By Dick Birley, President of Condor Rebar Consultants, Inc.
First published in Concrete International Magazine, December 2006
Alternative bottom beam reinforcement layouts can reduce congestion
Beam-column joints are frequently areas of congestion in reinforced concrete construction. Several factors can contribute to beam-column joint congestion, such as column verticals that terminate in the beam with hooks and excessive top steel over the column. By far the most common problem, however, is the lapping of bottom beam bars at a column to meet the structural integrity requirements in Section 7.13 of ACI 318-05,[1] especially if the column and beam are the same width. When the bottom beam bars are lapped at the column, the number of bottom bars that must pass through the joint doubles. The resulting congestion can contribute to poor consolidation of the concrete at a critical location where the concrete is under a complex state of stress and may also cause clearance problems with the column ties or beam stirrups. This congestion can be significantly reduced by using alternative splice locations and bar arrangements. The following discussion presents four general splice arrangements, along with the advantages and disadvantages of each from a constructibility point of view. Designers must also consider structural issues when selecting among the alternative locations for the bottom bar splices. Only continuous bottom beam bars are shown. All other bars are omitted for clarity.
Arrangement #1 — Splices located at Supports
The most common arrangement is to locate all bottom bar splices at the supports as shown in Fig. 1, but this arrangement also produces the most congestion in the joint.
Advantages:
- Simplest to detail;
- Good arrangement where beams are wider than the supporting columns; and
- No additional steel is required.
Disadvantages:
- Causes heavy congestion, especially if the column and beam are the same width or a large amount of reinforcement must be continuous;
- Installation of single-bay preassembled beam cages is difficult; and
- Installation of multiple-bay preassembled beam cages is almost impossible.
Arrangement #2 — 50% of Splices on Each Side of Supports
To ease congestion in the joints, half of the continuous bottom beam bars can be spliced on one side of the joint and the other half on the other side of the joint, as shown in Fig. 2. This arrangement eliminates half of the bars passing through the joint compared to Arrangement #1.
Advantages:
- With no splices over the supports, congestion is eased; and
- No additional steel is required.
Disadvantages:
- Detailing and preassembled cages are slightly more complex;
- Preassembled beam cages are longer and more awkward to install; and
- Installation of multiple-bay preassembled cages is very difficult.
Arrangement #3 — 100% of Splices Located on One Side of Supports
The third alternate arrangement is similar to Arrangement #2 except that all of the splices are located on one side of the joint, as shown in Fig. 3.
Advantages:
- Detailing and preassembled cages are relatively simple;
- Preassembled cages are the same length as in Arrangement #1, but easier to install because they pass through only one joint; and
- No additional steel is required.
Disadvantages:
- Care must be taken to ensure that the cages are oriented correctly if installation begins at the center of the beam and progresses both ways.
Arrangement #4 — Splice Bars Added Through Supports
The fourth alternate arrangement is to add splice bars passing through the joint that are spliced to the bottom beam bars on both sides of the joint. This arrangement is shown in Fig. 4.
Advantages:
- No congestion at columns because splice bars through columns are added later;
- Preassembled cages are shorter than any of the previous options;
- Very easy to install preassembled cages because no bottom bars pass through the columns during installation; and
- Best option for installation of multiple-bay preassembled cages.
Disadvantages:
- Additional steel required.
Design Considerations
Congestion should be considered when choosing the location of continuous bottom beam bar splices. Arrangements #2, #3, and #4 address this issue to varying extents. Even though Arrangement #4 increases the amount of steel required, it may be the most cost-effective in certain situations. The cost of the extra steel may be more than offset by the savings in labor or other costs. By permitting preassembly of the reinforcing steel cages on the ground, rather than “in the air,” Arrangement #4 increases safety and eliminates the need for special scaffolding to support bundles of steel while the cages are being assembled in place. The advantages of preassembled cages are illustrated by the example shown in Fig. 5, where the bottom steel in the preassembled beam cage was configured per Arrangement #4. The total time required to hook up to the cage, position the cage in the forms, and install the splice bars was only 30 minutes for two ironworkers. Safety was also increased by requiring only two ironworkers “in the air” during installation.
References
1. ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05),” American Concrete Institute, Farmington Hills, MI, 2005, 430 pp.
Civil Engineering 