Trussed Beam- Design Principles and Erection Practices

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A beam which is stiffened by a system of braces constituting a truss of which the beam is a chord is called trussed beam. It consists of steel sections (or wooden beams) and struts of steel rods. Trussed beams are used when there is great weight to be supported across wide space without supports from beneath. For instance in the case of industrial buildings where free space requirement are essential for more working areas. Number of design principles have been established to not only simplify the design process but also get efficient and economical trussed beam. The span of trussed beam varies from 10m to 100m depending type and requirement of the available spaces. Added to that, truss beam provides an economic solution for spans longer than 25m.

Trussed Beam Design Principles

1. Trussed Beam Geometry

In order to get a good structural performance, the ratio of span to truss depth should be chosen in the range 10 to 15.The architectural design of the building controls its external geometry and governs the slope given to the top chord of the truss. In order to get efficient layout of the truss beam between chords, it is recommended to use 35° to 55° diagonal member inclination, apply point loads at nodes, and the orientation of the diagonal members should be such that the longest members are subject to tension.

2. Trussed Beam Sections

Sections should be symmetrical for bending out of the vertical plane of the truss. For members in compression, the buckling resistance in the vertical plane of the truss should be similar to that out of the plane. Moreover, for large member forces, it is a good solution to use chords having IPE, HEA or HEB sections, or a section made up of two channels (UPE) and diagonals formed from two battened angles. Added to that, it could be a good practice to have the top chord with a vertical web, and the bottom chord with a horizontal web. Furthermore, it is recommended to employ hollow sections for chords and/or for internals. Finally, the top main chord (rafter) is divided into main divisions which in turn subdivided to suite the roof covering sheets.

3. Types of Connections

Connections are used to connect different types of trussed beam. There are two major types of connection which include:

1. Welding connections
2. Bolted Connections

4. Spacing of Trussed Beams

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 1/4 of span up to 15.0m and 1/5 of span up to 15m to 30m.

5. Design Procedure for Trussed Beam Structure

The following steps should be considered when designing a truss:

1. Selection of type and layout of structure.
2. Determination of loads on the structure.
3. Evaluate internal forces and moments in the structural components.
4. Selection of material and proportioning of members and connections for safety and economy.
5. Checking the performance of the structure under service conditions, and finally perform the final review.

6. Design Formula for Trussed Beams

Table 1 Design formula for trussed beams

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

7. Fabrications and Erection of Trussed Beam

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. To avoid bending of truss on either side during erection, 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 span- in mm).