Steel Braced Frame - Brace Locations

[JrStructuralEng]

Considering the following:
-square
-single storey
-conventionally framed structural steel braced frame
-OWSJ's & metal deck w/ diaphram action
-HSS columns and wide flange beams
-diagnol bracing on each of the four exterior walls

What is everyone's opinion on the ideal braced bay locations and the type of bracing if they can go anywhere and be anything?

Also, does anyone have any literature on best practices for such building stability design.

Thanks!

 

[gte447f]

I would go with a chevron brace centered along each exterior wall. There may be a more ideal solution, but this is what I would be most comfortable with, and it wouldn't be considered unusual. I would use HSS diagonals in the braced frames. You may need interior frames depending on the strength and span of the roof diaphragm.

 

[DaveAtkins]

I agree that the braced frames should be centered on each side--this eliminates any thermal expansion resistance from the frames.

I think an X-brace will be the cheapest, because the members will be tension only (and can be rods or small angles).

DaveAtkins

 

[KootK]

Where tension only braces are suitable, I've become quite fond of using small channels (single or double). Some of the advantages:

1) You don't have to deal with any significant self weight sag in the braces (rod/angle problem).

2) You can detail the two braces to bypass one another without inducing much eccentricity in the brace-column connection (rod problem).

3) There's hardly any shear lag to deal with (angle problem).

A channel brace usually requires a bit more steel but I've yet to hear anyone complain. In my experience, a double C8 channel will work in the vast majority of low rise bracing situations. It makes for easy preliminary design!

 

[ToadJones]

for a low rise building with squatty dimensions (aspect) (2)-C8's seems large....especially if you are counting on the tension only brace mechanism.
Of course, this a very vague conversation to begin with....so....

 

[KootK]

TJ:

They are large. No argument there. However, they're still relatively insignificant compared to the overall cost of the structure.

Where the loads justify it -- which is quite often -- I go with single C8's. Or C6's if I can get the job done with two or three bolts.

I think the geometry is slick. I center the connection plates on each column, place the two braces on either side of the plates, and connect the braces where they cross using a spacer plate of the same thickness.

It's not rocket science but it sure does make for nice clean brace. I mostly just hate to see sag in my braces when I visit the site. It offends my structural sensibilities.

I also worry that it introduces non-linearity and extra drift into my braced frames that I didn't bother to model. I've used some steel rod bracing in tall glulam frames in the past. Because the frames are wooden, you can't pretension the heck out of the brace from the get-go like you can with a steel structure. When I look at the sag in some of the these braces, I get the sense that the frames would have to deflect a foot before the braces would be fully engaged.

 

[ToadJones]

KK-
I typically use double angles with gussets as you describe and detail them such that they are short according to an old detailing reference I have. The guys in the field usually have to drift-pin them in, but I never have any sag that is readily visable.
Since double angles can get wide and take up wall space, I also use square HSS with knife plates. Very clean looking. On large braces I have used large HSS, pipes, and WF's. You're double channels might be something I could use in the future in place of WF's to simplify details.

 

[KootK]

Every engineer has their favorite bracing arrangement. I often walk under an old railroad bridge that uses double channels for the tension members and WF's for the vertical struts. I should get a photo of that, it looks spiffy.

I use a lot of HSS too. I use them exclusively when the brace is actually within a wall system. You can get some out of plane capacity out of an HSS brace which is good.

Do you use WT's at the ends of the HSS braces or a plate slotted into the walls of the HSS?

 

[ToadJones]

I have not used many HSS sections as columns (mostly industrial)
The HSS I use for braces are usually on WF columns.
I have used through plates to connect to HSS before, however.

 

[DaveAtkins]

When I have used tubes for diagonals, I have done it both ways--I sometimes slot the tube and insert a plate (I prefer this method because the weld design is easier), and I sometimes weld a WT to the end of the tube.

DaveAtkins

 

[JrStructuralEng]

and as far as locations along a wall? centered as Dave explains? one on each end?

 

[KootK]

I'm sure that you could position the braces anywhere along the walls and make it work. If you truly do have complete flexibility, which is pretty rare, then I'd say go with Dave's suggestion for the reason that he mentioned.

 

[ToadJones]

In industrial buildings that are long and narrow we always try to brace near the center in an attempt to mitigate the effects of expansion.
If you only have a few bays then the effects will not be as great. If only two bays, well, then it is obvious which bay to brace
If you really have this much flexibility, it sounds like a damn good job to have!!! The kind we all dream about.