Simplified Design for Columns - Simplified Design of Reinforced Concrete Buildings - page 170

5.7 PROCEDURE FOR SIMPLIFIED COLUMN DESIGN

The following procedure is suggested for design of a multistory column stack using the simplifications and
column design charts presented in this chapter. For sway frames with non-slender columns, both gravity and
wind loads must be considered in the design. Figs. 5-18 through 5-25 can be used to determine the required
reinforcement. For non-sway frames with shearwalls resisting the lateral loads and the columns sized so that
slenderness may be neglected, only gravity loads need to be considered; the reinforcement can be selected for
Figs. 5-18 through 5-25 as well.

STEP (1) LOAD DATA

(a) Gravity Loads:

Determine factored loads P u for each floor of the column stack being considered. Include a service dead
load of 4 kips per floor for column weight. Determine column moments due to gravity loads. For inte-
rior columns supporting a two-way floor system, maximum column moments may be computed by
ACI Eq. (13-7) (see Chapter 4, Section 4.5). Otherwise, a general analysis is required.

(b) Lateral Loads:

Determine axial loads and moments from the lateral loads for the column stack being considered.

STEP (2) LOAD COMBINATIONS

For gravity (dead + live) plus lateral loading, ACI 9.2 specifies five load combinations that need to be
considered (Table 2-6).

STEP (3) COLUMN SIZE AND REINFORCEMENT

Determine an initial column size based on the factored axial load P u in the first story using Fig. 5-2,
and use this size for the full height of building. Note that the dimensions of the column may be preset
by architectural (or other) requirements. Once a column size has been established, it should be deter-
mined if slenderness effects need to be considered (see Section 5.6). For columns with slenderness
ratios larger than the limits given in ACI 10.10, it may be advantageous to increase the column size (if
possible) so that slenderness effects may be neglected.

As noted earlier, for nonslender columns, Figs. 5-18 through 5-25 may be used to select the required
amount of reinforcement for a given P u and M u . Ideally, a column with a reinforcement ratio in the
range of 1% to 2% will result in maximum economy. Depending on the total number of stories,
differences in story heights, and magnitudes of lateral loads, 4% to 6% reinforcement may be required
in the first story columns. If the column bars are to be lap spliced, the percentage of reinforcement
should usually not exceed 4% (ACI R10.9.1). For overall economy, the amount of reinforcement can
be decreased at the upper levels of the building. In taller buildings, the concrete strength is usually
varied along the building height as well, with the largest › used in the lower level(s).