Roof Truss Design

DESIGN PRINCIPLES FOR ROOF STEEL TRUSS INTRODUCTION: Steel trusses are being used for both buildings and bridges. But the design principles are different for different uses. Many books are course oriented and not with a practical principles. Now an attempt has been made to gather information on the design principles from various references ON THE LAYOUT AND OTHER DESIGN PRINCIPLES. Steel roof trusses are used for mainly for the Industrial buildings where free space requirement are essential for more working areas. The span of truss varies from 10’-0” to 300’-0” depending on the type of requirement and the available spaces. The following steps should be considered when designing a truss: 1. Select the general layout of truss members and truss spacing. 2. Estimate external loads to be applied including self weight of truss, purlins and roof covering together with wind loads. 3. Determine critical (worst combination) loading. It is usual to consider Dead loads alone and then Dead and Imposed loads combined. 4. Analyze the frame work to find forces in all members. 5. Select material and section to produce in each member a stress value which does not exceed the permissible value. Particular care must be taken with compression members (struts) or members normally in tension but subject to stress reversal due to wind uplift. For span up to about 20.00 m, the spacing of steel trusses is likely to be about 4.00m i.e. 1/5 of span. A slope of 22Ø(degree) is common for corrugated steel and asbestos roofing sheets. For economic spacing of roof trusses, the cost of truss should be equal to twice the cost of purlins +the cost of roof covering. As a guide the spacing of the roof trusses can be kept : a. ¼ of span upto 15.0m. b. 1/5 of span upto 15m to 30m. Trusses with parallel chords are often referred to as LATTICE GIRDERS.

1

DIFFERENT SHAPES OF TRUSSES FOR DIFFERENT SPANS. 7.0M TO 11.0m

50) 2. that of the web member is greater than 100.(l/r>100). 3. All the members of the roof truss usually do not reach their limit state of collapse simultaneously. The design code suggest an effective length factor between 0.7 and 1.0 for the in-plane buckling of member depending upon this restraint and 1.0 for the out of plane buckling. Zin the case of roof trusses, a member normally UNDER TENSION due to gravity loads(DL+LL) may experience stress reversal into compression due to DL+WL combination. SLENDERNESS RATIO: The design standard (IS800) imposes max.slenderness ratio as given below: Sl.n Member type o

restrictions

on

the

Max limit

l/r

11

1. 2. 3. 4. 5.

Member under COMPRESSION under loads other than Wind/EQload TENSION members undergoing reversal due to loads other than WL Members normally under TENSION but may have to resist COMPRESSION under Wind load Members designed only for TENSION even though they may experience stress reversal Members always under TENSION

180 180 250 350 400

For smaller or where there is net uplift loading a WARREN truss will be lighter than PRATT-truss.

Ref: Design of Metal Structures by K.Mukhanov. Selection of sections and type of sections: The top chord if unequal angle section is chosen then the short leg should project downwards as shown to have more r xx. (outstanding

leg

is

smller

from

the

plane

of

the

truss)

Ly=2Lx

Equal angle section is preferred.

If each joint of the top chord is fixed in someway by ties or roof slab(Ly=Lx) equal stability of the chord is ensured by section formed of unequal leg angles installed with their short legs outstanding from the plane of the truss. The remaining compression diagonals and verticals between whose effective lengths there is insignificant differences(Lx=0.8L,Ly=L) are most frequently designed of equal leg angles. For tension members the type and arrangements of the angles is not so importance since here the determining factor is the net sectional area.

12

Other types of sections than angle are seldom employed and only if there are specific requirements for design. Thus for example, chord made from channels are employed when they are subjected not only to an axial force but also to a considerable local moments originated by a load applied between panel points of the trusses. The verticals of trusses are designed with a T section formed of tqo equal leg angles. EFFECTIVE LENGTH OF COMPRESSION ELEMENTS: The effective length of a compression top chord in the plane of truss is equal to its geometrical length(between panel point centres) Le=L. For diagonals(except for the support one which is considered as a continuation of chord) and verticals the effective length in the plane of truss is taken equal to Le=0.8L. When selecting angle sections for compression elements, the tendency should be to use angles of the smallest possible thickness since their radii of gyration have the relatively greatest value. Limiting slenderness ratio λ for compression and tension elements

NAME OF ELEMENT

Chords, support diagonals & verticals of trusses, transmitting support reactions Other truss elements Roofing ties(except brace rods)

COMPRES SION ELEMENT S

TENSION ELEMENTS UNDER DIRECT ACTION OF DYNAMIC LOADS

STATIC LOAD

IN BUILDING WITH HEAVY SERVICE CONDITIONS

120

250

400

250

150 200

350 400

400 400

300 300

Lecture notes by Dr.L.S.Jeyagopal, a leading structural consultant given below for further guidence:

is

1. A truss is a beam which is bent to the shape of the bending moment diagram in opposite direction.

13

The shape shown is better to take care of the Bending moment. H= rise of the truss which is 1/8 for AC sheet. 2. As suggested earlier the top main chord (rafter) is divided into main divisions which in turn subdivided to suite the roof covering sheets.

3. Nowadays it became obsolete to use the rivets but customary to make use of welding as well as high tension bolts. There are four types of bolts available. Bolt G98 means bolt is having a strength of 9 Mpa 8 is % of strength used for calculation. 4. The structural design procedure consists of six principal steps. a. Selection of type and layout of structure. b. Determination of loads on the structure.

14

c. Determination of internal forces and moments in the structural components. d. Selection of material and proportioning of members and connections for safety and economy. e. Checking the performance of the structure under service conditions, and f. Finial review. 5. Fabrication: Ease of fabrication and erection has an important influence on the economy of the design. In general, small and medium trusses of symmetrical design are lifted at the ridge during erection. In order to prevent buckling of the bottom chord, it is necessary to proportion it to carry the compressive stresses developed during hoisting. An empirical relation is given by b/L =1/125. where b is the width of the bottom chord at its centre and L the span length. For example a 50 m span truss shall have the top chord and bottom chord width =span/125.i.e. 50x1000/125=400mm. (≈8times spanin mm).

This is to avoid bending of truss on either side during erection. The shape of a truss :

15

TRUSSED BEAMS: FORMULAE FOR TRUSSED BEAMS-REF: Building engineers hand book Used for long spans and are built up of wooden beams and struts of steel rods. But the wooden beams may be replaced by steel sections.

16

Es=2.1x10^6Kg/sq.cm(steel) Design formulae. Sl.n

Description

o Uniformly distributed load –in Kg 1. Tension in rod 2. Compression in strut 3. Compression in beam Concentrated load over strut,Kg 1. Tension in rod 2. Compression in strut 3. Compression in beam

Single strut

Double strut

0.312Wh/r 0.625W 0.312WL/2r

Wh/3r W/3 WL/9r

Ph/2r P PL/4r

Ph/r P PL/3r

17