SCBF Configurations for Seismic Design

[chrisrosebud2001]

I am working on a non-building structure that I classified as being similar to a building. The structure falls under seismic design category D. I want to use an(SCBF) lateral force resisting system, but there has to be a large opening at the base. Because of this, the bottom bay cannot have bracing, or the bracing cannot go to the work point at the base of the column making the bottom bay essentially an (EBF). Can an SCBF still be used in either of these situations? Most examples I have seen indicate that this would not be allowed. Or, is there perhaps an (SCBF) configuration that would allow a large opening in the bay? The last option would be to go to a moment frame system, but I would prefer not to if it is not absolutely necessary. On another note, I have also looked at an (EBF) system. However, this was not an option because there is no way I can brace the ends of the link.

 

[BAretired]

Provide a sketch.

BA

 

[chrisrosebud2001]

Here is the requested sketch. The sketch on the left side would represent the situation originally presented. There is also a large opening at the top of the structure in the other orthogonal direction. That case is shown on the right side. Basically the purpose of this structure is to provide lateral support to a flue composed of refractory brick. The flue is not vertically supported by the steel.

 

[PUEngineer]

I'm not sure we understand the complete scenario without additional sketches or explanations. What does the plan view look like? Are there four sides with a bay of bracing on all four sides.

Obviously a concentrically braced frame must be concentric, so that would mean that no you couldn't offset the work point.

I think you're options would revolve around using struts or chevrons...however, this gets funky with AISC because you have to have a certain distribution of tension vs compression members in each of the orthogonal directions.

 

[chrisrosebud2001]

Yes, the structure is 4 sides. The North and South side elevations are the same and the East and West elevations are the same. Attached is an additional sketch. I'm not sure if chevron bracing would be an option either. A connection to the centerline of the base of the column could not be made due to the opening at the base.

 

[BAretired]

As I see it, the unbraced panels must be resisted by columns in flexure.

BA

 

[chrisrosebud2001]

Yes BA that would be correct, but I'm not sure if that meets the requirements of the Seismic Design Manual. I have designed structures like what you are suggesting, but only per R=3 design.

 

[PUEngineer]

I would say no, it does not meet AISC 341 for R>3...at least as a concentrically braced frame. How did you come to the conclusion that you needed to follow AISC 341 with R>3? What model code are you using?

 

[chrisrosebud2001]

PUEngineer, the seismic design category is D and my understanding is that R=3 construction would not be allowed. I'm basing this on the fact that under ASCE 7-05, ch. 12, Steel Systems not Specifically Detailed for Seismic Resistance is Not Permitted.