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CONCRETE Wall Shear question

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Ref.: "1997 UBC", "ACI 318-99" and "IBC2000"
and "Seismic Design Manual Vol. III Bldg. design Examples...Concrete..."

I have found that section 1921 can offer 30% more shear strength than that
in section 1911. I believe this may be an error. The shear wall design
example "Seismic Design Vol. III" uses 1921 and does not check the 1911
equation. Does anyone understand this condition? Is there an error in my
analysis? Are there changes to the codes that I do not know of?

I have a shear wall that has an hw/lw = 1.83 and I have set Nu=0 (no or
insignificant axial load).
I compared Section 1921.6.5 to section 1911.10.6. &.9
1921 allows a 30% higher shear for the same reinforcement. This is counter
intuitive as a wall is more challenged when exposed to larger seismic events
relying on ductility.
I found the equations similar. steel shear strength has a 10% advantage over
1911.
The most important difference is the use of d in the 1911.10.6 & .9 and the
full section depth in Acv is allowed for 1921.6.5.

If the two sections had used the same d or depth of section, then the
equations are nearly similar except 1911 allows one to take advantage of the
axial load to increase shear strength. I have no idea why the steel strength
was increased 10%.

in 1921 ALPHAc is simplified to = 6.0-2*hw/lw

in 1911 the critical section is lw/2 as per 1911.10.7 hence an equivalent
ALPHAc value is 0.6+1.25/((hw/lw)-1)

David Merrick, SE


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