SCBF Single Story Configuration 1

[Galambos]

Does anyone have an opinion on the most efficient SCBF single story configuration?

I was thinking of either a V sshaped brace (not an inverted V) or an inverted brace with a zipper column.

Thoughts?

 

[DCBII]

X-Bracing is more efficient if it doesn't get in the way. The point of intersection of the two braces is restained by the brace in tension reducing the unbraced length of the braces, and hence the required brace capacity. Since the design of the frame is based on the brace capacity this considerably helps the frame.

If you do use X-Bracing, pay special attention to the connection at the intersection. It's best to use a connection that doesn't rely entirely on catenary action if you can (i.e. avoid a single knife plate through the brace - square HSS members make use of dual cover plates easier).

 

[rscassar]

If there's space...brace

 

[sandman21]

An X requires 5 connections v. inverted chevron having 3 connections. Inverted chevron wins.

 

[Galambos]

Sandman, I would say that an inverted chevron wins in an R=3 structure.

From the little I know, in an SCBF single story inverted chevron, the beam must take the full yield strength of the tension brace, considering the compression brace has buckled, which is no small feat. The X brace and zipper column configuration allows you to avoid this penalty.

If Im wrong, anyone, please let me know.

 

[amecENG]

Efficiency really depends on the height to width ratio of your braced bay.

As a rule of thumb:

For H/W = 0.5-1.5 tension only braced frames

For H/W = 0.3-0.4 chevron braced frames

In terms of fabrication efficiency as sandman21 alludes to I'm not sure if chevron bracing does win over X bracing. With chevron bracing you may need to detail stiffeners and add bottom flange bracing on the supporting beam.

 

[Galambos]

Again, I am dealing with an SCBF. Tension only bracing is not permitted.

 

[DCBII]

Keep in mind these braces will yield/buckle. Think of what that does to the connected elements.

For inverted V-Bracing you will need to design the beam for the capacity of the braces (horizontal and vertical component). You usually end up with a huge beam.

V-Bracing would require the anchorage to be detailed for this load in a 1-story configuration. That will be tough to do.

 

[sandman21]

Why are you using SCBF for a single story building? Most of the time OCBF can be used, a little more information we could help more.

An X-brace will require ten reinforcement plates v. eight reinforcement plates. Five gusset plates with plate stiffeners that need to be installed properly v. three gusset plates. Keep in mind that the hinge zone tolerances are extremely tight, it’s not uncommon for errors that require extensive fixes. It is easy to provide a zipper column to take the unbalanced load.

 

[Galambos]

Sandman, thanks for the help.

The building is a small, L-shaped, two story school (cat III) with a one story portion. The one story portion is the small leg of the L, which projects only 70 feet from the main building

My thoughts were that the beam would be excessivley large to accomodate the brace force.

Thoughts?

 

[sandman21]

Using a zipper column takes care of the vertical component, the axial load in the beam is not very hard to accommodate. The issue we have with single story X-bracing is the gusset plates get extremely large to accommodate the hinge zone with the standard framing we see on single story buildings

I designed a performing arts center with a single story portion that has frames that are 26’ wide by 18’-6” T.O.S, the braces are HSS 6x6x3/8, zipper columns are HSS 8x8x1/2” and the beam is a W18x50. The only part that can get out of control is the zipper column there are things you can do, consider fix-pin, which we did not. We have moved away from SCBF and will be using BRBF when we need to use SCBF. From our research the BRBF will have cost savings over the SCBF.