Large Steel Podium - What Lateral Force Resisting System?

[manstrom]

I've spent the day trying to put together a lateral system for a large (400'x250') L-shaped steel podium. The podium is a composite steel system supporting 5 stories of wood residential. The structure is open on all sides, so I have been using a series of X-braces throughout the building. The braces take about 100k to 200k each. Both wind and seismic controls depending on the direction.

My structures typically use concrete retaining walls. This one is an oddball. I am detailing the braces and finding that the solution is not as elegant as I would like. Lots of bolts, odd looking details. It just doesn't look right.

Should I be looking at another option for lateral support?

Different type of frames?
Concrete walls?
Moment frames?

 

[JedClampett]

It seems that even as large as the structure is, 100 to 200 kips per brace is very large. You sure that you haven't made a math error?
If not, you're going to have some huge connections. Lots of bolts or welds. All those checks that we usually ignore, (net vs. gross area, etc.) will start to come into play.

 

[KootK]

Maybe the answer is just more bracing. I usually find that, anything that I can do with concrete shear walls, I can usually do with similar plan lengths of steel bracing.

I expect that you'd have a hard time getting the stiffness that you want out of moment frames. Are your cross braces assumed to be tension only? One of the things that I like about chevron systems is that the shear load gets dumped into two beam/column joints. It spreads the load around and lightens up the connections some.

I've never done a composite steel podium. Is it composite like deck & topping over beams and studs? Or more like 200 slab over beams and studs? Do you place a steel beam beneath each of your bearing walls? Is the ground floor CIP concrete?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[manstrom]

This steel podium has steel beams under bearing walls with a 5" concrete topping on a metal deck. Beams span to girders at gridlines over parking.

I prefer and urge concrete on these, but sometimes steel is supposedly cheaper.

The math is right. 400' long x 70' tall. Medium wind and seismic zone. About 6 braces each way. I am looking at adding more. There are a few stair towers that I will bring in.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller