Design Of Steel Roof Trusses 4j3p41
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Design of Steel Structures
Design of Steel Roof Truss - G. S. Deshmukh
INTRODUCTION Trusses are triangular
frame works in which the are subjected to essentially axial forces due to external load. External load and the
lie in the same plane.
In case of space trusses
2
are oriented in three dimensions and also the loads may act in any Design of Steel Roof Truss direction.
Lattice Girders lattice girders are the trusses which frequently used to
span long lengths in the place of solid web girders.
3
Design of Steel Roof Truss
Steel subjected to axial forces are generally
more efficient than in flexure since the cross section is nearly uniformly stressed. Trusses are extensively used, especially to large span structures.
4
Design of Steel Roof Truss
USES Trusses are used in Roofs of single storey industrial buildings Long span floors and roofs of multistory buildings, to resist gravity loads [Figs. (a) and (b)].
5
Design of Steel Roof Truss
Walls of multi-storey buildings and horizontal planes
of industrial buildings to resist lateral loads and give lateral stability [Figs. (c) and (d)].
long span bridges to carry
gravity loads and lateral loads [Fig. (e)].
6
Design of Steel Roof Truss
LOADS ACTING ON TRUSS Dead load- 1) Dead load of claddings
2) Dead load of purlins 3) Self weight of the trusses 4) weight of bracings Live load
As per IS:875-1987 (Reaffirmed 1992). Wind load Earthquake load
As per IS:1893-1985. 7
Design of Steel Roof Truss
ANALYSIS OF TRUSSES The loads are assumed to be acting only at the nodes of
the trusses. They are usually statically determinate. Can be analysed manually by the method of ts or
by the method of sections.
8
Design of Steel Roof Truss
CONFIGURATION OF TRUSSES Pitched Roof Truss
Parallel Chord Truss Trapezoidal Truss
9
Design of Steel Roof Truss
TRUSS The of trusses are made of either rolled steel
sections or built-up sections depending upon the span length, intensity of loading.
10
Design of Steel Roof Truss
DESIGN OF TRUSSES The design standard (IS: 800) imposes restrictions on
the maximum slenderness ratio, (l/r), as given below: Member type
11
Max l/r limit
under compression under loads other than wind/ earthquake load
180
Tension undergoing stress reversal due to loads other than wind load or seismic forces
180
normally under tension but may have to resist compression under wind load
250
Compression flange of a beam against lateral torsional buckling
300
designed only for tension even though they may experience experience stress reversal
350
always under tension (unless pre-tensioned to avoid sag) sag)
400
Design of Steel Roof Truss
These limits are imposed to ensure the following: Too slender a member is avoided which may be damaged during transportation and erection. do not sag excessively under self-weight during
service causing excessive deflection in truss. Compression do not sag greater than 1/1000th of
their length, which is beyond the imperfection limit assumed in the compressive strength calculation. It is a common practice to specify a minimum angle size of
50 X 50 X 6 in the case of roof trusses. 12
Design of Steel Roof Truss
13
Design of Steel Roof Truss
Design of Steel Roof Truss - G. S. Deshmukh
INTRODUCTION Trusses are triangular
frame works in which the are subjected to essentially axial forces due to external load. External load and the
lie in the same plane.
In case of space trusses
2
are oriented in three dimensions and also the loads may act in any Design of Steel Roof Truss direction.
Lattice Girders lattice girders are the trusses which frequently used to
span long lengths in the place of solid web girders.
3
Design of Steel Roof Truss
Steel subjected to axial forces are generally
more efficient than in flexure since the cross section is nearly uniformly stressed. Trusses are extensively used, especially to large span structures.
4
Design of Steel Roof Truss
USES Trusses are used in Roofs of single storey industrial buildings Long span floors and roofs of multistory buildings, to resist gravity loads [Figs. (a) and (b)].
5
Design of Steel Roof Truss
Walls of multi-storey buildings and horizontal planes
of industrial buildings to resist lateral loads and give lateral stability [Figs. (c) and (d)].
long span bridges to carry
gravity loads and lateral loads [Fig. (e)].
6
Design of Steel Roof Truss
LOADS ACTING ON TRUSS Dead load- 1) Dead load of claddings
2) Dead load of purlins 3) Self weight of the trusses 4) weight of bracings Live load
As per IS:875-1987 (Reaffirmed 1992). Wind load Earthquake load
As per IS:1893-1985. 7
Design of Steel Roof Truss
ANALYSIS OF TRUSSES The loads are assumed to be acting only at the nodes of
the trusses. They are usually statically determinate. Can be analysed manually by the method of ts or
by the method of sections.
8
Design of Steel Roof Truss
CONFIGURATION OF TRUSSES Pitched Roof Truss
Parallel Chord Truss Trapezoidal Truss
9
Design of Steel Roof Truss
TRUSS The of trusses are made of either rolled steel
sections or built-up sections depending upon the span length, intensity of loading.
10
Design of Steel Roof Truss
DESIGN OF TRUSSES The design standard (IS: 800) imposes restrictions on
the maximum slenderness ratio, (l/r), as given below: Member type
11
Max l/r limit
under compression under loads other than wind/ earthquake load
180
Tension undergoing stress reversal due to loads other than wind load or seismic forces
180
normally under tension but may have to resist compression under wind load
250
Compression flange of a beam against lateral torsional buckling
300
designed only for tension even though they may experience experience stress reversal
350
always under tension (unless pre-tensioned to avoid sag) sag)
400
Design of Steel Roof Truss
These limits are imposed to ensure the following: Too slender a member is avoided which may be damaged during transportation and erection. do not sag excessively under self-weight during
service causing excessive deflection in truss. Compression do not sag greater than 1/1000th of
their length, which is beyond the imperfection limit assumed in the compressive strength calculation. It is a common practice to specify a minimum angle size of
50 X 50 X 6 in the case of roof trusses. 12
Design of Steel Roof Truss
13
Design of Steel Roof Truss
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