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Bridge truss
Fig. 1-69.

expansion or contraction of the members due to a change in
temperature or application of loads.
Assumptions for Design. To design both the members and the
connections of a truss, it is necessary first to determine the force
developed in each member when the truss is subjected to a given
loading. To do this we will make two important assumptions:
• All loadings are applied at the joints. In most situations, such
as for bridge and roof trusses, this assumption is true. Frequently the
weight of the members is neglected because the force supported by
each member is usually much larger than its weight. However, if the
weight is to be included in the analysis, it is generally satisfactory to
apply it as a vertical force, with half of its magnitude applied at each
end of the member.
• The members are joined together by smooth pins. The joint
connections are usually formed by bolting or welding the ends of the
members to a common plate, called a gusset plate, as shown in Fig. 1-
70,a, or by simply passing a large bolt or pin through each of the
members, Fig. 1-70,b. We can assume these connections act as pins
provided the center lines of the joining members are concurrent, as in
Fig. 1-70.
Because of these two assumptions, each truss member will act as
a two-force member, and therefore the force acting at each end of the
member will be directed along the axis of the member. If the force
tends to elongate the member, it is a tensile force (T), Fig. 1-71,a;

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