Forming Tolerance Cloud
By Dick Birley, President of Condor Rebar Consultants, Inc.
First published in Concrete International Magazine, October 2005
In my last two articles, I discussed tolerance clouds associated with fabrication and placement of reinforcing bars.[1,2] While every builder strives to cast concrete to the precise dimensions indicated by the designer, the reasonable constraints of time, technology, and economy make this impractical. That's why I'd like to point out one more cloud associated with concrete construction—the forming tolerance cloud.
CONCERNS
Tolerances for forming concrete are found in Section 4 of ACI 117-90.[3] The tolerances for cross-sectional dimensions of cast-in-place members vary with the overall dimension (Table 1).
Using the example from my previous article of a 14 x 14 in. (350 x 350 mm) column, the tolerance is +1/2 in. (+13 mm) or –3/8 in. (–10 mm). Ignoring vertical alignment, this produces the forming tolerance cloud shown in Fig. 1, with a column having acceptable dimensions as large as 14-1/2 x 14-1/2 in. (363 x 363 mm) or as small as 13-5/8 x 13-5/8 in. (340 x 340 mm). While it is highly unlikely that these small variations would create any constructibility or design concerns with everything else being perfect, a very different picture arises when we consider them in conjunction with the other possible tolerances.
With 1-1/2 in. (40 mm) cover, the design width for the column ties is 11 in. (280 mm), and the tolerance is ±1/2 in (±13 mm). Combining the maximum acceptable tie dimensions with the minimum acceptable column dimensions produces the configuration shown in Fig. 2. With the reinforcing cage centered, the cover is reduced from the design value of 1-1/2 in. (38 mm) to 1-1/16 in. (27 mm) on all four sides. Recalling that the placement tolerances allow the cover to decrease to 1 in. (25 mm) minimum, the cage must be placed within ±1/16 in. (±2 mm) of the center of the column in both directions if it is to meet tolerance requirements. Considering the straightness of the bars and the straightness of the forms, this could be very difficult for the Contractor to do.
For the example of a 14-in.-thick (350 mm) wall that we previously examined, the situation is somewhat different because there are no tie tolerances to contend with. However, as we will see in the following example, other issues arise that must be dealt with.
The forming tolerance for the wall thickness allows the wall to be between 14-1/2 in. (363 mm) and 13-5/8 in. (340 mm) thick as shown in Fig. 3.
Reinforcing placement tolerances allow the 1-1/2 in. (38 mm) design cover on the outside face to be between 1 and 2 in. (25 and 50 mm) and the 3/4 in. (19 mm) design cover on the inside face to be between 1/2 and 1-1/4 in. (13 and 32 mm). The minimum wall thickness combined with the maximum cover on the outside face reinforcing is shown in Fig. 4.
In this situation, the original effective depth of 12 in. (305 mm) for the vertical No. 8 (25 mm) bars on the outside face has decreased to only 11-1/8 in. (283 mm). Assuming 4000-psi (28-MPa) concrete and Grade 60 (Grade 420) reinforcement, this reduction in effective depth would result in a decrease in nominal moment capacity from the original 45.1 kip•ft/ft (201 kN•m/m) to 41.6 kip•ft/ft (185 kN•m/m)—a 7.7% reduction due to forming and placement tolerances alone. The effect on moment strength would be even more drastic for thinner walls. To guard against this, Section 7.5.2.1 of ACI 318-05[4] places a tolerance on effective depth d of ±3/8 in. (±10 mm) for d ≤ 8 in. (200 mm) and ±1/2 in. (±13 mm) for d > 8 in. (200 mm). These tolerances would produce a 4.4% reduction in nominal moment strength for the example wall considered here; however, designers should realize that effective depth is not checked in the field. Bars are placed and tolerances checked relative to the formwork surfaces.
DESIGN CONSIDERATIONS
Casting of concrete always involves the fabrication, placement, and forming tolerance clouds. While these instances are not encountered every day, they occur frequently enough to create constructibility problems.
Any combination of these tolerance clouds working against each other has the potential to create a constructibility concern that quite often is difficult to reconcile, especially if it involves two different trades, each within their own acceptable tolerances. The designer must always assess the risk of this kind of problem arising in critical areas and consider options that mitigate or eliminate the possible constructibility problem.
Acknowledgments
Thanks to ACI member John H. Scarino for his contributions to this article.
References
1. Birley, D., "The Tolerance Cloud," Concrete International, V. 27, No. 6, June 2005, pp. 61-63.
2. Birley, D., "Placement Tolerance Clouds," Concrete International, V. 27, No. 7, July 2005, pp. 50-51.
3. ACI Committee 117, "Standard Specifications for Tolerances for Concrete Construction and Materials (ACI 117-90) and Commentary (117R-90) (Reapproved 2002)," American Concrete Institute, Farmington Hills, MI, 2002, 23 pp.
4. ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (318R-05)," American Concrete Institute, Farmington Hills, MI, 2005, 430 pp.
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