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Use of the 0.75 stiffness reduction factor in the Concrete Slender Wall module

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Why is the 0.75 stiffness reduction factor offered as an option in the Concrete Slender Wall module?

 

The ACI code refers to the 0.75 factor as a “Stiffness Reduction Factor”, and it is always applied in the equations that concern wall flexibility for deflections.  (They are applied in a moment equation but it is for the purpose of modifying wall stiffness for P-Delta effects.)

 

ACI says this about the 0.75 factor :

 

A factor of 0.75 is used to reduce the calculated bending

stiffness of the concrete section in accordance with ACI 318-08,

Chapters 10 and 14. It is intended to account for variations in

material properties and workmanship. Test results

(SEAOSC 1982) and analytical studies (Lai et al. 2005) have

indicated that small variations in the position of reinforcement

in the concrete section will have a significant effect on the

strength of the wall panel and the bending stiffness properties.

A reduction in bending stiffness is not limited to the provisions

of Section 14.8. It should also be employed where other

methods of slender wall analysis are used. Historically,

design methods for tilt-up panels have not specifically included

this requirement. The 0.75 reduction factor in bending stiffness

should be incorporated by all other alternate design methods

to comply with the requirements of ACI 318-08.

 

Although the code mentions it should be used, it is up to the engineer to decide whether it is applicable to any particular project.  There may be cases where very high tolerances and strict inspections will provide superb quality control, and using this factor might be overly conservative.

 

It is our opinion that the ACI has constantly been changing its tilt-up wall design processes over the last 25 years to attempt to calculate wall deflections by formulas that reflect the actual 1980’s SEAOC test results.  The 0.75 factor and 2/3 multipliers that you see are all fudge factors to try to get the wall to conform to test results.  Much of this is because the ACI equation for Ieffective does not give correct results for thin and lightly reinforced concrete bending members.

 

Because the engineer is the final judge of how a slender wall should be designed, we make the 0.75 factor an on/off toggle.