Reinforcing existing basement beams for longer spans

[faromic80]

I have a situation where the owner of a home wants to remove the basement posts supporting the beams. The beams run along the center of the basement. the house is approx 3000 sq ft. there are 2 beams which are 8" deep (didn't get the exact size yet) that span from the top of the foundation to the center post (there are 3 posts total). The beams meet at the center post and bear on a cap plate (typical basement construction). The owner wants to remove the two posts the beams are cantilevering over. I'm not sure if I'd want to sign off on something like this as I have never encountered this situation. He doesn't want do decrease head height significantly so a WT welded to the bottom is out of the question. What about welding a 1/2" cover plate to the bottom of the beam? I don't think you could weld it while the beam is being stresses in it's current state. would you have to try to shore up the joists at least a bit to relieve some of the stresses before welding? Not to mention you would have to remove the two posts before welding the cover plate to the underside and end shear reactions.

 

[DWHA]

It will be real tough to do with out having excessive deflection.

 

[civilperson]

Not a good idea. The original designer needed the posts to keep the depth of beams at a minimum and so do you. Adding steel to the bottom of the W shape will give two different maximums, compression and tension. The lessor controls. An idea is to lower the floor to keep the same headroom with deeper beams.

 

[jike]

A cover plate on the bottom only will not add much strength. You would have to have reinforcing on the top and bottom to achieve additional strength.

Another idea is to place additional 8" beams either side of the existing beam with additional columns and footings. If you have trusses bearing on the beams, be sure you modify them to accomodate a new bearing.

A larger issue might be the adequacy of the existing footing.

Good luck!

 

[diarmud]

Residential construction is covered by the state building code and possibly local amendments.

You will have to do an analysis of the entire building. The code requires you bring all the forces (LL + DL) through the building (from the roof), its' walls, columns, beams, etc to the foundation, which has to be adequate.

You have to analyze the beams, the columns and the footers. I'll bet you will need a new footer in the center. I doubt if you can remove the posts and reinforce the beams if you have little headroom. You might be able to remove the beams and use flitch plates. Remember, the code dictates a minimum headroom in the basement.

I doubt if the original design was overdesigned.

Regards,
Dermott

 

[msquared48]

You could always tell the client that he needs to seal the top story and fill it with helium. Then the two posts could be removed. However, they would have to live in the lower story. He otta love that as he quickly walks away from you...

Mike McCann
MMC Engineering

 

[ChipB]

Well, I kind of like the helium idea. I've always thought we should call out for helium entrained concrete in numerous occasions.

That being said, I've come across this bunches of times in the past. Realtors love to tell prospective buyers that it's "No big deal."

It can be done, but it gets expensive.
Forget about reinforcing the beam. It'll never work.
My preferred option, take the span from the center post to the outside wall, and divide it by 3 (It's divided by 2 now). Assuming the floor joists are on top of the existing beam, size a beam to span from outside wall to outside wall, perpendicular to the existing beam, at these third points. You want the deepest beam you can get to work out for your client. I typically get into the void of the floor joists. They'll have to shore the existing beam, & cut it for it to connect to the side of the new beam.

There's a whole bunch of other stuff to consider (footings, flange bracing, etc), but that's the jest of it.

Hope this helps.

 

[Einsteim]

One option would be to shore the floor joist and remove the beam line and posts. Cut the floor joist in the beam location to "let-in" and new deeper wide flange section. The floor joist will then require top-flange type joist hangers to support the existing joist. "Letting-in" the beam will allow for a deeper section that will span farther. The new section may or may not span the entire length of the basement, but may allow removal of (1) or (2) columns.