huangyhg

ltb bracing using web stiffeners?
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.
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ken - just throwing some initial thoughts your way - i would say your concept is appropriate...i just know that for bracing against ltb, you need sufficient strength and sufficient stiffness.
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.
sounds like a reasonable approach to me. i see the requirement in paragraph 2-2.2 of the document. looks like you have other factors that are working for you:
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.
there is an interesting statement "a load path from the slab to the foundation for this upward load does not need to be defined" in 2-2.2
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.
you will need to check the web distorsional stiffness in the aisc lrfd 3rd edition to use the concept you propose. it definitely can be done.
thank you all for the valuable input.
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.
don't weld web stifferners to a bottom flange. this is your tension flange under gravity loads. welding to a tension flange increases the susceptibility to fatigue or brittle fracture.
jmiec - fatigue is not an issue in buildings.
jae - i guess that brittle fracture isn't a design issue either. truth is, i lifted the reasoning out of a salmon & johnson textbook. many years ago, when i was right out of school, i was told by my supervisor that welding across a tension flange was a bad practice. the rule stuck, but it was so long ago, i've forgotton his rational. so, i would check with a welding expert.
well, welding acrossfff"> a tension flange, say across the bottom, when its under load, is truly a bad idea in that the weld heat could initiate a semi-molten/brittle fracture during the weld. but a vertical stiffner, such as that discussed in this thread, poses no such risk as the weld is up the web, and across the top half of one flange only.
though we never said so, i was assuming we were considering stiffeners on both sides of the web. if stiffeners are on one side only, then the stiffness analysis will have to consider flange bending, even if the stud and stiffener are in close proximity.

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