A Simplified Design Approach - Simplified Design of Reinforced Concrete Buildings - page 16

When increasing the overall slab thickness is not possible or feasible, drop panels or shear caps can be provided at
the column locations where two-way shear is critical. Chapter 4 gives ACI 318 provisions for minimum drop
panel dimensions.

In all cases, the slab thickness must be larger than the applicable minimum thickness given in ACI 9.5.3 for
deflection control. Figure 4-4 may be used to determine the minimum thickness as a function of the clear span
n for the various two-way systems shown.

1.8.2 Columns

For overall economy, the dimensions of a column should be determined for the load effects in the lowest story of
the structure and should remain constant for the entire height of the building; only the amounts of reinforcement
should vary with respect to height.* The most economical columns usually have reinforcement ratios in the range
of 1-2%. In general, it is more efficient to increase the column size than to increase the amount of reinforcement.
This approach is taken to eliminate congestion of column reinforcement, which has to be critically evaluated along
the lap splice length and to accommodate horizontal beam bars at the beam column intersections.

Columns in a frame that is braced by shearwalls (non-sway frame) are subjected to gravity loads only. Initial
column sizes may be obtained from design aids such as the one given in Fig. 5-2: assuming a reinforcement ratio
in the range of 1-2%, a square column size can be determined for the total factored axial load P u in the lowest
story. Once an initial size is obtained, it should be determined if the effects of slenderness need to be considered.
If feasible, the size of the column should be increased so as to be able to neglect slenderness effects.

When a frame is not braced by shearwalls (sway frame), the columns must be designed for the combined effects
of gravity and wind loads. In this case, a preliminary size can be obtained for a column in the lowest level from
Fig. 5-2 assuming that the column carries gravity loads only. The size can be chosen based on 1% reinforcement
in the column; in this way, when wind loads are considered, the area of steel can usually be increased without
having to change the column size. The design charts given in Figs. 5-18 through 5-25 may also be used to
determine the required column size and reinforcement of a given combination of factored axial loads and
moments. Note that slenderness effects can have a significant influence on the amount of reinforcement that is
required for a column in a sway frame; for this reason, the overall column size should be increased (if possible)
to minimize the effects of slenderness.

1.8.3 Shearwalls

For buildings of moderate size and height, a practical range for the thickness of shearwalls is 8 to 10 in.
The required thickness will depend on the length of the wall, the height of the building, and the tributary wind
area of the wall. In most cases, minimum amounts of vertical and horizontal reinforcement are sufficient for
both shear and moment resistance.

In the preliminary design stage, the shearwalls should be symmetrically located in the plan (if possible) so that
torsional effects on the structure due to lateral loads are minimized.

In tall buildings, › may vary along the height as well. *