A truss is a structure composed of slender members joined together at endpoints by bolting/Riveting or welding. Ends of the members are joined to a common plate called Gusset Plate.
- Roof trusses are often used as industrial building frames. The roof load is transmitted to the truss at the joints by means of a series of purlins. The roof truss along with its supporting columns is termed as bent.
- To keep the truss rigid, and thereby capable of resisting horizontal wind forces, knee braces are sometimes used at the supporting columns.
- The space between an adjacent truss is called a bay.
- Bays are often tied together using diagonal bracing in order to maintain rigidity of the building’s structure.
- Trusses used to support roofs are selected on the basis of the span, the slope, and the roof material.
- The scissors truss can be used for short spans that require overhead clearance.
- The Howe and Pratt trusses are used for roofs of moderate span about (18m) to (30m). For larger spans, the fan truss or Fink truss may be used.
- If a flat roof or nearly flat roof is to be selected, the Warren truss is used.
- Sawtooth trusses are used where column spacing is not objectionable and uniform lighting is important.
- The bowstring truss is sometimes used for garages and small airplane hangars.
- The arched truss is used for high rises and long spans such as field houses, gymnasiums, and so on.
- Load on the deck is first transmitted to stringers, then to floor beams and finally to the joints of the two supporting side trusses.
- The top and bottom cords of these side trusses are connected by top and bottom lateral bracing, which serves to resist the lateral forces caused by wind and the sideways caused by moving vehicles on the bridge.
- Additional stability is provided by the portal and sway bracing.
- The Pratt, Howe, and Warren trusses are normally used for spans up to 61m in length. The most common form is the Warren truss with verticals.
For larger span, Parker truss is used.
- The greatest economy of material is obtained if the diagonals have a slope between 45ο and 60ο with the horizontal.
Assumptions for Design of Truss Members and Connection
- The members are joined together by smooth pins.
- All loading is applied to joints.
- Self-weight of the members is negligible.
This assumption means that centre lines of joining members are concurrent. Thus, moment of F1, F2 and F3 about O is zero. i.e. joint acts as a pin.
Normally satisfied in roof and bridge trusses if purlin is at joints and in bridge trusses transferred at panel points.
Since force supported by members are large compared to its weight, the self-weight of members is neglected. For Example, if self-weight of a member of truss is 65kg it can even support 2400kg load.
Zero Force members
- If only two non-collinear members exist at a truss joint and no external force or support reaction is applied to the joint, the members must be zero-force members.
BC and CD are non-collinear members and no force exists at joint C, hence BC and CD are zero force members. Similarly, AB and AE are zero force members since A is not carrying any force, and AB and AE are non-colinear.
- If three members join at a point and out of them, two are colinear and also no external load acts at joint, third member is a zero-force member.
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The method consists in solving indirectly the equations of equilibrium as formulated in slope deflection method without finding the displacements. This is an iterative procedure. This is also known as relaxation method.
Slope deflection method is useful to analyse indeterminate structures like continues beams and plane frames.
For a given loading, the maximum stress and deflection of indeterminate structures are generally smaller than that in statically determinate structures.