Single steel beam in cavity to support both leafs of masonry design

[gmannix1000]

Hi All,

I got asked to look at a steel beam to the rear of domestic house with sliding doors below built in 2010 with no issues which supports two stories of floor load above on the inner leaf. It spans approximately 4.8m. It is in the cavity zone with a steel plate welded along the bottom flange to support the outer leaf and inner leaf plus concrete slab. There are vertical stiffeners welded on one side of the web at 800C/C. Outer leaf load is approx 5kn/m factored and inner leaf/concrete slab load is 45kn/m factored. My design preference for this would have been two steel beams, one under each leaf but this is already present onsite. Beam is a 305x171UB51.

In looking at this, I'd ideally like to prove this works in lateral torsion and buckling but it is failing significantly. Could anyone offer any thoughts or advice on this? Any help would be gratefully appreciated.

 

[human909]

The wall acts as a a continuous lateral and torsional restraint. Sure it isn't an ideal restraint but there. I can see your concern though but as you have already observed it is working.

Also was that a mistype? Did you mean 356x171UB51, your beam doesn't seem a normal product in justifications I'm aware of.

Standard practice here is to use welded T-sections to support double leaf's. This is so common that these are supplied off the shelf, galvanise and with span tables. (There is a clause in the flyer on the link about "equal loading" but that is never going to be the case.

All that said, give the gaps between the walls, if I was designing that I'd likely use two beam. Most likely channels.

 

[JAE]

The torsional stiffness of the beam is very low.
I think the vertical stiffeners are there to keep the inner flange from bending down due to the heavier load.

So what's resisting the twist?
Per human909 it must be the wall above and the slab to the side that have inherent stiffness that keeps the whole assembly from twisting around.

It's just not a good detail though in my view.
We typically use steel tubes and find a way to take out the torsion at the beam ends - usually with columns torsionally attached to the beam ends.

 

[Tomfh]

The twist will also be restrained by the flanges bending about their strong axes, aka warping torsion, which forms a couple to resist the eccentricity of the applied load.

For a 4.8m long beam with wide flanges I’d expect this to play a role.

 

[hokie66]

It might help if you would tell us why you were asked to look at it. Homeowners don't usually ask for advice about structural elements with "no issues". Is your client a prospective buyer?