Steel picture frame in masonry wall - deflection criteria?

[neil254]

The use of rectangular steel picture frames is common when forming openings in existing masonry walls. I would be interested to hear what deflection criteria people adopt when designing these given there will be deflection of both the top and bottom beams within the frame?

 

[BAretired]

How common is it? I don't believe I have ever seen picture frames used for such a purpose.

BA

 

[dik]

...depends. I usually use L/720, but have used more and less.

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-Dik

 

[r13]

"framing the opening"? Wasn't lintel a frequently seen solution for opening in the existing masonry wall? What is going into/through the opening that require framing?

 

[MIStructE_IRE]

Have done it plenty of times in domestic where the back wall (shear wall) is to be removed at ground level.

Typically i take vertical deflection dead + live < L/360. Lateral sway i take height/300.

Rightly or wrongly I ignore deflection in the foundation beam. I don’t believe it mobilises like an upside down version of a normal beam..

Google images Photo for reference just to make sure we’re all taking about the same thing!

 

[JoshPlumSE]

Some quick thoughts on this:
1) Yes, this is very common. I've seen it more for window openings than for the large opening that MIStructureE_IRE has seen. But, my experience is relatively limited.

2) Remember that there is "arching action" of the masonry such that the beam won't see the full weight of the masonry above it. It will actually see very little. Even the live loads and such applied above will be arched over the opening to a good extent.

3) The bottom beam won't really see any load.

4) I used a more severe deflection limit if I remember correctly. It wasn't as severe as the L/720 for dead load that DIK uses (although we might have used that for the dead load only). I think it was L/600 for DL + LL. I would not go for less than L/480. Masonry will show cracks more easily than other materials. So, a little extra steel is more than worth it to avoid the cracking.

 

[Celt83]

... 2) Remember that there is "arching action" of the masonry such that the beam won't see the full weight of the masonry above it. It will actually see very little. Even the live loads and such applied above will be arched over the opening to a good extent. ...

Want to add that the arching action will only occur if the thrust created by that action can be resisted.

For deflection I believe TMS 402 notes lower of L/600 or 0.3 inches

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[neil254]

Thank you all for your responses.

MIStructureE_IRE your picture is exactly what I am looking at as it is very common (in Scotland anyway) for residential projects where clients want large glazed sliding doors or to remove walls in traditional apartments. What has been on my mind is the load distribution through the bottom beam into whatever is below (either a few storeys of masonry if higher up a building or a substructure and RC footing if at gnd level). The load distribution through the beam will vary depending on the stiffness of the support as it is a beam on an elastic foundation and the beam must deflect for load to be transferred along it ie unless it is very stiff then more load must go down close to the columns.

I suspect it is something very difficult to accurately assess due to the many factors that contribute to the behaviour and a general rule of thumb such as L/600 can be used for design on the basis that it has been done for a long time and works! For what its worth my usual approach is height/300 for lateral sway and L/600 for vertical deflections (though would try and limit to 8mm or so avoid cracking in masonry and/or doors sticking.