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Determination of Pu/Ag in Concrete Slender Wall Module

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Section 14.8 “Alternative design of slender walls” requires that Pu/Ag at midheight shall not exceed 0.06f’c.  ENERCALC SEL presents a comparison of Pu/Ag with 0.06f’c, but the value of Pu/Ag is not determined at midheight.  How is SEL selecting the location to calculate Pu/Ag and why isn’t it necessarily at midheight?

 

Section 14.8 presents a highly simplified version of slender wall design that fits certain very specific criteria.  But in order to create these simplified provisions, ACI had to declare many requirements that tend to be inapplicable in many real world implementations of slender wall panels. 

 

The Section 14.8 methods are specifically for these conditions:

 

“wall panel shall be designed as simply supported”

“subjected to an out-of-plane uniform lateral load”

“maximum moments and deflections occurring at midspan”

“cross section shall be constant over the height of the panel”

 

ACI always leaves open the option to apply engineering judgment in a design, and that is what has been done in the development of the Concrete Slender Wall module.  As such, the module provides capabilities that are not covered by Section 14.8.  Our extension of slender wall principles follows the principles of structural mechanics appropriate to this type of analysis.

 

The Concrete Slender Wall module allows for these conditions:

 

different base fixity conditions

different loading configurations

conditions where the maximum moments and deflections occur at locations other than at midspan

variations in cross section (such as reveals) may be defined and incorporated into the analysis and design.

 

In order to properly implement these additional capabilities, the module has some adaptations to it.  One of those adaptations is the fact that it determines the value of Pu/Ag at the elevation where the maximum moment is found to occur, rather than automatically calculating it at midspan.  This seems most in keeping with the intent of Section 14.8, which stated that Pu/Ag was to be determined at midheight, but which also anticipated that the maximum moment would occur there, too. The intension of this check is to make sure the wall capacity is reached in a tension-controlled manner.