Temporary suspension of block wall 1

[ajk1]

Does the temporary suspension needling of an existing 8" block wall (which extends about 7') above the needling beams, as shown in the attached, need something to prevent the needling beams from lateral buckling of the compression flange? What is the usual practice in this regard?

The needling beams are to be installed at 3 foot centres. The load on each needling beam is about 1200 pounds. The reason for the needling is that the suspended slab on which the wall currently is supported, needs to be demolished and repoured, due to corrosion damage of rebar and concrete delamination top and bottom of the relatively thin slab.

There are numerous active electronic communications wires and cables attached to the face of the block wall, and that is the reason that the wall is not just demolished and rebuilt.

 

[bookowski]

Normally I've never braced a compression flange for needles - although you have a pretty unique detail.

Would it make sense to instead use a channel on each face parallel to the wall, less hangers but through bolted to slab instead of adhesive. Each pair of channels could replace ten needles - seems like it would be much less work.

 

[SkiisAndBikes]

Depending on the magnitude of the load, yes the beams may need some lateral restraint to the compression flange. However, the span is small and the Lu of a W8x14 is about 1930mm, which looks to be less than the distance support to support. I would typically select a beam that requires no lateral support in this scenario. I have not seen or specified any form of lateral support of the compression flange in any needling on projects I have been involved with.

Looking at the detail, is it not possible to through bolt the threaded rods to above? I am cautious/wary of adhesive anchors in this type of scenario.

 

[KootK]

I'd like to get a look at 31/SK18. The particular issue here in my mind is whether or not you have effective torsional restraint at the beam ends. If required, one could improve matters by:

1) Welding end plates to the needle beams.

2) Bolting plates to the end plates that extend a foot or so above the needle beam top flanges.

3) Attaching the hangers to the top of the plates in #2.

Another option would be to use square HSS needle beams with, obviously, no propensity to laterally torsionally buckle.

For what it's worth, my gut feel here is that you're just fine as is.



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.

 

[BAretired]

I agree with KootK that the torsional restraint at the ends of the needle beams is questionable. Without torsional restraint, the needle beams will tend to roll over. That is not a question of lateral torsional buckling of the needle beams. It is an instability if the load point and the suspension points are considered hinges.

Is the upper slab currently being supported by the existing wall? If so, how much deflection will occur when the support is removed and the slab is carrying its own dead load plus seven feet of masonry wall?

BA

 

[ajk1]

Than you all for your thoughtful comments.

First let me say that this is a partition, not a bearing wall, so the loads are quite small. My bad for not mentioning this in the original post. 8" hollow block partition wall at 45 psf x 7.5 feet high x 3 foot needle beam spacing = 1012 pds per beam, plus some allowance for all the wiring etc, say 1500 pounds max. Two hangers each end of each beam gives about 400 pounds per hanger. Why the shoring engineer's drawing shows 1" diameter hanger rods is a mystery to me, unless I am missing something. I believe that there is a deflection space at the top of he wall, but I will double check. If there is no deflection space, then perhaps some creep will have occurred and some added load may be on the wall...but how much added load would be hard to determine, and not sure if should design for that...

I too would prefer to use mechanical anchors for hangers. Adhesive anchors are too prone to workmanship issues, not cleaning the hole properly is a chronic issue. Although they are needed in some cases where edge distances etc are an issue, this is not one of those cases.

The span of the beam is not correctly shown on the shoring engineer's drawing, because the hangers must be located much farther out from the wall to clear the various things projecting out from the wall. The span will probably have to be more like 12 feet ±.

If I understand the proposal correctly about bolting channels to the wall, that is something I too had been initially considering, but it is not practical because of all the electrical items mounted on the wall (and running vertically top to bottom) that cannot be readily moved. It is the communications etc. for the entire superstructure. Also, the idea of thru bolting to the slab above would penetrate into the offices above (something that would not be well received; no pun intended).

To Kootk and BAretired, among my favourite people on the site, thanks for clarifying that it is an end torsional restraint issue. That was my gut feeling as well, although I did not express it in my post.

Suppose instead of welding the plate to the bottom flange of the beam (as shown on the shop drawing), it were welded to the top flange, would that solve the end torsional issue? That would involve no more materials, and no more work and no change in the beam size. The top flange would have to be checked bending due to the moment caused by the force times the distance from the weld to the flange to web fillet, but the loads are quite small.

 

[KootK]

The top flange connection would certainly improve matters. Whether that's enough is difficult to say. With both ends and the load application point considered torsional hinges at the same elevation, you'd still be mathematically unstable (or neutrally stable). That's why I pitched shifting the hanger attachment point decidedly above the top flange. Economics aside, another attractive option would be to run lengths of channel perpendicular to the needle beams and connecting their tips.

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.

 

[ajk1]

ok, thanks. I'll give it some more thought

 

[bookowski]

Your loads are so small that I would bet this works itself out. The rods themselves provide some torsional restraint - really small but so are your loads. The needle pocket itself provides something.

You could run a small channel, say a c6 or c8, flat along the tops of the w8's with a little weld to each - then maybe add a kicker or two up to the slab. At 3ft on center you wouldn't need much to provide torsional restraint by tying one to the next - maybe could even come up with something in cold formed.