What defines a "encased composite beam"
What defines a "encased composite beam"
(OP)
Hello,
I am graduate structural engineer helping out a senior structural engineer. We have a problem and can't seem to understand the reasoning for it. On a composite section, what dictates a "concrete encased beam". In the AISC 13th ed. it states "Beam totally encased in concrete cast integrally with the slab". So in our situation, we have a beams that are fully encased along the top flange and sides. The concrete stops even with the bottom flange. The question posed by my senior engineer is why is it that the bottom flange needs to be encased in concrete if the top flange and web are in compression and the bottom flange is in tension? He has called AISC, but was unable to talk to anyone with "experience". He is looking for a reason and or an article that states otherwise. We could really use the extra strength in the design.
Thanks for the help,
Michael
I am graduate structural engineer helping out a senior structural engineer. We have a problem and can't seem to understand the reasoning for it. On a composite section, what dictates a "concrete encased beam". In the AISC 13th ed. it states "Beam totally encased in concrete cast integrally with the slab". So in our situation, we have a beams that are fully encased along the top flange and sides. The concrete stops even with the bottom flange. The question posed by my senior engineer is why is it that the bottom flange needs to be encased in concrete if the top flange and web are in compression and the bottom flange is in tension? He has called AISC, but was unable to talk to anyone with "experience". He is looking for a reason and or an article that states otherwise. We could really use the extra strength in the design.
Thanks for the help,
Michael






RE: What defines a "encased composite beam"
1) Shear splitting failure between steel and concrete.
2) To slow or potentially prevent high temperature steel expansion from causing failure during fire.
I know that concrete encased beams tend to behave badly in fire if not well protected. The slabs, ironically, perform very well and in fact can act like a rubber skin, failing well after expected.
RE: What defines a "encased composite beam"
Isn't the additional strength provided by a more normal composite sections enough?
RE: What defines a "encased composite beam"
I was thinking it might had to do with the fire protection. In this case we have a tunnel with W36x302 (they vary in size) beams encased in 42" thick concrete slab. The reactions are roughly a shear of 550k and phiMn of 4800k-ft. The steel shops already have been approved, but now there is an increase in loading that puts us close to the limit. That's basically all I know about the project as I just jumped on it last last week and the original engineer left the company.
Thanks for the help and I'll relay the information and we can have a discussion on how to proceed next.
~Mike
RE: What defines a "encased composite beam"
How did the load increase? Please, please, please keep in mind that the Owner's problem is *not* your problem!
RE: What defines a "encased composite beam"
RE: What defines a "encased composite beam"
What a world we've found d ourselves in!