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Steel Designers' Manual - 6th Edition (2003)
The spacing of bridge trusses depends on the width of the carriageway for road
bridges and the required number of tracks for railway bridges, in addition to considerations
regarding lateral strength and rigidity. However, in general the spacing
should be limited to between 1 /18 and 1 /20 of the span, with a minimum of 4 m to 5 m
for through trusses and approximately 1 /15 of the span, with a minimum of 3 m to
4 m, for underslung trusses.
19.3 Effects of load reversal
For buildings with light pitched roofs, load reversal is often caused by wind suction
and internal pressure. Load reversal caused by wind load is of particular importance
as light sections normally acting as ties under dead and imposed loads may be
severely overstressed or even fail by buckling when required to act as struts. For
heavy pitched or flat roofs load reversal is rarely a problem because the dead load
usually exceeds the wind uplift forces.
For bridge trusses, load reversal in the component elements may be caused by the
erection technique adopted or by moving live loads, particularly in continuous
bridges. During the detailed design stage, consideration should be given to the
method of erection to ensure stability and adequacy of any member likely to experience
load reversal. For short-span simply-supported trusses erected whole, load
reversal in the chords and web members is attained if the crane pick-up points
during erection are at or near mid-span, Fig. 19.4(a) and (b). For large-span bridges,
erection by the cantilever method causes load reversal in the chords and web
members. Load reversal caused by moving loads is usually more significant in continuous
trusses. A convenient way of overcoming the problem of load reversal
in web elements which are likely to buckle is to provide either temporary or permanent
counter bracing, Fig. 19.4(c). This will ensure that the web elements are
always in tension under all load conditions and avoids the use of heavy compression
elements.
19.4 Selection of elements and connections
19.4.1 Elements
Selection of elements and connections 547
For light roof trusses in buildings the individual members are normally chosen from
rolled sections for economy; these are illustrated in Fig. 19.5(a). Structural hollow
sections are becoming more popular due to their efficiency in compression and their
neat and pleasing appearance in the case of exposed trusses. Structural hollow sections,
however, have higher fabrication costs and are only suited to welded construction.
For larger-span heavily-loaded roof trusses and small-span bridge trusses
it often becomes necessary to use heavier sections such as rolled universal beams