Technical Questions

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Reinforcement termination in wastewater concrete containment

Q. Does ACI 350-06 allow terminations of vertical reinforcing bars in a wastewater concrete containment tank wall? For example, my calculations show that a cantilevered wall requires vertical bars at 4 in. (102 mm) spacing at the connection to the tank base. At mid-height of the wall, however, the moment can be resisted using the same bar size at 8 in. (203 mm) spacing. Does ACI 350-06 allow for the termination of every other bar?

 

A. Development of flexural reinforcement in nonprestressed members is discussed in Section 12.10 of ACI 350-06. That section applies to your containment structure. You may terminate alternate bars as you describe, but the terminated bars must be fully developed at the base and extend at least d or 12db from the point in the moment diagram corresponding to the maximum moment strength of the remaining (nonterminated) reinforcing bars. This is illustrated in Fig. R12.10.2.

In your case, the remaining bars must extend to the end of the cantilever (less the cover depth). Note that you should also check additional reinforcement design considerations provided in Section 4.4 of ACI 350.4R-04, as they might affect reinforcing details for your containment structure.

Finally, note that Fig. R12.10.2 has a typo. The extension of the “Bar b” negative reinforcement beyond the inflection point should be labeled (d, 12 db, or ln/16), as the first subscript in the series should be a comma, not a 1.

 

References: ACI 350-06

Topics in Concrete: 318 Building CodeDesign; Tank

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