Uplift Bridging 2

[wompa1]

I am designing a building in a high wind zone and there is a tremendous amount of uplift on the roof members, specifically the beams. Are there any rescourses out there that talk about how to design the bridging for those members (basically I am bracing the beam members at midpoint)? I looked in the SJI specifications but it only talks about L/r limitations. Any advice would be greatly appreciated.

 

[SteelPE]

If you are looking into how much force the bridging must resist, then I would look at Appendix 6 of the AISC 13th edition (Stability Bracing For Columns and Beams). This section gives you the required strength and stiffness for bracing members.

 

[JedClampett]

What's your scope in designing the bridging? We always define the uplift loads and have the Steel Joist Supplier provide the necessary bridging. Do you need the amount of bridging or its loads for design?
Without knowing the bottom chord sizes (which is very hard to pin down), I don't think you could design the bridging, nor should you want to.

 

[wompa1]

Thanks guys,

Well the roof system is just Wide flange beams and girders. We are bracing the top flange with the roof deck for the gravity loading, but as we analyze the uplift (from the wind, 0.6DL+WL) the bottom flange is unbraced the entire span. So we are adding angle bridging to brace the bottom flange at mid points. I was wondering if there were any resources to find out how to design that bridging. I talked to some guys around my office and they talk about using 2% of the allowable compression capacity (given the unbraced moment of the beam section) but I can't find that in any liturature.

 

[wompa1]

Thanks for the update on App. 6. That is just what I was looking for. Thanks SteelPE.

 

[asixth]

Are you referring to purlin bridging or fly braces?

I always provide briging every 4000mm (12') for purlins regardless.

 

[hokie66]

The OP mentioned the SJI (Steel Joist Institute), so he is likely in the US and the term bridging refers to the bracing between the open web joists, not bridging between cold formed purlins. But then he says that the roof system is just beams and girders, so no wonder there is confusion about what the "bridging" is bracing.

 

[structuresguy]

the 2% rule is generally considered conservative. In fact, testing has shown that even 1% is adequate for uplift bracing.

Basically, find your maximum moment in the roof beam, calculate out the compression force in the compression flange, then take 2% of the force. Design a single angle for that compression (and tension) force over the span length of the angle.

If I have many beams side by side, like you say you have, I run a continuous angle across the bottoms of each. Then at the end (or also sometimes at some intermediate point), I X-brace the last bay, to bring the load up to the deck. If you do it this way, there is some debate as to whether the 2% at each beam is cumulative in the brace. Some say yes, some say no. Just to make sure I am convered, I add the forces up and design the angle for the total sum.

So lets say I have 8 W16 beams, spaced 5 feet apart, with a moment of 100 k-ft in each. So my compression flange force is 75 kip. 2% of that would be 1500 lb. So now if I only design a single brace from teh bottom chord up to the next adjacent top chord (L~6'), it would need to be L2x2x3/16. But if I run a continuous angle all the way across, with x-bracing at each end, the maximum force would be 8/2*1.5k, or 6 kip. Now the angle would need to be L3x3x1/4. So you can see, that it is not much of an increase in this case.

The last thing you need to worry about is how you resist the brace force at the end of the brace. Typically you would bring it back up to the deck diaphragm. So now you have to ensure your deck can take the point load, or figure out how to distribute it over a large enough area. And you should consider the normal deck stresses (uplift, diaphragm shear) due to the wind event at the same time as your uplift brace load. This can result in additional deck fastening in the area of the brace.

 

[minorchord2000]

Excellent treatment, structuresguy. Good Post.

 

[GalileoG]

Thank you structuresguy, that was very informative.

I am curious if people consider the 2% bracing force cumulative when running a continuous angle brace at the bottom flange of each beam.

 

[strainman]

Like the way you think structuresguy!
The steel joist institiute makes no differentiation in their bridging requirements (sizing) between one joist or a thousand.
Which would make one think that accumulation of forces for each member would not be required. But for the additional cost of a few pounds of angle, I like to sleep at night.

 

[structuresguy]

Glad to help out.

Clansman: there is definitely debate in articles I have read, and people I talked to, regarding additive nature of bracing requirement. Personally, I think it should be additive in some way. Let's say you connected the bottom chord of several joists together, but did not tie it off at the end. Now the entire group of joists could buckle together in the same direction. Well, if you brace one joist, it won't buckle, but the others still want to. So does each joist then "apply" a buckling load to the one brace, or does only one buckling load exist? Well, if you think that only one load exists, then the continuous brace from joist to joist would then be a zero force member, if all the buckling load was taken out only at the last joist. I don't know if this is really true behavior, but also don't want to take a chance.

The way I do it works well in most cases, where you don't have too many joists in a row. When I frame out a building, I like to use W-beams at the columns, and joists in between. So my uplift bracing is only carried over 4-6 joists at at time, typically. In the case of a long roof with only joists, say with tilt-up walls, then I X-brace between the joists every 5 or 6 joists. That way, my cumulative brace force never gets excessively large.

 

[ChipB]

2% in Literature:
Salmon & Johnson: Steel Structures, Design and Behavior

It's under lateral bracing design. I have the 2nd Edition and the 2% is on pg 514.