LFRS Load Path & Combinations

[StrEng007]

I'm taking a moment to ponder some design topics after reading a new text. I'm reading through the section on lateral force-resisting systems and diaphragm behavior in "Structural Loads - 2012 IBC & ASCE/SEI 7-10" by Fanella... In this section, they give the sequence of wind load path as it "travels" through the diaphragm to the foundation. The text mentions that wind loads begin with the application of wind at the windward wall. It goes on to trace the loads through the subsequent supporting members. It's straight forward.

The question that arises for me is why they don't mention any contribution from the leeward wall MWFRS wind loads. I typically consider the overall contribution from both the windward and leeward MWFRS wind, applied to the floor or roof diaphragm in mind. This is analogous to a beam with loads being applied transversely at the top and bottom flanges for an overall distributed load.

This chapter goes on to mention that bearing wall systems, similar to building frame systems, should be designed to consider the most critical combined load effects due to gravity and wind loads. However, it also mentions that for wood construction, most of the connecting elements are only designed for lateral and not gravity loads. I typically consider the combined effects through IBC load combinations, if the element is designed to support both applications of load regardless of material type.

Perhaps this should be two different topics, but I'd like to get some opinions from other engineers:

1. In this text, why does the wind load propagation from a diaphragm to the MWFRS only include effects from the windward wall?
2. For the different types of structural systems (ie Moment frames, Braced frames, bearing walls etc.), when should the combined effects of gravity and lateral loads be considered for designing the building elements and connections? Any text references that cover this in detail?

 

[KootK]

1) Laziness? Other than for the design of the walls themselves, I think that it's pretty common to just throw all of the net load onto one side or the other. I'll do this for the macroscopic aspects of building design. I'll use my judgment with respect to the attention to given to dragging leeward wind back back to the lateral system. It can be tedious work as you know. Simplifications beget sanity and endurance.

2) I think the generic answer would be to always consider the combined effects. But, then, I'm not sure that I understand the situation. Can you elaborate on the kind of connecting elements? Or the context of the statement?

How are you finding the book? I'm always on the lookout for stuff like this as I find that the biggest issue with new recruits is just understanding load path rigorously. I like to hand them a good book and ask them to return in month for comprehension testing. I've been using Loads and Load Paths in Buildings by Taly. It's okay. I've got some of Fanella's stuff and like it but I find him to be overly prolific which makes me doubt the content quality. Kind of like Taranath. Guy's got fifteen books on highrise design that seem to all more or less recycle the same content. Each on its own is admittedly pretty good.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[StrEng007]

KootK,

1) That's what I thought. It would have been beneficial for the text to mention that simplification.

2) I know some engineers that keep their gravity and lateral systems completely independent. Which is fine, for example, if they're looking at a brace that is assigned to ONLY support lateral loads. What I'm talking about are elements that support both load directions. Again, this goes back to the overall simplification of a project. I've always kept in mind which elements need to do dual duty. For instance, an exterior load bearing CMU shear wall. Do you combine, through the appropriate load combinations, in plane wind shear, out of plane side wall wind suction and all applicable gravity loads? Granted, gravity loads with minimal eccentricity will often improve a walls ability to support out of plane loads... but you get my point about considering the worst case scenario.

This book is a fairly good resource. The derivation of loads using the current ASCE 7-10 is also very helpful. A majority of texts out there use older codes as you already know (sounds like you have a library of resources already). This text walks you through the interpretation of ASCE 7-10, working examples in here and there to help guide the discussion. There are flow charts that will carry you through the steps required to calculate loads.

I think the text lacks the quantity of examples that would take it to the next level as a "go to" reference. To be fair, I know that would be a huge task for the writer to take on. Don't get me wrong, the examples that are provided are good.

I know that many engineers spin their wheels on how to calculate certain load situations that don't fall into a particular ASCE 7 section. Putting their opinion in print is risky business. It would be great to find a resource that gives a full list of design examples for various and unique situations. AISC does an excellent job with this in their 'Design Examples' guide.

 

[icundiff]

Sorry for the hijack.

KootK,

What are some of the books you have concerning load paths, that you would recommend?

 

[KootK]

What are some of the books you have concerning load paths, that you would recommend?

While I myself love a good hijack, it's not been going well for me lately. @SR007: you say the word and I'll yank all this down lickity split. With the recommendations below, I've specifically limited the list to books satisfying two requirements:

1) The get into load path not just load determination (W/S/E).
2) They deal with entire building load path in a macroscopic sense. Otherwise, I would have included Blodget etc.

The Taly book is actually the weakest of the bunch despite it having the most relevant title (that's why I bought it). The other books all deal with load path only in certain sections but do it exceptionally well in those sections. I started getting into this seriously when I realized that load path was the biggest issue for new grads joining my team.

On two separate occasions, I worked with rookies who designed gravity columns for me assuming that they were cantilevered from the foundation (K=2.0). W T F? Then I realized that isn't really such a goofy assumption for someone who doesn't know about diaphragms or core walls. It's wrong all day long of course, but not unreasonable given the knowledge starting point.

Putting their opinion in print is risky business.

Yes, that is exactly the problem. It's also the reason why a lot of the books that I've listed shine. They step out a bit in terms of their willingness to commit to how things are done in the real world of compromise and incomplete information. I still feel that there's a massive void in terms of the perfect structural load path book. And one could be assembled relatively easily by essentially just collating the information found in other areas. Up for a co-authorship / get rich very slowly plan?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.