Page 331 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FLOOR AND ROOF SYSTEMS

FLOOR AND ROOF SYSTEMS 7.15

FIGURE 7.16 Beam and girder with shear connectors for composite action with concrete slab.

shear connectors can be provided in the original design at minimal additional cost. When the
increased loadings must be accommodated, reinforcement plates need only be welded to the easily
accessible bottom flange of the beams and girders, since the added shear connectors have already
been installed.
Noncomposite design is generally found to be more economical for relatively short spans, where
the added cost of shear connectors tends not to justify the savings in steel framing, or for small pro-
jects where the cost of mobilization to provide shear connectors is not warranted.
Shored versus Unshored Construction. Composite floor framing can be designed as being either
shored or unshored during construction. In most cases, unshored construction is used, which allows
dead-load deflections to occur during the concrete placement and the floors to be finished with a level
surface. In such cases, the additional concrete dead load must be taken into account when designing
the beams and girders, and other components of the structure.
When unshored construction is used for moderate spans with relatively large dead-load deflec-
tions, the beams and girders can be cambered for the dead-load deflection, thereby resulting in a level
floor surface after placement of the concrete. Maximum and minimum values of camber, which are
dependent on the depth and length of the beam or girder, and permissible camber tolerances, are
given in the “Standard Mill Practice” section of the AISC Manual of Steel Construction.
When camber is specified, however, careful consideration should be given to the end restraint
of the beam (for example, whether the beam frames into girders or into columns), even if simple
connections are used throughout. End restraint reduces deflections, and camber that exceeds the
actual dead-load deflection can sometimes be troublesome, since it may affect the fire rating
(because of insufficient concrete-fill thickness over metal deck), the elevation of preset inserts in
an electrified floor system, or installation of interior finishes. It should also be kept in mind that
permissible camber tolerances are always positive, that is, will result in a camber greater than that
specified.
Shored construction will result in lighter or shallower beams and girders than unshored con-
struction, since the flexural members will act compositely with the floor deck in resisting the weight
of the concrete when the shores are removed. However, consideration must be given to the deflec-
tions that will occur after shore removal, and whether the resulting floor levelness will be acceptable.
Also, the effect on the construction schedule should be considered, since the shoring must be kept in
place until the concrete fill has reached sufficient strength. This is usually at least 75% of its specified
minimum 28-day compressive strength.

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