Steel Cranked Beam End Supports 1

[AFP0592]

Hi All, I would appreciate your thoughts and feedback on a cranked beam support over a masonry wall.

I'm designing a cracked beam to support a new suspended floor inside an existing roof. To avoid damaging the ceiling below and to maintain the lateral stability of the roof, we will need to keep the ceiling joists. Due to its height in relation to the existing masonry wall, the cranked beam seems the only solution in this case. See photos with the scheme and the assumed support diagram.

Now, I have a transverse beam that would provide lateral and vertical supports for the new cranked beam. The beam is embedded in the external walls.

By designing the cranked beam as a roller support, I have a small horizontal deflection of more or less 1mm in the cranked beam. I may be overthinking this, but just wondering about the cranked beam triggering friction against the wall and the overall stability and the reality of a roller in that case.

I can not have a pin there due to the lateral stability of the wall and the high thrust/axial load that is going to trigger on the cranked beam.

Thank you for your thoughts and views on this.

 

[GeorgeTheCivilEngineer]

I don't see much of an issue with that.

 

[milkshakelake]

I'm not sure the bottom reaction point would be a roller. Aren't your ceiling joists restraining the thrust? I would personally model it as a pin. The horizontal reaction (i.e. "shear" in the vertical part of the cranked beam) would add to the thrust. Therefore, I would reinforce the connection of the ceiling joist to the CMU wall.



I think the roof diaphragm itself can take some thrust, dumping it into the parallel shear walls, and it gets wishy-washy in wood design fashion. But I'd reinforce those ceiling joist connections just in case.

If it's a pain to reinforce those connections and you need to keep the ceiling in place, the thrust might just wishy-wash away (because you have a steel beam at the end of the cranked beam, and you have the diaphragm as well). So maybe it's not 100% necessary. I'm just giving something to think about.

 

[IceNine]

I've always done what you're proposing. Make the cranked beam stiff enough that the horizontal deflection with a roller support is minimal. Then you don't have to worry about the thrust, let the wall push out 1mm.

 

[XR250]

I would not worry about any of this.

 

[Eng16080]

I would also not worry about a 1 mm displacement from your analytical model. With both the ceiling and roof diaphragms, I doubt this movement will occur. If it does, I still wouldn't be too concerned.

Wood construction moves through shrinkage and seasonally by more than the 1 mm you're concerned about.

 

[driftLimiter]

I worry about this until I see the lateral support deflection. Get it small enough and I stop worrying about it.

 

[Greenalleycat]

Can't you just drop the beam down to the plane of the ceiling joists anyway?

 

[HTURKAK]

Have a look to the first picture , the top of 215 mm masonry wall , Is that RC capping beam ? If so , is this beam in a closed form all around and on the top of interior walls ?
If so , IMO this set up is OK. Can you post the plan ?
Check the masonry stresses and ensure that less than allowable. If necessary , you may consider adding pillars ( fin wall set up) under the cranked beam supports.