Civil Engineering Reference
In-Depth Information
Columns and hammerhead cap forms for the Gandy Bridge, Tampa, Florida. (Courtesy of
Economy Forms Corporation.)
9.5
CODE REQUIREMENTS FOR CAST-IN-PLACE COLUMNS
The ACI Code specifies quite a few limitations on the dimensions, reinforcing, lateral re-
straint, and other items pertaining to concrete columns. Some of the most important limi-
tations are as follows.
1.
The percentage of longitudinal reinforcement may not be less than 1% of the
gross cross-sectional area of a column (ACI Code 10.9.1). It is felt that if the amount of
steel is less than 1%, there will be a distinct possibility of a sudden nonductile failure, as
might occur in a plain concrete column. The 1% minimum steel value will also lessen
creep and shrinkage and provide some bending strength for the column. Actually, the
Code (10.8.4) does permit the use of less than 1% steel if the column has been made
larger than is necessary to carry the loads because of architectural or other reasons. In
other words, a column can be designed with 1% longitudinal steel to support the factored
load and then more concrete can be added with no increase in reinforcing and no increase
in calculated load-carrying capacity. In actual practice the steel percentage for such mem-
bers is kept to an absolute minimum of 0.005.
2.
The maximum percentage of steel may not be greater than 8% of the gross cross-
sectional area of the column (ACI Code 10.9.1). This maximum value is given to prevent
too much crowding of the bars. Practically, it is rather difficult to fit more than 4% or 5%
steel into the forms and still get the concrete down into the forms and around the bars.
When the percentage of steel is high, the chances of having honeycomb in the concrete is
decidedly increased. If this happens, there can be a substantial reduction in the column's
load-carrying capacity. Usually the percentage of reinforcement should not exceed 4%
