Steel Tower

[rittz]

Attached is a sketch of a free standing marque our client wishes to to erect at the entrance to his business premises. I have been working on a solution assuming portal frame behavior. The extreme aspect ratio has me a bit anxious and wondering if the usual assumptions which are made for a portal frame are appropriate without adjustment. I wanted to use cast in place piles but they are so close together they will interact. A concrete frost wall on a footing may be more logical. Any suggestions?

 

[Ron]

Your structure will act more like a cantilevered column. Yes, you will have frame action within the steel framing, but overall for foundation purposes, it is just a large cantilever.

Your overturning moment is high, so a footing will have to be quite large to react that moment x 1.5 or 2 depending on your code provisions. For that reason, a drilled pier or two large augered concrete piles with a cap to support the structure makes more sense. Yes, with two augered piles you'll get interaction, but as long as you account for that, it's ok.

Since this structure is stucco covered, advise your client that waterproofing the stucco facade is extremely important. The stucco system should be designed as a drain plane or rainscreen system, not a face barrier system.

 

[XR250]

Seems like you could make the members much more slender and add some x-bracing and forget the portal frame (unless it needs to be open)
Also, seems like a spread footing would work if you have enough real estate

 

[DST148]

@rittz> Assuming there is enough room on all sides, why not have one large pier which will grab all the four
steel columns and the hold-downs, and one single pad footing with enough soil overburden to counteract the uplift.
Alternatively, you may have four individiual piers for four columns connected at the top by tie beams and one common pad footing.
Bringing in another trade for deep foundations does not seem a good idea unless the soil is very poor.

 

[oldestguy]

In cold areas (presumed by your "frost wall") what you have show may lift by frost action on the"grade beam". Before deciding which way to go, what does the test boring show? Off hand, Ron, what about one larger caisson?

 

[Ron]

OG...agree a single caisson would be ideal; however, it is a marquee so space is likely limited for a large caisson rig. Even an Augercast rig might be a challenge for a small space leaving the foundation options to maybe just a big chunk of concrete!

 

[humanengr]

If you're using pile foundations, can you spread the piles out and tie them together with a pile cap / beam that can support the columns? (If the marquee columns are 2 ft. c/c - is there any reason the piles can be spaced further apart?)

 

[a2mfk]

Have you looked into using CMU instead of steel framing? The CMU could then be modeled as a tube/moncoque structure in bending, and with a moderately sized pad foundation the CMU weight alone may counteract most of the 20psf lateral loading... This would eliminate deep foundations.

Also, helical foundations can be installed by smaller tractors about the size of a bobcat. But I think you would need four, one at each corner.

 

[azcats]

Is the whole thing covered in stucco? If so, I would embed one larger cantilevered pipe column into one laterally loaded drilled shaft and frame the 3' x 6' x 24' out of angle. It's how 'sign' fabricators build these monuments. Do the soils have lateral capacity?

I like that or the idea of masonry above. Is a spread footing an option? If so, masonry would give you extra dead weight to resist OT. Not sure if seismic is a governing load though.

 

[TDIengineer]

I agree with excel. Utilize that 3'-0" depth to analyze your built up shape as one big column. You should be able to cantilever 20'-0" no problem with a 3'-0" deep truss element. Add some diagonals and check it like a chimney.