Use of seismic overstrength factor in light-frame construction 1

[IslandEngineer]

Basically, when is the overstrength factor required to be applied in light-frame wood design?

Previous thread176-244422 asked this question, JAE replied and referred to previous Eng-Tips discussion; consequently, I searched the site for the previous posts but came up empty.

I attended a seminar last year concerning new code requirements (2009 IBC & ASCE7-05) and was told that I had to apply the 2.5 overstrength factor on "everything" if I used ASCE7-05 Section 12.14 (Simplified Alternative Design for seismic design); consequently, I changed to the ASCE7-05 more complex "standard method" to compute seismic loads. Still, I remain confused when the code refers to use of the overstrength factor "when specifically required" (such as Section 12.4.1 Applicability and 12.4.3 Seismic Load Effect Including Overstrength Factor). I have been unable to find an explanation for "when specifically required."

My practice is mainly involved with residential and small commercial.

I would appreciate any guidance to a reference and explanation of "when specifically required".

Regards and Thanks,
IslandEngineer

 

[JAE]

I have been unable to find an explanation for "when specifically required."

ASCE 7-05 (and 10) have statements within the body of the text where they explicitly refer to the load combinations that include the overstrength factor.

Having said that, the intention of the overstrength factor is to kick up certain connections and non-redundant or brittle conditions such that the designer can help ensure that the structure will enter a ductile mode of failure.

So you are somewhat free to kick up connections, etc. to ensure that....speaking of this is easy with steel. With wood - not so sure but I think the principle is the same.

 

[IslandEngineer]

JAE,

Thanks for the input.

What I think I am understanding is that the overstrength factor does not need to be applied if the materials and connections, making up the seismic force resisting system, develop a redundant and ductile system.

Noting the many nails, screws, bolts, laminated lumber, sawn lumber, and multiple wall lines developed to resist lateral loading on most wood light-frame construction, I think I could consider those structures redundant and ductile. That said, I see no reason to apply the overstrength factor to these wood framed type of structures. Even in fairly complex bolted or mortise/tenon timber joints, we have multiple failure patterns and seldom is there a condition where one bolt's failure or one member's failure could result in an "explosive" or "brittle" failure. If there was such a connection, OK, apply the overstrength 2.5 factor; otherwise, the OS factor need not be applied.

If you know of any other investigation I might make to test the position I have outlined above, I would appreciate your guidance.

Regards,
Island Engineer

 

[JAE]

Perhaps I'm mis-reading what you just wrote, but just to make sure - if you are thinking that the OS factor is either applied to the structure or not to the structure as a whole, you are mistaken.

The OS factor is directed at certain parts of the structure that are not inherently ductile due to their material or geometry.

In a steel X-brace, for example, you would apply the OS factor to the end connection of the diagonal brace but not to the brace itself as you want the brace to yield BEFORE the connection ruptures in a brittle fashion.

Away from my codes here so I can't point to the OS application for wood. Just thought I'd make that point. You may understand that but the impression I got from your post was the opposite.

 

[IslandEngineer]

JAE

I sincerely appreciate your comment "if you are thinking that the OS factor is either applied to the structure or not to the structure as a whole, you are mistaken." Frankly I was applying the factor to the entire structure. This I did following my interpretation of the 2010 structural seminar leader's direction (I attended the seminar last year concerning new code requirements (2009 IBC & ASCE7-05) and was told that I had to apply the 2.5 overstrength factor on "everything" if I used ASCE7-05 Section 12.14 (Simplified Alternative Design for seismic design). The seminar leader was S.K. Ghosh, co-author of the 2009 IBC Structural Provisions Handbook, and certainly more knowledgeable than me. Perhaps his response had something to do with my use of Section 12.14 (Simplified Alternative Design); perhaps not; in any event, I asked the question twice and the response was the same). Consequently, I adopted use of OS factor on "everything". As you might imagine, the result, in my opinion, was significant overdesign. So, I am very happy to be "mistaken" in the "applies to everything" design approach.

Your comment "The OS factor is directed at certain parts of the structure that are not inherently ductile due to their material or geometry" is what I expected to hear in the seminar.

In conclusion, Thanks very much for your time and comment. I will proceed applying the OS factor only when I believe certain parts of the structure are not inherently ductile due to their material or geometry"; that certainly makes good sense to me.

Thanks Once More,
IslandEngineer