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DESIGN PRINCIPLES OF TRUSSED BEAMS


DESIGN PRINCIPLES OF TRUSSED BEAMS




A beam which is stiffened by a system of braces constituting atruss of which the beam is a chord is called as Trussed Beam. Trussed beams 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.



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


The shape shown is better to take care of the Bending moment.

H= rise of the truss which is 1/8 for AC sheet.


As suggested earlier the top main chord (rafter) is divided into main divisions which in turn subdivided to suite the roof covering sheets.




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.


The structural design procedure consists of six principal steps.


Selection of type and layout of structure.


Determination of loads on the structure.


Determination of internal forces and moments in the structural components.


Selection of material and proportioning of members and connections for safety and economy.


Checking the performance of the structure under service conditions, and


Finial review.


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. 50×1000/125=400mm. (?8times span- in mm).


This is to avoid bending of truss on either side during erection.

The shape of a truss :


TRUSSED BEAMS:

FORMULAE FOR TRUSSED BEAMS-REF: Building engineers hand book.

Trussed beams are 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.


Es=2.1×10^6Kg/sq.cm(steel)
Design formulae.
Sl.no
Description
Single strut
Double strut
Uniformly distributed load –in Kg
1.
Tension in rod
0.312Wh/r
Wh/3r
2.
Compression in strut
0.625W
W/3
3.
Compression in beam
0.312WL/2r
WL/9r
Concentrated load over strut,Kg
1.
Tension in rod
Ph/2r
Ph/r
2.
Compression in strut
P
P
3.
Compression in beam
PL/4r
PL/3r
Lecture notes by Dr.L.S.Jeyagopal, a leading structural consultant.
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