AASHTO vs ACI Effective shear depth differences

[engjg]

Looking for background information on difference in the effective shear depth definition between aashto and aci codes. I note aashto limits d to 0.9de. I want to know what is the reasoning behind this reduction?

 

[RPMG]

The shear depth, dv, is at least 0.9*de, like in a shear wall.

 

[engjg]

Thank you, but for one-way beam shear why d for aci and 0.9*de for aashto?

 

[RPMG]

dv = d - a/2, but at least 0.9d. I'm guessing it was just the formulation, and it all evens out with different load/reduction factors.

 

[rapt]

dv is not the effective depth, it is the flexural lever arm, the distance between the centroids of the tension and compression forces on the section. This is assumed to be .9 of the effective depth. This is part of the MCFT logic that AASSHTO is based on.

This is shown very clearly in several diagrams in AASHTO.

 

[r13]

engjg,

The depth for shear strength calculation of reinforced concrete section in ACI is "d". The calculated shear strength is reduced by a strength reduction factor, φ = 0.9, to account for the cracked section at the ultimate stage, and for the uncertainty of the depth of the crack.

Therefore the equation without consider contribution from steel (stirrups) is, Vu <= φVc = φ*2*sqt(fc')*b*d, Vu = factored shear force.

Where in AAHTO requires the use of 0.9*de? And, how d and de are defined?

 

[rapt]

Are we all reading the same AASHTO?

Read AASHTO 5th Edition 2010 section 5.8.2.9 for the definition of dv!

 

[BridgeSmith]

AASHTO gives an equation to calculate dv and then says that it need not be taken less than 0.9de or 0.7h (h = overall height of the concrete section), so it's taken as the greatest of the 3 values.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

rapt,

I have no exposure to AAHTO for now. Can you explain d and dv, and the uses. Thanks.

 

[r13]

BridgeSmith,

Interesting. So, as opposed to the ACI using a single parameter d, the AASHTO requires designer to choose the greatest depth from the envelop of three values (dv, 0.9de, 0.7h). Do you know the background(s) of AASHTO's approach?