33412Deflection Checks

The service load deflections of the stub girder are needed for several purposes. First, the overall dead load deflection is used to assess the camber requirements. Due to the long spans of typical stub girders, as well as the flexibility of the framing members and the connections during construction, it is important to end up with a floor system that is as level as possible by the time the structure is ready to be occupied. Thus, the girders must be built in the shored condition and the camber should be approximately equal to 75% of the dead load deflection.

Second, it is essential to bear in mind that each girder will be shored against a similar member at the level below the current construction floor. This member, in turn, is similarly shored, albeit against a girder whose stiffness is greater, due to the additional time of curing of the concrete slab. This has a cumulative effect for the structure as a whole and the dead load deflection computations must take this response into account.

In other words, the support for the shores is a flexible one and deflections therefore will occur in the girder as a result of floor system movements of the structure at levels in addition to the one under consideration. Although this is not unique to the stub girder system, the span lengths and the interaction with the frame accentuate the influence on the girder design.

Depending on the structural system, it is also likely that the flexibility of the columns and the connections will add to the vertical displacements of the stub girders. The deflection calculations should incorporate these effects, preferably by utilizing realistic modified Ec-values, and determining displacements as they occur in the frame. Thus, the curing process for the concrete might be considered, since the strength development as a function of time is directly related to the value of Ec [13]. This is a subject that is open for study, although similar criteria have been incorporated in studies of the strength and behavior of composite frames [16,17]. However, detailed evaluations of the influence of time-dependent stiffness still need to be made for a wide variety of floor systems and frames. The cumulative deflection effects can be significant for the construction of the building and consequently also must enter into the contractor's planning. This subject is addressed briefly in Section 33.5.

Third, the live load deflections must be determined to assess the serviceability of the floor system under normal operating conditions. Several studies have demonstrated that such displacements will be significantly smaller than the L/360 requirement that is normally associated with live load deflections [5-7,11]. It is therefore rarely possible to design a girder that meets the strength and the deflection criteria simultaneously. In other words, strength governs the overall design.

Finally, although they rarely play a role in the overall response of the stub girder, the deflections and end rotations of the slab across the openings of the girder should also be checked. This is primarily done to assess the potential for local cracking, especially at the stub ends and at the floor beams. However, proper placement of the longitudinal girder reinforcement is usually sufficient to prevent problems of this kind, since the deformations tend to be small.

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