Odd Vertical Bracing

[Nick6781]

The vertical bracing layout should follow the configuration shown in the snippet below. Since the compression force is continuous through the bracing members, gusset plates, and beam webs, I believe I need to verify whether the weak-axis stiffness of the beam is sufficient to brace the bracing member per Appendix 6. I'll also need to check beam shear.

Am I missing any other critical checks?

 

[HDStructural]

That is a unique bracing layout.

I agree that you need to consider the stiffness of the weak axis of the beams. Are there other beams running in and out of the page that frame into those beams, or a diaphragm? I'd recommend having stiff plates at each connection to strengthen the beam web, regardless of whether they are required or not.

 

[driftLimiter]

If resisting seismic loading in SDC D or above, I think this would be difficult using prescriptive design per AISC.
Do the beams in your diagram represent floor levels ?
The type and ductility of this frame needs to be addressed if SDC D or above. That is a pretty significant hurdle.

 

[Ron247]

I am not following the connectivity.

1. Is the brace centerline and beam centerline the same location or offset?
2. Are the beams also serving as floor beams?
3. Fixed or pinned connections?
4. Appendix 6 of what code and what year?

 

[Nick6781]

It's an open structure. The section shown is the central stair tower, with platforms connecting to the top level of the tower.

There are no intermediate diaphragms, as most of the load originates at the top level and can be transferred directly to the vertical framing through the diaphragm located there.

According to AISC 360 Appendix 6, the beam’s weak axis provides sufficient stiffness to brace the compression bracing. Therefore, using “l” as the effective compression length is justified, rather than using the full diagonal length of the rectangular bay. Does any code require the working point of a brace to be braced by a perpendicular element? If so, I can add some infill beams as shown in blue.

The structure is located in Seismic Design Category (SDC) C.

 

[HDStructural]

Why can't you use traditional x bracing?

 

[KootK]

Does any code require the working point of a brace to be braced by a perpendicular element? If so, I can add some infill beams as shown in blue.

No code requirement for low seismic that I know of. That said, the infill beams feel pretty good if they are, themselves, laterally restrained by diaphragms. Perhaps less work for you than mathing out the solution without the infill beams.

Two stability aspects need special attention here:

1) The lateral bracing of the diagonal bracing as you suggested.

2) Roll over bracing of the beam when it's getting hammered with axial from both sides by the bracing. This bracing probably gets done using some combination of;

a) The torsional capability of the beam and;

b) The OOP flexural capacity of the braces.

 

[KootK]

So what keeps you from doing a more conventional, concentrically braced design? By the time that you beef up the braces and beams to perform the bracing functions, I suspect that you'll have blown any $$$ saved by having shorter brace members several times over. If connection complexity is the concern, just make them slightly eccentric, as shown below. It won't make much of a difference to anything in SDC C.

The old adage usually holds true: there's a reason everybody always does it that way. But, then, maybe you have your own reasons that we are not yet privy to.

 

[Nick6781]

Thanks, everyone, for your input.

I'm not usually one for unconventional ideas—at least not when it comes to engineering—but the reason I'm proposing this is due to the stair access layout. It's something like this:




The alternative scheme (shown in blue) is another option, but it doesn't attract as much lateral shear.

 

[KootK]

I take it back, that makes nothing but sense now.

 

[Ron247]

The alternative scheme (shown in blue) is another option, but it doesn't attract as much lateral shear.

What do you mean by "attract as much lateral shear"? Is there another stiffer brace system elsewhere in the structure?

Are the stair stringers isolated from the beams in terms of taking load?