Overstrength Factor when Wind Governs
Overstrength Factor when Wind Governs
(OP)
Inexperienced California EIT here.
Lets say that you have low Seismic loads and High Wind loads, and you pick a system with an Omega factor.
All of your members are designed to behave elastically for the governing wind load combinations, including the "seismic fuse".
Now, however, you must design the elements surrounding the "fuse" to behave elastic. Lets say you have to multiply the earthquake load effects by a factor X to yield the "fuse".
If X < Omega, I understand that we can (always?) multiply the EQ load effects by X instead of by Omega, as only X is required to yield the "fuse" and the surrounding members are protected.
However, lets say X > Omega. Because the earthquake load effects were low, multiplying the Earthquake loads by Omega=3 is still lower than the force required to develop the expected yield strength of the "fuse" (which was sized based upon Wind loads, not EQ loads). We need to protect the members around the fuse, but is the Omega factor a maximum? In sizing the members surrounding the "fuse", do you multiply E by Omega, or are you required to multiply E by X?
Hopefully this makes sense.
This thought process leads to another question: if a building was governed by wind, is it more efficient to use a system with a lower Omega Factor?
Edit: If our system has R>=3, do we have the option of reducing R to 3.0 and Omega to 1.0 and following the Seismic Steel Manual R=3 chapter? That way we don't need to protect members around the "fuse".
Lets say that you have low Seismic loads and High Wind loads, and you pick a system with an Omega factor.
All of your members are designed to behave elastically for the governing wind load combinations, including the "seismic fuse".
Now, however, you must design the elements surrounding the "fuse" to behave elastic. Lets say you have to multiply the earthquake load effects by a factor X to yield the "fuse".
If X < Omega, I understand that we can (always?) multiply the EQ load effects by X instead of by Omega, as only X is required to yield the "fuse" and the surrounding members are protected.
However, lets say X > Omega. Because the earthquake load effects were low, multiplying the Earthquake loads by Omega=3 is still lower than the force required to develop the expected yield strength of the "fuse" (which was sized based upon Wind loads, not EQ loads). We need to protect the members around the fuse, but is the Omega factor a maximum? In sizing the members surrounding the "fuse", do you multiply E by Omega, or are you required to multiply E by X?
Hopefully this makes sense.
This thought process leads to another question: if a building was governed by wind, is it more efficient to use a system with a lower Omega Factor?
Edit: If our system has R>=3, do we have the option of reducing R to 3.0 and Omega to 1.0 and following the Seismic Steel Manual R=3 chapter? That way we don't need to protect members around the "fuse".






RE: Overstrength Factor when Wind Governs
Not necessarily. For drags/collectors in SDC C and above you're stuck with Omega, for instance.
Sometimes, sometimes not. Say for welds of braces to gussets in SCBF, you design that for the lesser of A) brace strength with an overstrength factor (dictated by AISC 341, not the omega from ASCE 7), and B) whatever the load the system is capable of transmitting to the brace. However for flexural design of columns in concrete moment frames, you design to force the hinge in the beam regardless of how overstrength the beam may be. If the beams are 10x stronger than they need to be for seismic, then the columns also need to be 10x stronger than they need to be for seismic (plus a 25% increase for concrete, so actually 12.5x stronger). This is one of the reasons that it's important to try and keep spans down on your moment frame beams. If they get too long their design will be driven by gravity, not seismic. And don't go crazy with the throwaway steel/PT in your beams/slabs. It can and does hurt your column design because you have to count all of that in your strong column-weak beam checks.
For the actual members it applies to, yes. Though for true seismic-controlled design these are typically accompanied with lower R factors (so higher design forces for everything else). At least in my experience there typically aren't that many elements that actually utilize ASCE 7's omega factor. Mainly drags/collectors, anything supporting discontinuous stiff elements (like walls), and some items with piles. AISC 341 tends to use their own overstrength factors (Ry/Rt). ACI 318 tends to specify their own as well, with the exception of anchor design in newer versions.
RE: Overstrength Factor when Wind Governs
Yes for moderate to low seismic (SDC C and below), but I'd vote no for SDC D and above as ASCE 7 prohibits the use of 'structural steel systems not specifically detailed for seismic resistance' for high seismic. And you can use the examples in the manual for design but would note that there's nothing in the actual provisions of AISC 341 about designing for R=3 because you're technically not even required to be using that standard for R=3 under ASCE 7. I believe the lowest R for any system with requirements in AISC 341 is 3.25 for OCBF.
But at long as using the R=3 system is okay under ASCE 7, I don't see why you couldn't design it that way. Just important to make sure you're consistent with your R/Cd/Omega factors and don't mix and match from various systems. That goes for any system really. There's also nothing stopping me from detailing my ordinary concrete moment frames as special concrete moment frames while still using the ordinary frame's R value for load determination. I'm not sure why I would do that, and I tend to think my client would by justified in being upset with me if I did that because I'm designing a needlessly complex and costly system. But it's technically allowed.