Structural Detailing of Reinforcement for Economy - Simplified Design of Reinforced Concrete Buildings - page 249

8.8 SPECIAL SPLICE REQUIREMENTS FOR COLUMNS

8.8.1 Construction and Placing Considerations

For columns in multistory buildings, one-story high preassembled reinforcement cages are usually used. It is
common practice to locate the splices for the vertical column bars just above the floor level. In certain
situations, it may be advantageous to use two-story high cages since this will reduce the number of splices and,
for lap splices, will reduce the amount of reinforcing steel. However, it is important to note that two-story high
cages are difficult to brace; the required guy wires or projecting bars may interfere with other construction
operations such as the movement of cranes for transporting equipment and material. Also, it is more difficult
and time-consuming to place the beam or girder bars at the intermediate floor level since they have to be
threaded through the column steel. These two reasons alone are usually more than sufficient to offset any
expected savings in steel that can be obtained by using two-story high cages. Thus, one-story high cages are
usually preferred.

8.8.2 Design Considerations

Special provisions for column splices are given in ACI 12.17. In general, column splices must satisfy
requirements for all load combinations for the column. For example, column design will frequently be
governed by the gravity load combination (all bars in compression). However, the load combination, which
includes wind loads, may produce tensile stresses in some of the bars. In this situation, a tension splice is
required even though the load combination governing the column design did not produce any tensile stresses.

When the bar stress due to factored loads is compressive, lap splices, butt-welded splices, mechanical
connections, and end-bearing splices are permitted. Table 8-4 may be used to determine the minimum
compression lap splice lengths for Grade 60 bars. Note that these lap splice lengths may be multiplied by 0.83
for columns with the minimum effective area of ties (throughout the splice length) given in ACI 12.17.2.4.
In no case shall the lap splice length be less than 12 in. Welded splices and mechanical connectors must meet
the requirements of ACI 12.14.3. A full welded splice, which is designed to develop in tension, at least
1.25 A b f y (A b = area of bar) will be adequate for compression as well. A full mechanical connection must
develop in compression (or tension) at least 1.25 A b f y . End-bearing splices transfer the compressive stresses by
bearing of square cut ends of the bars held in concentric contact by a suitable device (ACI 12.16.4). These types
of splices may be used provided the splices are staggered or additional bars are provided at splice locations (see
ACI 12.17.4 and the following discussion).

A minimum tensile strength is required for all compression splices. A compression lap splice with a length
greater than or equal to the minimum value given in ACI 12.16.1 has a tensile strength of at least 0.25 A b f y .
As noted above, full welded splices and full mechanical connectors develop at least 1.25 A b f y in tension. For
end-bearing splices, the continuing bars on each face of the column must have a tensile strength of 0.25 A s f y
where A s is the total area of steel on the face of the column. This implies that not more than three-quarters of
the bars can be spliced on each face of the column at any one location. Consequently, to ensure minimum
tensile strength, end-bearing splices must be staggered or additional bars must be added if more than three-
quarters of the bars are to be spliced at any one location.