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Column Failure 1

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PEinVA

PEinVA

Structural
Nov 15, 2006
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I'm currently working on a building which is primarily PT slab and RC column construction. It is 6 stories and the roof is structural steel. The exterior walls contain mansard roofs that start to come back into the room at 3' from the floor.

The structure of the mansard is framed by welding a W8 vertical member that sits on a baseplate to a stub column to a W8 member at approximately 40 degrees to a W14 horizontal member that is actually supporting the beams that the roof structure sits on. I know it sounds complicated, and I've included a link where you can see the image of a typical framing condition.

There are concrete columns that extend 2'6" above the 6th floor. They are typically 24" wide and 14" deep. There are 31 columns that we've surveyed and 13 of them have failed. There is a large crack that splits the column and most of the members on top of the columns have kicked out up to 1". After scanning for steel, we found only vertical bars and no ties, we believe this is ultimately the reason they have failed. The loads found in RISA are about 11 kips. These baseplates land at the back corner of the column top, and back center of the column tops.

We have to detail a fix for this poorly thought out connection. I was thinking of welding a member on the angled W8 and bring it straight down to the front face of the concrete column and attach there.


I'm sorry if this is very confusing but I'm trying to look for some direction here. Any ideas? Can I clarify anything for you? Please feel free to critisize my description.

Thanks a bunch guys

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 
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The crack direction is paramount to the fix. Concrete is beautiful in that it describes to you exactly its stress state when it cracks. Your principle tension is perpendicular to that crack, and from what it sounds like it must be angled 60 degrees or more from the plane of the slab. That being the case, ties are indeed the issue, though still verify the vertical steel is developed or else you still have a problem after fixing the ties. I wasn't sure how your proposed fix addressed the shear capacity issue.

I would start by unloading the column for the fix. Weld a plate onto the roof member to use as a jacking point and lift off of a frame on the slab (after verifying its shear capacity next to the column). With the column unloaded I would pull the crack closed tight and install a composite fiber jacket on the column to replace the required calculated ties. An actual good use for all of those FRP articles you've read, wahoo!
 
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SteelMover
Thanks for the ideas, I know I'm going to have to jack this thing, and unfortunately the slab was only designed for 40 PSF LL, it is a hotel, so its understandable I suppose. Would I apply FRP simply perpendicular to the crack, which like you stated above, matches that angle. However, Some locations are framed (anchor bolts) within 3" of the edge in both directions (perpendicular and parallel) to the load. I was thinking of welding an extension to the angled member and rerouting the load to a more central portion of the column, and still fixing it with a fiber jacket.

What do you guys think?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 
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Another thought just occurred to me, what if I put plates on the face of the columns perpendicular to the direction of the force and throughbolt enough threaded rods to replace the needed ties? What do you think?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 
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RCraine,
I haven’t full understood why ”The exterior walls contain mansard roofs that start to come back into the room at 3' from the floor”. Is this caused by horizontal load at the top of the stub columns from the roof load?

Reinforcing the columns with plates and bolts through may not be good enough for the vertical rebar restraint in other direction. Fiber jacket as proposed by SteelMover seems nicer.
 
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RC,
I think you already have enough ideas about how to fix these stub columns. I just wonder about what really caused them to crack in the first place. The links didn't work for me, so just trying to visualise possibilities.

11 kips horizontal on 24" x 14" concrete is only 33psi, which doesn't sound like much. Or are the cracks just the corners cracking off because load is at the back side?

The worst thing that can happen is to repair the columns and then have it happen again. So need to make sure of forces.

Does your analysis include thermal loading? The steel structure will expand and contract as it wants. The short columns are very stiff, and if subject to forces from the steel expanding, you may not be able to design for this without allowing some horizontal movement of the steel.
 
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I can't use any fiber because of fireproofing issues. These obviously don't work under normal loading conditions and as far as I know, that is a requirement to be able to use any carbon fibre reinforcement. Thanks.


RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 
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Mr. Craine,

"Another thought just occurred to me, what if I put plates on the face of the columns perpendicular to the direction of the force and throughbolt enough threaded rods to replace the needed ties? What do you think? "

Your idea of dropping down in front of the pedestal and attaching to a plate through bolted to the pedestal sounds like it may work. You are putting the lateral force down closer to the floor diaphragm I would think would act as a tie to resist lateral forces. Maybe the reinforcement in the slab would then act as the needed ties? I think to be on the safe side I would remove the damaged section of the pedestals, re-align the anchor bolts, add back ties, and reset your baseplates. With the lateral force accounted for at the pedestal base, the new pedestal top should only see partial lateral (maybe none?) As Hokie stated, it does sound like thermal expansion of the steel "popped" out the top of the pedestal.

I don't know if I would sleep well after "fixing" this one though :).

good luck please update us with your solution.
SD
 
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