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Existing Column Removal 2

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SteelPE

Structural
Mar 9, 2006
2,759
I have a project where the client would like to remove an existing column from a building in two locations (see attached). The original idea (which had the new support beam framed 90 degrees to what is currently shown) was to frame a new joist girder to the underside of the existing joist and use X bridging to stabilize the top of the new girder. With the new girder direction the options are less desirable.

1) Remove the existing girder and install a new joist girder that can span desired length (sort of the existing columns if necessary).
2) Frame a new girder to the underside of the existing girder utilizing kickers to stabilize the top chord of the new girder. This girder is to run from existing column to existing column
3) Same as option #2 but run the new girder the necessary length (short of the column to column location (as shown in the attached sketch)

I like #1 but I think the client is going to want #3. This is going to require me to figure out a way to stabilize the top of the new column in the middle of the new girder.

Has anyone done something similar before?

Please keep in mind I am aware of the foundation concerns hence the different options.
 
 http://files.engineering.com/getfile.aspx?folder=4a23ebde-8c08-4932-a5a3-b4a0d2894a57&file=img008.pdf
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SteelPE said:
There isn't enough space to slide the girder up that high plus we have to deal the rerouting some roof drains.

Got it.

SteelPE said:
The issue I have with that is the existing joist girders slope causing conflicts with the bracing for the new girders.

Not sure I get how the slope impacts the bracing. Can you elaborate? Regardless, if you're just picking up the column, as I'd recommend, the slope shouldn't cause any issues that can think of.

How are you handling bottom chord bracing for the new girders? Running the bottom chords out to the columns and tying in laterally with slots?

SteelPE said:
And I don't see adding the kickers which are only attached to the top chord of the existing joists causing a "moment connection" in the joist. The idea is to apply the kickers in-between the existing joist and then use a L5x5 to take the stabilization forces back to the joist.

See sketches below for the problem, to the extent that there is one, and a cost neutral improvement (in my opinion). The 5x5 introduces some beneficial flexibility, I agree.

Capture_nzxbqw.jpg

Capture2_nxofss.jpg


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.
 
The bottom chord bracing issue is more of a filed issue but I will try to briefly explain. The existing girders are 40' long and sloping 1/4"/foot. If we place the top chord of the new girder 10" above the bottom chord of the existing girder there will become a point where I can't get the angle in I show on the top chord of the two girders (if the new girder has parallel chords). This length will be pretty significant as the slopes are slight, therefore we would need to create multiple details for the TC bracing. We could eliminate this problem if we slope the top chord of the new girders to match the existing slope of the roof (1/4"/foot).

How am I going to handle the bracing of the BC of the new girder, the same way they joist manufacturer would normal do this, brace it back to the TC of the joists. Actually haven't given it much thought as the manufacturer typically handles this. I might put a feeler out to them if the is the direction that we go.

Interesting sketch. I will give it some thought, not sure I like the vertical slots (although I know why they are there).
 
SteelPE said:
The bottom chord bracing issue

Ah. Is it at least a single slope? Or does one existing girder slope up and the other down? Some options:

1) My proposed detail eliminates the issue.

2) Use a pair of mated, long leg angles back to back to form a zee shape. Adjustable.

SteelPE said:
Actually haven't given it much thought as the manufacturer typically handles this. I might put a feeler out to them if the is the direction that we go.

That sounds like quite the forest of long bracing angles. Some alternatives that the joist supplier may be able to help with:

1) My detail eliminates this issue also as the bottom chord is also effectively braced.

2) Internally cross brace the two new girders.

3) Make the girder botttom chords laterally stiff enough that they can span between columns without bracing.

4) Make the bottom chords heavy enough that they're self stabilizing without any help.

I'd look into supporting the column at the bottom of the new girders. You're inherently more stable that way.

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.
 
But yeah, daydreaming aide, I think that your concept is pretty solid. KootK approved.

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.
 
KootK

I really appreciate you opinion on this. You made me go back and check the beam alternative. Upon doing so I realized the only way to do this is to use a joist girder. If I use 2-40x149 beams my total load deflection is 4 inches at 298#/foot. If I use a 2-72G girders my total load deflection is 2 inches at approx 140#/foot weight and it's easier to "grab some extra moment of inertia" with the joist girder.

I also like your idea of "supporting" the column on the bottom chord of the truss. This will give some added stability to the system.
 
That's some fine looking open space. Thanks for reporting back. Any unforeseen hiccups along the way?

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.
 
KootK,

Just a few minor things here and there mostly relating to coordination with the joist mfr and the steel fabricator not "pricing" things correctly. Once everyone was on the same page it was an easy process. Remember, we had a meeting onsite with the fabricator prior to final engineering. What is shown is my initial design interpreted by the fabricator and then put through final engineering. Time will tell if the design is good or not.

Of note: I calculated initial deflection of the system under dead load of 5/8" so we had the fabricator jack the girders 3/8" (attach the column to the girders then shim each end 3/8" then remove the column). As measured in the field (using the best methods I could without spending tons of money on equipment) I measured a deflection at one location of 1/4" and the other at 1/8" (2 columns were removed). So it appears as if we are on the right track.
 
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