Bottom Flange Bracings?

[RBS01]

On a gabled steel moment frame, my architect requested me to take out all bottom flange bracings making it fully unbraced. Top flange is effectively braced by purlins. With wind uplift controlling, I’m always using the full width of the frame (sloping) as my compression flange unbraced length. I’ve seen some designers and not realizing that node pt. at ridge is not a braced point unless provided with other positive means. Any one has a different approach to this condition?

 

[SteelPE]

Can you rephrase the question and post a sketch of your situation?

 

[kslee1000]

Don't you have a ridge beam (perpendicular to gable frame)?

 

[RBS01]

There's not a ridge beam so the full rafter span is unbraced at bottom flange. It is 100' span symm. single gable at 3:12 slope, I'm using about 103' unbraced length of compression flange for wind uplift. Node pt. at ridge should not be assumed as braced pt.

 

[DaveAtkins]

You are correct. It is unbraced for its full length.

DaveAtkins

 

[kslee1000]

So, was your question Lb = 103' or 206'?
Forget above the ridge beam, it is immaterial for this case. I agree 103' is correct, because the true unbraced length of concern is shorter than that.

 

[RBS01]

Thanks guys. It's just that other designers can sometimes overlook this without specifying the correct Lb in their model,i.e., using Lb=51.5' instead of 103'

 

[BAretired]

I assume you told your architect you can't comply.

BA

 

[hokie66]

The other guys are correct that the full span length must be used if the bottom flange is totally unbraced, which probably makes your design very unwieldy.

There are proposed design methods which use the strength of the purlins, bolted to cleats with at least two bolts, and combined with web stiffeners, to brace the bottom flange indirectly. There have been some discussions on this site about using stiffeners as braces, but I don't have a specific reference.

 

[jhardy1]

Australian Steel Institute (ASI - formerly known as Australian Institute of Steel Construction - AISC) publishes a book called "Limit State Design of Portal Frame Buildings" by Woolcock, Kitipornchai & Bradford. I have the first edition (1991), but I know it has been updated since then.

A couple of quotes from the first edition:

From 5.3.2.3.2 "Without Fly Bracing":

"... theoretical and experimental studies have confirmed that translational restraint alone acting at the level of the tension flange, such as that provided by purlins, is virtually ineffective [to increase the lateral buckling capacity] ... It is possible to design the purlin-rafter connection for some rotational capacity by providing two or four friction bolts to the cleat ..."

From 5.8 "Fly Braces":

"In this case, the bottom flange should be braced by using a wider purlin cleat and four high strength bolts, and a web stiffener on one or both sides to prevent distortion ... There is some evidence that the stiffeners are unnecessary. However, until testing confirms this, it is recommended that at least one side of the web be stiffened."

Hope this helps!

 

[enginerding]

I would recommend looking into using Torsional Bracing so that your shape does not get needlessly large. See Appendix 6 of the AISC 360-05 (American Institute of Steel Construction) Specification.

"2. Torsional Bracing - It is permitted to provide either nodal or continuous torsional bracing along the beam length. It is permitted to attach the bracing at any cross-sectional location and it need not be attached near the compression flange. The connection between a torsional brace and the beam shall be able to support the required moment given below..."

 

[hokie66]

RBS01,

You will need a truss in the plane of the roof to brace the building in the direction parallel to the ridge. If you locate this at or near the bottom flange, it will brace the rafters at the node points. You could place a truss near each end, and run struts between to brace the rafters.

 

[kslee1000]

That was my question, a ridge beam, or truss. Amazed to know there is none.

 

[271828]

"There are proposed design methods which use the strength of the purlins, bolted to cleats with at least two bolts, and combined with web stiffeners, to brace the bottom flange indirectly. There have been some discussions on this site about using stiffeners as braces, but I don't have a specific reference. "

One idea relaed to this is to use a Cb equation other than the one given in the AISC Manual. The Yura/Helwig bracing seminar notes show a Cb equation for the case of the top flange braced, bottom flange unbraced and in compression. Cb can end up being 5 or 6.

 

[csd72]

As mentioned above the stiffeners and using the purlins as 'torsional bracing' is the way to go.

apply the required bracing force at the botom flange and check that the bolts in the purlins are able to take the moments.

Only option other than that would be to put the purlins between the rafters (single span).

 

[kslee1000]

"Only option other than that would be to put the purlins between the rafters (single span). "

This is the better way.

 

[hokie66]

That idea has limited practicality. Using single span purlins adds a lot of mass to the purlin system. If the purlins are not coped, the connections cleats are cumbersome. It also creates a problem in fastening the roof sheeting where it occurs over the rafter.