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Steel Designers' Manual - 6th Edition (2003)
In some instances there may be a requirement for alternate columns to be omitted
for planning requirements. In this instance load transmission to the foundations is
effected by the use of long-span eaves beams carrying the gravity loads of the intermediate
truss to the columns: lateral loading from the intermediate truss is transmitted
to points of vertical bracing, or indeed vertical cantilevers by means of
longitudinal bracing as detailed in Fig. 1.12. The adjacent frames must be designed
for the additional loads.
Considering the truss and stanchion frame shown in Fig. 1.13, the initial assumption
is that all joints are pinned, i.e. they have no capacity to resist bending moment.
The frame is modelled in a structural analysis package or by hand calculation, and,
for the load cases considered, applied loads are assumed to act at the node points.
It is clear from Fig. 1.13 that the purlin positions and nodes are not coincident;
consequently, due account must be taken of the bending moment induced in the
rafter section. The rafter section is analysed as a continuous member from eaves to
apex, the node points being assumed as the supports, and the purlin positions as the
points of load application (Fig. 1.14).
The rafter is sized by accounting for bending moment and axial loads, the web
members and bottom chord of the truss being initially sized on the basis of axial
load alone.
Use of structural analysis packages allows the engineer to rapidly analyse any
number of load combinations. Typically, dead load, live load and wind load cases are
analysed separately, and their factored combinations are then investigated to determine
the worst loading case for each individual member. Most software packages
provide an envelope of forces on the truss for all load combinations, giving
truss for horizontal load
T/VNJNV
truss for vertical load
Fig. 1.12 Additional framing where edge column is omitted
Design of common structural forms 19