Another "when does omega apply" question (IBC, ASCE-7)

[AaronMcD]

Regarding elements supporting discontinuous lateral systems, I've read tons of prior threads and I think I even made a thread a while back. And it appears just as murky.

This question is specifically about a lateral system supporting a lateral system.

Wood frame shear wall, offset in either direction (in plane or out of plane) from the wood frame shear wall below. Typically I would not use overstrength on the shear wall below, but use it on the transfer beam or diaphragm.

Another common thing is a OMF supporting a wood frame shear wall. Since the OMF is itself a lateral system, it seems it does not require overstrength.

Thoughts?

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[strucbells]

1. Out of plane is a horizontal irregularity, type 4 (see ASCE 7-16 Table 12.3-1). In plane is a vertical irregularity, type 4 (see Table 12.3-2). Depending on your Seismic Design Category, there are punitive measures you need to incorporate within the load path. The Tables reference you to various requirements depending on SDC. Generally, yes, it is the transfer load path that needs to be amplified by overstrength but there are some other requirements as well.

2. Wood Frame Shear Wall over OMF is a vertical combination, see ASCE 7-16 12.2.3.1. Basically, since the R factor for a steel OMF is 3.5 and the wood shear walls is 6.5 (or 7 if building frame), you can design the upper lateral system for its own R factor, but you have to amplify the transfer forces by R_upper/R_lower. I would also check if you have a soft story irregularity as well.

 

[WesternJeb]

Intuitively, this would appear to meet the intent of what overstrength is for, code required or not. You are providing a stiff element in a ductile system that all of the force ends up transferring through.

 

[AaronMcD]

^ yes of course I amplify by Rupper/Rlower, but typically not overstrength, considering the lower frame part of the seismic system.

I've read chapter 12 probably 100 times and it's just as ambiguous when it comes to lateral system supporting gravity system supporting lateral system. The commentary leaves this out as well. And according to all the previous threads on here, people have different stances on a lot of this. I haven't seen this particular question asked yet, so was just wondering if there is consensus on, for example, overstrength stops at a lateral system with it's own R value.

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[strucbells]

Sounds like you have a clear picture in your head of a scenario that you've done nothing to explain in this thread. Hard to give you input when there is no picture or particulars.

 

[Eng16080]

Typically I would not use overstrength on the shear wall below, but use it on the transfer beam or diaphragm.

I believe that's correct. I always refer back to ASCE 7-16 Commentary Figure C12.3-5 on p.554 whenever this comes up. Like stated above by strucbells, it's basically the load path necessary to transfer loads from the lateral system above to the lateral system below. From the commentary, the intent is to protect the gravity members from a potential overload from the lateral system.

 

[AaronMcD]

Many very common scenarios. Literally anywhere the structural system below is not directly in line (any type 4 horizontal or vertical irregularity). In these situations I typically only design "gravity" systems (in quotes because they really aren't gravity only anymore) supporting the loads from above for overstrength, and NOT the lateral system below that in turn supports. Example below. Exact situation may vary.

In addition, a wall post landing on a moment frame beam could be considered in line and therefore not an irregularity, but maybe some consider it an irregularity where the moment frame beam needs omega load from the post above.

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[AaronMcD]

@Eng16080
In that image, I would personally design the beam end connections for overstrength. I would amplify the left post unless it is part of a designated shear wall (if it is standalone or only sheathed in gyp board). Seems like that's what other engineers do as well?

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[Eng16080]

Another common thing is a OMF supporting a wood frame shear wall. Since the OMF is itself a lateral system, it seems it does not require overstrength.

I believe this is correct, assuming that the shear wall is directly above the OMF and the shear wall chords (end posts) are aligned with the columns of the OMF below.

 

[strucbells]

I would agree with everything stated above, except that in the case of a wood shear wall post (end of shear wall) landing on a moment frame beam NOT in line with the moment frame column would require the use of the overstrength factor for the reaction on the moment frame beam for the analysis of the moment frame beam.

 

[Eng16080]

In that image, I would personally design the beam end connections for overstrength. I would amplify the left post unless it is part of a designated shear wall (if it is standalone or only sheathed in gyp board). Seems like that's what other engineers do as well?

I've contemplated this as well, but it seems this is in disagreement with the code commentary: "For wood light-frame shear wall construction, the final sentence of Section 12.3.3.3 results in the shear and overturning connections at the base of a discontinued shear wall (i.e., shear fasteners and tie-downs) being designed using the load combinations of Section 2.3 and 2.4 rather than the load combinations with overstrength of Section 12.4.2 (Fig. C12.3-5)."

Of course, if the hold-down for the shear wall was being anchored into concrete, then ACI 318, Ch. 17 would apply, and one of the options in designing the anchorage is to use the overstrength factor, so this is potentially in conflict with ASCE 7, or at least is confusing. I suppose in that case, the connection of the hold-down to the wood shear wall would be designed without overstrength but the connection of the hold-down to the concrete could use overstrength.

Of course, then we get into the whole thing where you don't necessarily need to use the overstrength factor if it can be shown that a ductile system cannot transmit the amplified (omega) forces.

 

[AaronMcD]

What I mean is I would design the beam end connections to columns for overstrength, not the holdowns/straps connecting the wall above to the beam. Even with concrete, I almost always design for ductile failure of the holdown rod and inspect them on site for 8d exposed out of the concrete.

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