Lateral System Options

[MacGruber22]

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?

 

[JAE]

Are the moment frames near the middle of the building? In other words, are you using the tilt panels as shear walls?

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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[MacGruber22]

Yes - in other words, yes.

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.

 

[msquared48]

Works well for small pole buildings.

Mike McCann, PE, SE (WA)

 

[KootK]

I've renovated a few large malls where every interior column was part of a moment frame in each direction. Basically a permutation of the joist girder moment frame system that you mentioned. Apparently it allows for a great deal of future renovation flexibility. These buildings often seem to be flexible laterally and clad in precast which makes me wonder what the real lateral system is when the wind blows. Column heights are closer to 20'.

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.

 

[JAE]

40 to 45 feet is very tall (too tall) for cantilevered columns in my opinion.
For drift limits you might need a W36. Then what do you do in the other direction.



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[structSU10]

I would think, if you have a lot of columns, and were trying to get cantilevered columns to work, you would alternate their orientation within the building, so half are strong axis orientation for each direction. I would think for a tall building, unless forces are relatively small, the base moment connection would be prohibitive, not only for the base plate but for the footing supporting the column.

 

[Brad805]

I have seen this many times on pre-eng steel buildings when the lateral loads are small, but the height is usually in the range of 25'.

 

[MacGruber22]

We tend to use 12" square HSS for the gravity columns, which would preclude the need to have the alternating orientations if those became part of the MWFRS.

Although, as many have suggested, drift still might be a problem given the 40 - 45-ft story height.

 

[structSU10]

even for the stiffest 12" HSS, the 'spring constant' is .3 kips/in of deflection. holding an h/500 deflection say, and 100 effective columns, your maximum load would be like 33 kips if evenly distributed, and that does not include any additional drift due to foundation rotation and such. Now if you take a much shorter building, say 25' tall, you could handle 105 kips at the same criteria.

 

[Brad805]

If you want economy and the architect will allow the end column size, it is hard to beat a Pre-eng frame. Costco does this frequently. Nucor will supply parts.

 

[MacGruber22]

At say 15 psf MWFRS, working, I usually need to resist a total of 180 to 200 kips (worst case direction). Even if 25% of that is resisted by each end concrete shear wall, that still leaves around 100 kips of load, which would be L/150 with your 0.3 k/in stiffness.

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.

 

[JAE]

Don't forget the flexibility of the footings - very hard to estimate accurately - but they would have a tremendous effect on these columns' drift I would think.



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[MacGruber22]

JAE - yup, definitely. There is a method in the PCA manual I have used a few times.

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.

 

[Brad805]

Too bad Vulcraft or one of the others will not work with you on the joist girders. They are so practical and would make for easier column connections.

 

[PEFLWI]

Unless the roof diaphragm is flexible, won't most of the lateral load be resisted by the wall panels? I would do a quick model to see where the lateral load is resisted.

 

[clarke1973]

Surely a fully braced frame, using the wall panels as shear walls would be the most practical solution.
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.

 

[MacGruber22]

PEFLWI - Good point.

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.

 

[SteelPE]

Why are you keeping your column sizes to a W14 (is there even a W14x293 section)? With these building I will tend to use larger columns a W18x192 will save about 100 pounds per foot and only sacrifice 4" of depth.... but you are pushing the KL/ry limit of 200

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.