What defines a "encased composite beam"

[syckoman]

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

 

[CELinOttawa]

Good question... Someone around here is likely to know. My money is on to prevent one of two issues:

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.

 

[JedClampett]

I took the AISC refresher course. During some of the time I wans't asleep, I remember some extensive requirements for encased composite members having to do with shear transfer. It's just a type of composite member that AISC decided needed design guidance. Engineers were asking, so they accomodated. There's other composite members with just the top flange encased or even just shear studs between the flange and the concrete deck.
Isn't the additional strength provided by a more normal composite sections enough?

 

[syckoman]

Thanks CELinOttawa and JedClampett,

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

 

[CELinOttawa]

Hey Mike,

How did the load increase? Please, please, please keep in mind that the Owner's problem is *not* your problem!

 

[syckoman]

CELinOttawa, yes I am aware of it being the owners problem, and they know it too. Basically it's an atom smasher research facility. The scientist came back and wanted to increase the soil fill above the tunnel from the agreed upon 125psf to 140psf, I believe due to radiation concerns.

 

[CELinOttawa]

Interesting project... I've been the EOR for some weird ones in my career as well. In a precious life I was the Canadian government's Blast Hazard Engineer. In this day and age I'll let you figure out what a Structural Engineer does for a national government relating to hazards from explosives. *sigh*

What a world we've found d ourselves in!