SERM 6

[McEngr]

Alan Williams is doing a better job on the SERM. He has recognized that the lateral analysis of his previous SERM's has been lacking particularly in the wind design areas. He has completely revised the chapter that once had a split between UBC 97 and IBC 2003. He now has a chapter called lateral forces and has categorized it down to:

1.) Lateral Resisting Systems - vertical and horizontal resisting elements.

2.) Seismic Analysis

3.) Wind Analysis

If you're looking for an in-depth look at each material as it relates to seismic design, you will not find it in this chapter. I would recommend the SEAOC Seismic Design Manuals and the Alan Williams book entitled "Seismic and Wind Forces - Structural Design Examples, 3rd Edition". There is also AISC 341 and PCA Notes that will help with steel and concrete.

The book is redesigned and typifies a lot of information that can be retained quickly. I wouldn't call it a complete overhaul as it in a lot of respects is just a code update from the 4th and 5th ediitons.

Overall, I'm glad I purchased it simply because there is no replacement for some of the lateral information and the bridge design overview.

 

[STEEL MAN]

Anybody here can give comments based on the last Oct 2011 SE exam and the SERM 6 outline?, I wanted to know before buying it. Is it close to NCEES SE exam?

 

[Rhoadies]

I hate to bring up an old thread, but my question is right down this alley.

Is anyone else a little distraught over the SERM 6th and it sounding like it is the authority on how these items should be done. I began here for the lateral load portion of my studying as it seems to roll all of the needed codes into a handy little reference. However, specifically in regard to wood shear walls, I believe this book falls short. At the bottom of page 7-7 of the review manual it states "Since the shear wall is considered non-rigid, gravity loads along the top of the wall do not provide a restoring moment to the wall."

I have no issue with this if it is true as it makes our tie down force a little bigger and we do not have to determine the resisting moment of the wall or walls above, saving a step and much needed time on the test. I do have an issue with the book sounding like that is the way it should be done and then having the sample NCEES test suggest that we should take the resisting moment and determine our hold down forces that way.

What do you think? I just stumbled onto this site today, but I've been studying for a couple of months now. I'm worried I learned and reinforced the wrong method for this.

 

[dakota_79]

I noticed the same thing (though not from NCEES - in SEAOC V1 they do a wood shear wall tie down example where they use the gravity load at the top of the wall to help resist against net uplift on the tie down, also in contradiction to the SERM approach).

However, Williams didn't pull the more conservative approach out of thin air as you imply - if you continue that paragraph to page 7-8, you'll see he cites that the approach is based on SDPWS Eq 4.3-7. If you look at SDPWS section 4.3.6.1.1 (and it's associated commentary), you'll see he seems to correctly cite this. Since SDPWS is a required reference standard for the exam, it would seem to be valid to use this conservative and simple approach on the exam if it comes up and cite "SDPWS Eq 4.3-7" in your calcs if it's on one of the essay questions. If it's on a multiple choice, that's trickier - guess I'd follow whatever approach the question steers you towards. Not sure how it's done IRL, since I don't do wood.

Either way, don't over-think things. If the exam question tells you to approach something in a certain way....approach it in that way. It is the ultimate authority for us for now!

 

[Rhoadies]

I saw his reference as well, I just didn't see anywhere in the SDPWS that says you can neglect the gravity loads above. I think that I was getting confused by section 4.3.6.4.2 that implies if you have enough of a resisting moment that the tie downs are not necessary. I just found the NCEES afternoon question a lot simpler when I didn't have to spend the time calculating the resisting moment and could spend the extra time detailing the wall section.

I noticed in the morning problems it seems that they do not want us to use conservative approaches. I can't remember which morning vertical problem had the standard "size the base plate" problem, but if you conservatively take lambda equal to 1 (as is done in the SERM) in the lambda-n' thickness check, you're pretty much guaranteed to get it wrong.

Do we know if they are sticking us to the most efficient designs in the afternoon or just showing whether we know what we are or are not doing, even if it means we were a little conservative in our responses?

 

[dakota_79]

Good catch. Nothing in the SDPWS commentary on that section. I checked another source: "Design of Wood Structures, 6th ed." by Breyer et al, which is generally considered a good source on wood design. On page 10.35 regarding anchorage forces he says, "It will be recalled that the (shear wall in-plane overturning) moment is carried by the chords, and the chord forces are obtained by resolving the moment into a couple. The chord forces may be calculated from the gross o.t.m., or the resisting moment provided by the dead load may be considered. Both procedures are illustrated...". Granted, this isn't codified language.

But it seems to be a judgment call: if you have a tall wall and/or small dead load as in a non-bearing wall, it's conservative-but-not-overly-so to just ignore the uniform load. If you have a squat wall and/or large dead loads, it's advantageous and correct to use it. Any wood design guys feel free to override me there. Either way I'm coming around to your original point that Williams pulled his rationale out without a good explanation or citation. I'm trying not to look at the NCEES sample exam ahead of time (before using it in a couple weeks for a full, timed exam simulation) so can't comment on the specific problem you're citing from it.

You're right on the morning problems: when there's only 1 correct answer, you have to do it pretty much exactly the way they want it unless it's very minor shortcuts/approximations that you know will get you close enough. For the afternoon: not positive since the scoring criteria are a tightly held secret, but I'm just approaching it such that if they aren't using the essay format as an opportunity to flex our Sound Engineering Judgement muscles, then what's the point?

 

[McEngr]

Good discussion! I don't have time to discuss, but on most issues of judgement, I take Alan Williams SERM with a grain of salt. For concrete columns it is also highly lacking. I usually do what you guys did and over write the serm with the governing material spec.

Unfortunately, the SERM is still geared towards the civil pe and not requiring much knowledge to master in my opinion.

 

[dakota_79]

@Rhoadies: FWIW, just ran across an example problem in "Structural Engineering Solved Problems, 5th ed.". Problem (Ch.6, problem 1.3) deals with seismic load path into, through, and out of a 2-story light frame system of diaphragms, collectors, blocking, and shear walls, and asks to determine design forces in all horizontal and vertical ties.

This treatment ignores gravity loads for determining all of the vertical tie forces in the system. Seems pretty clear based on all of this evidence that this conservative approach is a common design practice.

Again, any people who actually design wood structures IRL feel free to chime in.

 

[McEngr]

In practice, I have always used the (0.6-0.14SDS)D+rhoQe for shearwalls. Never used the live load in the combination.

 

[McEngr]

These are a couple of spreadsheets I created to deal with such issues. Others were involved, but I created the force transfer from scratch.

Feel free to pick apart if you like...

Lat Wood IBC 2006.xls

Rational_Shearwall_Openings.xls

 

[Chosen One]

I think I may have located an error in the SERM 6th Edition: on page 1-2 the phi factor for spiral columns is 0.7, but in the 2008 ACI (pg 118) this appears to have changed to 0.75. Must be an update from 2005 ACI, anyone else confirm?

 

[McEngr]

Chosen One: You are correct. Good catch!