Column Fixed Base On Tall Pilaster

[BadgerPE]

Since it is Friday, I need a bit of a sanity check.

I am working on a steel portal framed 3-story + basement project. Due to the amount of glazing and small brick bands on the building, I am attempting to limit drift of my steel portal frame to H/500 with a 50 year MRI per ASCE 7-10. This may be a little stout, but it is a very architecturally appealing structure that will be a featured business in our town. I'm getting a bit beat up on the drift because my overall frame is 42' tall (3- 14' stories), but only 28' wide. I am utilizing several bents to limit the amount of load on each individual frame and interior bracing is not an option. Furthermore, end frames can't be braced either due to openings in the wall.

My question regards "fixing" the base to eliminate the drift on the building. Based upon the heavy dead loads encountered, I end up having no net tension in my base plate. It seems really simple to design the pilaster/pier for gravity & shear loading, but it would appear that "bending" of the pier/footing would be negated by the large gravity loading. Is it really this simple?

Attached is a sketch and I missed adding that it is a W14 column with a 17" bolt gage. Also, there will be a floor diaphragm near the base of the column elevation.

 

[KootK]

t seems really simple to design the pilaster/pier for gravity & shear loading, but it would appear that "bending" of the pier/footing would be negated by the large gravity loading. Is it really this simple?

It's not clear to me how bending would be negated. I still see the 35 k-ft entering the top of the pier. Oodles of dead load may mean that the pier never cracks in tension but the moment will stil be there for consideration.

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.

 

[kc9508]

I agree with KootK; regardless of the amount of dead load, the moment is still there and must be accounted for. I assume this load case is with minimum dead load (0.6D for ASD or 0.9D for LRFD). Or were you perhaps referring to the overturning moment on the entire pier/foundation assembly? Either way, the pier itself still needs to be designed for the moment.

 

[BadgerPE]

Sorry all. My question was not as clear as I intended, but thanks to both of you for your responses. I agree that the pier would be subjected to the bending moment and should be designed as such.

I know that in multiple threads, several others have said that it is hard to achieve true fixity due to footing rotation, but in this situation, my eccentricity from the moment is quite low and I have a diaphragm at the top of pier elevation. Knowing those two things I feel that the base could be considered fixed and help me limit the drift of my building. Thoughts?

 

[RHTPE]

Having dealt with a few PEMB foundations, I thought I would add my thoughts to the problem.

If the PEMB manufacturer assumed the base of the columns to be pinned (I.e. no horizontal movement were possible, but rotation was possible), how much outward movement (of the foundation) would it take to negate that design assumption?

Similarly, if the assumption was completely fixed, what effect would a small amount of outward movement AND rotation have on that design assumption?

I would think that even a small amount of "give" in the foundation could result in serious redistribution of the stresses experienced by the building frame. While this particular problem does not involve a PEMB, the situation is somewhat analogous.


Ralph
Structures Consulting
Northeast USA

 

[KootK]

Sure, this is a pretty good candidate for base fixity. I think that accrues mostly from that great spread that you'll get between the two diaphragm forces that will form the resisting couple. I'd model the pier as part of the frame to account for it's flexibility. Maybe throw something in there for the base plate too. I doubt it matters much but, in the age of unlimited computing power, why not? I doubt it adds ten minutes to the exercise.

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.