Beam-Column Joints
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.
Continue to Avoiding the Dead Zone -->
|
