Trusses:

It can be seen from the stress distribution of a loaded beam that the greatest stress occurs at the top and bottom extremities of the beam .This led to the improvement on a rectangular section by introducing the l-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 rising a framework design.

Before discussing more trusses let us know some basic parts and types of trusses.

considering very simple truss the vertical member is called web and the side members are called upper cords and horizontal members can be termed as lower cords for better understanding of the truss components check this figure out.

   For enlarges view click this Fig.

Now as we know the mail components of a truss let us gain some knowledge about some types of the truss. Flowing Figs can very easily explain some very basic types of trusses.

               CLICK PAGE 1                                                       CLICK PAGE 2

There are also some specific types of trusses mainly named as

Light Frame Trusses
Light frame trusses are made from dimension lumber of various sizes. The chords and webs are connected together by the use of toothed connector plates which transfer the tensile and shear forces. The connector plates are stamped from galvanized steel sheet metal of different grades and gauge thicknesses to provide different grip values. Design of these trusses is usually done by the truss manufacturers using computer software developed by the truss plate manufacturers. Other custom developed structural analysis software is also used.

Heavy Timber Trusses
Heavy timber trusses are made from timbers, or from manufactured wood products, such as glulam or parallel strand Iumber (PSL), havlng the dimensions of timbers. Connections for members are made by using bolts and plates, split rings and special brackets and hangers. Heavy timber trusses provide long spans for applications where the space required for the depth of the trusses can be accommodated into a building plan.

For their more details 

Now as we know all basic components about truss let us discuss how load is supported by the truss.

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 x h = T x h, where C = T in parallel cords 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, 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, made up of 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 rain water. Roof trusses consist of sloping rafters which 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 purling, which are members laid longitudinally across the rafters, the roof covering being attached to the purling. The arrangement of 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, since 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, be arranged so that long members are in tension and compression members are short to avoid buckling problems.