LTB Bracing Using Web Stiffeners?
LTB Bracing Using Web Stiffeners?
(OP)
I am designing a building that needs to meet the requirements of UFC 4-023-03 "Design of Buildings to Resist Progressive Collapse".
One of the provisions in this manual requires that the floor framing be designed for a net uplift load equal to the self wt. DL + 0.5LL.
This results in huge beam sizes due to the unbraced length of the bottom flange, unless some type of bottom flange bracing is used.
I really don't want to require the installation of a gazillion angle braces. This would be labor-intensive as well as a major headache for the HVAC boys.
The floors are metal deck and concrete on steel beams with studs for composite action. I figure that if I use full-height web stiffener periodically along the length of the beams, I can assume these as brace points for LTB.
My theory is that the top flange is well-restrained against torsion due to the studs extending into the slab, and I can provide lateral support to the bottom flange by "cantilevering" off the top flange with stiffener plates.
Anybody have any comments or words of wisdom regarding this issue?
Thanks in advance.
Ken Kilzer, P.E.
One of the provisions in this manual requires that the floor framing be designed for a net uplift load equal to the self wt. DL + 0.5LL.
This results in huge beam sizes due to the unbraced length of the bottom flange, unless some type of bottom flange bracing is used.
I really don't want to require the installation of a gazillion angle braces. This would be labor-intensive as well as a major headache for the HVAC boys.
The floors are metal deck and concrete on steel beams with studs for composite action. I figure that if I use full-height web stiffener periodically along the length of the beams, I can assume these as brace points for LTB.
My theory is that the top flange is well-restrained against torsion due to the studs extending into the slab, and I can provide lateral support to the bottom flange by "cantilevering" off the top flange with stiffener plates.
Anybody have any comments or words of wisdom regarding this issue?
Thanks in advance.
Ken Kilzer, P.E.






RE: LTB Bracing Using Web Stiffeners?
The strength is probably OK but you should check the added tension in the studs due to a lateral load (this is provided in the Third Edition AISC LRFD Manual for nodal bracing).
As the beam tries to bend sideways (bottom flange kicking out) the top flange will be in bending unless the stiffener and the stud are in close proximity - which I would think could be specified on your plans. You might consider first calculating the studs required for normal composite action, and then ADDING more studs - in line with the sitffener plates, to get the desired behavior.
As for stiffness - I think that, again, using the AISC LRFD provisions - Section C3.4a - would give you what you need. You might want to model this in a detailed way to determine a "typical" condition and ensure that you have adequate strength and stiffness for the general system.
RE: LTB Bracing Using Web Stiffeners?
1. Load applied to one bay at a time (depending on floor continuity, could the dead load from adjacent bays help to "hold things down"?)
2. Allowable strength reduction & over-strength factors.
3. Steel connections are probably much "better" than simple supports. If you have not already done so, perhaps you can assume that the beams have partially or totally fixed ends, that would help.
I would put in far more that the minimum number of stiffeners that you calculate, since the assumption that they are "true" braced points is probably only partly correct.
www.SlideRuleEra.net
RE: LTB Bracing Using Web Stiffeners?
In the OK City bombing I thought the biggest reason for the collapse was the perimeter beam rolled off of the column when the slab was lifted in the blast. Is this type of failure considered taken care of in the tie requirements?
I haven't used this document before, I know the TI-809 makes me pull my hair out for essential buildings in high seismic areas.
RE: LTB Bracing Using Web Stiffeners?
RE: LTB Bracing Using Web Stiffeners?
I am going to add studs above that required for composite, specify their location adjacent to the stiffener, and do a detailed analysis of the required load and stiffness.
Haynewp, my project is classified as IIIE, so I am slogging my way through the TI 809. Not a lot of fun. Interestingly, I am in SDC "C", and so hoped to be able to use an R-value of 3 and dispense with alot of the required seismic detailing. But since I have a IIIE building, all that detailing is basically required, so I might as well detail the thing for seismic and take advantage of the higher R-value.
Thank you all again,
Ken Kilzer, P.E.
RE: LTB Bracing Using Web Stiffeners?
RE: LTB Bracing Using Web Stiffeners?
RE: LTB Bracing Using Web Stiffeners?
RE: LTB Bracing Using Web Stiffeners?
RE: LTB Bracing Using Web Stiffeners?
RE: LTB Bracing Using Web Stiffeners?
RE: LTB Bracing Using Web Stiffeners?
The composite steel/concrete provides plenty of bracing for the top flange (everyday loading). The channel would provide increased bottom flange bracing for the uplift situation. As a bonus, the channel would increase I and S for the entire beam, perhaps allowing the W section to be a little lighter.
Just thinking "out loud".
www.SlideRuleEra.net
RE: LTB Bracing Using Web Stiffeners?
If you're concerned about welding to the bottom flange then bolt to the bottom flange with a clip angle, accounting for the loss in section. Just something I have done in the past for certain bridges.
Regards
VOD
RE: LTB Bracing Using Web Stiffeners?
If the design incorporates the stiffness of the top flange to concrete connection, his idea may well work. Perhaps testing is a good idea to gain faith in it. In railway bridges we use a similar concept, an old concept, called u-frames which provide LTB and torsional stiffness for the girders when the train travels longitudinally between two girders and thus no bracing can be placed.
Regards
VOD
RE: LTB Bracing Using Web Stiffeners?
RE: LTB Bracing Using Web Stiffeners?
My $0.02
RE: LTB Bracing Using Web Stiffeners?
I only have xerox copies from old textbooks that reference work by Muller-Breslau and Zimmerman. The solution to the required stiffness of intermediate lateral supports to an unsupported flange was a theory developed in 1893 by a German engineer F. Engesser. The solution was referred to as the "Engesser Formula".
RE: LTB Bracing Using Web Stiffeners?