Simplified Design for Columns - Simplified Design of Reinforced Concrete Buildings - page 150

ε t (transition, 0.005 > ε t > 0.002 ) φ

3. For other values of is calculated as follows:

φ ε t - 0.002)(250/3) for columns with tie reinforcement

= 0.65 + (

φ ε t - 0.002)(250) for columns with spiral reinforcement

= 0.75 + (

φ φ

It is important to note that the value for P n should not exceed the value of P n , max as calculate from ACI

equation (10-1) or (10-2) above.

For columns subjected to combined axial load and biaxial bending (P u , M ux and M uy ), the design strength is
expressed by three-dimensional interaction surface. A simplified design method for this case is introduced in
Section 5.5.3.

5.4 PRELIMINARY COLUMN SIZING

It is necessary to select a preliminary column size for cost estimating and/or frame analysis. The initial selection
can be very important when considering overall design time. In general, a preliminary column size should be
determined using a low percentage of reinforcement; it is then possible to provide any additional reinforcement
required for the final design (including applicable slenderness effects) without having to change the column size.
Columns which have reinforcement ratios in the range of 1% to 2% will usually be the most economical.

The design charts presented in Figures 5-2 and 5-3, are based on ACI Eq. (10-2). These charts can be used for
nonslender tied square columns loaded at an eccentricity of no more than 0.1h, where h is the size of the
column. Design axial load strengths φ P n(max) for column sizes from 10 in. to 24 in. with reinforcement ratios

between 1 and 8% are presented in Fig. 5-2. For other columns sizes and shapes, and concrete strengths the
chart in Fig. 5-3, based on ACI Eq (10-2) can be used for preliminary column sizing.

These design charts will provide quick estimates for a column size required to support a factored load P u within
the allowable limits of the reinforcement ratio (ACI 10.9). Using the total tributary factored load P u for
the lowest story of a multistory column stack, a column size should be selected with a low percentage of
reinforcement. This will allow some leeway to increase the amount of steel for the final design, if required.