Lateral restraint for top of needle beam props

[demayeng]

Hi,

I am having trouble visualizing this..

I got this image off a prop-hire business website

Is that ok as pictured, or do you need to laterally restrain the top of the props?

Thanks

 

[ishvaaag]

It seems to correspond to a case of making an opening by just cutting a wall by sawing it whilst the props are already set in place.

In this kind of work (in Spain more commonly seen to make opening in masonry bearing walls) those making the works use to be almost systematically quite optimistic on the overall behaviour of the construction system.

In particular, they usually assume that whilst retiring the stiffness of the demolished part, the building will be staying working non-translationally, the translational failures still being prevented by what remaining of the building. Even if experience proves it so be the case, not checking the thing is taking a card for disaster.

In your case, even with the structure remaining non-traslational, you still have the issue of the apparition of relative displacement between feet and head of the props (something they will argue is prevented by the friction on compression at the feet and restraint at head) and the standing capacity in LTB of the beam on the props. This they will say control through selection of some capable enough member. Disgusting that the sight may be, it may work if not pushing the use of the capacities very high.

In short, they have adopted a leaning column approach. A check for the kind of structure used should start with a model with enough consideration for initial imperfections.

It must be noted that for the new openings on bearing masonry walls, a far greater number of pairs of props are used typically along the span as well to later install the then required new bearing member that substitutes the bearing capacity of the removed part of the wall. So here, again, they have been optimistic in expecting some arcing of the loads above occurring to safeguard the span opening. This arcing may be occurring not just in the hanging wall-now beam- standing, but from far well above for the stories above, and just for the inmmediate floor above just on the "beam" formed.

 

[demayeng]

Yeah, in my case I am happy that the structure itself won't translate, it is more the ends of the needle beams that could potentially kick out in opposite directions

I think you are right, that in most cases it probably works if not loading the system too high, however from an engineering standpoint I can't justify not putting in a lateral restraint at the top.

 

[csd72]

I believe the theory is that if there is load there is friction and therefore lateral restraint provided by the wall bearing against the top flange. Also the stiffeners at the end of the beam will help, though not sure how much.