Serviceability - Design of Reinforced Concrete - page 151

Georgia Dome, Atlanta, Georgia. (Courtesy of Economy Forms Corporation.)

6.2 IMPORTANCE OF DEFLECTIONS

The adoption of the strength design method, together with the use of higher-strength con-
cretes and steels, has permitted the use of relatively slender members. As a result, deflec-
tions and deflection cracking have become more severe problems than they were a few
decades ago.
The magnitudes of deflections for concrete members can be quite important. Exces-
sive deflections of beams and slabs may cause sagging floors, ponding on flat roofs, exces-
sive vibrations, and even interference with the proper operation of supported machinery.
Such deflections may damage partitions and cause poor fitting of doors and windows. In
addition, they may damage a structure's appearance or frighten the occupants of the build-
ing, even though the building may be perfectly safe. Any structure used by people should
be quite rigid and relatively vibration-free so as to provide a sense of security.
Perhaps the most common type of deflection damage in reinforced concrete structures
is the damage to light masonry partitions. They are particularly subject to injury due to
concrete's long-term creep. When the floors above and below deflect, the relatively rigid
masonry partitions do not bend easily and are often severely damaged. On the other hand,
the more flexible gypsum board partitions are much more adaptable to such distortions.

6.3 CONTROL OF DEFLECTIONS

One of the best ways to reduce deflections is by increasing member depths—but designers
are always under pressure to keep members as shallow as possible. (As you can see, shal-
lower members mean thinner floors, and thinner floors mean buildings with less height, with
consequent reductions in many costs such as plumbing, wiring, elevators, outside materials