It can be seen from the stress distribution of a loaded beam that the greater stress occurs at the top and bottom extremities of the beam.
This lead to the improvement on a rectangular section by introducing the I-section in which the large flanges were situated at a distance from the neutral axis. In effect, the flanges carried the bending in the form of tension stress in one flange and compression stress in the other, while the shear was carried by the web.
For these situations where bending is high but shear is low, for example in roof design, material can be saved by raising a framework design. A truss is a pinpointed framework.
A truss concentrates the maximum amount of materials as far away as possible from the neutral axis. With the resulting greater moment arm (h), much larger moments can be resisted.
Resistance of a truss at a section is provided by:
M = C*H = T*h
Where,
C = T in parallel chords and:
C = compression in the top chord of the truss.
T = tension in bottom chord of a simply supported truss.
h = vertical height of truss section.
If either C or T or h can be increased, then the truss will be capable of resisting heavier loads. The value of h can be increased by making a deeper truss.
Allowable C- or T-stresses can be increased by choosing a larger cross-section for the chords of the truss, or by changing to a stronger material.
A framework or truss can be considered as a beam with the major part of the web removed. This is possible where bending stresses are more significant than shear stresses. The simple beam has a constant section along its length, yet the bending and shear stresses vary. The truss, comprising a number of simple members, can be fabricated to take into account this change in stress along its length.
The pitched-roof truss is the best example of this, although the original shape was probably designed to shed rainwater. Roof trussess consist of sloping rafters that meet at the ridge, a main tie connecting the feet of the rafters and internal bracing members. They are used to support a roof covering in conjunction with purlins, which are laid longitudinally across the rafters, with the roof cover attached to the purlin. The arrangement of the internal bracing depends on the span.
Rafters are normally divided into equal lengths and, ideally the purlins are supported at the joints so that the rafters are only subjected to axial forces. This is not always practicable because because purlin spacing is dependent on the type of roof covering. When the purlins are not supported at the panel joints, the rafter members must be designed for bending as well as axial force.
The internal bracing members of a truss should be triangulated and as far as possible, arranged so that long members are in tension and compression members are intention and compression members are short avoid buckling problems.
The lattice girder, also called a truss, is a plane frame of open web construction, usually with parallel chords or booms at top and bottom. There are two main types, the N-(or pratt) girder and the Warren girder. They are very useful in long-span construction., in which their small depth-to-span ratio, generally about 1/10 to 1/14, gives them a distinct advantage over roof trusses.
Steel and timber trusses are usually designed assuming pin-jointed members, In practice timber trusses are assembled with bolts, nails or special connectors, and steel trusses are bolted , riveted or welded. Although these rigid joints impose secondary stresses, It is seldom necessary to consider them in the design procedure.
The following steps should be considered when designing a truss.
1. Select 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 combinations) loading, It usual to consider dead loads alone, and then dead and imposed loads combined.
4. Analyse the framework to find forces in all members.
5. Select the material and section to produce in each member a stress value that 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 caused by wind uplift.
Unless there are particular constructional requirements, roof trusses should, as far as possible, be spaced to achieve minimum weight and economy of materials used in the total roof structure. As the distance between trusses is increased, the weight of the purlins tends to increase more rapidly than that of the trusses. For spans up to around 20 m, the spacing of steel trusses is likely to be about 4 metres and, in the case of timber, 2 metres.
The pitch slope, of a roof depends on locality, imposed loading and type of covering. Heavy rainfall may require steep slopes for rapid drainage: a slope of 22 degree is common for corrugated steel and asbestos roofing sheets. Manufacture of roofing material usually make recommendations regarding suitable slopes and fixings.
Types of trusses
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