AASHTO Concrete Shear

[rsaunders]

I'm studying for the Structural I PE exam and need some help with the concrete provisions of AASHTO. If anyone has any experience with this, some pointers would be much appreciated.

Here's what it boils down to: I've been studying the concrete provisions and comparing them to the similar ACI design provisions in 318-05. I've come to the section on shear and torsion (section 5.8) and cannot make sense out of the Vc equation. The AASHTO provision lists Vc on pg 5-61 as follows: Vc=0.0316*(Beta)*(f'c^1/2)*bv*dv. The definition lists Vc in units of kips on page 5-55. I understand that fc^1/2 should always be in psi, so is there some conversion in the 0.0316? If I work this out with a typical beam problem, I get shear values that are extremely large. This equation does not seem to be close to any of the Vc equations given by ACI in chapter 11 on pages 151-152.

I wasn't sure if this was an error, but I've searched for errata on this version of the AASHTO code and can't find any. I found errata for the 2nd and 4th editions, but that's not very helpful.

Any insight would be much appreciated.

 

[rsaunders]

Rats...I figured it out. I read further into other subjects and figured out that AASHTO is using f'c in ksi, even under the radical. All of ACI is based on f'c in psi. Anytime f'c is beneath the radical in ACI, the units must be in pounds and inches for the units of the answer to work out. If f'c isn't under the radical, then it doesn't matter as long as you're consistent.

Lets set 0.0316*beta*(f'c^1/2)*bv*dv equal to the ACI equation 2*(f'c^1/2)*b*d. I'll use the following values:

f'c = 5000 psi

bv=b= 12 inches

dv=d=20 inches

beta = 2

0.0316*(5^1/2)*12*20 = 2*(5000^1/2)*12*20

33.917 kips = 33,941 lbs.

Bravo. Any idea why they decided to rewrite these equations this way? You'd think that they'd stick with the notation used by ACI as the preeminent concrete research and design authority in the United States, but apparently they want it to be more confusing than it needs to be. AASHTO has it such that anybody who already knows something about concrete design through ACI feels completely lost upon first use of this manual. :screwloose:

This may have turned into a gripe session about AASHTO here at the end, but I can't see the logic in re-writing code provisions that serve the same purpose. :smileyballs:

 

[MA_PE]

As you pointed out the equations are identical, except that AASHTO specifies f'c in ksi and ACI calls for psi. I'm not disagreeing that a direct comparison might be confusing (and I can't think of a good reason for it) , but your example demonstrates the need to pay attention to the codes and CHECK YOUR UNITS.

good luck on the test.