Page 340 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FLOOR AND ROOF SYSTEMS

7.24 CHAPTER SEVEN

7.16 DEAD-LOAD DEFLECTION

Although, in general, building codes restrict the magnitude of live-load deflections, they do not con-
tain criteria or limitations relating to dead-load deflections.
The dead-load deflection of the floor framing system will not affect the levelness of the floor surface
if the concrete is finished level despite the deflection or if the floor framing members are cambered
for deflection due to the concrete dead load. In cases where the concrete is finished with a level surface,
the slab will be thicker at midspan due to ponding and, hence, the floor framing members should be
designed for the additional concrete dead load. In cases where the floor framing members are
cambered, care must be taken to avoid providing too much camber. (See “Shored versus Unshored
Construction” in Art 7.8.) Some designers find it prudent to establish a maximum camber limit,
such as 1/240 of the span, above which a heavier or deeper beam should be selected in order to avoid
potential serviceability problems.
When shored construction is used, or when the concrete floor thickness is kept constant, that is, the
top surface follows the deflected shape of the framing members to avoid the placement of additional
concrete, the dead-load deflection of the floor-framing system should be evaluated to determine
whether the resulting floor levelness will be acceptable.

7.17 FIRE PROTECTION

There are several methods by which fire ratings can be readily achieved for structural-steel floor fram-
ing systems. These methods include application of spray-on fire protection, encasement of the framing
members in a fire-rated assembly, or installation of a fire-rated ceiling system below the framing. For
open-web joists and lightweight steel framing, the last two options are usually more practical because
spray-on fire protection of such members tends to be difficult. (See also Art. 4.12.)

7.18 VIBRATIONS

Although a floor system may be adequately designed from a strength standpoint, a serviceability
problem will result if unacceptable vibrations occur during normal usage of the floor. Perceptibility
to vibrations is significantly affected by the amount of damping, or energy-dissipating capability,
provided by the structural and nonstructural components of a floor system.
The anticipated performance of floor systems designed prior to the 1980s could be analyzed by
computing the first natural frequency and the amplitude, that is, deflection when subjected to a heel-
drop impact, of the floor framing member and plotting the result on a modified Reiher-Meister scale
to determine the degree of perceptibility to vibrations. This method was generally accurate for con-
crete slab (including concrete fill on metal deck) floor systems framed with steel joists or steel
beams. However, these floor systems typically had much higher damping than modern floor systems.
For example, full-height partitions used in older office spaces resulted in damping ratios of 5–7%
compared to modern open-office layouts, which result in damping ratios of only 2–3%. As a result,
the modified Reiher-Meister scale is not recommended for the evaluation of modern floor systems
with low damping characteristics, such as open-office layouts.
Modern floor systems can be evaluated for walking-induced vibration by using AISC Steel
Design Guide Series No. 11, “Floor Vibrations Due to Human Activity.” Design Guide 11 provides
recommended damping ratios and actual live loadings for various functional uses, and a procedure
for computing the predicted acceleration of the floor system and comparing it to a tolerable acceler-
ation. (See example in Art. 6.13.)
It is sometimes prudent to design for a more conservative condition, such as a lower damping
ratio corresponding to an electronic office as opposed to a paper office, to account for a potential
change in usage in the future. It is generally much more difficult and costly to rectify a vibration
problem in an existing structure than to eliminate the problem in the original design.

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