concrete encased beams
concrete encased beams
(OP)
Hi all,
I am working on modifying an old building ('31), which had the beams fireproofed by concrete encasement. We are increasing the loads and the beams by themselves no longer cut it. Can anyone point to me an appropriate procedure as to how these beams can be analysed as composite? (or if somewhere it specifically says that this should not be done!)
Thanks,
-G.S
I am working on modifying an old building ('31), which had the beams fireproofed by concrete encasement. We are increasing the loads and the beams by themselves no longer cut it. Can anyone point to me an appropriate procedure as to how these beams can be analysed as composite? (or if somewhere it specifically says that this should not be done!)
Thanks,
-G.S






RE: concrete encased beams
RE: concrete encased beams
Most of the beams like this that I've seen have minimial, if any, reinforcement, stirrups, etc. and no studs or other means to integrate the two materials to take interface shear.
The most I've usually seen are some cases where some minor welded wire fabric was used. Rarely have seen studs - especially from 1931 era.
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RE: concrete encased beams
You could possibly chip off the bottom concrete and fasten new steel (cover plates, tubes, channels, etc.) to the bottom flange and then re-cover with fireproofing.
Additional columns or cross beams might also be an option but I certainly don't know or understand all your limiting parameters.
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RE: concrete encased beams
RE: concrete encased beams
Are these composite beams (steel beam and concrete slab)?
RE: concrete encased beams
No, I do not believe the system was designed as composite - our drawings do not indicate any shear transfer mechanism. The beams are encased in 2+ inches of concrete on each side. I am thinking it would be just too much work to break away the concrete, weld studs, and replace concrete. We are just going to add new beams underneath to carry the whole load.
RE: concrete encased beams
For design / construction in 1931, completely agree.
One aspect you could investigate, in 1931 structural steel allowable stress was 18 KSI. If appropriate steel samples, from the beams, can be laboratory tested and results are satisfactory, a modestly higher allowable stress could be considered.
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RE: concrete encased beams
RE: concrete encased beams
RE: concrete encased beams
RE: concrete encased beams
Sure it does. You just have to design to elastic stress distributions if you want to include composite behavior without providing a positive mechanism for horizontal shear transfer. Whether or not composite elastic design yields better results than bare steel plastic design is a question for you and your calculator.
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RE: concrete encased beams
I am looking at AISC 360-10, Sec 16, under force transfer mechanisms (dated 2010) - "the force transfer mechanism of direct bond interaction shall not be used for encased composite members"... what am I missing?
RE: concrete encased beams
They're speaking to external loads applied to axially loaded members where those loads must be transferred over relatively short distances. See I6.1.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: concrete encased beams
RE: concrete encased beams
I3 gives us three methods for designing encased beams (two composite) and only one of them requires steel anchors. Is that not implicit support for a direct bond mechanism? The I3 commentary would certainly seem to suggest this.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: concrete encased beams
RE: concrete encased beams
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.