If this is an infill of an existing building, it is likely that the lateral system of the steel building was not designed with the extra seismic weight of the infill framing in mind. I usually design the wood framing with its own lateral system and don't apply any of the lateral load from the wood into the steel building.

The steel building should have wind and seismic lateral loads on it, but the wood "building" inside will likely only see seismic loads. I would locate and design shear walls as needed assuming that the wood building is not getting any lateral support from the steel building. This might mean installing wood sheathing on interior walls (which architects and builders usually oppose) but I think that's the direction you have to go (unless you are allowed to and can get comfortable with relying on gypsum wall board sheathed shear walls instead of wood sheathed).

I partially agree with D.E.N. That MBS probably didn't consider the seismic weight of the buildout - but does that matter? Are you in a region with high seismic loads? Where I am, it wouldn't matter for strength. Our wind loads are a whole lot stronger than any earthquake would be and all steel is designed with R=3.

Regardless, I disagree that you can just split the loads out. This only works if you go all the way and provide a seismic separation joint between the two. If you don't, you'll either a) add mass to the portal frames anyway or b) cause significant damage to the wood framing either through impact damage between the two structures or inadvertently making the wood too stiff such that it attracts load from the MBS and fails the wood lateral system.

Pay close attention to how much drift that MBS has. In the states, most of them are designed to be H/60 - which is not sufficient for what you're doing here. Service level wind loads in my area would cause large cracks in the drywall going on the light frame infill.

(MBS = Metal Building System)

phamENG - I completely agree with your point on seismic separation. Ideally, you would be able to separate the structures where they wouldn't load each other at all, but that is not always possible. Of course it becomes even more difficult to isolate the two when there are shared openings between the wood and steel (ie exterior doors and windows). And I would also agree that the wood structure would likely be stiffer than the steel since as you said PEMBs are allowed to drift quite a bit usually.

I would be hesitant to apply any additional load to the PEMB since they are typically designed with no additional capacity. From a liability aspect, how would you make yourself comfortable with the lateral capacity of the PEMB with the additional load from the wood infill?

If you did go down the path of designing the wood structure to contribute to the lateral support of part of the PEMB, it seems like the load path would be challenging to detail. Wouldn't you risk altering the lateral load distribution of the PEMB too if you started connecting stiffer lateral elements to it (wood shearwalls vs mainframes with large drifts)?

I don't disagree with you, just trying to think of the implications of connecting the two instead of isolating.


PEMB = pre engineered metal building (another term for MBS)

Quote (D.E.N.)

I don't disagree with you

Maybe you should. I certainly don't know everything (or even most things...probably not even very much...)

These are site specific environmental loads, so context here is incredibly important. I practice in eastern Virginia. Plenty of wind and the occasional hurricane, an earthquake you feel once every few generations. I see you're in Missouri...less wind, more earthquakes. So I get where you're coming from.

For me, that wood framing would not be enough to elevate the seismic load to exceed wind the wind load. So the lateral force resistance wouldn't be challenged by that at all. Again, that's here - probably not where you are and I have no idea about the OP.

I would not try to mix wood shear walls with a metal building system. That was just an example of what would happen if they were connected but the connection was not accounted for.

I don't like to call anything "Pre-Engineered." With the exception of some smaller ag sheds and car ports, we stopped picking metal buildings and trusses out of catalogs a long time ago.

Quote (phamENG)

we stopped picking metal buildings and trusses out of catalogs a long time ago

I think this is the best argument I have heard to stop calling them PEMBs. We are in a transitional phase in our office to refer to them as Metal Buildings instead. I think that terminology matches the code a little better too. We usually end up designing the foundations for these before we get reactions so if anything they are "post-engineered" lol.