Bottom Flange Brace Uplift

[BH6]

What would be the "best"/standard detail for bracing the bottom flange of a W beam when you need to brace for uplift (steel deck on compression flange). I am seeing some options but not really any standard.

 

[jayrod12]

It depends often on the application and the adjacent framing. For a load bearing beam that has the joists framing into it, I'm a fan of a diagonal angle brace to the first interior panel point of the TOP CHORD, if you do it to the bottom chord I'm told it increases the bottom chord size quite a bit more than the effect on the top chord size of the joists.

If it is running parallel to joists, then diagonal bracing up to the adjacent joists, usually provide a horizontal angle at the top chord as well to allow the horizontal component to grab more structure.

If it's a series of beams, essentially beam joists, I'm more partial to continuous bottom chord bridging with the odd diagonal brace to take it up to the roof deck.

 

[BH6]

I have W beams (47' span) with the roof deck perpendicular to them but no other beams spanning into it.

 

[KootK]

X2 for bridging for that setup.

 

[BH6]

Is there any advantage to taking the shear plate full depth for an x brace in this situation or would you stay with a plate at the top connection and another at the bottom? -- If that makes sense

 

[KootK]

The forces in the bracing are usually plenty small enough that you can omit full depth stiffeners without fear of web distortion issues. In my market, there would usually be no plate whatsoever, not even a partial one. Rather, the bridging angles would have their ends coped, bent, and field welded to the beam flanges. That said, a partial height connection plate is pretty slick mechanically and I doubt that anybody would begrudge your having them at only the cross braced locations.

 

[BH6]

Okay thanks. I was thinking of using a WT not an angle would there be any problems with that or any reasons why that's not a good idea?

 

[KootK]

- A WT can be thought to be more concentrically loaded by the bracing forces which, obviously, is nice mechanically.

- I believe that a WT has to be cut from a W-section which may make it less cost effective.

 

[BH6]

Thank you!!

 

[r13]

I prefer the use of back to back angles at every other span with connection plates on top and bottom. Unless it is required for beam shear, full depth stiffener seems an over kill.

 

[BH6]

Thank you--Just wondering---If angles are used so often why don't programs usually allow for the design of a single angle? Most that I have used don't design a tension only member and they don't allow the use of a single angle in most cases.

 

[KootK]

When you introduce a single angle into a run of the mill FEM package, all of a sudden you have to contend with:

1) Are axial loads delivered through the shear center, the centriod, or somewhere else?

2) How do other load effects combine with #1?

3) What kind of nutty BS are users going to do with this if we make it available.

4) It's also something of a problem not in need of solving. The AISC manual tabulates axial capacities for angles loaded through their legs which is the case most of the time. That's the tool that I'd be using for this.

 

[BH6]

Haha--I guess that's true!
Thanks

 

[r13]

Use two angles in X pattern.

 

[jayrod12]

Use two angles in X pattern.

Why? Seems like added expense for minimal to no benefit.

Single angle brace from bottom flange of beam in question to top flange of adjacent joist/beam. Simple, cheap and extremely common.

 

[r13]

jayrod,

Not much difference. A single angle arrangement needs to be applied to every single span (aisle), as opposed to two angles (double) on alternate span. Also, the effective length of the two (double) angle arrangement can be shortened by bolting the mid-span with separator.

 

[jayrod12]

R13,

If you need these for every beam, then yes sure, however as I understand his question, it's a single beam.

However, if it is in fact multiple beams, I'm more partial to continuous bottom chord bridging with bracing every few bays. Keeps more of the spaces open for services.

 

[dik]

with cross-bracing in the end panels only.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BH6]

Sorry I didn't see the last few comments. There are multiple beams its a canopy.

If you need these for every beam, then yes sure, however as I understand his question, it's a single beam.

However, if it is in fact multiple beams, I'm more partial to continuous bottom chord bridging with bracing every few bays. Keeps more of the spaces open for services.

Continuous bottom chord bridging with bracing every few bays? So an angle along the bottom? And every few bays? So every other or can you go further and how do you determine that? How do you determine the loads going to every few bays? And are you talking about X bracing?

And then make sure you have an X brace at the end span or

with cross-bracing in the end panels only.

X brace only in the end span and an angle along the bottom everywhere else? So the end brace would take the entire load across the whole building/canopy?

Thanks

 

[r13]

For outdoor canopy, an addition row of transvers beams and columns should be considered, as for aesthetic and economic points of view.