Wood Beam for Special Concentric Braced Frame

[dtking]

I'm designing a steel braced frame in a building with steel columns and wood beams. The wood beams are currently acting as collectors in most places. I have conditions at my braces that would make it advantageous to use a wood beam in the frame, but I can't find anything in AISC 341-16 precluding or allowing this. Since the braces are where the SFRS is getting it's capacity from, would it be reasonable to design a wood beam using this code and still call it a steel SCBF?

Specifically the code requires the beam and columns to be highly ductile. Direction is given in chapter 4 for steel, concrete filled, and concrete sections. No direction is given for wood (since it's pretty atypical).

Any advice would be great!

Thanks,
D

 

[phamENG]

I hope you have a large budget. What you're proposing is probably so atypical that there's no accepted procedure to do it for a special braced frame.

I would say no, you can't use 341 to design a steelwood beam. You'll have to design it, and then have the connections tested to make sure they'll behave as expected, and then specify them for your building.

 

[r13]

I think you can, if the connections can be detailed to meet code requirements. By the way, I understand AISC is the abbreviation of the "American Institute of Steel Construction", its manual has chapter for composite design, unfortunately, wood is not included.

 

[NJ92]

I agree with phamENG. A system like what you are proposing would need to be tested in some manner to show it behaves as expected since it doesn't fit the mold of the established systems. With the beams being made of wood, I don't think the SCBF R=6 would be applicable so your testing would likely need to establish a new R-value for this system.

As far as what terminology would specifically prohibit this condition, it may come down to how you treat AISC 341. Since AISC 341 is based largely on the results of testing, I interpret it as an inclusive set of rules that need to be met in order to use it for design (ie: meet all of the applicable requirements to ensure that the established test data is representative of your condition or use some other reference for design). For comparison, I see AISC 360 as more of a guide that provides the rules for some but not all possible conditions and gives you the option to apply reasonable engineering judgement/design to address conditions that aren't explicitly covered. With this in mind, since AISC 341 - section D1.1a for highly ductile members doesn't explicitly list wood as an option to be considered highly ductile, I interpret that as wood being prohibited.

 

[Deker]

I could be persuaded to use a wood beam IF it was detailed similar to what's shown below AND you could prove that the connection could accommodate 2.5% drift as required by AISC 341 F2.6b(a). A few thougts:

1. If the intent of the moderately and highly ductile compactness requirements is to mitigate local buckling for members that will experience incidental (as a result of imposed story drifts) or intentional (fuse elements) inelasticity, then it follows that those requirements wouldn't apply to a beam that has been decoupled from the frame such that it will not experience moment as the buildings drifts. If the wood beam connection could accommodate 2.5% drift, you could argue that the beam will never see any seismic moment and compactness requirements wouldn't apply. It would essentially be acting as a collector.

2. Given the potential lack of ductility in the wood beam connection, at a minimum you'll want to design for an axial force consistent with the yield mechanism of your braces. More on that here: Link. You may find that difficult to achieve with wood connectors given the inherent overstrength in a SCBF system that's sized based on the compression capacity of the braces. If that's the case, consider switching to BRBF.

3. You may find that it's easier to clad a steel beam with wood to achieve your desired aesthetic.