Steel Building with 80% floor opening

[CCox]

I have a unique situation that I'd like people's input on. I am providing the structural design on an elevated steel deck with a huge opening in it. A PEMB sits down on top of the outer columns. The steel deck supports multiple pieces of heavy log merchandising equipment. Equipment being conveyors, indexers, transfer deck, and saw carriage. Total self weight of building, deck, and equipment is 1,000 kip. Base shear is 125 kip. Most of the weight is from the equipment because it is concrete filled. Can't shrink opening and can't add any additional horizontal bracing, because of residuals equipment below. In the attached RISA graphics, the area loads you see on the third drawing is the only location of a floor consisting of concrete over steel deck.

The first picture is the main steel deck. The second picture is the mechanical deck. The mech deck is really seven different pieces of log handling equipment that sit down on my steel members.

Outside of a subdiaphragm on the left, I have no reasonable diaphragm. The heavy equipment which contributes to most of the base shear is directly connected to my steel beams and columns.

I can justify not having an actual floor diaphragm because the equipment is directly connected to the steel structure. However, what is really going to control this design is building drift. I expect there to be fixed beam-column, beam-beam, and column-foundation connections in order to control drifting. It is just a matter of iterating on this until my model is stiff enough to overcome P-Delta instabilities.

Anyone have any thoughts regarding my approach?

Thanks

 

[BAretired]

The approach looks good to me. If you don't have a diaphragm, then each portion must be capable of standing on its own.

BA

 

[CANPRO]

What are the dimensions of the building? Do you have large horizontal reactions from the PEMB to deal with at the connection to your structure?

Even if you had a proper diaphragm, I think I would still prefer your approach with a brace bay on each column line...if for no reason other than to give the PEMB reactions a direct load path.

 

[CCox]

Thanks for the input.

building is 66'x120'. Horizontal reactions are not huge. The largest is from wind and it is 14.7 kips.

I didn't even consider not having braced frames at each PEMB line. I bet my model wouldn't even converge if I started wiping frames out. Right now, my model runs with P-delta considered, I am just looking at drift ratios to make sure I am within code requirements for both wind and seismic.

I actually am factoring up the collector loads 25% because this is technically a Type 3 horizontally irregular structure. Irregardless of this, wind still controls.