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Beam Uplift at Cantilever
3

Beam Uplift at Cantilever

Beam Uplift at Cantilever

(OP)
I'm strengthening a 28' wood beam, single span, by adding a column at 6 feet from one end.
This creates an uplift condition at the existing end support, a brick exterior wall.

The usage is floor loading, assembly, 100 psf.
DL is about 15 psf (I'm using 20).
Beam is 12x16, 8 foot trib.

We want to avoid moving the new post to the center, but possibly could move it a little farther away from the wall. Flitch plates, added LVL's, etc., don't seem easier or simpler than adding a new post and footing.

Several ideas I've considered.
  • Ignore it. It will never feel the full live load, or only for short duration.
  • Modify the brick beam pocket, at existing support, to allow upward movement (about 1"). This might allow unwanted movement effect at the short end of the beam.
  • Modify the brick beam pocket to restrain upward movement so that unwanted upward deflection is not felt by occupants.
  • Cut the beam to hinge at the new post, or the zero moment point.
  • Use a spring support "tuned" to the loading/deflection conditions.
Thanks for you time and interest.

RE: Beam Uplift at Cantilever

Quote (FloydLloyd)

Cut the beam to hinge at the new post...

I like that solution, if done right (to maintain beam's stability), has several advantages:

1) Uplift is eliminated.

2) Load is better proportioned among the three supports. For the proposed 2-span-continuous beam, the new post carries 85% + of the total load. After the cut, the new post has 50% of the total load.

3) Existing supports are not modified... minimizing the chance of unintended consequences of doing so.

www.SlideRuleEra.net idea

RE: Beam Uplift at Cantilever

(OP)
I appreciate your input.

And even though it was my suggestion, the thought of cutting up these beautiful 120 y.o., 12 x 16 x 28' wood beams just doesn't seem right (there are about 8 of them).

I also advised the owner that, with the approval of the building official, an exception to the load rating could be made for this historic (or any existing) structure. We could post a sign indicating maximum allowable live load. But there is some, perceptible vibration, in the floor so the 'do nothing' alternative is out.

I think I like the 'short end restraint' option (3). The dead load of structure over that end of the beam is one-story 13" brick wall up to roof framing -
brick wall (13"/4* 40 psf/ft)*(4'x10')
flat roof, wood framed (20 psf)*(6'x14')
5200+1680 pounds which exceeds the uplift reaction of -4200 lbs.

Thanks for your input. Its good to get some feedback.

RE: Beam Uplift at Cantilever

Do you think kickers would be helpful to reduce the length of the longer span, and help to bring down the uplift end back to bearing, if situation permits.

RE: Beam Uplift at Cantilever

(OP)

Quote (retired13)

kickers would be helpful to reduce the length of the longer span
That's a great idea, and we're doing that in a different wing of the building where the framing is slightly different.
I'm detailing the connections for those braces, and new posts bolted to the exterior brick wall to take the horizontal force from the brace.

RE: Beam Uplift at Cantilever

For me, the uplift you're talking about is too much for an old masonry wall. (I'd calculate the area of wall that resists the uplift to be a triangular shape) The repeated loading will be an ongoing effort to loosen the mortar joints over the beam pocket. I don't know the use of the building, but I imagine that any time people dance on the floor bits of mortar dust will sprinkle down from the wall.

With the geometry you described, I'd lean toward allowing the upward deflection within the pocket. This creates some problems, too: you probably want to rely on the beams to provide restraint for the wall, and that's tough to do if the beam is not connected even by friction. You also want to be sure that the deflection is always reversible, such that no bits of brick or dirt can fall under the beam when it deflects upward.

If you can, I'd recommend that you move the post toward the center of the span a couple feet. It seems that a 20' span makes your problems much more manageable. You get an uplift in the hundreds or pounds, not thousands, (with a FOS closer to 10 than it is to 1) or you get a minimal upward deflection. You also get more capacity and stiffness out of the existing beam, so it will feel noticeably better and nobody's talking about occupancy limits.

I can see how you'd not want to cut the existing beam. Not only is this type of esoteric solution harder to justify to an owner, but you also might want to leave the historic fabric of the structure intact for future flexibility.

RE: Beam Uplift at Cantilever

2
Do you have the headroom to do something like the photo below? Kinda exotic but the span is similar to yours. They used cable/wire rope and tensioned them w/ turnbuckles.

RE: Beam Uplift at Cantilever

Can you put the posts in just snug against the bottom of the beams (leave the beams in their deflected shape under dead load, so that they're only carrying live load? This would minimize the live load deflection/vibration, but also minimize the uplift at the end.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Beam Uplift at Cantilever

(OP)

Quote (kipfoot)

repeated loading will be an ongoing effort to loosen the mortar joints over the beam pocket, ...beams to provide restraint for the wall,...20' span makes your problems much more manageable, ...leave the historic fabric of the structure intact
Those are all excellent points. I was ignoring the wall.

Quote (azcats)

cable/wire rope and tensioned them w/ turnbuckles
Another great idea, and yes, we do have headroom. I was looking at a tension tie solution, bottom chord, ea. beam, with queen posts. The ties get connected through steel plates bolted to the sides of the beam, near the ends. The connection is challenging because the tension forces are very high, but maybe I can increase the depth, of the 24" queen posts.
There's a great article on this solution in "Wood Engineering and Construction Handbook", Faherty and Willliamson.

RE: Beam Uplift at Cantilever

(OP)

Quote (BridgeSmith)

put the posts in just snug against the bottom of the beams (leave the beams in their deflected shape under dead load)
That suggestion helps greatly in how to analyze the "tune the support" solution.
I'm not confident I can detail this correctly to achieve our desired effect, reduced deflection response, adequate LL capacity.
I have not separated the (existing) dead load deflection and live load response in my analysis. Thank you for pointing that out because that is something that always comes up in beam strengthening.

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