slender masonry walls 3.3.5

[Laura SE April 2012]

You're likely familiar with the current NCEES list of references.

Does anyone understand the reasoning behind footnote 2 (copied below)?

"Examinees will use only the Allowable Stress Design (ASD) method, except strength design Section 3.3.5 may be used for tall slender walls with out-of-plane loads from wind and seismic."

From the wording, I gather that we are able to use ASD for absolutely everything if we want to, but it would seem there's some motivation for candidates to use strength design section 3.3.5 in the case aforementioned. I personally don't understand why a candidate would want to learn and remember a separate design method just for that particular case. Any information?

Thank you!

 

[dakota_79]

Yeah, I found (and still find) that statement a little confusing.

I guess my approach was to not overthink it, and if anything came up on the exam where it seemed 2nd order effects would be non-negligible, used the 3.3.5 strength method which accounts for those effects. Not sure if that was the correct approach/interpretation, but I do think it's the conservative one (you can never be wrong to account for 2nd order, only waste time....but you can be wrong to ignore it).

If I had a hunch, I'd guess they want to tell you you're supposed to use 3.3.5 for slender walls with OOP loads, but don't want to just give it away with an explicit direction like that so they snuck in that word "may". Just a hunch though.

 

[kevo_55]

I personally don't understand why a candidate would want to learn and remember a separate design method just for that particular case. Any information?

Thank you!

Simply put, there is no moment magnification for P-delta effects with ASD design. LRFD is the only way to get this.

I hope this helps.

 

[McEngr]

I think I understand Laura's confusion. I've always been told that ASD and LRFD don't mix. This can also be understood to mean that you should not mix the two - either go all LRFD or go all ASD. I think that's where the exam spec's could do a better job of drawing the line.

All I can say (without giving away too much info) is that you won't have to mix the two design methodologies on the same problem. I personally feel that a slender wall on an ASD problem for the afternoon portion is a poorly written problem. One should recognize that the problem requires chapter 3 in lieu of chapter 2 from the get go.

When I design to ASD for steel design, it's all in the same 13th edition manual, so I do cheat on this at times... so I guess I'm a hypocrit.

 

[Laura SE April 2012]

All 3 of these responses are helpful to me. Thank you!

I am still looking into finding a hard-fast rule from an "exam-authoritative" source for how to determine whether a wall is "slender" or how to determine whether P-Delta effects should be a concern. Some codes are pretty explicit.

Unless I find otherwise, I might use the "h/t exceeds 30" mentioned in section 3.3.5. (I wouldn't necessarily make this conclusion based on the text, but I was in a review course where the teacher checked a wall with h/t = 30 using ASD, and then used 3.3.5 for the same wall, only with a 2' height increase, bringing h/t to 33). If anyone has any other ideas, please share. Thanks!

 

[McEngr]

Laura, check your messages...