By Dick Birley, President of Condor Rebar Consultants, Inc.
First published in Concrete International Magazine, June 2005

Design engineers often lack the practical field experience necessary to provide construction documents that reflect job-site requirements. Detailing and installation conflicts can therefore occur, leading to requests for information, delays, and additional costs. As a result, a variety of ad hoc, job-site solutions (“cowboy tricks”) have evolved. While many of these job-site remedies provide good, workable solutions, others could be improved through practical design review.

To improve the constructibility of reinforced concrete structures, ACI Committee 315-B, Details of Concrete Reinforcement-Constructibility, is working to identify constructibility concerns; offer practical, job-site-based solutions; and solicit input and comments from the design community. As part of the committee’s efforts, I and other members will author a series of articles, with a primary focus on constructibility concerns that involve the assembly of reinforcing bar components. Further, we will address how other trades affect reinforcing bar installation, and we will attempt to identify design details that could be deficient, ambiguous, or simply missing. What better place to start our discussion of these issues than where the installation of reinforcing starts — with a single reinforcing bar and the tolerance envelope that exists at that single bar — what I call the tolerance cloud.

Concerns
Designers, reinforcing steel detailers, and software programmers often fail to factor the impact of fabrication tolerances into their work. As a simple example, let’s consider the fabrication tolerances for a simple reinforcing bar with 90-degree bends (Fig. 1).

For the purposes of our example, let’s assume that the bar is a No. 8 (25 mm) bar and that Side A is anchored in
the (idealized) plane ABG. For this bar size, the standard hook is 16 in. (400 mm) long, and the linear and angular tolerances are ±1 in. (±25 mm) and ±2.5 degrees, respectively.1

Now, let’s examine the potential effects of these tolerances. First, note that Sides A and G can be as short as 15 in. (375 mm) (red to black zone interface) or as long as 17 in. (425 mm) (end of blue zone) and still be within tolerances (Fig. 2).

Because we have assumed Side A to be anchored in ABG, we will not need to consider out-of-plane angular deviation for Side A. However, we will need to consider in-plane angular deviation. When we add this angular deviation of ±2.5 degrees to Side A, the tolerance envelope (cloud) will appear as shown in Fig. 3. (Note: To simplify the illustrations, the effects of the angular tolerances are shown as one-bar-diameter deviations in the position of the ends of the 16 in. hooks. Actual deviations will be about 70% of a bar diameter.)

Next, we add the dimensional tolerance of ±1 in. for Side B (Fig. 4)

and the in-plane angular deviation of ±2.5 degrees to Side G (Fig. 5).

Finally, we add the out-of-plane angular deviation of ±2.5 degrees to Side G. The resulting tolerance cloud is as shown in Fig. 6.

Design Considerations
Clearly, the fabricated bar arriving on the construction site can be quite different from the bar the designer or software programmer might have envisioned. Keeping this in mind during design could significantly reduce constructibility problems. For instance, if our example bar were replaced with two hooked bars lapped in the middle, the only tolerance that might introduce problems would be in-plane angular deviation. Because both hooks could be rotated, there would be no out-of-plane deviations. Further, because the lap could be adjusted slightly, there would be little chance of problems with the length of Side B.

Consideration of tolerances becomes even more of an issue when two or more bars are being assembled together in a structure. In such cases, one must deal with an accumulation of tolerances. Future articles in this series will examine these and other constructibility issues related to reinforcing steel.

References
1. ACI Committee 117, “Standard Specifications for Tolerances for Concrete Construction and Materials (ACI 117-90) and Commentary (117R-90),” American Concrete Institute, Farmington Hills, MI, 2002, 22 pp.

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