Design & Detail to BS 8110-1997

BQ7I

Brifish Ceiiieri Associahon

Designed and detailed (BS 8110: 1997) J. B. Higgins and B. R. Rogers MA, CEng, MICE

This document contains 32 pages OFC

For a worldwide and up-todate literaturesearch Ofl any aspect of concrete design or construction and related topics, contact the BCAs Centre for Concrete Informationon Dl 344 762676.

43.501 First published 1973 Second edition 1986 Third edition 1998

Published by

British CementAssociation

Century House. Telford Avenue Crowihorne.BerksRG45 ÔYS

ISBN 07210 1541 7 Price Group F © British Cement Association 1998

Tel: 01344 762676

Fax: 01344 761214 Website: www.bca.org.uk

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and limitations ol its contents and take responsibilityfor its use and application.No liability (including that for negligence) for any loss resulting from such advice or informationis accepted. Readers should note that all BCA publicationsare subjectto revisionfromtime to time and should therefore ensure that they are in possession of the latest version.

IFC

Designed and detailed (BS 8110: 1997) J. B. Higgins and B. R. Rogers MA. CFng, MI(I

Contents

Foreword This third edition ofDesignedand detailedhas been revised to BS 8110 : Part I: 1997, and the amendment dated 15 September 1998. Althoughthere havebeen several amendments to the code since 1985, the latest and most significant change in the partial safety factorfor reinforcement m from 1.15 has been the reduction . . to 1 .05. With higher stresses, less steel is required. However, the total saving may not be fully realised becausethere are other considerations such as choosinga practical arrangement of bars, and the deflection in the case of shallower

2

Introduction

3

BS 8110 and limit state design

6

Design information

7

Structural summary sheet

8

Floor slab

members.

10

First-floormain beam

The calculations have also been revised for the loading requirements ofBS 6399 Part 1: 1996 and Part 2: 1995.

16

Edge beam

18

Columns

22

Foundation

Designcharts in BS 8110: Part 3: 1985 may still be used to providea conservative solution, and one ofthese charts has been includedfor the design of columns. Lap lengths for these members have also been takenfrom BS 8110, Table 3.27, but adjusted for the design stress of 087f.

24

Shear wall

26

Staircase

28

Columndesignchart

29

Further information

.

The tie reinforcement for robustness is designed at its characteristic strength. If the characteristic bond stress is used for calculating laps and anchoragelengths, then the values in Table 3.27 may be multiplied by I 05/l4. This publication takes a conservative practical approach and usesdirectly the values given in Table3.27. Observant users of previous editionswill appreciate the skill that is evidentin the setting out of the calculations and the drawings. This is the work of the late Jim Higgins, whose care in the production of the original artworkwas meticulous. Sadly, he never saw the second edition in print. I hope that my amendments to this thirdedition will not detract from his fine workmanship. Special thanks are due to Tony Threlfall for his advice and suggestions for this edition.

Railton Rogers

Introduction

The purposeof this publication is to apply the principles of limit stale design given in BS 8110 by means of a simple worked example for a reinforced concrete building frame. The calculations and details arc presented in a form suitable for design office purposes and are generally in accordance with the following pLihIications.

BRITISHSTANDARDS INSTITtJTION Siructural use 0/concrete. Part I . Code of practice/ordesign and construction. Milton Keynes, BSI. 1997. 120 pp. BS 8110

Part

I:

1997.

H MSTATIONERY OFFICE. Building and buildings. The Building Regnlation.v 1991 (Amended 1994). HMSO, London. 21 pp. Statutory Instruments No. 2768. BRITISHSTANDARDS INSTITUTION Loading/or buildings. Part I . Code 0/practice

br deadand inposed loads. Milton Keynes. BSI. 1996. It) pp. BS 6399 : Part

I

1996. BRITISH STANDARDSINSTITUTION.Loading/or

buildings. Part 2. Code 0/practice

Jiir wind loads.Milton Keynes,BSI. 1995.82 pP BS 6399 : Part 2:

1995.

/ir

BRITISH STANDARDS INSTITUTION.Loading buildings. Part 3. (ode 0/practice loads. Milton BSI. 1988. 23 pp. BS 6399 : Part 3: 1988. roo/ imposed Keynes.

/r

BRITISH STANDARDS INSTlThTIO'J. Specification /or scheduling, dimensioning, bendin' (111(1 cutiin' steelrein/irceinent/r concrete. Milton Keynes, BSI. 1989. 20 PP BS 4466 : 1989.

/ir

IIIE C()NCR VIE SOCIETY. Modelprocedure the presentation0/ calculation,r. London (now Slough). 1981 . Technical Report 5, second edition. 18 pp. THE CONCRETE SOCIETY ANDTHE INSTITUTIONOF STRUCTURAL F.NGINEERS, 5iandardmethodo/detailnig structural concrete. London. The Institution. 1989. 138 pp.

BS 8110 and limit

c:)bjective

state design

To serve its purpose, a structure must be safe against collapse and be serviceablein use. Calculations alone do not produce safe, serviceableand durable structures. Equally important are the suitability of the materials, quality control and supervisionof the workmanship. Limit state design admits that a structure may become unsatisfactory through a number of ways which all have to be considered independently against defined limits of satisfactory behaviour. It admits that there is an inherent variability in loads, materials and methods of design and construction which makes it impossible to achieve complete safety against any possible shortcoming. By providing sufficient margins of safety, the aim of limit state design is to provide an acceptable probability that the structure will perform satisfactorilyduring its intended life. Limit states can he classified into two main groups: (I) the ultimate limit state, which is concerned with the provision of adequate safety; (2) the serviceability limit states, which are essentially concerned with durability. Generally, in practice, there are three limit states which are normally considered for reinforced concrete and these are given in the Table below.

Serviceabilitylimit states

Ultimate

Objective

limit state

Deflection

Cracking

Provision of adequate safety

Structure should not deflect so as to impair use of structure

Cracking should not be such as to damage finishes or . otherwise

Design ultimate

Loading regime

loads

Design service load

Performance limit

Deflection should Crack width Structure should not exceed should not not fail exceed 03 mm specified limits

Characteristicvalues

impair usage

generally

For the testing of materials, a statistical approach can be applied to the variations within materials which occur in practice. A normal or Gaussian distribution curve is assumed to represent the results of the tests and a value known as the characteristic value can be chosen below which not more than 5% of the test results may be expected to lie. The characteristic strength is given by the equation: Characteristicstrength = Mean or Averagestrength — L64 X Standarddeviation Ideally, a characteristic load should be similarly defined, as a load with a 5% probability of being exceeded during the lifetime of the structure. Flowever, it is not yet possible to-expressloading in statistical terms, so the Code uses the loads defined in BS 6399: Parts 1, 2 and 3. 3

Desiqn toads

The design load is given by the equation: Design load = Characteristic load X

where 'r is a partial safety factor for loading. This factor takes into account the possibility that the loads acting on the structure may be greater than the characteristic values. It also takes into account the assumptions made in the method of analysis, and the seriousnessof failure to meet the design criteria for a particular limit state. The consequence of collapse is much more serious than exceeding the serviceability limits and so this is reflected in the higher values of the partial safety factors. Components of load have to he considered in their most unfavourable combinations, Sc) sets of values of for minimum and maximum design loads are required. For example, the worst situation for a structure being checked for overturning under the action of wind load will he where the maximum wind load is combined with the minimum vertical dead load. Lower values of ;' are used for the combination of wind, imposed and dead loads than for the combinations of wind and dead, and dead and imposed loads, as the probability Df three independent design loads achievingtheir maximum value at the same time is less. The table below gives the partial load factors for the ultimate limit state.

Partial safety factor to be applied to Combination of loads

dead load

14 14 12

adverse heneficEal

10

16

10

—

—

12

12

12

1)

with imposed

Deiiçn strenqths

wind

load

when effect of load is adverse beneficial

1 Dead and imposed 2 Dead and wind 3 Dead and wind

imposed load —

—

14 12

The designstrengthis given by the equation: Characteristic strength [)esign strength = —______________________ is a partial safetyfactor on the material strength. This factor takes into account the variation in workmanship and quality control that may normally be expectedto occur in the manufacture of the materials. The values of to he used for the two materials when designingfor the ultimate limit state are given below: where

Values of

,

for theultimate limit state

Reinforcement

(oncrete Flexure or axial load Shear strength without shear reinforcement Bond strength Others (e.g. bearing stress)

iOLisiuest

4

I .05

IS 125

14 15

In addition to providing a structure that is capable of carrying the design loads, the layout should be such that damage to small areas of a structure or failure of single elementswill not lead to a major collapse. The Code requires that in all buildings the structural members should be linked together in the followingmanner: (a) by effectively continuous peripheral ties at each floor and roof level:

(b) by internal ties in two directions approximately at right-angles, effectively continuous throughout their length and anchored to the peripheral ties at each end (unless continuing as horizontal ties to columns or walls); (c) by external column and wall ties anchored or tied horizontally into the structure at each floor and roof level; (d) by continuous vertical ties from foundation to the roof level in all columns and walls carriing vertical loads.

In the design of the ties, the reinforcementmay be assumed to be acting at its characteristic strength with no other forces present but the tie forces. Reinforcementprovided for other purposes can often be used to form part or the whole of these ties, so that in the design process, when the required reinforcementfor the usual dead, imposed and wind loading has been found, a check can be made to see whether modifications or additions to the reinforcementare required to fulfil the tie requirements.

Durabflty and re resislance

At thecommencementof the design, the following should be considered: — the climate and environmental conditions to which the concrete will be — —

exposed; the concrete quality; the cover to the reinforcement.

It should also be noted that the quality of the construction process and the Iirst hours after casting of the concrete have a major influenceupon the subsequent durability of the structure. The cover for protection against corrosion may not be sufficient for fire protection, so this should be considered at the onset of the design, and also the dimensionsof the members. The Code gives maximum water/cement ratios, minimum cement contents and minimum characteristic strengths for concretes suitable for use in various environments with specified covers and using 20 mm nominal maximum size aggregate. The minimumgrades will generallyensure that the limits on free water/cement ratio and cement content will be met without further checking.

Appflcation

Durability and fire resistance requirements are considered at the onset of the design process because this determines the grade of concrete, the cover, and the size of the members. Usually,for most structures, Part 1 of the Code will be used in which it is assumed that the ultimate limit state will be the most critical limit state. Design will therefore be carried out at this limit state, followed by checks to ensure that the serviceabilitylimit states of deflection and cracking are not reached. In special circumstances,other limit states, such as vibration or the effects of fatigue, may require consideration. Should it be necessary to calculate deflectionsand crack widths, methods are given in Part 2 of the Code. The serviceability limit state of deflection may be the limiting requirement for floor slabs with large span/effective-depthratios. This can he checked before the reinforcementis determined, although some engineers may prefer to followthe procedure where the check is made after thereinforcementhas been found. Simplifieddetailing requirements for the curtailment of the reinforcement may be used for beams and slabs which fulfil certain design conditions. Nowever, for other situations, the curtailments should be taken from a bending moment envelopeand be in accordance with the general recommendationsof the Code.

5

Design information

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2

Commentaryon bar arrangement US 8110 ref

—

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I

3.3.1.2 3.12.9.1 3.12.8.14

—

2

3.12.3.4

— 3.12.11.1 3.12.9.1

'4

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3 —

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'

loose I—bars are bxcd insidc the column bars and provide continuitS for column and internal ties. ('heck minimum distance between tension bars 25 mm (aggregate si/c 5 mm). 30)) — 200 -— 100 mm 25 mm — OK. Top legs propect from centre-line into span. minimum dimensions shown in the curtailment diagram.

f

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3.12.8.14 3.12.8.3 —

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5

10

-

'

—4-

3.12.8.3 —

6.9

link hangerbars arc same length as bar marks I and 4. Bar is onesize largerthan links(n' inimum 12 mm).

3.12.9.1

7.8

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3.12.4.1 — 3.12.8.12

II

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Bar marks

Notes

I

Horizontal bars in this member provide the peripheral tie. Minimum lap = 300 mm. The two tension bars are stopped 50 mm from the column lace to avoid clashing with the column bars shown in section A-A.

3.12.8.11

3.12.10.2

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A

Figure 3.24 Table3.27 3.12.10.2 Figure 3.24

3

3.12.10.2 Figure 3.24

4 5

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c247

(tOGOIvw)

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(1 0

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ok, ok. 19

BS 8110

CALCULATIONS

ref.

(Foo

EXTRJAL COLUMt4

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k

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TOTAL LDAO t.

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Commentary on bar arrangement BS 8110 ref

3.12.5.3

3.12.6.2 3.12.8.15 Table3.27 3.12.7.2 3.12.7.1 3.12.8.12 —

Bar marks

1

2

3

Table 3.27 —

4

3.12.8.13b Table3.27 3.12.8.14

-

—

5

—

6

Notes

The presentation shownabove is schematic. This tabular method adapts readily to element repetition. The sections are shown in their relativepositions adjacent to the vertical reinforcement. Main bars,area> minimum 04%bh. = Slope of crank at lower end = 1:10 maximum.Crank offset 50 + 10% =55 mm.

Minimumcrank length = 350 mm (140). Length of short projection beyond crank = compressionlap +. say, 75 mm for tolerance. Reinforcementarea at laps < 10% bh. Bars project above first-floorslab level to provide a compressionlap above the kicker. Bar projection= 35 x 087/095 x 25 mm + 75 mm for kicker= 875 mm, i.e. compressionlap = 800 mm. A single link is provided, since each verticalbar is restrained by a corner. Minimum size = 25/4, use 8 mm. Maximum spacing = 12 x 25 = 300 mm. (R8 @ 300.) Cover to vertical bar = 40 mm> 15 x 25 = 375 mm. Linksextendto undersideof floor slab. Normally, starter bars are detailed with the footing, as column F2. It can be economic to detail starters withthe columnabove as shown.In this caseit is advisableto schedulethe starter bars so that they can be processedtogetherwiththe footing.Note with this detail that the sectionat mid-height also applies to the starterbar arrangement. The starter bars would be shown dotted on the footing detail together with a suitable cross-reference. Bars project abovethe top of the baseto providea compression lap above the kicker = 35 x 087/095 x 25 + 75 = 875 mm, i.e. lap = 800 mm. As barmark 1, but bars provide a tension lap above 1st floor kicker. Cover = 50 mm. Cleardistancebetween adjacent laps = 100mm< 175 = 438 mrn'Im. (T 2 @ 300 EF

= 754 mm2/m.j

T 12 bars provide reasonable rigidity for handling and help stabilize the cage during erection.

Table 3.27

3.12.3.4 —

4,5,6

7,8 9

Minimum projection above top of first—floor level is a compression lap + kicker = say 25 mm. Lap = 450 mm. Minimum horiiontal reinforcement. Area = 438 mni7m. (T10 @ 200 EF = 786 mm'/m.) Provide at least a tension lap = 35 x 0 = 350 mm. say 450 mm to satisfy shrinkageand thermal requirements. Bars are placed outside vertical reinfircenient to provide maximum control against shrinkageand thermal cracking. Those bars in the wall 05 in below first—floor slab act also as interna] ties. Tension lap 6)r tie = 35 >< 10 = 350 mm, say 450 mm. Peripheraltie at first floor. 1,—bars at either end provide continuity with edge beams. Laps. say 450 mm. Wall spacers maintain location of each face of reinforcement. 25

Staircase end-span continuous slab

3500 175

5060

BS 8110 ref.

3?

T.$Ie

•3

o.

4

RLVrt

4

Ave.rMe

ctb L

OUTPUT

ri )

c'-

RE.1STAI4C.

.toor

LoA4 z

CALCULATIONS

c.e.cc o.k...

=

=

cZt.c ce4 SoaA

..

(14GS 1.4.o)5.o

.4'I

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2

4FOR.CMeNT

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-

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2

3

ck

32.34

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= T.b\3'tO

=

kN/

4.Okt/

43.

OSx46OxO.Sx'49

r

Q,S

k

0.040

A5

344

k

43.1< 571 = 286 mmlrn. Use 110 @ 15(1 = 524 mill/ni to match spacing of span bars. 1.ap, say450 mm. Optional ruinfoicLnlent Minimum ULd = 228 mm Simil u for h ii maik 16 27

Column design chart

50 45 40

35 CJ

E E 30

z 25

0

z

20 15 10

5

0

1

23 -

2 44

4 :3

4

5

6

7

8

9

10

11

M/bh 2 —N/mm2

Rectangular columns fcu

40 460

d/h

28

080

12

nformation from the Reinforced Concrete Council Spreadsheets Many of the design principles used in this publication will be covered by spreadsheets for reinforced concrete design now being developedby the Reinforced Concrete Council. Versions for both BS 8110and EC2 are in preparation. For detailswrite to the RCC at Century House, Telford Avenue, Crowthorne, Berks RG45 6YS.

Buildability and whole building economics It should be stressedthat the structural solution presented in this publication has been chosenfor the purpose:of illustrating analysis, designand reinforcement detailingprinciples. A typicalbuildingframe accounts for only 10% of the wholeconstruction cost, but affects foundations, cladding and service provision. The choice and details of a building's structure shouldreflect both buildability and overall building economics. Analysis of these factors using a structural optimisation program* or chartsfrom a publication** suggests that a flat slab alternative may save around 2% ofoverall building costs and ten days' construction time. Similarly, rationalisation and simplification ofreinforcement will normally speedconstruction and hence reduce overall construction costs and programmetime. Excessive curtailment and tailoring of reinforcement to save material at theexpenseofrationalisation will provecounter-productive. These aspects are currentlybeing investigated at the EuropeanConcreteBuildingProject at Cardington, and will result in the publication of best practiceguidance. With increasing emphasis on the cost in use ofbuildings, there is a trend towards the use of exposed soffits for passive cooling. This move to whole life costs will modify the optimum solution, and deep ribbedor coffereci slabs area favoured option to meet daylighting, thermal mass, ventilation and acoustic requirements. *Concept - a computerprogramthatallows the rapid semi-automated choice of concrete frame while considering wholebuildingcosts. Produced by the Reinforced Concrete Council. Available from the RCC on 01344 725733. ** Economic concrete frame elements - a pre-scheme design handbook, basedon BS 8110, that helps designers choose the most viableconcreteoptions. Produced by the Reinforced Concrete Council. Available from the ECA on 01344 7257U4.

IBC

Designed and detailed (BS 8110: 1997)

Cl/SfB (28)

J. B. Higgins and B. R. Rogers

q4

(K)

UDC 624.073.33.012.45:

BRITISH CEMENT ASSOCIATION PUBLICATION 43.501

(Ofl@rete OBC

624.04.001.3