Possible combination of SMF and OMF?

[X4vier]

Hello,
Is it possible to combine SMF and OMF in the following way,
The two stories to the right are concrete floors.

Thank you for your comments.

 

[haynewp]

That's a steel OMF and all concrete SMFs? Or all steel frames and only concrete floors at the SMFs?

I think the relevant ASCE 7-16 sections are 12.2.3, 12.2.3.3 and 12.2.4. It doesn't look like you can approach it as intended and it's of no advantage if using all steel frames.

 

[X4vier]

All steel frames, only concrete floors.

 

[Luceid]

Can you give some explanation on why you wouldn't use an SMF on the left as well?

ASCE chapter 12 says that different lateral systems in the same direction have to use the lowest R-value between the systems, so you would have to design the SMF for the much lower R-value of the OMF which would probably be tricky considering you already have much more restrictive design requirements on the SMF. Having to balance those with ~2x the force you would normally design the SMF for seems like a tough design path, but I'm curious if there is another reason to use the OMF...maybe it's hard to do SMF's with sloped beams? (Not an expert here, just throwing ideas out)

 

[driftLimiter]

Its possible to do sloped SMF beams but it isn't fun. I agree the connections of the SMF are going to get out of control if the beams/columns are proportioned based on the OMF R factor.

 

[HTURKAK]

Yes .. it is possible as long as you use the most stringent applicable limitations in Table 12.2-1.

The following clause 12.2.3.3 explains the limitation

(12.2.3.3 R, Cd , and Ω0 Values for Horizontal Combinations.The value of the response modification coefficient, R, used for
design in the direction under consideration shall not be greater than the least value of R for any of the systems used in that
direction. The deflection amplification factor, Cd, and the overstrength factor, Ω0, shall be consistent with R required in
that direction..)








I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[haynewp]

If you have to use the most stringent then what’s the point?

 

[X4vier]

The left side is just a standing seam roof, so, guaranteeing the lateral bracing for the SMF connection will be difficult.
I'm getting similar sizes for beams/columns with parameters for the SMF (R=8 and cd=5.5) than with the parameters for OMF (R=3.5 cd=3). Frames are controlled by deflection limits.
No problems with the shared columns? It must comply with all the SMF and OMF requirements, and that's all?

Thank you.

 

[MotorCity]

Most of the lateral load will be resisted by the OMF since it is expected to remain elastic and not be anywhere near as ductile as the SMF.
The SMF beam to column connections will involve extensive design, detailing, and fabrication in order to achieve their required rotation.
Because the SMF and OMF share a column, it may not even be possible to obtain the required drifts and rotations because the two systems will be fighting each other.
As such, the cost will increase but you won't be able to take advantage of any benefits a SMF offers.
My first solution would be to use the less costly OMF and design the adjacent portion as a "lean to" or "leaning column".
My second solution would be to introduce another column line between the two structures and use an expansion joint (essentially two independent structures).

 

[haynewp]

I would have used all OMF.

 

[Luceid]

Yeah, now that I think of it, if you are choosing to use an OMF in the left bay, is there any reason at all to use the SMF in the right bay? You don't get to take advantage of the higher R, so why add complexity over there?

 

[driftLimiter]

It would be very penalizing on the SMF connections. Your beams are sized for R = 3 and your connections are based on max expected strength of the beams. It wont be pretty.