ACI 318-11 Shear Wall (phi) factor 1

[chamokinawan]

ACI 318-11 9.3.4(a) states that phi for shear "shall be .6 if the nominal shear strength of the member is less than the shear corresponding to the development of the nominal flexural strength of the member." Does this mean that if flexure governs then I have to use .6 for shear instead of .75? Does anyone have a better clarification for this? Thanks.

 

[KootK]

This is probably best illustrated via example. Imagine that your shear wall is cantilevered from the ground and is 10' long and 50' tall. The only load is a factored lateral load of 30 kip at the top.

1) Mf = 30 kip x 50' = 150 kip*ft

2) Let's say that your design Mn = 200 kip*ft. Capacity rarely matches demand exactly.

3) Vf = 30 kip. Given.

4) V_Mn = 200 kip*ft / 150 kip*ft * 30 kip = 40 kip. Amplified shear that would correspond to Mn.

5) IF [Vn < V_Mn (40 kip)] THEN phi = 0.6 ELSE phi = 0.75.

The concept is that shear demand is rarely expected to exceed that which would correspond to flexural yielding because flexural yielding would preclude the development of such a shear demand. Since a design where Vn > V_Mn is safer in this respect, phi can rationally be lower.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

phi can rationally be lower.

Correction: phi can rationally be higher.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.