Ordinary Concentrically Braced Frames Modification

[Charred]

I am working on a project where I am using HSS tube columns and a concentrically braced frame for shear resistance. The steel horizontal braces were originally going to be steel tubes that ONLY resisted lateral loads because the architect wanted an exposed wood truss and glu-lam support beam floor system. The glu-lam beams would pass along the face of the columns where it would be supported with a seated connection and the steel horizontal braces would tie into the face of the column as usual with a diagonal tube brace and gusset connection. The reason for this is that the building will have a lot of openings and for a modern/industrial look, the architect wanted to see the steel braced frames from the outside windows looking in. After digging into the design, I started wondering if the glu-lam beam could act as the horizontal brace and resist the gravity and lateral loads. It would greatly help the economy of the project if I did not have the redundancy of the steel beam if the glu-lam could be used. I looked through the code and it does not necessarily state this is acceptable one way or another - or at least from what I have read. It gives ductility requirements and that may be my answer, but just from a thought process - it seems that these large glu-lam beams would be able to resist the axial forces from the lateral loads. Have any of you ever had this situation or come across something similar? Any input would be helpful.

 

[KootK]

If I understand your situation correctly, you'll be delivering shear to the face of your columns via the glulam beams and resisting that shear near the center of your columns via the vertical bracing. I expect that, baring some very special detailing, this condition would result in some terrible cross grain bending in your columns at the joint. And that, of course, is often a deal breaker in wood.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

steel columns Koot, only wood beams. I could see this working ut the connection of the beams to the columns will likely be the weak point.

 

[KootK]

Ah... I see. What would have been cross grain bending will now be some significant torsion that may have to be dealt with in the columns and at the base plate. Or via weak axis bending in the glulam beams I suppose.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Could there be a floor infill beam behind the column that would run right out to the column? Maybe hang the spandrel beams from that? I could see some improved detailing that way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

- how many stories?

- tension only bracing?

- any weird detailing required for the thermal break?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Charred]

There will not be any thermal breaks, but there will be some tricky detailing to connect the wood beam to the gusset plate for the applied force. The braces are being designed for tension and compression. The building is a three story building.

 

[KootK]

The braces are being designed for tension and compression. The building is a three story building.

This may present an interesting opportunity. With T&C braces, you don't actually need your horizontal spandrel beams to be part of the vertical truss that forms your bracing system. As such, each of those tricky connections would only need to transfer the shear delivered at that diaphragm level. That would reduce your peak connection force by about a factor of three. It might also be a better reflection of the expected force distribution since the connection eccentricity between the braced frame and spandrel beams will introduce additional flexibility.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.