Trusses - 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 per- manent 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.

Effects of load reversal. (a) Normal loading; (b) reversal during erection; (c) counter bracing
Fig. 19.4 Effects of load reversal. (a) Normal loading; (b) reversal during erection; (c)
counter bracing

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