concrete encased beams

[sklgleb]

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

 

[mike20793]

In the US, the analysis of concrete encased or filled members is found under AISC 360-10 in Chapter I.

 

[JAE]

I wouldn't try to use the concrete as composite unless you really know how they were built - even then I'd be sheepish about doing it.

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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[JAE]

It seems to me that the best response here would be to add structural framing below or on either side of the beams in question.

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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[sklgleb]

I think AISC 360-10 covers what I had in mind. We were assuming there was going to be some 'direct bonding' between concrete and steel, as we do have 16 gauge wire at 8" intervals; however, looks like AISC does not allow for direct bonding in encased member design. Thanks for the prompt replies!

 

[DETstru]

I've run into a similar situation where I was able to remove the concrete encasement and replace it with spray applied fireproofing. That reduced the weight enough to allow the added load but I don't know if that will be enough for your case.

Are these composite beams (steel beam and concrete slab)?

 

[sklgleb]

DETstru,

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.

 

[SlideRuleEra]

I do not believe the system was designed as composite - our drawings do not indicate any shear transfer mechanism.

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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[bookowski]

New beams to carry the entire load seems inefficient unless there's a good reason to not chip concrete. We see these often and usually weld a wt to the bottom.

 

[sklgleb]

welp, as far as inefficiency, I am not sure that with the labor cost factored in it will really come out much different - I am by no means an expert on this so please correct me if I'm wrong. We have the option to to just plop some beams in, or have someone come in, chip away the concrete, bring in a welder, reform the concrete/add new fireproofing...The welding would be upside down, and as there are too many complaints with this, I would expect that there would be a surcharge.

 

[bookowski]

Probably depends on your particular situation. If you only need a little extra then a bottom plate would do, if you need more then a wt is nice. If the connections are not over capacity then you can do a partial length wt (or plate), say 75% - would depend on your situation. This would be a substantial savings in steel which may not add up to much, but the handling/logistics might be worth a lot depending on your site. This saves on chopping into column, putting new connections, connecting new to old, more headroom, less new fireproofing etc.

 

[KootK]

looks like AISC does not allow for direct bonding in encased member design.

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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[sklgleb]

KootK,

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?

 

[KootK]

what am I missing?

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.

 

[Lomarandil]

No, KootK, I'm not sure that's right. The same language (prohibiting bond transfer for encased members) is used at the end of I6.3

 

[KootK]

I6.3 is the very clause that I was speaking to above. It falls within section I6 which deals exclusively with load transfer in externally loaded axial members per I6.1.

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.

 

[sklgleb]

Hmm, I'd say this spec is vague... I've come across a British spec that says the only value of the concrete is to provide continual lateral bracing, all load is to be carried by the beam. Also, I think any paint at the interface will not be conducive to shear transfer (and it looks like, in my case, we got the good ol' red lead paint)...

 

[KootK]

Yeah, the direct bond method gives me the heebeejeebees to start with. The paint would be enough to throw me off as well. Come to think of it, I'm not sure where the requirements for surface prep / lack of paint actually reside in AISC. You'd think that there would be something.

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.