ASD vs. LRFD NCEES sample exam prob. 801

[TehMightyEngineer]

So, in continuing my last minute cramming totally thought out review for the exam next weekend I decided to run through the afternoon problems of the lateral section. Problem 801 has us design a masonry wall using the ASD combinations from the 2006 IBC. If you follow through that then you get the wall as unacceptable for flexure. However, if you do it by LRFD (which we should be allowed to do on the actual exam I hope) then you actually get it being satisfactory. Now, I know that LRFD in ACI 530 gives you more credit than ASD does but I want to make sure I didn't goof somewhere.

Here's my math for 801. c)

Given 4' parapet with 100 PSF wind, #5 at 48" o.c. at centerline of wall. f'm = 1500 psi, fy = 60000 psi. Weight of wall = 60 PSF.

Design in 1 ft widths.

Mu = 0.9D + 1.6W = 0.9*(0) + 1.6*(800 lb-ft) = 1280 lb-ft

As = 0.31 in^2 / 4ft = 0.0775 in^2 / ft

Pu = 0.9D = 216 lb/ft

Ase = (Pu + As*fy) / fy = 0.0811 in^2/ft

a = (Ase*fy) / (0.80*f'm*b) = 0.3379 in

d = 3.82 in

Mn = Ase*fy*(d - a/2) = 17,766 lb-in = 1480.5 lb-ft

ϕMn = 0.9*Mn = 1332 lb-ft > Mu OK!

Check max steel = OK!

The ASD solution given by NCEES shows that the wall is insufficient for the load with the reinforcement given.

 

[TehMightyEngineer]

[cricket noise]

Man it's dead in here... you would think it would be going nuts what with the exam less than a week away.

 

[Agostage]

I don't have the example in front of me but wanted to say that I came across the same thing. As you noted the ASD and LRFD for masonry are not even comparable. I'm not sure about using LRFD for the PM, they state to use ASD for the exam except for slender walls but I imagine a grader would fine with it. I'm used to ASD so that's my horse.

 

[TehMightyEngineer]

Yeah, I'm used to ACI 318 LRFD so I find that LRFD for ACI 350 is easier to use for me but both LRFD and ASD for masonry make sense to me. I guess it all comes down to would you consider a parapet to be a "slender wall"?

 

[kevo_55]

You guys/gals might want to check the design standards for the SE exam.

They are pretty explicit with respect to what you are supposed to use with masonry.

 

[Agostage]

Right, you're supposed to use ASD for everything but slender walls. Personally, I wouldn't risk using LRFD beyond slender walls, and if the examples are any indication they will just tell you to consider 'slenderness' or 'p-Delta' effects or flat out say use section 3 if that's their intent. My thought was that if you solved a PM problem with LRFD and it was a good design it would be okay, but maybe that's just too risky on second thought.

 

[cajone5]

1. You should be using ASD

2. A 4-ft cantilever wall would not typically be considered slender. Slender walls are affected by P-delta by definition. Your solution, first of all, does not account for the P-delta effect of dead load on the moment demand. Second, a uniformly loaded cantilever will respond similar to a uniformly loaded simply supported beam with twice the cantilever length. I would highly doubt an 8-ft CMU tall wall would be considered slender (span to depth ratio of about 12). So you shouldn't be using LRFD per the explicit recommendations of the exam.

I would like to strongly urge you don't "go rogue" and assume that because you can do another method it will be acceptable to a grader. If they have explicitly told you what method to use, you should stick to that method. Good or bad. Right or wrong. You should do everything you can to do things as they want to see them on the exam. Don't let your grade be up to the judgement call of a grader. Give them what they are expecting. Play it safe. Get through the damn thing and do LRFD after you get your SE. Good luck man!

 

[TehMightyEngineer]

Oh, yes, I know that I should be using ASD for all but slender walls (h/t > 30) I just wasn't sure if I should use the same h/t for a cantilever and I was also wondering if people find that there is a large disparity between LRFD and ASD. I'm well aware of the design standard referenced by NCEES for the exam.

As for P-Delta, a 4 ft cantilevered parapet wouldn't really have any P-delta affects worth considering, correct?

 

[cajone5]

Oh, yes, I know that I should be using ASD for all but slender walls (h/t > 30) I just wasn't sure if I should use the same h/t for a cantilever and I was also wondering if people find that there is a large disparity between LRFD and ASD. I'm well aware of the design standard referenced by NCEES for the exam.

As for P-Delta, a 4 ft cantilevered parapet wouldn't really have any P-delta affects worth considering, correct?

Typically, yes you are correct.

And as I said in my previous post you can think of a cantilever as being similar to a simply supported member of twice the length. Feel free to derive the relationship but they have exactly the same moment and similar deflections. Anyway, that said, you can look at an equivalent span-to-depth ratio for the cantilever wall as 2h/t ~ 12 which is much less than 30 so it is not slender. In addition, the problems usually ask specifically for you to use LRFD or ACI 530 Section 3.whatever when the problem warrants it so keep that in mind.