Civil Engineering Reference
In-Depth Information
(2) The truss is loaded and supported only at its joints.
(3) The forces in the members of the truss are purely axial.
Assumptions (2) and (3) are interdependent since the application of a load at some
point along a truss member would, in effect, convert the member into a simply
supported beam and, as we have seen in Chapter 3, generate, in addition to axial
loads, shear forces and bending moments; the truss would then become statically
indeterminate.
4.3 I
DEALIZATION OF A
T
RUSS
In practice trusses are not pin-jointed but are constructed, in the case of steel
trusses, by bolting, riveting or welding the ends of the members to gusset plates
as shown in Fig. 4.4. In a timber roof truss the members are connected using
spiked plates driven into their vertical surfaces on each side of a joint. The joints
in trusses are therefore semi-rigid and can transmit moments, unlike a friction-
less pinned joint. Furthermore, if the loads are applied at points on a member
away from its ends, that member behaves as a fixed or built-in beam with unknown
moments and shear forces as well as axial loads at its ends. Such a truss would pos-
sess a high degree of statical indeterminacy and would require a computer-based
analysis.
However, if such a truss is built up using the basic triangular unit and the loads and
support points coincide with the member joints then, even assuming rigid joints, a
computer-based analysis would show that the shear forces and bending moments
in the members are extremely small compared to the axial forces which, themselves,
would be very close in magnitude to those obtained from an analysis based on the
assumption of pinned joints.
Two angle sections
back to back
Rivets
Gusset
plate
Single
angle
Centroidal axes
F
IGURE
4.4
Actual truss
construction
