Beam too Near Window?

[NAFTALI-HAKOHEN]

hi

i have a design situation with a concrete wall , with a window opening, and a concrete beam that ends quite close to the window, about 10cm horizontally,

see diagram:

my concern is that the load from the beam going into the wall will cause cracks around the window area - as the load spread of 45 degrees hits the window area below it...

are my concerns founded? the beam end support reaction into the wall is about 50 tons,

thanks

naftali

 

[jayrod12]

50 tons is substantial. I think you do have some concerns there, only because the amount of wall available to support that load is truncated because of the window opening. Does the wall have enough capacity accounting for the truncated width of wall?

I don't personally feel that the vertical load from the beam will affect the spandrel over the window opening, so as long as the spandrel has enough capacity to span accordingly over the opening it should be fine in that regard. But you've got a lot of load coming down on that window pier. I'm more just worried about global capacity of that chunk of wall.

 

[dik]

With a load that high, it will likely cause a diagonal shear/tension crack from the top right corner of the window to the bottom left part of the beam. If OK structurally, a bunch of rebar will help. This is a good example where FEM modelling may be of use. First 'kick at the cat' would be to move the window...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JoelTXCive]

How thick is the concrete wall that supports the beam?

I think you need some FEM modelling too, but maybe you could do an integral column in the wall? (if the wall is thick enough)

 

[NAFTALI-HAKOHEN]

Hi.

The Wall is 30cm thick concrete

I was thinking about a 'column strip' in the wall under the beam as Joel suggested, will this certainly solve the crack issue?

Does anyone know how I can calculate the stress concentrations in such a situation without FEM?

Dik, the FEM calculation doesn't consider rebar concentrations in the wall so how accurate will the results be?

 

[jayrod12]

Personally I really don't see the issue with the window as long as the wall part is designed appropriately. Stirrups (links, ties, whatever your local nomenclature is) over the window, have your bending steel for the window spandrel extend beyond the edge of opening. Since the bottom of beam is so close to the top of opening vertically, I don't believe you will end up with significant stresses at the window corner from the point load.

But perhaps I'm being too flippant.

 

[KootK]

1) I think that one of the significant risk factors in this is how the load distributes out to the supporting structure / foundation. And that may well be largely out of your control. If you're bearing on rock, you may see no cracking at all. If you're bearing on a more flexible material, then you may wind up with a situation like I've sketched below where the anticipation of vierendeel panel action across the window might be a good guide to anticipated cracking and potential reinforcement strategies. Seek out the locations where you've got both flexural tension and significant shear.

2) I'd consider either closed ties below the beam or u-bars as shown below to help out with any local bursting stresses.

3) The vierendeel beam that you've got below the window is pretty deep and may serve as a good ally in preventing cracks from developing. It may be deep enough to not crack itself and its stiffness may shield the upper beam from cracking. For sport you might investigate that beam as plain concrete to get a sense for the potential for cracking.

4) Short of introducing a movement joint at the right side of the window, it's pretty tough to guarantee uncracked performance in a situation like this.

 

[Brad805]

What is the support conditions?

 

[PMR06]

How about we start at 50 tons (100 kip) shear in a 30cm x 60cm beam (~12" x 24")?! Is that correct? Is the beam poured monolithically with the wall?

 

[jayrod12]

Even if it were poured monolithically, you make a great point. At a quick and dirty calculation you'd need 10m stirrups @ 100 on-centre just to make that work away from the wall. That's a substantial amount of shear in a relatively small beam. But I guess technically it calculates out. Even for bearing capacity it appears to work without much additional effort.