Detailing Concrete Columns: Designers of concrete structures are typically concerned about the final design and not necessarily about how a specific concrete element gets built. In other words, engineers tend to avoid issues that are traditionally contractor “means and methods.” Yet, decisions made during the design process can have major impacts on the project cost, schedule, change orders, number of requests for information (RFIs), and overall constructability. Knowledge of how a concrete element gets built can help ensure it matches the engineer’s design so the listed impacts are minimized. From a reinforcing bar detailing and placing standpoint, understanding the constructability aspects can perhaps expedite placing drawing review or reinforcing bar inspections in the field. (Note: detailed reinforcing bar drawings are known as “placing drawings,” as opposed to “shop drawings.”)Detailing Concrete ColumnsDesigners of concrete structures are typically concerned about the final design and not necessarily about how a specific concrete element gets built. In other words, engineers tend to avoid issues that are traditionally contractor “means and methods.” Yet, decisions made during the design process can have major impacts on the project cost, schedule, change orders, number of requests for information (RFIs), and overall constructability. Knowledge of how a concrete element gets built can help ensure it matches the engineer’s design so the listed impacts are minimized. From a reinforcing bar detailing and placing standpoint, understanding the constructability aspects can perhaps expedite placing drawing review or reinforcing bar inspections in the field. (Note: detailed reinforcing bar drawings are known as “placing drawings,” as opposed to “shop drawings.”)
This Detailing Corner concerns various aspects of reinforced concrete column design and construction,
including lap splicing of the longitudinal bars, placement of the column dowels, and offset bends. The recommendations presented herein are based on years of experience and they have proved helpful in keeping many a project on time and under budget.
Lap Splice Location
Section 1.2.1 of ACI 318-081 lists important informational items that must be included on design drawings, details, or specifications, including anchorage length of reinforcement and location and length of lap splices. One note concerning tension lap splices of longitudinal bars in a column: common industry practice is to categorize the bars as “other” types of bar, and not “top” bars.
A contract column schedule from the engineer may show the columns to be lap-spliced every floor. Depending on the size of the column vertical reinforcing bars for the particular project, the tension lap splices may be fairly lengthy. In some instances, the laps could approach the entire story height. This essentially results in a doubling of the column bars, which may unduly increase congestion in the column and make concrete placement more difficult. If the length of a lap splice is more than about one-third to one-half the story height, it may be more economical to save on laps and lap-splice the bars every other floor, if possible.
Ideally, the column cage should be stable enough to stand on its own so that cable guying or pipe bracing (Fig. 1) is avoided, because it can obstruct construction
Joint ACI-CRSI Committee 315-B, Details of Concrete Reinforcement Constructability, has developed forums dealing with constructability issues for reinforced concrete. To assist the Committee with disseminating this information, staff at the Concrete Reinforcing Steel Institute (CRSI) are presenting these topics in a regular series of articles. If you have a detailing question you would like to see covered in a future article, please send an e-mail to Neal Anderson, CRSI’s Vice President of Engineering, at email@example.com with the subject line “Detailing Corner.”
activities. Inadvertent, “temporary” releases by other trades can result in instability, so coordination is
required. A slightly more robust column cage design by the engineer may eliminate the need for guying or bracing. Certain factors will affect the stability of a column cage:
- Size of column—The larger a column is in cross section, the larger the moment of inertia of the reinforcement arrangement. The column cage is thus more stable as a freestanding unit.
- Quantity and size of bars—Larger bars are more rigid and stable than smaller bars. Similarly, a large number of bars arranged around the column perimeter would be more stable than a small number of bars (Fig. 2). Again, these factors influence the moment of inertia of the bar group.
Floor-to-floor height—The greater the floor-to-floor height, the less stable the column cage becomes because the unbraced length is greater
Bar Orientation and Location
A number of changes can occur to a specific longitudinal bar run as it traverses from the footing up to the roof;
the bar can change in size as it is spliced to another bar or it can be offset bent to a slightly different location. For these reasons, the reinforcing bar detailer (Detailer) will assign an identification or bar mark for every bar in the column. One way to maintain bar identification on a placing drawing is to assign a unique symbol for each bar, as shown in Details A and B in Fig. 3. In this figure, a unique circular mark representing a longitudinal bar run is shown in plan for pairs of bars (Detail B) and labeled below the footing in Detail A.
On the placing drawings, the Detailer will likely show a “North” orientation arrow on any plan views cut
through rectangular columns (illustrated in Detail B in Fig. 3). The reference North arrow indicates the proper column orientation, which aids in the correct placement of the north-south face and east-west face reinforcing bars in the column cage by the ironworker. It also gives a point of proper reference in the column cross section should two opposite faces have a different quantity of bars than the other two sides. Although this may seem obvious, control points or column lines may not be yet established at the project site; thus, a “North” reference on the column cage may be the only sensible reference, given the point or stage of construction progress.
Erecting Column Cages
Although column cages are always shown on the design and placing drawings in their proper vertical
orientation, they are almost always assembled horizontally on the ground on horses (Fig. 4). The cage is then hoisted into place. To ensure the ironworker properly constructs the cage, the first and last ties must be properly located at the ends of the cage while it’s being fabricated on the ground. Referencing the tie location relative to a floor elevation or a beam soffit is meaningless to the ironworker at this stage because these control points are nowhere to be found on the cage while it’s being built. The Detailer
will thus provide distances from the end ties to the ends of the longitudinal bars, as shown in Detail A in Fig. 5. Once the location of these key end ties has been established, the remaining ties can be accurately placed along the length of the cage.
In spacing column ties, it’s considered good practice to work with tie spacing, rather than the number of ties. Hence, this will be shown on a placing drawing by the Detailer. Multiplying the number of spaces by the spacing distance results in a hard dimension (from the first tie to the last), which is usable to the ironworker. Multiplying the number of ties by the tie spacing doesn’t result in a usable dimension to the ironworker. Using tie spacing also allows the ironworker to mark or “tick off” the tie locations on the longitudinal column bars while building the cage.
Mechanical Splices | Detailing Concrete Columns
Sheared reinforcing bars usually result in burrs or shear lips at the cut ends. Certain mechanical splices
require special preparation at the bar end, such as a square saw cut, tapered thread, straight thread, or upset end. Other mechanical splices can couple bars without any special end preparation. A Detailer should be familiar with the numerous mechanical splice and headed bar systems on the market and make notes if special end preparations are required. The CRSI publication Reinforcing Bars: Anchorages and Splices2 contains.
information on various types of mechanical splices, including those that require special end preparation.
Detail A of Fig. 3 depicts the required mechanical splices for this particular column. Each type of mechanical splice is properly located on the drawings by dimension lines and/or elevation references. Specific mark numbers are typically referenced back to a schedule, which may contain additional information on the splice device.
ACI 318-081 requires mechanical splices be staggered, which results in both short and long vertical bars in the column run. To reduce bar placement errors, the Detailer must properly indicate the placement of these short and long bars on the placing drawing with bar marks. As an example, consider an eight-bar column configured with three bars in each face, which reduces to a four-bar column higher up in building elevation; obviously the four bars would be in the corners of the column cage. It’s incumbent on the Detailer to properly and clearly indicate the locations of the different length bars on the placing
drawing through the bar mark numbers. If the drawings are unclear or ambiguous to this subtle length difference, the four vertical bars at the higher column elevation will be short should the ironworker accidently reverse the bar positions at the lower elevations.