)

W1 = 1.82 kN/m2

W = 0.35 kN/m2 for board cladding (lower storey) W = 0.25 kN/m2 for plasterboard internal wall

For L 7.8 rn s 0.6 m, A = 4.2 m, B = 3.6 m, h 2.55 m (2.4 m for internal wall) Front wall:

.

7.8

L

Load per stud point at eaves

= WrxSx=i.97xO.6x__

Load at foot of each upper storey

= 4.61 +(w

Load per stud point at 1St floor level (near wall)

=5.93+IWfxSx 2

h)= 4.61 + (0.86 x 0.6x 2.55)

=5.93+11.82 x 0.6 Load at foot of each lower storey stud

Internal wall:

5.93 kN

= 7.9 kN

2

= 7.9 +(W x s x h) = 7.9 +(0.35 x 0.6 x 2.55) =WfXSX

Load at foot of each internal wall

= 4.26 +(W x s x h) = 4.26 +(0.25 x 0.6 x 2.4)

2

= 8.44 kN

=1.82x0.6x72

Load per stud point at 1S floor level stud

= 4.61 kN

= 4.26 kN = 4.62 kN

H

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TRADA Technology Timber frame housing: UK Structural recommendations

1.2.1.2 Roof and floor spanning between cross-walls (discontinuous joists supported by load bearing internal wall) Roof and ceiling load Wr kN/m2 total

Typical loading Inner support spacing = d Stud and joist spacing = s Load at each inner roof support point

Shaded areas carry roof or loads

L

W xdx— 2

Average load in each wall stud (spaced s) Floor load Wf kN/m2

[W1xsx+(Wxsxh) Careful design required unless d = s

Load at each interior joist support = WfXSX (A + B)

2

Load in each stud

l

\Nr xsx +(w xsxh) +Wf xsx (A+B) +(w xsxh)

Stud supporting beam, designed as for lintel support

Self weight of wall W kN/m2

For weights of materials and construction, see Section 1.3. Wr = 1 .97 kN/m2

W = 0.86 kN/m2 for tile hanging (upper storey)

Wr = 1.82 kN/m2

W, = 0.35 kN/m2 for board cladding (lower storey) W = 0.25 kN/m2 for plasterboard internal wall

For D = 8.4 m, s = d = 0.6 m, A = 1.8 m, B = 3.0 m, L = 4.8 m, h = 2.55 m for lower storey, 3.15 m (average) for upper storey, 2.4 m for internal wall. End wall

Load per stud point on spandrel panel

=W xdx—=1.97x0.6x———

= 2.84 kN

Load at foot of each upper storey stud

= 2.84 +(W x S x h)= 2.84 +(0.86 xO.6 x 3.15)

= 4.47 kN

Load per stud point at 1st floor level (near end)

=4.47+IWxSx =4.47+11.82x0.6xi 2

= 5.45 kN

Load at foot of each lower storey stud

= 5.45 +(W x s x h )= 5.45 +(0.35 x 0.6 x 2.55)

= 5.99 kN

Load at foot of each internal wall stud

=Wf XSX (A+B)J (w xsxh)

2

2

2)

= 1.82x0.6x—I+(0.25x0.6x2.4)

2)

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=2.98kN

TRADA Technology Timber frame housing: UK Structural recommendations

1.2.1.3 Other arrangements Other arrangements are also possible. For example the roof may span front to rear as in 1.1.1.1 and the floor span between cross-walls as in 1.1.1.2, or the roof may span between cross-walls as in 1.1.1.2 and the floor span front to

rearasin 1.1.1.1.

1.2.2 Calculation of wind loads Wind pressure is the main horizontal force causing racking and bending deformations. If adequate provision is made to resist the wind loadings, all other racking forces can usually be ignored for two-storey domestic buildings. The design wind pressures will depend on the locality, the degree of exposure, and the overall height and proportions of the building.

1.2.2.1 Wind pressures on the timber frame The dynamic wind pressures and pressure coefficients for the elements of a timber frame building are calculated, as for other buildings, according to BS 6399-2. An example of a calculation for a two-storey house can be found in BRE Digest 436 Part 2. Where masonry walls provide an external shell, the resulting wind loads should be based on the overall dimensions of the masonry walls, not those of the load bearing timber frame. However BS 5268-6.1 allows the wind load transferred to the timber frame to be reduced by a factor K100 where masonry walls conforming to Clause 3.2.2 provide a shielding effect, for the calculation of racking forces and overall building stability only (ie not for stud design). This reduction should not be applied to the spandrels of gable walls, because the masonry walls in this area have no returns to make them self-supporting in a direction perpendicular to their plane.

1.2.2.2 Horizontal wind loads The figure below shows the overall dimensions of a semi-detached timber frame house with masonry cladding built on a foundation 0.3 m above ground level. The racking forces on each pair of walls are shown as R1, R2 etc. 1.3

2.5

1.3

Ri

N

Ro

N

Firstfloor I

R3

N

R4

N

5.0 2.5

0.3 -- _________________ ____________________ — ___________________ : 0.3 Front elevation

—

End elevation

wall

I I

6.8

.

Dimensions in metres

Plan When calculating the racking loads on the wall panels in a particular storey, it is assumed that the wind load on the upper half of the storey is applied as a racking load to the top of the panels, and the wind on the lower half of the storey is applied to the bottom of the panels where it is resisted either by the

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TRADA Technology Timber frame housing: UK Structural recommendations

panels in the storey below or by the foundations. Therefore the total racking load on a timber frame wall is calculated as the racking load transferred from the roof or the storey above it, plus half the wind load applied to the same storey. The next figure shows how the diagonal dimension, a, is calculated for each diaphragm and wall. This dimension is used to determine the corresponding size effect factor, Ca, (BS 6399-2 Clause 2.1.3.4). Since the lowest value is 0.95, it will normally be simpler to assume a value of 1.0 for Ca, and calculate the exact values only if it proves necessary.

Front and rear walls

Gable walls Diaphragms First floor diaphragm

Roof diaphragm 6

—---s----- t

First floor diaphragm

-

--——--- 7.2 m

-72m

68m

a4 42.52 +6.82

a2 =/2.52 + 7.22

a1 41.252 +7.22

= 7.62 Ca = 0.963

= 7.31

Ca = 0.968

= 7.62 Ca = 0.963

= 7.26 Ca = 0.968

Racking First floor panels

Ground floor panels

v

First floor panels

-A y

E

Ground floor panels

/-

,,'

—7

72m

a5 =V1.252 +7.22 = 7.31 Ca = 0.968

72m

a6 3752 +7.22 = 8.12 Ca = 0.957

a7 =2.552 +6.82

a8 5Q52 +6.82

= 7.26 Ca = 0.968

= 8.47 Ca = 0.953

First floor panels

-d

7m a9 = 2.52 + 7.22 = 7.62 Ca = 0.963

6.8 m

+ 7.22 a10 = = 8.77

a11 43.82 +6.82 = 7.79 Ca = 0.960

Ca = 0.950

Wall stud For the wind load on a wall stud, Ca = 1.0.

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= 9.27 Ca = 0.945

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TRADA Technology Timber frame housing: UK Structural recommendations

Some typical values for the dynamic pressures, q, to be used during the normal lifetime of a building, are shown below in Pascals (N/rn2). For buildings with masonry cladding, BS 5268-6.1 Table 1 allows a reduction in the wind loads on the timber frame and allows part of the load on the timber frame to be transferred back into the brickwork through specified wall ties, but the building must also be stable during construction, when there is no cladding. BS 6399-2 Annex D allows a reduction in the normal 50 year wind load for shorter periods of construction. Hence there are two design situations in which the building's strength and stability have to be checked: (a) the construction period when there is neither a masonry shielding effect nor a roof load to resist overturning but the normal design wind load is reduced, and (b) the constructed period when the shielding and load-sharing of any masonry may be taken into account, but the full 50 year return wind load is applicable. (a) is particularly important in the construction of multi-storey buildings when overturning forces can be significant. The wind pressures shown in the diagram have been reduced where applicable by the appropriate value of K100, the reduction factor for the masonry shielding effect given in Table 1 of BS 5268-6. Note that this factor is not applied to the spandrel area of the gable walls, for reasons given previously (see Section 1.2.2.1). The pressure coefficients, Cpe and C, for the example building are also shown, obtained from BS 6399-2, Tables 5, 10 and 16. For a semi-detached house the external wind pressure on the party wall may be taken as zero. The overall dimensions of the two dwellings should be used to calculate the pressure coefficients.

Wind on gable wall —

Wind on rearwall

________________

+027

-1.17

-05

qs = 756 Q.4 qs = 756 = 658 +084 _________________ -0.5 ____________________

qs = 658 x 1.0

qs—0

qs=658x0.5

= 329 +084 -0.3

0.11

3.49

0.11 3.49

-0.3 -0.5 qs=590x

qs=590x 0.8=472 -0.3 -0.5

0.82=484

+082 -0.3

_________________ Gable end

Front

Separating wall

Reference values for BS 6399-2: Table 5: D/H=1.03 Table 10: W=7.2, bw7.2

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Rear

Gable end

TableS: D/H=1.36

Front

Table 10:L=13.6 bL=10.6

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TRADA Technology Timber frame housing: UK Structural recommendations

1.2.2.3 Loads on diaphragms and walls In order to calculate the loads on the elements of timber frame buildings, it is convenient to rewrite BS 6399-2 Equation (7) in Clause 2.1.3.6 as:

(1)

Pfinal

= O.85x x(1+Cr)xCa

where Pfinal

= final load on a diaphragm or the walls in one storey in one direction

0 85

= factor to allow for the fact that maximum gust speeds on front and rear walls are not simultaneous

P

= nett wind load on an element of the building which contributes to the final load, unfactored for dynamic augmentation or size = proportion of the element which contributes to the final I

load

= dynamic augmentation factor (BS 6399-2 Clause 1.6.1), taken as 0.02 in this example = relevant size effect factor (see 1.2.2.2) For detached houses or wind blowing on the front or rear wall (2)

P

= (qs,front,iCpe,front,i — qs,rear,iCpe,rear,i)Ai

where A

= area of the element on which the wind blows.

For semi-detached and terraced houses with wind blowing on the gable wall, the external pressure on the rear wall is zero, so

(3) Pi

= qs.front,iCpe,front,iAi

To check the strength and stiffness of wall studs, the nett pressure on the wall panel depends on the difference between the external and internal pressures, so P

(4) = (qs,tront,iCpe,i — qS,fIOI,ICPI,I)A

(a) Wind on gable end Equation (3)

Gable: P gable

= 658 x 0.84 x 7.2 xl .3 x 0.5

2587 N

= 329 X 0.84 x 7.2 x 2.5

= 4974 N

= (329 x 0.84 _(_)329 X 0.3)x 7.2 x 2.5

= 6751 N

= 0.85 x (2587 X 0.667 + 4974 X 0.5)x 1.02 x 0.968

= 3535 N

= 0.85 x (4974 X 0.5 + 4974 x O.5)x 1.02 x 0.963

= 4153 N

Racking load on first floor panels P racking, 1

= 0.85 (2587 xl + 4974 x O.5)x 1 .02 x 0.968

= 4258 N

Racking load on ground floor panels P racking, 0

= 0.85 x(2587 xl + 4974 xl .5)x 1.02 x 0.957

8337 N

Each storey: P siorey Equation (4)

Walls: P wail Equation (1)

Roof diaphragm load P ceiling First floor diaphragm load P fioor, 1

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TRADA Technology Timber frame housing: UK Structural recommendations

Shear force on first floor panels

Pshear,i

0.85x(2587x1+4974x1)xl.02x0.963

=6313 N

Shear force on ground floor panels

Pshear,0

0.85x(2587x1+4974x2)xl.02x0.950

= 10324 N

Nett load on a wall stud, assuming 0.6 centres

Pstud

= 0.85 x 6751 x 0.6 xl .02 xl .0

=488N

7.2

(b) Wind on front and rear walls Equation (2)

Roof: P roof Each storey: P storey

= 756 x(0.4 xO.68 + 0.27 x2.92 —(—)1 .17 xO.68—

(—)0.5x 2.92) xtan20°x6.8

= 6205 N

= (484 x 0.82 —(—)472 x 0.5)x 6.8 x 2.5

= 10759 N

= (484 x 0.82— (—)484 x 0.3)x 6.8 x 2.5

= 9215 N

= 0.85 x (6205 x 1 + 10759 x 0.5) x 1.02 x 0.968

= 9722 N

Equation (4)

Walls P wall Equation (1) Roof diaphragm load

ceiling

First floor diaphragm load

P floor,!

= 0.85 x (10759 x 0.5 + 10759 x 0.5) x 1.02 x 0.963

= 8983 N

Racking load on first floor panels

P racking, 1

= 0.85 x(6205 xl + 10759 x 0.5)x 1.02 x 0.968

= 9722 N

Racking load on ground floor panels

P racking, 0

= 0.85x(6205x1+10759x1.5)xl.02x0.953

= 18461 N

Shear force on first floor panels

P shear, 1

= 0.85x(6205x1+10759x1)xl.02x0.960

14119 N

Shear force on ground floor panels

P shear, 0

= 0.85x(6205x1+10759x2)xl.Q2xQ.945

= 22714 N

Neff load on a wall stud, assuming 0.6 m centres

P stud

—

0.85x 9215 xO.6x 1.02 xl .0 6.8

= 705 N

The results are shown in diagram below

Pceiling = 9.72 kN —

Pceiling

= 3.17 kN

— Pracking,1

4.26 kN

Pracking,1 = 9.72 kN

—k

Pfloor, 1

= 4.15

Pshear,1

Pfloor, 1

—

6.31 kN

= 8.98 kN Pstud

Pracking,o = 8.34 kN

Pstud

= 0.705 kN

—s

= 0.488

—

Pracking,o = 18.46 kN

—s

—k

Wind on front and rear walls

Wind on gable end

TRADA Technology Ltd 2006

—s

Pshear,0 = 22.71 kN

Pshear,o = 10.32 kN

©

Psyear,i = 14.12 kN

16

TRADA Technology Timber frame housing: UK Structural recommendations

The shear force is used to calculate the required fixing of the wall panels to the floor or sole plate. With a shear force of Va Newtons on one wall, the maximum fastener spacing is given by: Smax

= Lfadm/Va (mm)

where L

= wall length (mm)

fadm

= permissible fastener load (N)

With recommended fixing to the foundation or floor it is not usually necessary to check panels against rotation, but this may be necessary where there are low vertical loads, such as in the upper storey of a building with a flat roof, or where particularly narrow or specially stiff panels are installed. This should be carried out for the end panels in each wall and any narrower panels, using an appropriate proportion of the racking load on the complete wall as a moment force at panel height, and the vertical load on the panel (normally a UDL) as a restraining moment, as shown in the diagram in Section 2.3.3. For each load case the minimum vertical load, normally occurring beneath the part of the roof where there is maximum wind uplift, should be used. The vertical load on the panel should include the weight of the panel itself. For details on designing overturning restraint, see Section 2.3.3 (Overturning effects).

1.2.2.4 Asymmetrical buildings In the example shown above, the wind loads are resisted by two equally stiff racking walls so half the load is applied to each. If the resistance of two racking walls differs greatly then the horizontal diaphragms may usually be assumed to redistribute the load so that the stiffer and therefore stronger wall takes a proportionately higher share of the load. All that is necessary is to determine that the total wind loads do not exceed the combined resistance of the walls. For the same reason the resistance of internal load bearing walls may be added to that of the external walls, regardless of their position in the building. However, for buildings with dimensions outside the limits specified in BS 5268-6.2 Clause 6.5, and for unconventional building forms or unconventional types of floor or roof, the assumption about load redistribution may not hold true. In this case, if the building is asymmetrical then the racking and shear resistance of each wall should be checked separately.

1.2.2.5 Racking resistance in terraced housing In some end terrace houses it is difficult to provide adequate racking resistance to wind blowing directly onto the flank wall. In this case part of the load may be transferred to the adjacent house via steel party wall straps. In order to maintain a low level of sound transmission the Building Regulations specify a maximum density of one row of straps at 1200 mm centres installed at each storey height. However to ensure adequate load distribution the spacing needs to be not greater than 1200 mm, so this spacing should always be used. The thickness of the steel is usually 2 to 3 mm and the width of the straps is typically 25 or 30 mm. The cross-sectional dimensions should not exceed 3 mm x 40 mm. It is recommended that each strap be fastened to the underside of the top plate of the party wall panel, with two nails or screws at each end, maintaining the BS 5268-2 minimum edge distances of 5d. (For the design of party walls, see TRADA's Timber Frame Construction.) The loadcarrying capacity should be calculated as the minimum of the permissible load on two fasteners and the buckling strength of the steel.

The following points should be noted. Continuous tiling battens can generally transfer all the horizontal load from the roof of an end terrace house to the roof of the adjoining house. Where the remaining racking load at ceiling level, or racking loads at intermediate floors, are partly transferred via party wall straps to the adjoining house, there will be a corresponding reduction in the racking loads transferred to lower floors in the end terrace house itself. This permits large openings in the front and rear racking walls on the ground floor of an end terrace house.

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(R

TRADA Technology Timber frame housing: UK Structural recommendations

Example Consider a Grade 43 (S275) galvanised steel strap 250 mm x 25 mm x 3 mm thick, fastened with a total of four 3.75 mm square twist nails, 50 mm long in C16 framing members. The free length of the strap between the inner nails, L, is 140 mm. From BS 5268-2: 2002 Table 61 the basic single shear load F is: F

= 377+O.35x(453—377) 0.4

=443N

(As the headside member is steel the specified headside penetration of 46 mm may be ignored.) K44 for square twist nails

= 1.2

K46 for steel to timber joints

= 1.25

K48 for short- and very short-term loads

= 1.25

Permissible load on 2 nails = 2 x F x K44 x K46 x K48

= 1661 N

From BS 5268-2: 2002 Table 21 the effective length of the strap Le = 0.7L.

= Le = Le

Slenderness ratio

I

b

=

0.7x140x3.464 3

113 mm

From BS 449-2: 1969 Table 17a the allowable compression stress: 68 N/mm2.

Pc

Hence permissible compression load on steel strap:

= 68x3x25

PCXA

5100> 1661, hence permissible load on one strap

5100 N. = 1661 N

With straps at 1200 mm centres at eaves level on a 7.2 m wide house the maximum racking load which can be transmitted at eaves level: = 1661x 7200

Fadm

9.97 kN

(1 200 1000) In the example above, the racking load at ceiling level on the end house, excluding the component from the roof = 0.85 x 4974 x 0.5 x 1 .02 x 0.968/1000 = 2.09 kN, so all of this could be transferred to the adjacent property if necessary. The remaining racking load on the ground floor panels 4.15 kN, which again could be transferred at first floor level to the next house, leaving no requirement for racking resistance in the ground floor walls of the end house! Obviously the racking loads have to reach the foundation eventually, so with n houses in a terrace no more than (n-2)/n of the racking load at any one level should be transferred to the adjacent property, starting from the roof. The designer should specify precisely the required materials, dimensions and positions of the party wall straps and fasteners.

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TRADA Technology Timber frame housing: UK Structural recommendations

1.2.3 Calculation of roof loads The connections between the roof trusses and the head binders should be adequate to resist the nett horizontal wind forces on each truss after allowing for frictional resistance, and the nett uplift forces on them after allowing for the weight of the roof. The uplift forces are also required for stability calculations to determine the overturning moment. The method for calculating these forces is not unique to timber frame buildings, but is illustrated in the example of calculations for a complete dwelling in Chapter 7. As mentioned in Section 1.1.2 the use of truss clips on every truss is recommended to ensure adequate diaphragm action in the plane of the ceiling. Therefore, the truss clips could be called on, if required, to provide any additional resistance to roof uplift or horizontal wind loads. Manufacturers of proprietary truss clips should provide safe working loads or characteristic values for vertical loading and for horizontal loading in the plane of the truss and the wall. It is not safe to use the load-carrying capacity of the nails, because TRADA tests on various makes of truss clip demonstrated that failure usually occurs in the clip itself, at a lower load than that predicted by calculating the permissible load on the nails. However, if manufacturers' data are not available it may be assumed, based on the tests mentioned above, that a truss clip with at least three 3.25 mm round wire nails in each member can resist a vertical or horizontal wind load of 1.1 kN. (Some kinds of clip can resist significantly more than this in certain directions.) Where, as is normally the case, the clip resists components of load in two directions, it should be verified that 2

Fva 2+1 Fha Fvadm

where

Fhadm

Fv,a and Fh,a

= the vertical and horizontal forces applied to each connection respectively

and

Fv,acjm and Fh,adm = the corresponding permissible vertical and

horizontal loads. When a value of 1.1 kN is used for Fv,adm and Fhadm the formula above

simplifies to Fva2 + Fha2

with Fva and Fh,3 in kN.

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1.2

TRADA Technology Timber frame housing: UK Structural recommendations

1.2.4 Checking strength and stability Care must be taken to check every relevant loading situation and potential area of instability when designing a timber frame house. During the construction period no allowance should be made for the shielding or load-sharing effect of any masonry walls which may be added later, nor for the weight of the roof in resisting overturning and sliding. Internal walls may not be present initially, and unless the external wall panels are prefabricated with internal linings there may be a period when their plasterboard linings are not attached. Party walls consisting only of two layers of plasterboard and timber framing need particular consideration, but in general their calculated racking resistance may be assumed during the construction period provided that they are adequately braced on erection in accordance with BS 5268-6.1 Clause 4.7.5. To offset the effects of these conditions, BS 6399-2 permits a reduction in the normal design value of the wind load for periods of less than 50 years. For a 1 year maximum construction period, the probability factor described in BS 6399-2 Annex D is 0.749, which means that the dynamic wind pressure for the construction period may be reduced by a factor of 0.7492 or 0.561.

Where wall panels are built in situ there may be a period during which neither the structural sheathing nor the plasterboard linings are attached to the studs, but the studs may nevertheless have to support temporary loads such as packs of OSB or plasterboard. Under these circumstances the studs will not be braced against buckling in their weaker plane. The engineer is therefore advised either to check their load-carrying capacity in these circumstances, or to specify a construction procedure which will ensure that neither heavy construction loads, nor roof weights are applied before the structural sheathing is fully attached, or to specify that the sheathing be attached to wall panels before their erection. During the service life of the building the full value of the design wind loads must of course be used. However, if masonry walls are present, BS 5268-6 allows a reduction in the wind loads on the timber frame racking walls as a result of the shielding effect of the masonry and, provided that the masonry and timber frame walls are tied together in a specified manner, a further contribution from the masonry towards the racking resistance.

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TRADA Technology Timber frame housing: UK Structural recommendations

I 3 Weights of materials The weights of materials can be obtained from sample tests, information from manufacturers or by reference to BS EN 1991-1-1 or BS 648 Schedule of weights of building materials.

Weights of materials commonly employed in house construction are given below as a convenient reference. These may be taken as a general guide, though the density of individual products can vary considerably. For detailed calculations the manufacturer's figures should be used.

Aluminium

kg/rn2 1.5—3.4

flat: 0.6 mm to 1.2 mm corrugated 0.6 mm to 1.2 mm (including 20% for added laps)

2.0—4.4

Battens, tiling

38mmxl9mm:

Bituminous roofing felt

3 layer felt bonded together, 13 mm granite chippings

Blockwork, concrete

Typical values. More accurate figures may be obtained from manufacturers' literature.

Brickwork

3.4

loOmmgauge

34.0—37.0

Hollow, stone aggregate:

100 mm thick

139.0

Solid stone aggregate:

100mm thick

219.0

Aerated:

100 mm thick

82.0

Clay, medium density:

103 mm thick

221.8

Concrete

103 mm thick

237.6

Sand lime:

103 mm thick

206.0

6 mm thick

Calcium silicate and cement

5.5—7.5

fibreboards

Concrete

100 mm thick

Reinforced:

0.6 mm as laid

6.5

Board, semi-compressed:

25 mm thick

4.9

Flooring:

25 mm thick

9.8

Bitumen impregnated insulating board:

13mm thick

3.1 —4.5

Hardboard:

6.3 mm thick

4.2

Insulating board:

12.7mm thick

3.4

Copper roofing

Cork

Fibre building board

240.0

5.0—7.2

Medium board:

9.0 mm thick

Glass

Plate:

6.4 mm cast clear and armoured

Gypsum panels and partitions

Building panels:

75mm to 150mm

43.9—63.5

Dry partitions:

57 mm to 63 mm

20.5—25.9

Glass fibre acoustic insulation for floating floors:

25 mm thick

Glass fibre thermal insulation:

100mm

2.3

Mineral fibre

100mm

1.2—2.4

Insulation

Internal stud partitions

thermal insulation:

2.0

29.6

12 .5mm plasterboard 2 sides

20.4 — 33.6

Proprietary

Lead

16.1

1.7 mm to 3.0 mm

Sheet:

19.5—34.2

OSB (oriented strand board)

9mm to 18mm

Pitch mastic

Flooring:

25 mm thick

Plasterboard

Gypsum:

9.5 mm

6.1

Gypsum:

12.5mm

8.0

Gypsum-

15mm

9.8

Gypsum

19mm

Flexible pvc:

1.6 mm to 3.2 mm

2.4—4.9

Pvc vinyl:

1.6 mm to 4.8 mm

3.4— 10.3

American construction and industrial plywood:

12.5mm to 19mm

7.3—10.9

Canadian Douglas fir and softwood plywood:

12.5 mm to 18.5 mm

7.2— 10.6

Plastic flooring

Plywood

© TRADA Technology Ltd 2006

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5.7— 12.8

14.9—18.1

15.0

TRADA Technology Timber frame housing: UK Structural recommendations

Plywood (continued)

Finnish birch:

12mm to 18mm

kg/rn2 8.4— 12.4

Finnish birch faced:

12mm to 18mm

8.1 —11.6

Swedish softwood:

12.5mm to 19mm

Tropical hardwood:

12 mm to 19 mm

PSL (Parallel strand lumber)

5.3—8.0 7.3—11.6 730 - 750

12.5mm

Rendering

Portland cement: sand (1:3):

Roofing felt

3 layers felt and chippings

37.0

20 mm asphalt

46.0 3.2 mm to 9.5 mm

Rubber flooring Sand

Dry per cubic metre

Shingles

Cedar

Slating

Welsh:

5.4— 16.1 1522—1682

7.3

Westmorland:

Cornish:

Tiling, roof

29.3

thin

24.4

thick

48.8

thin

48.8

thick

78.1

thin

29.3

thick

48.8

Typical values. More accurate figures may be obtained from manufacturer's literature Clay,

machine made:

100 gauge

63.5

hand made:

100 gauge

70.8

100 gauge

68.4

Concrete, plain:

41.5—56.1

Interlocking, single lap:

Tiling wall

Concrete, plain

68.0

19mm

Weatherboarding Flooring grades, P4 and PS

Wood chipboard

Hardwood: beech, oak

Wood flooring

Softwood: pitch pine

Softwood: redwood

Wood wool

Slabs:

Zinc

Sheet 12 to 16 zinc gauge (0.63— 1.04 mm)

7.3

15mm thick

10.2—11.2

18mm thick

12.8—14.0

22 mm thick

14.3— 15.7

16mm thick

11.7

19mm thick

13.9

21 mm thick

15.3

28 mm thick

20.3

16mm thick

11.4

19mm thick

13.5

21 mm thick

14.9

28 mm thick

19.8

16mm thick

8.3

19mm thick

9.8

21 mm thick

10.8

28 mm thick

14.4

25 mm thick

14.6

4.4—7.8

To convert kg/rn2 into N/rn2 multiply by 9.81 LSL (laminated strand lumber)*

kg/rn3 610—670

LVL (laminated veneer lumber)*

530— 710

PSL (Parallel strand lumber)*

730 — 750

* The density of structural timber composites depends on the moisture content, species or grade. For more accurate values see the manufacturers' literature.

To convert kg/rn3 into N/mrn multiply by 9.8lbh x io- where the breadth and depth, band h, are in mm.

© TRADA Technology Ltd 2006

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TRADA Technology Timber frame housing: UK Structural recommendations

1.4 Weights of typical constructions Imposed loads from BS 6399-1: 1996

1.4.1 Roofs To obtain load per square metre on plan of a pitched roof surface, divide the weight of roof surface measured on slope by the cosine of the pitch, ie

k

Plan load w kg/rn2 =

cosO

where k is the weight of roofing (tiles and battens) in kg/m2 measured on slope and B is the pitch angle in degrees.

Example (1) Pitched roof

kg/rn2

kN/m2

Medium weight tile and batten (on plan)

59.5

0.57

Allow for self weight of structure

15.0

0.15

Imposed load on roof structure

0.75

Total design load for roof surface:

1.47 25.5

Ceiling dead

0.25 0.25

Ceiling imposed

200 mm mineral wool quilt

2.4

0.02

Total ceiling design load

0.50

Total design load for roof structure

1.99

Example (2) Flat roof (cold deck type) Bituminous roofing felt with chippings 9 mm OSB

35.5 5.8

12.5 mm plasterboard

10.0

Self-weight of joists

11.5

200 mm mineral wool quilt

2.4

65.2

0.64

Imposed

0.75

Total design load for roof

1.39

1.4.2 Floors Example (3) Floor

kg/rn2

22 mm tongued and grooved chipboard

14.3

12.5 mm plasterboard

10.0

140 mm phenolic foamboard Self-weight of joists

4.2

11.0 39.5

©

TRADA Technology Ltd 2006

kN/m2

0.39

Imposed

1.50

Total design load for floor

1.89

23

ci:13,

TRADA Technology Timber frame housing: UK Structural recommendations

1.4.3 Walls Example (4) WaIl with tile hanging

kg

Clay tiles and battens

64.5

9 mm OSB sheathing

5.8

140 mm mineral wool batt

3.5

Self-weight of framework

7.0

12.5 mm plasterboard

kN

10.0 89.8

0.88

Example (5) WaIl with board cladding 25 mm boards and battens

12.5

9 mm OSB sheathing

5.8

150 mm glass fibre batt insulation

5.0

Self-weight of framework

7.0

12.5 mm plasterboard

10.0

40.3

0.40

Example (6) Interior wall 12.5 mm plasterboard both sides

Self-weight of framework

20.0

5.0 25.0

0.25

1.5 General requirements for prefabricated wall, floor and roof elements BS EN 14372 Timber structures — Prefabricated walls, floor and roof elements, will, when published, specify the product, performance, production and testing requirements for timber frame walls, floor and ceiling elements where these are prefabricated from 'joists" which are fixed to a panel product on one or both sides made of or from timber or gypsum plasterboard. The fixing may be by mechanical fasteners or glue. For glued fixings detailed requirements are given for the permitted types of adhesive, the methods of application and the quality control systems which must be in place.

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TRADA Technology Timber frame housing: UK Structural recommendations

2 WaIls 2.1 WaIl studs All load bearing studs and rails must be of strength graded timber. Most commonly studs are softwood of Strength Class C16 or C24 in accordance with BS EN 338 Structural timber. Strength classes.

2.1.1 Minimum desirable stud sizes Stud widths should allow for the satisfactory butt jointing of sheathing and plasterboard. BS 5268-6.1 recommends a minimum cross-section of 38 mm x 72 mm for external walls, 38 mm x 63 mm for internal walls. However, when calculations produce small sizes of stud, the resulting walls may appear too flexible to the occupants of the house, although the studs are safe against axial and wind loading. The depth of wall studs is often governed by the thickness of insulation required within the wall panels to meet the requirements of Building Regulations. For example a stud 89 mm deep with 90 mm of insulation material will only meet requirement Li of the 2004 England and Wales Regulations with a reflective breather membrane or the installation of a high efficiency boiler. A 140 mm deep stud with 140 mm of insulation will meet the same requirement without any special measures. Some companies use prefabricated timber I-joists as wall studs, both to obtain the required thickness of insulation and because their thin webs have much lower thermal conductivity than studs made of solid timber.

2.1.2 Multiple studs In domestic-scale buildings studs are normally spaced at 400 mm or 600 mm centres. Most wall panels are factory-produced but for on-site framing studs spaced at 600 mm are preferred to allow workmen room to walk between them. With high axial loads it may be necessary to specify double, triple or even more studs. In order to prevent buckling in their weaker direction, multiple studs should be fastened together in such a way that a total lateral force of 2% of the axial load on each stud can be applied uniformly to each stud. This may be achieved either by nailing through the sheathing material from both sides into the secondary studs with sufficient nails to provide the required force in lateral loading, or by nailing the multiple studs together with sufficient nails to provide the required force in axial (withdrawal) loading. In the latter case ringed shank nails or screws should be specified. The fasteners should be spaced at 300 mm or less. If three studs are fastened together it is preferable to nail or screw the outer sets from the outside of the panel, since the axial fastener load will be greatest in the outer studs. With the intermediate studs the central one in each set should be fastened to the panel.

When checking the combined bending and compression stresses in multiple studs or their deflection, Emin may be increased by the appropriate factor given in BS 5268-2 Table 20 (see Clause 2.11.5). To ensure vertical load sharing, multiple studs should be cut square with a tolerance on their length of +1- 0.5 mm.

Since the vertical load-carrying capacity of studs is often limited by the bearing strength of the bottom rail, it may be possible to reduce the number of studs required by using a higher grade of timber in the top and bottom rails, or deeper studs.

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TRADA Technology Timber frame housing: UK Structural recommendations

2.1.3 Lateral support According to BS 5268-6.1, solid timber studs covered on one or both sides with a specified board material and fixed as recommended in the standard may be assumed to be fully restrained against buckling in their weaker direction. However there is evidence from North America that for studs braced on one side only by a sheet material, as in a separating wall, the load-carrying capacity is reduced by about a quarter. Caution is therefore advised particularly when it comes to buildings of more than the four storeys covered by BS 5268-6.1. Where necessary nogging pieces may be used to restore full lateral support, provided that they are combined with some form of diagonal bracing to prevent all the studs buckling simultaneously in the same direction. United States recommendations indicate that one row of noggings at midheight is adequate for nominal 50 x 75 mm and 50 x 100 mm studs, and two equally-spaced rows for 50 x 150 mm studs. The sheet material should be nailed to the noggings as well as the studs.

2.1.4 Load capacities Tables 2.1 and 2.2 relate to timber of strength class C16 and C 24 respectively in service class 1. They give the permissible axial load in kN per stud (not per metre) for studs in panels 2400 mm high. It is assumed that there is adequate bracing against buckling in the plane of the wall, as described above. The end loading is assumed to be nominally axial. Where there is significant eccentricity, reduced figures should be calculated.

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TRADA Technology Timber frame housing: UK Structural recommendations

Table 21 Permissible axial load per stud in kN — Timber strength class C16 (a) With wind loading: very short term load case Spacing Breadth mm mm 400

Depth mm

Wind pressure in Pa (N/rn2) 0

100

200

300

400

500

600

700

800

900

1000

38

72

11.03

10.03

9.08

8.08

382

1 69

0.00

000

000

0.00

38

89

1

17.60

16.41

1529

5.95 1421

13.18

12.20

11.25

10.29

8.16

38

97

19.24

18.08

16.97

15.91

1488

1388

6.03 11.99

38

114

22.54

1292 21.45

—

--

600

?

120

38

140

38 44

145 72

44_

97

44

120

44

145

47

72

47

97

47

120

A

12.77

11.76

13.64

47

145

38

72

3_

89

12.63

97

38

114

38

- 140

11.67

9.90

10.75

8.23

938

610

397

1.84

0.00

000

20.52

19.42

18.36

17.33

16 33

7.25

5 11

2.98

0.85

0.00

21.19

20.12

19.07

18.06

000

0.00

1.76

0.00

0.00 0.00

38

145

72

44

120

4._

12.77

4.88 13.69

1.69

000

000

1220

10.79

8.16

0.00 4.96

19.24

17.52

15.91

1438

12.92

11.53

9.82

6.62

3.43

21.45

1988

18.36

16.90

15.49

23.12

21.56

2005

18.59

0.00

0.00

0.00

0.00

0.00

16.33

14.89

13.50

12.16

9.01

25.37

23.85

0.00

9.90

7.17

3 97 19.42

13.64

12.15

10.75

8.31

5.11

1.92

0.00

000

0.00

0.00

21.19

19.59

18.06

16.59

15.17

1380

11.81 26.51

1p 145

Values for a wind pressure of zero are given to permit interpolation between 0 and 1 00 Pa. iShaded values are governed by bearing stress in the bottom rail Values in italics are governed by a deflection limit of 0.003L. Other values are governed by buckling in the stud.

(b) Without wind loading: medium — and long term load cases Spacing mm

Breadth

400/600

38 38 38

72

38

114

38 38 38 44

120

44

97

44

120

44

145

©

0.78 17.84

145

97

47_ 47 NB

11.28

97__ --

72

—

0.00 1707

8.08 15.29

120

44 44

0.00 15.36

I

9.55 17.00

11.03

38

10.81

20.40 23.66

mm

Depth mm

89 97

140 145 72

47

72

47

97

47

120

47

145

Load duration

Iw I.

i eaiurn

I/ Iw 'iWa JLong

tk I

TRADA Technology Ltd 2006

27

TRADA Technology Timber frame housing: UK Structural recommendations

Table 22 Permissible axial load per stud in kN — Timber strength class C24 (a) With wind loading: very short-term load case Wind pressure in Pa (N/rn2)

mm

Depth mm

0

100

200

38

72

13.43

12.55

11.71

38

89

19.28

38 38

97 114

21.01

38

120

38

140

38

145

3O32 31.40

.40

44

72

15.55

14.67

44

97

22.91

44

120

44

145

28.34 34.

.25

47 47

72

16.61

15.73

14.88

14.06

97

23.72

47 47

120

29

145

35.46

38 38

72 89

13.43

12.13

10.91

19.28

8.17 18.19

38 38

97

21.01

.01

114

24

38

120

38 38

140

44 44

72

25.99 30.32 31.40 15.55 22.91

44 44 47

120

34

145

34.25

72

16.61

47

97

47

120

23.72 29.35 35.46

Spacing mm

Breadth

400

600

—

145

97

47

1300

10.91

500

600

700

800

900

1000

9.24

7.11

4.98

0.71

0.00

0.00

18.66

17.73

16.83

2.85 15.96

15.11

14.29

12.24

20.46

19.54

18.66

17.79

31.40 3.52

1.39

24 .32

40 13.00

13.82

12.04

9.91

.40

7.78

1.40

5.65

.25

.

34.25 4.92

.40 22.87

0.00 21.97

.25 2.79

0.66

13.26

11.31

9.18

7.05

4.98 16.83

1.78

0.00

0.00

15.53

14.29

0.00 11.17

7.98

0.00 4.78

20.00

18.66

17.36

16.12

14.66

.46

35.

31.

.46

0.00 1.58 11.47

23.60

1.40 14.24

.25 15.30

.

.32 31.40

31.40

13.00

10.98

7.78

34.

34

14.06

12.38

9.18

1.40 4.58

5.99

.40 1.39

0.00

22.87

21.53

0.00 20.24

2.79

0.00 23.63

0.00 22.32

.40 0.00

0.00

18.99

17.77

25

.25 0.00 19.81

145 .46 . .46 Values for a wind pressure of zero are given to permit interpolation between 0 and 1 00 Pa. I Shaded values are governed by bearing stress in the bottom rail Values in italics are governed by a deflection limit of 0.003L. Other values are governed by buckling in the stud.

NB

(b) Without wind loading: medium — and long term load cases Spacing

Breadth

mrn

rnm

Depth rnm

400/600

38 38

72 89

1114, 13.77

38

97

15.01

38 38 38 38

114

17.64

120 145

i4 21.66 . 22.

44 44

72

12.15

97

16.36

44 44 47

120

20.24

145

2

72

12.58

47 47 47

©

400

140

- 97

I.

Iivieaiurn

Long

,

,.

.

, r4c -vt 9,

,. 7.

rr

.. —

72

.

. t..

09

.

c

19

.

. ,

1

25.33

TRADA Technology Ltd 2006

I

.1891

,

11

,

120 145

I

Load duration

.

, ,. ,, '. , ,

?7 .27

28

TRADA Technology Timber frame housing: UK Structural recommendations

The tables show the permissible axial load per stud, checking combined bending and compression, bearing stress in the bottom rail assuming no wane, and deflection. It is assumed that the strength class of the top and bottom rails is the same as that of the studs. The bearing stress in the bottom rail is calculated using the BS 5268-2 bearing factor K.4, as this is applicable to the studs which are not at the ends of each panel, and these studs normally take more load. For bearing stress the load-sharing factor K8 has not been used because there is only one member involved. In the formulae for combined bending and compression and for deflection the effective length, Le, was calculated as 0.85L, as recommended by BS 5268-6.1. For the actual stud length, L, a value of 2324 mm = 2400 — (2 x 38) was used. The formula used to calculate deflection was:

Z = 0.005 Aca + öma X (öemean — öca)

where

A

=

Le 0.85L1if _______ I

h 2

a e,mean

andZ

mean A2

bh2

6 This gives the approximate deflection of a stud subjected to both compressive and bending forces. BS 5268-6.1 states that when calculating defiections the effects of axial loading should be ignored. However for some small studs with high wind loads ignoring the axial deflection can result in excessive deflections, hence axial load has been taken into account in these tables. A maximum deflection of 0.003L has been allowed. Where no value is given in the tables, this deflection is exceeded even without axial load. In particular situations a designer may choose to vary the deflection limit from 0.003L.

2.1.5 Design procedure The following load cases should be checked using Table 2.1 or 2.2: 1.

Dead + imposed + wind:

very short-term

2.

Dead + imposed:

medium term

3.

Dead + imposed floor and ceiling loads:

long-term

The very short-term axial loads are given in conjunction with a series of nett design wind pressures on the walls. The method of obtaining these is given in Section 1.2.2.3. In determining the design wind pressure, the worst combination of external and internal pressure and suction should be used. When calculating the vertical load on a stud, half the weight of the wall panel supported by the stud should be included. BS 6399-1 Table 2 permits a reduction of 10(n-1) % in the axial load carried by a stud due to imposed floor loads where there are n storeys above it for a maximum value of n = 5. This reduction is particularly important in the lower floors of multi-storey buildings. For the very short-term load case the vertical component of the wind load should be included in the axial load on the wall stud. As shown in grey in the tables, the load per stud is governed by the permissible stress in the bottom rail, except in the case of smaller studs under very short-term loading. Therefore in many cases the vertical load-carrying capacity of a timber frame wall panel can be increased by using higher strength timber or a structural timber composite for the top and bottom rails. Calculations for a stud wall supporting a vertical load and horizontal wind load are given in the example of calculations for a complete dwelling in Section 7.5.

©

TRADA Technology Ltd 2006

29

TRADA Technology Timber frame housing: UK Structural recommendations

2.1.6 Drilling of studs Unless otherwise justified by calculation, drilling of studs should conform to the following requirements:

Stud

Location

Load bearing

Non-load bearing partition

Maximum size

Centre line of section. Between 150mm from one end and 0.25 of the length measured from the same end. Holes to be spaced (centre to centre) at a minimum of 4 hole diameters. Centre line of section. Between 150 mm from one end and 0.40 of the length measured from the same end. Holes to be spaced (centre to centre) at a minimum of 4 hole diameters.

0.25 x depth of stud.

0.25 x depth of stud.

Notching anywhere in the stud is prohibited.

2.2 Framing around openings Where openings occur, they should be spanned by suitably designed lintels as indicated below and the load on the lintels should be transmitted to the foundations by sufficient stud material at all the levels concerned. This stud material is introduced in the form of cripple studs of the same cross-section and at least the same number as those removed from the opening, as shown in the diagram

II II

II II

Stud material from opening shared equally in framing each side

H Lintel: solid timber, structural timber composite or fitch beam.

ii

II

Ii

Lintel below first floor designed to support load from double stud to side of window above, including weight of construction from other studs.

H

© TRADA Technology Ltd 2006

30

H

[

TRADA Technology Timber frame housing: UK Structural recommendations

2.2.1 Lintel capacities Tables 2.3 to 2.10 which follow give the permissible load P (kN) per stud point (NOT per metre run) on solid timber lintels of nominal spans 1200mm, 1800 mm and 2400 mm, for studs spaced at 400 mm and 600 mm centres. For larger openings, or where the loading is too great for the lintels considered below, stronger types of beam may have to be built into the wall above the opening.

The load tables are for strength classes 016, 024, D40 and D50. Where the required span cannot be achieved in a softwood, it is now more common to specify LVL than a hardwood. However there are various makes of LVL which have different mechanical properties, so to obtain these properties the designer must refer to the appropriate manufacturer's certification literature (usually a BBA or BM TRADA Q-mark certificate) and calculate the required size accordingly. Where roof loads from similarly loaded trusses at no more than 600 mm centres are applied, it is generally adequate to convert them to a udl provided that the lintel is fastened to the top rail of the wall panel and the top rail of the wall panel is fastened to the wall plate with a minimum of two 2.85 mm diameter nails at 600 mm centres. In this situation the values in the tables, which assume that the loads act as point loads, may be conservative by up to 10%. However, where larger point loads from cripple studs on the floor above are supported by a lintel it is unsafe to regard the loads on them as udls. It should be noted that the load P, for a stud or floor joist bearing on a lintel, could be limited by the compressive stress perpendicular to the grain of the lintel (bearing stress). The total load on the lintel must be safely transferred to the foundation by means of cripple studs. For cripple studs, extra stud material may be provided by the principle illustrated in the previous diagram, and normally this will be adequate for bearing. A calculation for a cripple stud is given in the example of calculations for a complete dwelling in Section 7.8. The following arrangements of load points on lintels have been allowed for in the load tables. Bending, shear and deflection, have been calculated for the worst value of 'a' within the ranges shown for each case. A maximum initial deflection of 0.003L under dead and imposed load has been allowed. Separate tables are provided for medium term load, ie loads from roofs, and for long term load, ie loads from floors.

©

TRADA Technology Ltd 2006

31

TRADA Technology Timber frame housing: UK Structural recommendations

Loads at 600 mm centres

600

k

I

600 1200

1800

600 2400

1200

a 1 mm to 599 mm

Loads at 400 mm centres

_

A400

T

A

1200

00 4

A 1800

1800

4

a 1 mm to 399 mm

© TRADA Technology Ltd 2006

_rrrrr

T TA400

400

T

32

2400

2400

TRADA Technology Timber frame housing: UK Structural recommendations

Table 2.3 Permissible loads on lintels: Timber strength class C16 — Long-term loading Point loads at 600 mm spacing Span

No of loads

1200

1

2

Span

No of loads

1800

2

3

Span

Noof loads

2400

3

Depth Breadth 72 97 120 72

9!

'1 21

202

2.26 3.04 3.7

1 64

120

145

201

120

1.50

2.7 3.35

Depth Breadth 72 97 120 72 97 120

72

97

120

45

0.33 0.45 0.55 0.29 0.39 0.48 72

0.79

1.47

2.41

1.07 1.32

1 98

0.13 0.18

72 97 120

0.21

10

195

220

245

3.26 4. 5 2

4.23 5.69

5.31 7.1

6.51

245

0.69 0.93

1 27

1.93

1 71

2.60

3.51

1.14 97

2 12 120

3.22

4.35

141)

70

0.33

0.61

0.44 0.54 0.30

0,82

8.77 .84

7.

105

220

245

(

1.4 'I 7j,,, 098i

0.50

0.57 0.76 0.94

72

97

0.72 0.97

189

120

1.21

28

72 97

0.63 0.84

120

1.04

26

Depth Breadth 72 97 120 72 97 120

72

97

0.20 0.27

0.48

0.

0.64

1.1 9,

1.91

0.33 0.20 0.27 0.33 72

0.80 0.48 0.64 0.80 97

lÀ 8

2.36

0. 38

1.38 1.86

0.41

13.01

171)

3.25 4.02

1.01

245

17.53 .69

1

1.21

0.22 0.13 0.17

21)

195

8

0.90

Depth Breadth 72 97

170

3

97

120

4

72

15 2.1

32j

I 'f

2.59J

120

145

170

195

220

245

'1

488

170

195

220

245

Point loads at 400 mm_spacing Span

Noof loads

1200

1

2

Span

Noof loads

1800

2

3

No of

Span

loads

2400

Depth Breadth 72 97

Depth Breadth

3

0.21

4

0.28 0.35 0.20 0.28 0.34

0.15

______

©

deflection governs ______

TRADA Technology Ltd 2006

2

8

4

120

1.

0.39 0.53 0.66 0.38

145

811'ft91a48 a1J35 4.7 ')

1.4

2.30

120

145

2.58 3.19

3.34 4.13

4.20 5.19

1.86 2.51 3.11 175

2.26 3.04 3.77

2.55 3.43 4.25

5.91 2.84 3.83 4.73

1I5

220

24

1.08 1.46 1.80 1.05

0.51

0.68 0.92 1.14 0.66 0.89

0,64

1.10

1.75

1.41

= bending governs

1.41

1.90 2.35 1.39 1.87 2.32

1.77 2.38

2.17 2.92

2 95 1.75

3.61

2.35

2.14 2.88

2.91

3.56

— sh-?r .)c.virft

-

TRADA Technology Timber frame housing: UK Structural recommendations

Table 2.4 Permissible loads on lintels: Timber strength class C16 — Medium-term loading Point loads at 600 mm spacing No of 72 Span Depth loads 1200

1

2

Span

loads 1800

2

3

Span

No of loads

2400

72 97

1.21

3

120

2.02 0.90

145

170

220

195

245 13.01

1

.

17.53 1

1.21

1.50

Depth Breadth

72

97

120

145

170

72 97 120 72 97 120

0.33

0,79 1.07

1 47 1 98

1.32

245

2.41 3 4

26

0.45 0.55 0.29

1.27

0.39

0.69 0.93

0.48

1.14

3

Depth Breadth

72

97

2.12 120

72

0.33 0.44 0.54 0.30

0.61

1 05

1

0.82

1.42

2

120 72 97

0.13 0.18 0.22 0.13 0.17

1.01

1.76

2.

3.52

0.98

1.55

2.

1 32

120

0.21

0.50

0.57 0.76 0.94

2.09 2 59

2.7 3.42

97 4

120

1.64

72 97 120

Noof

97

Breadth

195

4

220

245

5.31

6.51

.1

8.77 10.84

1.71

0.41

70

145

1 64

195

220

2.11

3 4.42 57 3.47

246

3.25 4.38 5.42

4.

Point loads at 400 mm spacing Span

No of loads

1200

1

Depth Breadth 72 120

72 97 120

Span

Noof loads

1800

2

3

Span

No. of loads

2400

3

Depth Breadth 72

F

©

145!

170!

195!

220!

245!

97

120

145

17

195

220

245

0.20

1

1

0.48

0.89

97 ______

0.27

______

0.64

1.19

120

0.33

0.80

148

72

0.20

0.48

088

1.

2

1

1.

97

0.27

0.64

1.19

0.33

0.80

1.47

Depth Breadth

72

97

120

145

17C

195

72

0.09

0.21

08

1.41

0.12 0.15 0.08

0.28 0.35 0.20 0.28 0.34

0.39 0.53 0.66 0.38

0.68

97

16

1.90

1.14

80 1.05

2.35

72 97 120

0,11

0.14

= deflection governs

TRADA Technology Ltd 2006

j

I

3.1

.91

120

120

4

120!

0.72 0.97 121 0.63 0.84 1.04 72

97 2

97!

72!

2.

0.51

0.66 0.89

0,64

1.10

= bending governs

34

.11

-

1.41

[1

175

220

2.38 2.95

245

2.17 2.92 3.611

1.39 1.87

1.75

2.141

2.35

2.32

2.91

2.8 3.561

:hear qoverrv,

TRADA Technology Timber frame housing: UK Structural recommendations

Table 2.5 Permissible loads on lintels: Timber strength grading class C24 — Long-term loading Point loads at 600 mm spacing No of Span Depth 72! loads 1200

72 97

1

120

2

72 97 120

No of

Span

loads

2

1800

3

No. of load

Span 2400

3

971

120!

4.78 6.44

1.51

3.20

2.03

4.31

2.51 1.12 1 50 1 86

5

7

120

72 _____ 0.41 _____ 0.99 97 _____ 0.55 _____ 1.33 120 0.68 1.64 72 ______ 0,36 ______ 0.85 97 ______ 0.48 ______ 1.15 120 0.59 1.42 72 97 Depth Breadth

1.83 2.46 _____ 3.04 1.58

2

1800

3

No. of

Span

loads

2400

3

©

245

13

4.61

422

6

522

7

5.

145

95

175 I

220 I

97

120

'.44

4.03 4.98

245 I

0.63

2.11

3.

2,84

4

120

1.50

3.51

72 97

0.78

5 06 6.26

4.60 6.20 7.67

93

2.66 3. 3,

21

4.43

145

170

2 59 3. 4 4,50 5.00

5.57

19.

20

245

170

95

220

245

220

45

2.50 3.37 4.17

3.07 4.13

1.05 1.29

Depth Breadth 72 97

7.

97

120

45

0.25 0.33

120

0.41

1.10 1.48 1.83

1 89 2.54 3.14

72 97

0.25 0.33

120

0.41 72

0.59 0.80 0.99 0.59 0.80 0.99 97

145

75

105

0.11

026 U9 255

1..4

98

035

066

14

1,81

' 67

0.44

0;2

2 24

.3.30

047

0.14

0.25 0.34

1.42 0.82

0.64

1.11

0.17

0.42

0.79

1,37

Depth Breadth 72 97 72 97 120

______

2 63 120

1.21

120

4

'20

2.12

0.90

120

loads

170

Breadth

72 97

No. of

I'S

2 78

Point loads at 400 mm spacing No. of 72 Span Depth

Span

ic5

11

2 07[ 2.99 3.76

0.51

120

6 9.21

1.76 2.18 1.22 -. 1.64 2.03

0.26

72 97

2

245

1.31

0.21

120

1

2201

1.02 1.26 0.70 0.95 1.17

0.40 0.54 0.67 0.38

1200

195!

0.76

0.17 0.23 0.28 0.16

loads

170!

1

07

Depth Breadth

72 97

4

145!

Breadth

0.15 0.18 0.10

1.48 1.83 120

= bending governs

5.11

1.30

1 75 '

deflection governs ______

TRADA Technology Ltd 2006

110

17

= shear governs

TRADA Technology Timber frame housing: UK Structural recommendations

Table 2.6 Permissible loads on lintels: Timber strength class C24 — Medium-term loading Point loads at 600 mm spacing

No.of

Span

loads

1200

1

2

72

1.51

97

2.03

120

2.51

72

1.1 2

97

No. of loads 2

Span 1800

4

4.7 6.44

9.21

220

245

220

245

'f.5

220

245

195 11.

7.

12.

16.1

11.

1

97

120

145

170

19

72

0.41

0.99

183

3 13

4.61

1.33

2.46

4 22 5.22

6.22 7

5.98 8

0.55 0.68 0.36 0.48 0.59

1.64

304

0.85

1 58

1,15 1.42

212 263

Depth Breadth

72

97

120

145

' 73

72

0.17

0.40

0.76

1.31

97

0.54

1.02

1.76

2.07 2

120

0.23 0.28

0.67

1.26

2.18

3.

72

0.16

0.38

0.70

1.22

1

97

0.21

0.51

0.95

1.64

2.59

120

0.26

0.63

1.17

2.03

3.21

97

120

145

170

195

220

245

145

170

195

220

245

170

195

220

245

1 'j8

2.50 3.37 4.17

3.07 4.13

1 90 2 14

2 39 3.22 3.98

97

3

1

170

72

120

2400

145

1.L

72

No. of loads

120

Depth Breadth

97

Span

97

120

120

3

72

Depth Breadth

9.

Point loads at 400 mm spacing Span

No. of oads

1200

1

72 97

2

No,of

Span

loads

2

1800

3

No.of

Span

loads

3

2400

120

1.50

72

0.78

97

135

120

1,29

3. 51

72

97

120

72

0.25

1.10 1,48 1,83 1.10 1.48 1.83 120

97

0.25 0.33

0.59 0.80 0.99 0.59 0.80

120

0.41

0.99

97

0.33

120 72

0.41

72

Depth Breadth

97

1.89

2.54 3,14

145

0.49

0.85

0.66

1.14 ____

0.18

0.44

0.82

1.42

0 10 97 0.14 _____ _____

0 25

047

0 82

72

______ _____

3.

Depth Breadth

120

©

211 090 _______ 121 2.8

72 ______ 0,11 ______ 0.26 ______ 97 0.15 0.35 ______ ______ ______

4

______

72

Depth Breadth

120

0.17

0.64 _____ 1.11 0.34 _____ _____ 0.42

deflection governs ______

TRADA Technology Ltd 2006

1.8

2

I 30[ 1 75

2 7 3 30'

2.56

2.89

2 ilL 317 3.57 = bending governs 1' — shr 0.79

1.37

36

5.11

TRADA Technology Timber frame housing: UK Structural recommendations

Table 2.7 Permissible loads on lintels: Timber strength class D40 — Long-term loading Point loads at 600 mm_spacing No of loads

Span

1200

1

2

No of

Span

loads

2

1800

3

Noof

Span

loads 2400

3

97

145

120

170!

195!

2201

245

I

72

1.57

97

3.65 4.92

6.52

2.12

879

1072 1444

120

262

608

72

1.16

2.74

1087 496

786 829

19.941 25.04j 30.69 15.381 41.34 20.721 26.86j 2Ø4I $,323j 41 7$! 5115

17

97

1.57

3.69

6.69

1

120

1.94

4.56

8.27

3 81

Depth Breadth

72

97

120

145

110

15

220

245

72

0.43

103

1.90

3.26

5!J9

(.40

10.21

13 9

97

0.58

1,38

2.56

4.39

6.85

99

13.75

18 1Q

120

0.71

1.71

3,17

5.43

8.48

12.33

17.01

22 51

72

0.37

0.89

1.64

2.81

4.37

6,34

8.72

11.51

97

0.50

1.20

2.21

3.78

5.89

8.54

11.75

15.50

7.28

10.57

14.54

19.18

120

0.62

1.48

2.74

4.68

Depth Breadth

72

97

120

145

170

195

220

245

72

0.17

0.23 0.29 0.16 0.22 0.27

0.42 0,57 0.70 0.39 0.53

0.79

97

1.06

1.36 1.84

3.17 4.28

4,44 5.98

8.03

1.31

2.27

2.15 2.90 3.59

5.29

7.40

0.73 0.99

1.27

2.01

2.96

4.15

1.71

2.70

3.99

0.65

1.22

2.12

3.34

4.94

5.59 6.92

72

97

120

145

170

195

220

0.94

2.20 2.96

3,97

14.02

18.€

5.35

6.60 8.90

9.97

1.26

13.43

18.89

25.11

120

1.56

3.66

6.61

11.01

1'

2337 .-.. -. 31.1

72

0.81

1.89

3,41

5.E

97

1.09 1.35 72

2.55 3.15 97

4.59 5.68 120

7,61 9.41 14

1 1..,

195

220

245

0.26 0.35 0.43 0.26 0.35 0.43 72

0.62 0.83

1.15 1.54

3.06

4.46

4.13

6.01

1.03 0.62 0.83 1.03 97

1,91

1.97 2.65 3.28 1.96 2.63 3.26 145

5.11

7.43 4.42 5.96 7.37 195

6.15 8.28 10.25 6.09

8.13 10.96 13.56 8.04 10.83 13.40 245

0.11

0.27 0.37 0.46 0.26 0.36 0.44

0.51

120 72 97 120

4

72

Depth Breadth

5.96 9.94 5.58 7.52 9.31

Point loads at 400 mm spacing

Noof

Span

loads 1200

1

2

Depth Breadth 72 97

120

No of

Span

loads

2

1800

Depth Breadth 72 97 120

72 97

3

120

Noof

Span

loads 2400

3

4

Depth Breadth 72 97 120 72 97 120

______

©

0.15 0.19 0.11

0.15 0.18

deflection governs ______

TRADA Technology Ltd 2006

1.14 1.54 1.90 120

0.69

0.85 0.49 0.67 0.82

0.88 1.19 1.47

0.86 1.15 1.43

= bending governs

3.05 4.10 5.08 170 1.40

1.88

2.33 1.35 1.82 2.26

2.06 2.78 3.44 2.00 2.69 3.33

8.21

10.15 220

2.89 3.89 4.81

2.80 3.77 4.67

245

3.88 5.22 6.46 3.77 5.07 6.28

= shear governs

TRADA Technology Timber frame housing: UK Structural recommendations

Table 2.8 Permissible loads on lintels: Timber strength class D40 — Medium-term loading Point toads at 600 mm spacing 72

97

120

157 212 262 116 157 194

365 492

6.52

10.7

879

608 274

1087

1444 1786

496

8.2

3,69

1117

456

669 827

Depth Breadth

72

97

120

145

170

195

220

245

72 97

043 058

103 138

326

509

7.40

171

12.33

17.01

13.50 18.19 22.51

72 97

0.37 0.50 0.62 72

0.89 1,20 1.48 97

1.64

2.81

8.72

11.51

221

3.78

11.75

2.74

4.68

120

145

6.85 8.48 4.37 5.89 7.28 170

9.97

071

4,39 5,43

1021 1375

120

190 256 317

15.50 19.18 245

0.17 0.23

0.42 0.57 0.70 0.39 0.53 0.65

0.79 1.06

1.36 1.84

1.31

2,27

0.73 0.99

1,27

1.22

2.12

Span 1200

1

Breadth 72 97 120

2

72 97 120

Span

Noof loads

1800

2

3

120

Span

Noof loads

2400

3

4

Depth Breadth 72 97 120 72 97

0.29 0.16 0.22

120

0.27

1381

1,71

Point loads at 400 mm spacing Span

No. of loads

1200

1

2

No of loads

1800

2

120

145

170

195

72 97 120

0.94

2.20 2.96 3.66

3.97 5.35

6.60

9.97

1.26 1.56

14.02 18.89 23.37

72

0.81

1.89

3.41

5.65

97

1.09 1.35 72

2.55 3.15 97

4,59 5.68 120

7,61 9.41 145

0.26 0.35 0.43 0.26

0.62 0.83 1.03 0.62 0.83 1.03 97

1.15 1.54 1,91

0.27 0.37 0.46 0.26 0.36 0.44

0.51

0.88 1.19 1.47

Depth Breadth 72

72 97

120

2400

Noof

Depth

loads

Breadth

3

0.35 0.43 72

72

0.11

97

0.15 0,19

120

4

72 97 120

0,11

0,15 0,18

1T11 = deflection governs

©

5.29 2,96 3.99 4.94

97

120

Span

428

72

97

3

:

3.17

Depth Breadth

120 Span

2.15 2.90 3.59 2.01 2.70 3.34

6.34 8.54 10.57 195

TRADA Technology Ltd 2006

8.

6.61

11.(

14.54

220 4.44 5.98 7.40

5.96

4.15

5.58

5.59 6.92

7.52

8.03 9.94

9.31

18t 25.1 31.1

170

195

220

245

1.97

3.06

4.46

8.13

2.65

4.13

6.01

6.15 8.28

3.28

5.11

7,43

10.25

1.14

1,96

4.42 5.96 7.37 195

8.04

2.63 3.26 145

3.05 4.10 5.08 170

6.09

1.54 1.90 120

8.21

10,83 13,40 245

1.40 1.88 2.33 1.35 1,82

2.06 2.78 3.44 2.00 2.69 3.33

0.69

0.85 0.49 0.67 0.82

0.86 1.15 1.43

= bending governs

38

2.26

10.15 220 2.89 3.89 4.81

2.80 3.77 4.67

10,96 13.56

3.88 5.22 6.46

3.77 5.07 6.28

= shear governs

TRADA Technology Timber frame housing: UK Structural recommendations

Table 2.9 Permissible loads on lintels: Timber strength class D50 — Long-term loading Point loads at 600 mm_spacing Span

No of

loads 1200

1

Depth Breadth

72

97

72 97

2.64 3.56 4.40 2.63 3.26

6 13 8.26 10.22 4.60 6.20 7.66

72

97

120

2

72 97 120

1.95

120

145 14.

13.

1 .64

1123 1389 '20

l4

1800

2

72 0 72 1 72 3 19 97 0,97 2.32 4.30 120 1.20 2.87 5.32 913 4 /2 72 0.62 1.49 2.76 97 0.84 2.01 3.72 6.36 7.87 120 1.04 2.49 4.60 45 72 97 120 Depth Breadth 72 0.29 0.71 1.32 2.20 97 0.39 0.95 1.78 3,09 120 0.49 1.18 2.20 3.82' 2.13 72 0.27 0.66 1.23 97 0.37 0.89 1.66 2.87 120 0.45 1.10 2.05 3.55

loads

2400

3

4

245 39.28 .92

.7

Depth Breadth

Noof

220

16.99 8.34

No of loads

Span

19

9

10.

Span

3

170

/i

"

95

1 73

-— 1243 16 75 20.72 10.21 13.75 17.01

14.24 7.34 9.89 12,2 t

720

16 03 21.59 2671

i7 ' 221 ' 'i

3

$1

/ 18

1 )3 4.54 5.62

8.89 4.98 6.70 8.29

145

170

195

14.73

14.77 19.89 24.61

3/

245

1964 .46 7

245

7.46

9.82

10 05 12 43

13.23 16,37

6.93 9.40

'26'

38

11.63

6—

220

245

Point loads at 400 mm spacing Span

Noof loads

1200

1

72 97 120

2

72 97 120

Span

Noof loads

1800

2

Depth Breadth 72 97

loads 2400

3

©

2.62 1.36 1.83 2.26 72

6.15 3.18

4.28 5.29 97

6.(

8.98f 11.11 5.72 7.7 1 120

145

170

195

220

3.30 4.45 5.50 3.29 4.43

5.1E'

7.49 10.09 12.48 7.29 9.82

941

11.531

12.67 15.68

15.53

8.37

9.32

5.48 145

8.512.15 170

195

220

245

1.49

2.35 3.16

3.46 4.67 5.77 3.36 4.53 5.60

4.85 6.53 8.08

6.51

245j .

2.59

1.73

3.21

1.04

1.92

1.40

Depth Breadth

72

97

2.58 3.20 120

72

0.46 0.62

72

0.19 0.26 0.32 0.18

97

0.25

120

0.31

72 97

18.22

19.14 23.23 25.87 25.78 31.30 3486 31.90 3872 43.12

7.67 8.99 10.31 11.83 12.95 10.33 12.11 13.89 15.61 1745 9.0. A 12.78 14.98 17.18 19.38 21.59

1.40

120

______

3.69

4.97

1.93

97 4

1.57

2.12

120

1.04

120

Noof

Span

97

0.43 0.58 0.72 0.43 0.58 0.72

120

3

72

Depth Breadth

1.73

0.76

0.86 1.15 1.43

0.44

0.83

0.60 0.74

1.12 1.38

deflection governs ______

TRADA Technology Ltd 2006

2.00 2.48 1.44 1.94

2.40

= bending governs

6.9 8. 58 5.1

6.89

3.91

2.28 3.07 3.79

19.21

11.28 12.56

la.95 15.54

4.71

6.34 7.84

8.78 10.86

6.33 8.53

10.55 = shear governs

cC

TRADA Technology Timber frame housing: UK Structural recommendations

Table 2.10 Permissible loads on lintels: Timber strength class D50 Medium-term loading Point loads at 600 mm spacing No of 72 Span Depth loads

1200

1

2

Span

Noof loads

1800

2

Span 2400

10.19

72

1.95

460

8.34

97

2.63 3.26 72

6.20 7.66

11.23

120

97

120

145

1.72

3.19 4.30

5.48 7.38 9.13 4.72 6.36 7.87 145

Depth Breadth

0.72 0.97

120

1.20

72

97

0.62 0.84

120

1.04

Noof

Depth

72

loads

Breadth 72 97 120

4

613 826

1db

10.22

72

3

120

2,64 3.56 4.40

97 3

97

Breadth 72 97 120

72 97 120

0.29 0.39 0.49 0.27 0.37 0.45

2.32 2.87 1.49 2.01

170

195

220

245

14

.92

1

1389

5.32 2.76 3.72

2.49 97

4.60

0.71

1.32 1.78 2.20 1.23 1.66

120

95

220

170

195

220

3.62 4.87 6.03 3.37 5.62

5.33 7.18 8.89 4,98 6.70 8.29

9.40 11.63

195

220

170

74c

54

0.95 1.18 0.66 0.89 1.10

2.05

2.29 3.09 3.82 2.13 2.87 3.55

97

120

145

170

1.57

3.69

6.67

10.93

2.12 2.62

4.97 6.15 3.18 4.28 5.29 97

0 98 14.73

14.77 19.89 24.61

4.54

7.

ici

Fl 05l

12.-.13

6.98

2451

16.37j 9.38 12.64

1564

Point loads at 400 mm_spacing Span

Noof loads

1200

1

2

Span

Noof loads

1800

2

3

Span

No of

loads 2400

3

Depth Breadth 72 97 120 72 97 120 Depth Breadth 72 97 120 72

2.26 72

1.04 1.40 1.73

97

11.11

3.21 1,92

1.40

2.58

1.73

97

3.20 120

72

72 97

0.19 0.26 0.32 0.18 0.25

120

0.31

0.46 0.62 0.76 0.44 0.60 0.74

0.86 1.15 1.43 0.83 1.12 1.38

8.99 IO31 11,63 12.9 170

195

220

245

2.35 3.16

3.46 4.67

4.85 6.53

6.51

3.91

577 336

8.08

10.86

4.71

4.53 5.60

6.34 7.84

6.33 8.53

3.30 4.45 5.50 3.29 4.43 5.48 145

1.93

72

© TRADA Technology Ltd 2006

145

2.59

1.04

19.14 2323 25.87 25.78 3130 3486 3190 38r2 43.12

12.18 14.98 17.18 19.38 21

9.54 120

0.72

I

767

245

13 1211 13. 15.67 17.4

7.71

120

deflection governs

18.22

5.72

Depth Breadth

120

______

1.36 1.83

0.43 0.58 0.72 0.43 0.58

97 4

72

1.49 2.00 2.48 1.44 1.94

2.40

= bending governs

40

2.28 3.07 3.79

8.78

10.55

= shear governs

TRADA Technology Timber frame housing: UK Structural recommendations

2.3 Racking resistance of timber frame walls 2.3.1 Basic design and materials 2.3.1.1 Standard wall panels BS 5268-6 provides a method of assessing the racking resistance of a timber frame wall made of standard wall panels which are sheathed with one of a given list of materials nailed to the frame. To comply with the racking values in the Standard, the frame must consist of grade 016 softwood or better with a minimum cross-section of 38 mm x 72 mm. For internal walls the section may be reduced to 38 mm x 63 mm with a 15% reduction in racking resistance. The sheathing materials listed in the 1996 edition of BS 5268-6.1 refer to standards now withdrawn. All wood-based panels used for sheathing must now comply with the relevant technical specification requirements laid down for wall sheathing in BS EN 13986 Wood-based panels for use in construction. Characteristics, evaluation of conformity and marking. Further information is given in the TRADA Wood Information Sheets 0 - 5 Timber frame building: materials specification, 2/3-56 CE marking: Implications for timber products and 2/3 - 57 Specifying wood-based panels for structural use. Materials selected must be appropriate for use in Service Class 2 (BS 52682). OSB/3 is the most commonly used material. If gypsum plasterboard is assumed to make a structural contribution it must be manufactured in accordance with BS 1230-1: 1995 Gypsum plasterboard — Specification for plasterboard excluding materials submitted to secondary operations. BS 5268-6.1 gives a basic racking resistance in kN per metre length for a specified thickness of each sheathing material, fixed with nails of a specified diameter and spacing, in a 2.4 m square panel without openings or vertical load. The additional contribution of a secondary board, nailed independently of the primary board, is allowed for by another, lower, set of figures. The racking resistance thus obtained is modified as necessary by multiplying it by the following factors to obtain a value for the wall which should be similar to results from test data. K101

Variation in nail diameter

K102

Variation in nail spacing

K103

Variation in board thickness

K104

Height of wall panel

K105

Length of wall

K106

Fully framed openings

K107

Vertical load

The final result is multiplied by K108 an interaction factor of 1.1, which accounts for interaction between the walls, floors and the stiffening effects of corners in the actual building.

The construction of separating walls, which consist of two or more layers of plasterboard nailed to one side of the timber frame, is laid down in detail in the standard, and a basic racking resistance of 0.9 kN/m is given for such walls. The maximum allowable total contribution of plasterboard on other walls to the racking resistance of a building in a given direction is one third of the total racking resistance in that direction. No contribution to racking resistance may be assumed for glazed areas.

©

TRADA Technology Ltd 2006

41

(A TECHNOLOGY((7

TRADA Technology Timber frame housing: UK Structural recommendations

A worked example of the assessment of the racking strength of a house is given in Section 7.4.

2.3.1.2 Other ways to increase the racking resistance The figures given in BS 5268-6 are based on the strength of nailed joints in C16 timber framing and, in the case of plywood, on plywoods in group I as defined in Table 63 of BS 5268-2. Examination of this table shows that if C24 framing is used with a group II plywood then an increase in strength of 10% can be obtained. Another way to obtain higher racking values is to determine them by testing. Particular materials may have a basic racking resistance which is higher than the generalised figures provided by the standard: and different materials and forms of construction for which figures are not given may be suitable for sheathing. In such cases manufacturers may produce more appropriate figures by specific tests described in Section 5 of the Standard. The figures thus obtained may be used only for panels which are identical in all respects to the panels used in the test. Hence they may not be modified by factors K101, K102 or K103

BS 5268-6.1 allows for an additional contribution to racking resistance from masonry veneer, provided that is connected to the timber frame with specified wall tying and its contribution does not exceed 25% of the racking resistance provided by the timber frame. Special calculations or tests will be required if the designer wishes to take into account any additional contribution to racking resistance which may be provided by cladding material other than masonry.

2.3.1.3 Staples Some wall panel manufacturers use staples in place of nails for fixing sheathing. Eurocode 5 provides a method for calculating the strength of a stapled connection by means of which a designer could determine the equivalent nail diameter and hence obtain an appropriate value for K101 to determine the racking resistance of a stapled panel. However, there are different kinds of staple, some of which are designed to splay out on insertion, and it may be difficult to meet the minimum edge distance of 20d specified in Eurocode 5. Hence it is safer to obtain the racking strength of stapled panels by testing rather than by calculation. Eurocode 5 recommends a minimum corrosion protection of Fe/Zn 12c or Z275 for staples used in Service Class 2.

2.3.2 Adhesively bonded panels Within the limits specified in BS 5268-6.1 the racking resistance of a standard wall panel may be increased by increasing the nail diameter, reducing the nail spacing or increasing the board thickness. If the required racking resistance cannot be achieved by these means, shear panels of very high rigidity and strength can be obtained by attaching a structural sheathing material to the framing with a suitable structural adhesive. The manufacture of such panels must be carried out under a fully approved factory production control quality assurance system to ensure consistency of bond strength and structural characteristics. (See TRADA Wood Information Sheet 2/3-31 Adhesively bonded timber connections.) The strength and stiffness of such panels should be determined by testing in accordance with BS EN 594 (see Section 2.3.2.3).

2.3.2.1 Panel shear Assuming a single sheet of sheathing material on one side only, the maximum applied panel shear stress may be calculated as: = l.SFa N/mm2 tL

where

Fa

= total applied shear load (N)

= thickness of panel product (mm) L

©

TRADA Technology Ltd 2006

= length of panel (mm) 42

TRADA Technology Timber frame housing: UK Structural recommendations

This should not exceed the permissible panel shear stress. For plywood the grade stress for panel shear is given in BS 5268-2 Tables 40 to 56. Generic values for the characteristic planar shear strength of OSB, particleboards and fibreboards are given in BS EN 12369-1 Wood-based panels. Characteristic values for structural design. OSB, particleboards and fibreboards. These may be converted to permissible stresses using the method given in BS 5268-2 Clause 5.3. Normally the grade stress would be multiplied by K35 = 1.5 for wind loading, but US recommendations suggest that an increase of 1.33 is more appropriate for the response of timber frame wall panels within a building to a gust of wind. 1.33 is the value of K36 given in BS 5268-2 Table 39 for mediumterm loading in Service Classes 1 and 2. For OSB/3 under medium-term loading the permissible planar shear stress in Service class 2 is 2.48 N/mm2.

2.3.2.2 RoIling shear Assuming the worst case, when all the shear load is applied to the top rail of the panel, the applied rolling shear stress in the glued joint is:

Ta

where

=

N/mm2 Lh

Fa

= total applied shear load (N)

L

= length of panel (mm)

h

= depth of top rail (N)

Ta should not exceed the grade rolling shear stress for the sheathing material multiplied by 1.33 x 0.5.

The factor of 0.5 is required by Clause 4.7 of BS 5268-2 to allow for stress concentrations. A further reduction of 0.9 should be applied if the bonding pressure is applied only by nails or staples (Clause 6.10.1.4). These reductions are not required in designs to Eurocode 5 which provides little guidance on the design of adhesively bonded joints, preferring tests to determine their strength. For panel products other than plywood, the permissible planar shear stress maybe used, calculated by the method given in 2.3.2.1.

2.3.2.3. Values from tests Alternatively the racking strength of glued panels may be obtained by testing as described in BS 5268-6. This involves the test method BS EN 594: 1996 Timber structures — Test methods — Racking strength and stiffness of timber frame wall panels. Structural insulated panel systems (SIPS) may be tested in accordance with ETAG 019: 2005 Pre-fabricated wood-based loadbearing stressed skin panels. As previously stated, the racking resistances obtained from tests may not be modified by factors K101, K102 or K103.

2.3.3 Overturning effects Whether adhesive bonding is used or not, a panel must be restrained against overturning by means of:

• • • • •

its attachment to a panel on the windward side the vertical load applied to it by the weight of the building above it, less any wind uplift holding down the end studs to the foundation in some way holding down the bottom rail to the foundation with sufficient fasteners into the foundation and the structural sheathing a combination of these methods.

TECMNOLOGY(17

©

TRADA Technology Ltd 2006

TRADA Technology Timber frame housing: UK Structural recommendations

(a) Restraint forces applied at corners

V

F

(b) Restraint forces applied along base

w F

h

..

V

w

h

///// /// F

Rv

fh=F1L per unit length

U fvimax

L

Fh W

R _EflW

VL 2

f1max=

3'Fh W' per unit length

As shown in diagram (a) above, when the racking load is sufficient to cause rotation the reactions at the two bottom corners are in opposite directions. This means that if there is an adjacent panel on the windward side its connection to that panel can restrain it against uplift with a total restraint force of Fh/L + W12. For panels of similar length this is greater than the total restraint required against uplift, Fh/L - W/2. Hence no further restraint is necessary provided it can be shown that the connections between the panels are strong enough to transfer this force between the panels. For the end panels however, or for panels where further restraint is needed, other approaches should be considered. Diagram (a) shows restraint to overturning provided by a metal holding down strap at each of the bottom corners. With a total vertical load of W and a racking load of F the restraint strap must resist a force of

R

— Fh — W

L

2

A similar strap is needed at both ends to cater for a reversal in the wind direction. These are nailed, screwed or bolted to the wide face of the outer studs and anchored to the concrete foundation. The strength of the strap and its connection at each end should be calculated and exceed R. In addition the fasteners connecting the bottom rail to the binder or sole plate or foundation must provide a total lateral resistance of F, so their maximum spacing is FL 5

=—mm adm

where

fadm

= permissible load for one fastener for short- or very short-term loading = basic load x 1.25 N (BS 5268-2 Clause 6.4.9)

© TRADA Technology Ltd 2006

TRADA Technology Timber frame housing: UK Structural recommendations

A second approach is to attach the outer studs securely to the bottom rail by means of metal straps which wrap around the outside of the panel at each corner. Suitable straps are available in several proprietary forms, either as pierced plates for nailing to the framing or plates with integral teeth for application by hydraulic press. In this case all the overturning restraint is provided by ringed-shank nails or screws or bolts fastened through the bottom rail along its full length. These fasteners must provide a lateral resistance of fh = F/L per unit length and a resistance to axial withdrawal of: fvmax

- 3(Fh W —

per unit length as shown in diagram (b). BS 5268 does not provide a method to allow for the effects of combined axial and lateral loading, but Eurocode 5 states that for annular ringed shank and helically threaded nails, and by implication for wood screws, the requirement +

----

2

1 must be satisfied

1\Raxd) R1ad

where Faxd and Flad may be taken as the axial and lateral load respectively applied to each connection, and Rax,d and Rla,d may be taken as the permissible withdrawal and lateral load respectively for each connection. For nails and screws, the requirement in BS 5268 terms becomes 2

+

ax,ajm where

1

. facim)

s

= fastener spacing (mm)

fv,max

= maximum applied vertical load per unit length, as above (N/mm)

faxacim

= permissible withdrawal load for one fastener (N)

= basic load x 1.25 fh

= applied horizontal load per unit length = F/L (N/mm)

facirn

= permissible lateral load for one fastener (N)

= basic load x 1.25

1

Hence Smax

v,max 2 + fax,acim)

12 . acim)

With bolts it is sufficient to ensure that the resistance of each connection to the lateral loads is adequate and, in the case of anchor bolts to foundations, that the manufacturer's safe working load on each bolt is not exceeded. This is because, with bolts of at least 8 mm diameter, failure in two-member laterally loaded softwood connections occurs in the timber rather than the bolt. It may be possible to transmit the overturning moment from the structural sheathing into the bottom rail without any restraining straps. In this case the shear connection between the sheathing and the framing resists the applied loads. These loads comprise (i) a horizontal load of fh = F/L per unit length, which produces in an adhesively bonded connection an applied shear stress of ta as previously calculated, and (ii) a maximum vertical force per unit length

of fvmax

=

3(Fh W

N/mm

—--——-—j

which produces an applied shear stress of

T where h

©

TRADA Technology Ltd 2006

= v,max N/mm2 h

= depth of the bottom rail (mm).

TRADA Technology Timber frame housing: UK Structural recommendations

In mechanically fastened joints the fasteners must provide a total resistance of fres, a

= h2 + v,max2 N/mm

giving a maximum spacing of S

=mmm res,a

In adhesively bonded joints the resultant applied shear stress in the sheathing and the framing is

/2

Tresa = Ta +T

2

For the framing the permissible shear stress may be calculated from the formula given in BS 5268-2 Clause 6.10.1.3 as: = Tadm,ll(1_0.67510) Tadma

where

Tadm a = permissible shear stress in the timber

Tadmjl = permissible timber shear stress parallel to the grain = 1.33 times grade stress as above a

angle between the grain and direction of Tresa Z =tan —1—

Ta

For OSB the applied shear stress should be compared with the permissible planar shear stress. This can be obtained from the characteristic planar shear strength given in BS EN 12369-1 converted to a permissible stress using the method given in BS 5268-2. For OSB/3 under very short-term loading the permissible planar shear stress in service class 2 is 0.496 N/mm2. For plywood the angle of the shear stress will vary in alternate plies. Assuming that the rolling shear strength is one third of the shear strength parallel to the fibres, as implied by the above formula, it may be adapted as Tadma = Tadm,90(3_25ifl0)

where

Tadm a = permissible shear stress in the plywood Tadm 90

permissible rolling shear stress in the plywood = 1.33 times grade stress as above

a

= maximum of tan1 -- and tan1 .Tv

Where straps are not used the tensile strength of the sheathing should be checked for an applied tensile stress of fv,max/t where t is the sheathing thickness, and a permissible stress calculated as the grade stress parallel or perpendicular to the grain as appropriate x 1.33 as above.

2.3.4 Deflection It is difficult to calculate the horizontal deflection of a timber frame wall because: • the connection between panels is not rigid and manufacturing tolerances permit small gaps between the abutting studs, so the panels act independently to some extent, but not entirely • the sheathing tends to bow along the compression diagonal, increasing the total deflection • fastener slip between the top and bottom rails of the panel and the sheathing is difficult to calculate accurately • the framing makes an indeterminate contribution to the racking stiffness • the effect of openings is difficult to quantify.

TRADA ©

TRADATechnology Ltd 2006

46

TRADA Technology Timber frame housing: UK Structural recommendations

In general, however, it may be assumed that if a wall designed to BS 5268-6.1 can resist the applied racking loads then its deflection will not be excessive. One reason for this is that the racking resistance of wall panels calculated according to the Standard is based on tests which limit the amount by which a panel may deflect before it is deemed to have failed.

2.3.2.4 Buckling Timber frame wall panels designed to BS 5268-6.1 within the height range of 2.1 — 2.7 m do not require a separate check for the buckling resistance of the sheathing.

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TRADA Technology Ltd 2006

(R ci:13,

TRADA Technology Timber frame housing: UK Structural recommendations

3 Floors 3.1 Joists and beams Span tables for floor, ceiling and roof joists are provided in TRADA's Span fables for solid timber members in floors, ceilings and roofs (excluding trussed rafter roofs) for dwellings. This provides important information about the design and installation of timber floors using softwood of Strength Classes C16 and C24. BS 8103-3 provides similar tables but the spans are a little lower in some cases because they were calculated for the former SC3 and SC4 Strength Classes. Finally suitable member sizes for any weight of floor and imposed load may be obtained quickly using TRADA's Timber Sizer software (visit the Software Toolbox at www.trada.co.uk). When the distance between load bearing walls becomes so great that it is not economical to span directly with solid softwood floor joists, it may be possible to use prefabricated engineered timber joists, hardwood, or a structural timber composite such as LVL (laminated veneer lumber), PSL (parallel strand lumber), or LSL (laminated strand lumber). Typical spans which can be achieved economically are: • Softwoods: up to 5 m • Hardwoods and timber composites: 5 — 7 m • Engineered timber joists: 4—7 m or more.

If even longer spans are required, or if conventional joists in commonly available sizes are preferred, then intermediate floor beams may be used to support the joists. These may be deeper sections of hardwoods or structural timber composites, doubled engineered joists or steel flitched beams. Doubled engineered joists should be connected together in pairs by fixing web stiffeners between them or by using proprietary fasteners or fixing methods, always in accordance with the manufacturer's instructions. Strutting between floor joists in the form of solid timber blocking, timber herringbone strutting, or proprietary metal herringbone strutting should be specified in accordance with the recommendations given in the publications cited above or see TRADA's Wood Information Sheet 1-41 Strutting in timber floors. For engineered joists the manufacturers may provide other recommendations or strutting methods, such as the strong-backs used with metal open web joists. Currently the National House-Building Council (NHBC) recommends a deflection limit of 12 mm for all domestic floors made without strutting, because tests have shown that properly installed strutting significantly improves the vibrational performance of a floor, which is an important part of structural design.

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TICHNOLOGV((5

TRADA Technology Timber frame housing: UK Structural recommendations

3.1.1 Reactions, moments and deflections in a single span, simply supported beam 3.1.1.1 Uniformly distributed loads

Rot

t PA

w = load per unit length

Reaction at A

RA=

Reaction at B

w(b—a)(2L —a—b)

R6=

2L

Value of x

Bending moment at a distance x from RA

x L/2 b> L12

b2

—w

15L

Yb —------———C

2 L2

L2

El

(L — a)2

2 L2

C2

32 48

El

Yb =

—a

32

—

Shear deflection at centre (1)

48

Ys

a2 2 L2—— —a

(L — a)2

TRADA Technology Ltd 2006

3w 1 b2 —a2

a2

32 48 J 32 48

—2

L2

—

c2

32 48

For engineered timber joists, consult the manufacturer

©

2L

Maximum bending moment

w(x—a)2 2

Configuration

b1

For 25 x 100 C16 bracing: (A = 2500 mm2; Z = 41670 mm3; i,= 28.87 mm) Max BM = 180,200 Nmm

0m,a

0c,a

M Z

=

41670

583 2500

=

100

3400 28.87

=

= 4.32 N/mm2

= 0.233 N/mm2

5.3 x 1.75 xl

0m,adm =

Take

— — 180,200

= 10.47 N/mm2

= 118

For very short term load K12

=

0.1972

Gcadm = ae

6.8 x 1.75 x 0.1972 rr2E

=

— 9.87x5800 — (118)2

[LJ2

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TRADA Technology Ltd 2006

66

= 2.35 N/mm2

=4.11 N/mm2

TRADA Technology Timber frame housing: UK Structural recommendations

0c.a

Gma

K Gcadm

Gmadm[1_'

)

4.32

+

1.5x0.233 4.11

10471

0.233

2.35

x0.1972 = 0.519

= 0.420 + 0.099

For brace in tension

(300h1

Gtadm = Gta + 0m,adm 0t,adm 0m,a

3.2 xl .75 xl

I

= 6.32

100) 4.32

=

10.47

+

0.233

6.32

= 0.413 + 0.037

= 0.450 < 1.0 as required.

4.2 Connections

1

2 Nailed plywood or OSB gussets.

Circular or square toothed plate connectors used with bolts in lap-jointed members.

3 Proprietary pierced plates attached by tightly-

4 Proprietary metal gusset plates with integral teeth, applied by large hydraulic or roller pressures.

fitting nails.

The first three of the connection methods shown above are suitable for use in site workshops without special equipment. Designs for nailed plywood gussets may be obtained from plywood manufacturers associations and designs for method 4 from system owners who supply proprietary jointing plates to their licensees undertaking the work of fabrication. Details of systems owners and their licensees can be viewed on the TRADA website (www.trada.co.uk).

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TRADA Technology Ltd 2006

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TRADA Technology Timber frame housing: UK Structural recommendations

5 Foundations 5.1 Loading Total loads on foundations for the external walls of two storey timber frame houses vary between 6.3 and 30 kNIm run. The types of foundation commonly employed are illustrated in the TRADA publication Timber frame construction. Loads on the foundations of timber frame houses with timber claddings are lighter than those of masonry construction and, therefore, have distinct advantages where the soil bearing pressures are poor. Table 5.1 shows some typical foundation weights on the footings, depending on their size and shape.

Table 5.1 Foundation loads in kN/m

Exterior finish Load

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TRADA Technology Ltd 2006

Brick veneer

Tile hanging

Timber boarding

Dead

16-20

4.5—8.5

3.5—7.5

Dead and imposed

19-30

8-18

6.3-17

68

TRADA Technology Timber frame housing: UK Structural recommendations

6 Multi-storey buildings 6.1 General design considerations The Timber Frame 2000 project, begun in 1995, was carried out by the Building Research Establishment and TRADA Technology Ltd in collaboration with the British Government and the timber frame industry. It demonstrated beyond doubt that conventional timber frame construction can be used to build economical, safe and serviceable multi-storey dwelling units. The report Multi-storey timber frame buildings — a design guide concluded that the use of BS 5268-6 can be extended to the design of platform frame buildings up to eight storeys without excessive deflection. Particular attention should be given to the following issues:

• • •

•

•

each storey should have sufficient strength and stiffness to resist a horizontal long-term force of 2.5% of the vertical load + live load overturning forces should be carefully checked where party walls separate the structure into separate units, the engineer should ensure that horizontal forces can be taken by each unit independently or be transferred across the party walls where additional stiffness has to be provided, eg by the introduction of portal frames, the deflection limit should be appropriate for the structure and finishes, but may be no more than height/500 resistance to disproportionate collapse should be checked.

6.2 Construction The stability of the building during construction, ie before vertical loads are applied and plasterboard is fixed, must be considered as part of the design process. It is acceptable to reduce the wind load in accordance with BS 6399-2 for the construction process. Particular attention should be given to the buckling resistance of studs in party walls, which may have neither plasterboard nor sheathing attached during construction. Significant vertical loads can result from the storage of construction materials such as plasterboard packs, so normally it will be necessary to specify requirements for temporary bracing (see BS 5268-6.1).

6.3 Disproportionate collapse While the Timber Frame 2000 project demonstrated that timber frame construction is remarkably resilient to disproportionate collapse, specific design checks against this possibility may be required under the Building Regulations. Guidance is expected in a planned revision to BS 5268-2: 2002. Meanwhile, guidance has been published in the UK Timber Frame Association Technical Bulletin 3 Design guidance for disproportionate collapse. This specifies minimum nailing between the lower rails of wall panels through the interfaces to the upper rails of the panels beneath as follows: 3.1 mm diameter nails at 300 mm centres for Class 1 and Class 2A buildings 3.1 mm diameter nails at 200 mm centres for Class 2B buildings.

© TRADA Technology Ltd 2006

69

TECIINOLOQV(t7

TRADA Technology Timber frame housing: UK Structural recommendations

7 Example of calculations for a complete dwelling

3.9m

P

T8T1I! I

I

4

I

I

I

9.5m

Ground floor plan

First floor plan

_1L

to N-

r Rear elevation

Front elevation

Side elevation

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TRADA Technology Ltd 2006

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\ TECHNOLOGY1

TRADA Technology Timber frame housing: UK Structural recommendations

General description and specification • • •

Site snow load 0.64 kN/m2 Platform frame construction Brick veneer

• Trussed rafters at 600 mm centres • Concrete roof tiles weighing 49 kg/m2

• • • • •

Solid concrete ground floor All ground floor internal walls are load bearing Plasterboard internal linings

All wall panels 2400 mm high Window clear openings 2400 mm or 1200 mm wide, 1200 mm high Door clear openings 900 x 2000

•

Head binder to top of panels.

Note: The plans and elevations do not necessarily represent a typical house layout but provide a basis for the following calculations. The calculations rely on the assumptions that:

• •

•

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TRADA Technology Ltd 2006

The vertical loads described in Section 1.2.1 are all carried by the timber frame wall studs The wind loads described in section 1.2.2, are resisted by the horizontal diaphragms and timber frame walls; however a proportion of these loads can be transferred back into the brickwork via the wall ties in accordance with BS 5268-6.1 Clause 4.10 The wind loads have been previously calculated in accordance with BS 6339-2 and section 1.2.2.

71

YECMNOLOOY(

TRADA Technology Timber frame housing: UK Structural recommendations

7.1 Vertical loads Roof (350 pitch):

Concrete nterlocking tiles 1

49

kg/rn2

3

Battens

Felt 53

cos35° Trusses

53

kg/rn2

65

kg/rn2 on plan

16

kg/rn2

200 mm insulation quilt

2

12.5 mm plasterboard

10

94

So total dead load

0.92

imposed load 2

0.67

ceiling imposed load

0.25

18mm OSB deck

Floor:

47 x 194mm C16 joists at 400 mm centres

1.84

kN/m2

12

kg/rn2

9 10

12. 5 mm plasterboard

140 mm phenolic foam board

4 35

So dead load

1.50

Framing 47 x 97 mm at 600 mm centres 12.5 mm plasterboard

6

kg/rn2

6

Insulation quilt

3

Framing 38 x 63 rnrn at 600 mm centres

So dead load

25

kg/rn2

0.24

kN/rn2

4

kg/rn2

20

24

kg/rn2

0.24

kN/rn2

Concrete tiles can weigh from 42 to 58 kg/rn2

2 BS 6399-3: 1988 Clause 4.3.2 BS6399-1: 1996 Clause 5.2 BS 6399-1: 1996 Clause 5.1.1

©

kN/rn2

9.0 mm OSB sheathing

2/12.5 rnrn plasterboard

1

1.84

10

So dead load Partitions:

kg/rn2

0.34

imposed load

External walls:

kg/rn2

TRADA Technology Ltd 2006

72

(0.64 x 1.25) (60 - 35) / 30

= 0.67)

TRADA Technology Timber frame housing: UK Structural recommendations

7.2 Horizontal loads Building dimensions 9.5 m

Overall length (I) =

Overall width (w)

7.0 m 5.0 m

Height to eaves (h)

=

Height to ridge (H)

7.5 m

A dynamic wind pressure of q = 0.800 kN/m2 for both directions is assumed to have been calculated from BS 6399-2. As stated in Section 1.2.4 this may be reduced for a maximum 1 year construction period to

q=

= 0.449 kN/m2.

0.800 x 0.56 1

When timber frame walls are shielded from the wind by masonry walls, the load transferred to them is reduced by a factor K100 which depends on the proportion of the loaded wall occupied by openings. (In other types of construction the full wind load is taken by the timber frame walls, so K100 = 1.0) Values of K100 are given in Table 1 of BS 5268-6.1 and are applied to an entire wall measured to the eaves. K100 is not applied to the spandrel in a gable wall, which is not selfsupporting. For the dimensions given in the specification a table is made of the values of P, where =

area of openings x 100 area of walls including openings

Wall

P

K100

Front wall

22.0

0.81

Rear wall

27.3

0.82

Gable wall, door end

5.2

0.48

Gable wall, other end

0.0

0.45

Wind pressures on timber frame in kNIm2 in 50 year constructed period, walls shielded by masonry Wind on; Roof and spandrel

©

Front wall

Rear wall

Gable wall,

K100

1.0

0.81

0.82

door end 0.48

q x K100

0.800

0.648

0.656

0.384

TRADA Technology Ltd 2006

Gable wall, other end

0.45 0.360

(R

TRADA Technology Timber frame housing: UK Structural recommendations

BS 6399-2 Tables 5, 10 and 16 give the following external and internal pressure coefficients for the house illustrated. c0= -0.5 9.5

I' 2.55

-0.4

Cpe 0.83 Cpe.nlex

-0.7

0.951

0.8

Wind

__________

7.0

-1

2

1.75

.1

1

.1

1

-

6.0

-0.6

-0.5

-0.6 0.95

1.75

+0.8 or -0.3 bL= 9.5

-0.5

1 2

bw= 7.0

All Cp values = -0.3

Cpe 0.82

Cpe -0.5

_______ __________________

—

1.75

+05 or -0.1

2.55

1.75

3.5

0.8

Cpe,r,I

Wind All measurements in metres

For stability checks, the external roof coefficients shown above have been simplified as follows: 1. Overturning

-055

02

—,. Wind

4. Roof Uplift

2. Sliding & racking

3. Racking

-0 13

-0.5

-0.55

-0.5

-0

±05

-0.55

-0.25

Wind

Wind

Wind

-1.15 Wind

-1.15

Wind

Key 1.

2.

3.

Overturning

Sliding and racking

Racking

-0.55

= weighted mean of -0.4 and -0.7

-0.2

= weighted mean of -0.3 and -0.1

-0.6

= weighted mean of -1.1 -1.2, -0.6 and -0.5

-0.5

= weighted mean of -0.4 and -0.7

+0.6

= weighted mean of +0.5 and +0.8

-0.55

= weighted mean of-il, -1.2, -0.6 and -0.5 for vertical loads in racking strength calculations

4.

Roof uplift

-0.5

= weighted mean of -0.4 and -0.7

-0.25

= weighted mean of -0.1 and -0.3

-1.15

mean of -1.1 and -1.2 for roof uplift

For this example a value of 0.02 has been assumed for the dynamic augmentation factor, Cr, and a value of 1.00 for Ca in all cases. As shown in Section 1.2.2.2 the calculation of Ca values is time-consuming and is unlikely to reduce the design value of the wind load by more than 5%.

© TRADA Technology Ltd 2006

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TRADA Technology Timber frame housing: UK Structural recommendations

7.3 Overall stability calculations It is necessary to check the stability of the building both during and after construction. In the construction period a lower design wind pressure may be used, but there may be no roof weight to add stability nor masonry walls to reduce the nett wind loads. In the constructed period a roof and in some cases masonry walls contribute to stability, but the design wind pressure is higher. For the purpose of this example, the stability checks will be carried out for the constructed period only.

7.3.1 Wind pressures and self-weight In general the wind pressure on a surface is calculated from BS 6399-3 as p

= 0.85 x q x Cp (1+Cr) Ca

= 0.85 qC x 1.02 x 1.0

= 0.867 qC

(7.3.1)

Where reversing the wind direction produces different nett pressures, the worse case occurs when the higher pressure is combined with the higher value of Cpe. Hence the wind on the rear wall and the door end of the gable wall are calculated. Wind on rear wall For nett horizontal pressure expression (7.3.1) becomes Pnett

= 0.867 (qs,windward Cpe,wjndward — S, leeward Cpe, leeward

Nett pressure on walls

= 0.867{0656 x 0.82 - 0.648 x -0.5} = 0.747 kN/m2

Effective horizontal pressure on roof (overturning) = 0.867{0.80 x (-0.2) — 0.80 x (-0.55)}

= 0.243 kN/m2 Effective horizontal pressure on roof (sliding & racking)

= 0.867{0.80 x 0.6 - 0.80 x (-0.5)} = 0.763 kN/m2 For nett vertical pressures instability calculations expression (7.3.1) becomes = 0.867 (q Cpe)

Pnett

The internal pressure coefficients cancel out. Effective vertical pressure on windward side of roof (overturning)

= 0.867{0.80 x (-0.2))

= -0.139 kN/m2

Effective vertical pressure on leeward side of roof (overturning)

= 0.867{0.80 x (-0.55))

= -0.381 kN/m2

For nett vertical pressures relating to vertical loads on racking panels expression (7.3.1) becomes Pnett

0.867 (q Cpe — q C)

Effective vertical pressure on windward side of roof (racking vertical load)

= 0.867{0.80 x 0.6 — 0.80 x (-0.3)} = 0.624 kN/m2 Effective vertical pressure on leeward side of roof (racking vertical load) = 0.867{0.80 x (-0.5)— 0.80 x (-0.3))

= -0.139kN/m2

© TRADA Technology Ltd 2006

TRADA Technology Timber frame housing: UK Structural recommendations

Wind on gable wall, door end Nett pressure on walls

= 0.867{0.384 x 0.83 - 0.36 x -0.5} = 0.433 kN/m2 0.922 kN/m2

Nett pressure on spandrels = O.867{O.80 x (0.83 - (-)O.5)}

Effective vertical pressure on roof (overturning) (Internal coefficients cancel out) -0.416 kN/m2

= O.867{O.80 x (-O.6)}

Effective vertical pressure on roof (racking vertical load)

= 0.867(0.80 x (-0.55 - (-)0.3)}

= -0.173kN/m2

9.5 x 5.0 x 0.24 x 2

= 22.8 kN

Total building self-weight Front and rear walls

Gable walls without spandrels 7.0 x 5.0 x 0.24 x 2

= 16.8kN

Spandrels

7.0 x 2.5 x 0.5 x 0.24 x 2

= 4.2kN

First floor

9.5 x 7.0 x 0.34

= 22.6 kN

Roof including overhang

9.5 x 7.6 x 0.92

= 66.4 kN = 132.8 kN

Total

7.3.2 Overturning 7.3.2.1 Wind on front or rear wall

x

Overturning moment about xx due to wind load is:

9.5[(5 x 0.747 x 2.5) + (2.5 x 0.243 x 6.25) + (3.5 x 0.139 x 5.25) + (3.5 x 0.381 x 1.75)]

= 171.2 kNm

The overturning resistance due to building self-weight

= 132.8 x 3.5 Factor of safety against overturning = 464.8/171.2

= 464.8 kNm =

2.71

Required factor of safety against overturning (BS 5268-6.1 Clause 4.4)

1.20 (When checking overturning during the construction period, the roof weight should be omitted.)

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TRADA Technology Ltd 2006

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TRADA Technology Timber frame housing: UK Structural recommendations

7.3.2.2 Wind on gable wall

Th

2.5 m

2.5 m

2.5 m

L

Overturning moment about xx due to wind load is

7.0{(5 x 0.433 x 2.5) + (2.5 x 0.5 x 0.922 x 5.83) + (9.5 x 0.416 x 4.75)} = 216.3 kNm The overturning resistance due to building self-weight = 630.8 kNm

= 132.8 x 4.75 Factor of safety against overturning = 630.8/216.3

= 2.92

Required factor of safety against overturning (BS 5268-6.1 Clause 4.4) = 1.20

7.3.3 Sliding In this example the maximum sliding force results from wind on the rear wall. Applied sliding force

= 53.60 kN

= 9.5{(5.0 x 0.747) + (2.5 x 0.763)} Therefore required sliding resistance

1.2 x 53.60

= 64.32 kN

Assuming a coefficient of friction of 0.25 the resistance to sliding

= 33.20 kN

= 132.8 x 0.25

Therefore sole plate straps or other fixing methods are needed to provide a total resistance of = 31.l2kN. 64.32 - 33.20 For sole plate fixing straps use at least 2 No 3.25 mm diameter nails per strap. Assuming 016 timber and, initially, two members 38 mm thick, BS 5268-2 Annex G or TRADA's fastener software (visit www.trada.co.uk) gives a basic single shear lateral load of 327 N. Therefore the permissible load per strap

= 327x2x1.25x1.25x103

=

1.O2kN

(K45) (K48)

No of fixing straps needed = 31.12/1.02

©

TRADA Technology Ltd 2006

= 30.51

= 31

= 31/((7.0 x 2) + (9.5 x 2))

=

= 1000/0.939

= 1064 mm centres

0.939 per m

TRADA Technology Timber frame housing: UK Structural recommendations

Since fixing straps or hammered sleeve fixings are normally used at 600 mm centres to locate the sole plates during setting out, no additional restraint is necessary to augment the sliding resistance provided normal practice is followed. Adequate sliding resistance at first floor level will normally be provided by recommendations made by the UK Timber Frame Association to meet requirements against disproportionate collapse. These specify that the minimum fastening between the bottom rail of the wall panels and the floor deck or rim beam and between the rim beam and the head binder or top rail of the wall panel below should be as follows: 3.1 mm diameter nails at 300 mm centres for Class 1 and Class 2A buildings 3.1 mm diameter nails at 200 mm centres for Class 2B buildings.

7.3.4 Resistance to wind uplift The maximum uplift force occurs with wind on the gable walls. The maximum roof uplift pressure

0.867x0.80x 1.15x3.5

2.79 kN/m per side

Roof dead load

= 0.92x7.6/2

= 3.50 kN/m per side

Factor of safety against roof uplift

= 3.50/2.79

= 1.25

Required factor of safety against uplift (BS 5268-6.1 Clause 4.4)

= 1.20 Section 1.2.3 explains how to calculate the resistance of truss clips to combined lateral and vertical wind loads should this be necessary.

7.4 Racking calculations The method illustrated is in accordance with BS 5268-6.1.

7.4.1 Wind loads 7.4.1.1 Wind on front or rear wall

Horizontal load on roof F1

= 2.5 x 9.5 x 0.763

= 18.12 kN

With the wind on the rear wall

F2=F3 =2.5x9.5x0.747

© TRADA Technology Ltd 2006

78

= 17.74 kN

(fRADAgJ OV/

TRADA Technology Timber frame housing: UK Structural recommendations

7.4.1.2 Wind on gable wall

With wind on gable wall, door end F4

= 2.5 x 7.0 x 0.5 x 0.922

F5 = F6 = 2.5 x 7.0 x 0.433

= 8.068 kN = 7.578 kN

7.4.2 Racking forces The wind loads F are transferred as racking forces Q to the timber frame walls at right angles to them. Qa the applied force to be resisted by the first floor front and rear walls and by internal partitions parallel to them = F4 + 0.5 F5

= 11.86 kN

Force on ground floor front and rear walls and parallel partitions = F4 + F5 + 0.5 F6

= 19.44 kN

Force on first floor gable walls and parallel partitions = F1 ÷ 0.5 F2

= 26.99 kN

Force on ground floor gable walls and parallel partitions = F1 + F2 + 0.5 F3

= 44.73 kN

(When opposite wind directions produce different racking forces the higher value is used).

7.4.3 Design method for racking It may be possible to demonstrate that external timber frame walls designed to a minimum specification can support the actual racking forces, without having to calculate the contribution of other elements such as internal partitions. However, if the calculation shows the strength of the external walls alone to be insufficient, then as many as necessary of these other elements will have to be considered. The complete order to be followed is: 1. Strength of external timber frame walls without vertical load allowance. 2. Increase in strength due to vertical loads on top of such walls. 3. Contribution of any exterior masonry walls, via wall ties. 4. Contribution of internal partitions.

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TRADA Technology Timber frame housing: UK Structural recommendations

If the racking resistance thus calculated is still less than the racking force, then a stronger construction for the external timber frame walls must be chosen and the procedure repeated. The construction can be strengthened by: 5. 6. 7. 8.

Decreasing the nail spacing. Increasing the nail diameter. Increasing the thickness of the sheathing. Increasing the thickness of the lining. (This step would be unusual if the normal 12.5 mm thick plasterboard lining were used).

If there is still insufficient resistance, then

9. Apply a structural sheathing to the internal partitions. The racking force on the ground floor gable walls will be considered first, since it is likely to be the most onerous condition.

7.4.3.1 Strength of external walls without vertical load allowance Clause 4.7.2 of BS 5268-6.1 gives a formula for calculating the permissible racking force on a timber frame wall. This can be rewritten as:-

=(R+R1)XLXK

Qadm

where R and R1

= the basic racking resistance of sheathing and lining respectively in kN/m, modified by the product of the material modification factors for nail diameter, nail spacing and panel thickness = K101 x K102 x K103 = KM

L

= wall length in m

K

= product of wall modification factors for panel height, wall length, openings, vertical load and masonry contribution = K104 x K105 x K106 x K107 x K108

This version of the formula allows for the possibility that KM may be different for the sheathing and lining. The first step is to determine whether a construction of minimum strength is adequate. Denoting the ground floor gable wall with a door by the suffix d and the opposite gable wall by o'. =7.Om Ld =L0 Note: The most commonly used sheathing material is 9 mm thick OSB Grade 3 to BS EN 300. In order to illustrate the full design procedure, however, this example uses 7.5 mm thick plywood because this lower specification requires the use of various enhancements to meet the design requirements.

Table 2 in the Standard gives basic racking resistances of Rs,basic = 1.68 kN/m for 9 mm OSB or 9.5 mm plywood sheathing RI,basic = 0.28 kN/m for 12.5 mm plasterboard lining. (Higher values of basic racking resistance may be used if figures for the particular types of material which are specified are available from tests). K101 Nail diameter (Clause 4.8.2.1) The Standard permits the use of nails from 2.25 mm to 3.75 mm in diameter. For a diameter of d mm, the basic racking resistance must be multiplied by K101 where d K101

K101 is not applicable to plasterboard, which must be fixed with nails of at least 2.65 mm diameter in order to contribute to calculated racking resistance.

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TRADA Technology Timber frame housing: UK Structural recommendations

Using a 2.25 mm diameter nail for the sheathing

K101=---

=0.75

K102 Nail spacing (Clause 4.8.2.2) Table 2 in the Standard specifies the maximum nail spacings on the internal studs of timber frame panels which are permitted when racking calculations are based on values given in the Table. It also specifies standard nail spacings on the perimeter for various materials, s mm. Nail spacings should not be less than the minimum spacings recommended in BS 5268-2, eg 14 diameters for plywood-to-timber or OSB-to-timber joints, without pre-drilled holes. Although the Standard no longer specifies a maximum spacing of 300 mm, for reasons of wall stud stability and in harmony with recommendations for flooring it is recommended that the perimeter nail spacing should not exceed 300 mm, or 200 mm with sheathing less than 10 mm thick.

For a proposed nail spacing of S on the perimeter, the basic racking resistance must be multiplied by K102, where = _____________ S

K102

O.6x—- +0.4 sp

K102 is not applicable to plasterboard, which must be nailed throughout at centres not exceeding 150 mm.

In the case of plywood nailed at 200 mm centres on the perimeter 1

=

K102

= 0.833

I0.6x') + 0.4 150)

K103 Sheathing thickness (Clause 4.8.2.3) For 7.5 mm thick plywood the basic resistance of the standard 9.5 mm thick material must be modified by K103 (These are nominal thicknesses). = (2.8B - B2 —0.8)

K103

Where B

= proposed board thickness

Hence K103,5

= 0.789 for plywood

K103,1

standard board thickness

= 0.789 9.5

= 1.0 for plasterboard

Note that B is limited by BS 5268-6 to 0.75 B 1.25 R and R1 Hence if R5 and R1 are the racking resistance of the sheathing and lining respectively, R5

= 1.68 x K101 x K102 x K103

= 1.68 x 0.75 x 0.833 x 0.789 R1

= 0.28 x K103

=0.28 x 1.0

= 0.828 kN/m = 0.28 kN/m

K (Clause 4.9) K104 Height of wall panels (Clause 4.9.1) Panel height

= 2.4 m

:. K104 = 1

K105 Length of walls (Clause 4.9.2) Both walls exceed 4.8 m in length

:. K105 = 1.32

Note: Clause 4.9.3 states that in determining K105 for walls with framed openings which have a height in excess of half the panel height and are less than 300 mm from a corner of the building, the length of wall from the corner to the farther edge of the opening should be disregarded.

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TRADA Technology Timber frame housing: UK Structural recommendations

K106 Framed openings (Clause 4.9.3) K106

where

=(1_1.3.8)2

Aa

= aggregate area of framed openings in a wall

At

= total area of wall including openings

For the ground floor gable wall, door end Aa/At

= 0.103, hence

K106,d = 0.75.

For the ground floor gable wall, other end Aa/At

= 0.0, hence

K1060 = 1.00.

Note: Clause 4.9.3 states that where framed openings are separated by less than 300 mm and their heights are each greater than half the panel height, then the area of the opening should be taken as that of the rectangle which encloses both openings. Clause 4.9.4 explains that certain openings not more than 250 mm wide need not be framed. Its intention is that such openings may be discounted when calculating K106.

K107 Vertical loading (Clause 4.9.5) At this stage K107 will be taken as 1.0

K108 Interaction (Clause 4.9.6)

K

K104xK105xK106xK107xK106

1.0

.. K10

K108 = 1.1

:.KWd =

K0 = 1.45 Permissible racking force Qadm Qadm

=(R+Ri)xLxK

Hence Qadm,d = (0.828 + 0.28) x 7 x 1.09

= 8.45 kN

Qadm,o = (0.828 + 0.28)x 7 x 1.45

= 11.25 kN

Qadm, total = Qadm,d + Qadm,o

= 19.70 kN

If the two walls were of very dissimilar strength then the weaker wall alone would have to be considered with an appropriate share of the total racking load. From Section 7.4.2

= 44.73 kN

Qa Qadm



(m)

Length of storey height masonry wall

(kN).Mustbe>Qa

Permissible racking force = x K107 (kN) Total permissible racking force on the floor level and in wind direction shown

Uniformly distributed vertical load on external walls (10.5 kN/m maximum) Modification factor for vertical load

V

K107

Explanation

Symbol

Racking design figures Front

Rear

7

Wind on gable walls

62 99

8

87

82

87

9 02

9.02

9 10

3.50

4.96

3.50

7 .0

1.08

2.9

50.07 (>44.73) Strength adequate

2.23

0.871

0.792

1.9

7.00

1.33

1.21

3.9

11

TRADA'

2.23

0.871

0.792

1.9

34.50 (