By Dick Birley, President of Condor Rebar Consultants, Inc.
First published in Concrete International Magazine, July 2005
In my previous article “The Tolerance Cloud,” I identified the potential tolerance envelopes for a single reinforcing bar fabricated with a 90-degree bend at each end.1 Just as there are tolerances in the fabrication of a bar, there are also tolerances in the placement of a bar in a concrete member—creating potential “placement tolerance clouds.” Because designers and software programmers may overlook the impact of placement tolerances on constructibility, it’s worthwhile to use a couple of examples to take a brief look at what can occur..
Concerns
The tolerances for reinforcement location are found in Section 2.2 of ACI 117-90.2 Cover tolerances vary from 1/4 in. (6 mm) for member sizes of 4 in. (100 mm) or less to 1 in. (25 mm) when member size is over 2 ft (600 mm). The maximum reduction in cover is limited to 1/3 of the specified cover. In slabs and walls, the spacing tolerance is 3 in. (76 mm) for reinforcement other than stirrups and ties.
As an example, let’s consider the simple 14 x 14 in. (350 x 350 mm) concrete column shown in Fig. 1.
The column is reinforced with four No. 8 (25 mm) bars enclosed within No. 4 (13 mm) ties. The normal concrete cover to the ties of this column would be 1-1/2 in. (40 mm). The cover tolerance is ±1/2 in. (±13 mm). If the reinforcement was placed to the minimum tolerance in two directions, the column could appear as in Fig. 2.
However, the reinforcement could be placed to minimum tolerance in any of the four directions. Thus, the placement tolerance clouds would appear as in Fig. 3.
This could be quite a different image than the precise image one might have had in mind at the outset.
For a second example, let’s take the case of a simple 14-in.-thick (350 mm) wall reinforced with No. 8 (25 mm) vertical bars at 12 in. (300 mm) on center each face and No. 4 (13 mm) horizontal bars at 12 in. (300 mm) on center each face (Fig. 4).
The outside face cover is 1-1/2 in. (40 mm) and inside face cover is 3/4 in. (19 mm).
The cover tolerance for the bars on the outside face is ±1/2 in. (±13 mm). For the inside face cover, the maximum cover reduction is limited to 1/3 of the specified cover, resulting in a cover tolerance of +1/2 or –1/4 in. (+13 or –6 mm). Thus the outside face cover could be as little as 1 in. (25 mm) and the inside face cover as little as 1/2 in. (13 mm) (Fig. 5).
If we also consider that any one of the vertical and horizontal bars may be located as far as
3 in. (75 mm) either way from its designated location, the tolerance cloud would appear as in Fig 6.
Design Considerations
As in the instance of the fabrication tolerance cloud of a single bar, the placement cloud of a group of placed bars presents quite a different image than the one probably envisioned by the designer or software programmer. If the placement tolerances are factored into the design, they would realize that the available space they expected (to pass beam bars through a column or to place a vertical embed in a wall) might not be what is actually available, especially if they consider that the beam bars and the embed also have fabrication and placement tolerances of their own. Awareness of placement clouds may lead to design options that make these tolerances no longer a factor.
References
1. Birley, D., “The Tolerance Cloud,” Concrete International, V. 27, No. 6, June 2005, pp. 61-63.
2. 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, 1990, 22 pp.
Continue to Forming Tolerance Cloud
-->
|
