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RE: CONCRETE Wall Shear question

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Section 1921 takes precedence to the provisions of Sections 1901 through 1918, as explicitly stated in 1921.2.1.2. The language in 1921.2.1.4 follows that from the corrresponding section in the 1995 ACI 318 Code. Note that ACI 318-99 and -02 state the requirement differently. If you feel comfortable taking the lower value and multiplying by 0.6, that would be according to the letter of UBC 97 Sect. 1921.2.1.4. We do not believe that this was ever intended by ACI 318 or SEAOC. However, we agree that it is not proper for Sect. 1911 to yield higher shear strengths than Sect. 1921 for shear walls. This needs to be sorted out within ACI 318. This will be brought to their attention.
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Section 1921.2.1.4 clearly states that the wall must comply with the
1911.10.6 (11-32) shear strength. I have demonstrated cases where that shear
strength is about 30% reduced for high seismic areas, in section 1921.6.5.3
(21-7). Unless someone can tell me different, we must always check if
equation 11-32 controls. This concerns me because I have found recommended
shear wall designs not including the code required check for equation 11-32
for a high seismic zone. I then wonder how many proprietary concrete shear
wall programs also do not check this. This would lead to conclude an
under-reinforced shear wall by as much as 30%! Now, please tell me I am
wrong. If so, how?

The phi factor is the same for both equations. I hear what you are saying
that there is testing to prove the equations for section 1921. The minimum
strength requirement is controlled by the 1911 shear equation. I see example
calculations failing to comply with this requirement. We are allowed to use
scientific research to justify exceeding the code, not to undermine it.
Leave that to the code builders and legislative powers. I see this as
alarming and want to do diligence. I must test for if I have been mistaken,
if not then I can go ahead and peacefully do designs with 11-32 controlling
some shear wall strengths in high seismic zones.

The difference between the two equations is that the 21-7 uses the full
length of the wall and 11-32 reduces the length limited by the position of
the tension steel. This is allowed to be assumed as 80% of the wall length.
this affects both steel and concrete shear. The other 10% comes from the
maximum variance in shear steel strength.


_______________________

David A. Fanella, Ph.D., S.E., P.E.
Director of Engineering
S.K. Ghosh Associates, Inc.
1856 Walters Avenue, Suite 200
Northbrook, IL 60062
Ph: 847.291.9864
Fax: 847.291.9865
E-mail: dfanella(--nospam--at)skghoshassociates.com
Web Page: skghoshassociates.com 


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