Lateral System Options
Lateral System Options
(OP)
Does anyone use cantilevered steel columns for your lateral systems? Obviously, this system becomes impractical the greater the loads are(or not code-permitted over certain story heights). I ask because: I design many very large single story tilt-up/precast warehouses with sometimes 100 interior steel columns. Our office "policy" is to provide 2 or 3 interior moment frames in one direction (claimed to provide construction economy); however, many times this results in (what I think to be) massive moment frames (W14x293 columns, etc.)and huge base plates.
With OSHA requiring (4) anchor bolts at all columns, regardless of gravity or lateral, it seems like much of this lateral load could be distributed more uniformly through the roof diaphragm, and then to the slew of column bays elsewhere. I know you take a hit on the response modification with cantilevered columns - but with a building in SDC A or B with non-hurricane wind loads it seems that economy could be made by what I suggest above.
Thoughts?
With OSHA requiring (4) anchor bolts at all columns, regardless of gravity or lateral, it seems like much of this lateral load could be distributed more uniformly through the roof diaphragm, and then to the slew of column bays elsewhere. I know you take a hit on the response modification with cantilevered columns - but with a building in SDC A or B with non-hurricane wind loads it seems that economy could be made by what I suggest above.
Thoughts?






RE: Lateral System Options
Usually for large warehouses the roof is quite high and cantilevered columns just wouldn't have enough stiffness to keep the overall building sway down.
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RE: Lateral System Options
Yeah, they are usually in the 40 to 45 feet range for me.
I'd be curious to see what kind of drift I would have if I spread the lateral load to most of the columns.
I have suggested using more of the joist girders for moment frames, but apparently the joist people hate doing that.
RE: Lateral System Options
Mike McCann, PE, SE (WA)
RE: Lateral System Options
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Lateral System Options
For drift limits you might need a W36. Then what do you do in the other direction.
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RE: Lateral System Options
RE: Lateral System Options
RE: Lateral System Options
Although, as many have suggested, drift still might be a problem given the 40 - 45-ft story height.
RE: Lateral System Options
RE: Lateral System Options
RE: Lateral System Options
We tend to limit wind drift for these buildings to L/200 so it would be "close". Although, that does seem like a lot of extra tonnage of steel for little effect.
RE: Lateral System Options
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RE: Lateral System Options
Brad805 - of course they never ever want end columns. You could still do pre-engineered frame without end columns by having drag struts (structural steel or truss girders) at each end connecting to the conc. walls.
RE: Lateral System Options
RE: Lateral System Options
RE: Lateral System Options
It would mean a lot more horizontal roof bracing but would have savings in the column sizes and remove moments in the footings.
I wouldn't even consider cantilever columns or a portal frame approach for a building of that height.
RE: Lateral System Options
clarke - We *try* to encourage a partial height braced frame with 18 ft or so of clear height below the brace, but most of the time they want the full clear height throughout. They tend to be OK with the portal frames at the moment.
RE: Lateral System Options
There are joist manufacturers that will work with you on the joist girder idea. One in particular has been very accommodating over the years however, they tend to be the least economical. From the projects that I have done, I find the joist company mentioned above to be the most economical..... however they are extremely rigid in their ways and have recently become very combative to the point that I don't like working with them.
Are you in an area where you need to deal with seismic loads? I find my tilt panel buildings tend to be governed by seismic loads because the panels tend to be thick.... a cantilever column system even more so. Only in rare instances (which you may have) have I found wind loads to control. I would be interested in seeing what you find, but I suspect that the moment frames are still going to be cheaper than providing cantilever columns.