Rafter unbraced length

[GalileoG]

I have rafters spanning approximately 15 meters (50 ft) and was wondering if I should have bracing (I believe they're called kicker bracing, or kick bracing, or something like that) between the rafter bottom flange and the purlin to reduce the unbraced length when the rafter is subject to uplift. Now, typically, would this bracing connection be specified for each rafter to purlin connection, or every other rafter to purlin connection, or sometimes none at all? I suppose what I'm wondering is when do you decide to use this particular bracing connection? Thanks

 

[csd72]

apsix,

I dont know about the current version of AS4100, but previous versions used specific factors for the location of the loading in the section giving a lower (better) factor for bottom flange loading.

asixth,

Doesnt the 1.4 factor reduce to 1 for loading at or below the centroid?

It has been a few years but I am sure these are correct.

 

[civilperson]

hokie66,
From the "Steel Interchange" 05/01/1997:
"Does an unbraced trolley beam that is loaded on the bottom flange have the same buckling characteristics as an unbraced beam loaded on the top flange?"

Answers:
Recommended approximate solutions to estimate a beam's critical capacity under concentrated loads have been presented in a July 1971 issue of the Structural Engineer in Nethhercot and Rockey's A Unified Approach to the Elastic Buckling of Beams. The content of this article was later referenced in the text of Chen and Lui's Theory and Implementation, 1987, Elsevier with comparison to theoretical solutions of Timoshenko and Gere. The approximate solutions for centrally loaded simple beams with tip flange, shear center and bottom flange loading shows close agreement using Cb values.

........
Barry P. Gahagan, PE
Forte and Tablada, Inc.
Baton Rouge, LA
___________________________________

07/01/1997

An unbraced trolley beam that is loaded on the bottom flange has increased resistance to buckling compared to a similar beam loaded on the top flange. When a load on the bottom flange moves with the beam during buckling, it causes an additional moment about the shear center of the beam, and therfore, resists the tendency of the beam to buckle. ........

Brian J. Bidonde
Baker & Associates
Pittsburg, PA
_________________
reference

http://www.aisc.org/MSCTemplate.cfm?Section=Steel_Interchange2&Template=/CustomSource/..

 

[csd72]

There are two components to lateral torsional buckling the lateral and the torsional.

Bottom flange loading helps with the torsional but not the lateral.

 

[hokie66]

civilperson,

Thanks for the further information. I understand that tension flange loading reduces the tendency to buckle, but in the real world, I will continue not to depend on it. Reduces is not eliminates, and I am no hero.

 

[miecz]

Last time I did one of these, I took advantage of the increased resistance to buckling, and the deflection criteria ended up governing the design.

 

[Tomfh]

miecz,

How did you calculate the beneficial effects?

 

[asixth]

csd72,

That's correct, kl=1.4 above the centroid, kl=1 below the centroid. Good desciption of lateral-torsional buckling. It's always nice when someone brings some simplicity to engineering.

Clansman,

I ended up using three fly braces in my end span, located atound the quarter points. I'll have to wait for the tech to build up the model before I specify which purlins these are.

 

[miecz]

Tomfh

At the time, I was using the LRFD 3rd Edition. Formula F1-6 has an X₂ term that applies to tension flange loading. That is, the commentary to F1 implies that X₂ = 0 for "top" flange loading. I merely cranked X₂ through formula F1-6, which increased L_r. Looking back, I see that it had a minimal effect, increasing my L_r from 14.0 feet to 14.9 feet.

My old Guide to Stability Design Criteria for Metal Structures has the derivation of the critical load formulas that appeared in the 9th Ed. AISC Codes. The formulas for "Bottom Flange Load" and "Top Flange Load" were identical, except for a C₂ term that was negative for top flange loading, positive for bottom flange loading, and zero for load at the centroid. The Guide explained that load placed on the top flange had a tipping effect that reduces the critical load; and conversely, if the load is suspended from the bottom flange, there is a stabilizing effect that increases the critical load. I believe the Guide formulas were simplified for the ASD Specification by eliminating the C₂ term. With the dawn of the LRFD Specs, the C₂ term seems to have been resurrected.