Bracing
Longitudinal and transverse bracing forces are dictated by the minimum two percent of the dead load requirement or the sum of the actual horizontal loads. Two by six lumber is a common
choice of material for bracing when wood posts are used. Spikes, thrubolts or lag bolts may be used as fasteners. X bracing is usually fastened at the intersection of the members to reduce the
length of the compression member.
Cable bracing can be used with either steel or wood posts. This bracing is attached to the steel caps and pretensioned prior to
taking load. When top and bottom caps are not parallel, the bracing cables will have nonsymmetrical geometry. This means that each
cable will have different preload forces to yield equal elongation and equal horizontal forces.Unequal preload forces can produce distortion in the bent contributing to internal collapse;
therefore, in addition to calculating the preload forces, the cables must be pretensioned simultaneously.
In choosing beams for stingers, continuous long spans and cantilevered ends should be avoided due to the sequential deflection caused by the placing sequence. The second span
will not fully recover from the initial negative deflection resulting in unwanted soffi t variations.In calculating camber strips to compensate for beam deflection, the entire dead load must be considered including the weight of the forms
and the weight of the beam along with any residual camber for the structure. In box girders with deep sections, the initial deflection caused by the first pour, soffit and stem, is usually the total deflection for the system. This reasoning recognizes the load carrying capacity of the soffit and stem section at or near f ′c and the relatively light load of the deck. Consideration must also be given to the beams carrying the deck-finishing
machine regarding the finished line and grade and designed depth of the top slab.
In prestressed multi span bridges, the dead load distribution to the hinge is considerable and will require bents heavier than those required to hold the falsework span dead load.