Steel Beam Cast in Slab on Grade

[BubbaJ]

I have a client (contractor)whose building clien wants to cast 6" steel beams into a 6" reinforced concrete slab-on-grade. They want to weld loops/hooks/etc. to the steel beams and use them as an anchor when using come-alongs to straighten bent/racked trailer frames. There will be one beam parallel to each side of a repair bay, ten bays total, thus 20 beams.
I suggested welding rebar to the flange of the beam to tie to the rebar mat in the slab. Am I being too conservative?
Has anyone done something like this? What magnitude of forces do you design for?
Contractor is obviously concerned that the slab will sustain excessive cracking etc and client will come back on them.

 

[SlideRuleEra]

Since the slab is reinforced, I don't believe that you need to worry about anchoring the embedded beams - they can't go anywhere without taking the floor with them.

In industrial plants steel beams are frequently embedded in concrete floors, without special anchorage, to serve as support for electrical switch gear and other components that are welded to them.

Also, a historical use of embedded steel beams was grillage foundations, which perform nicely, even under very heavy loads.

http://www.SlideRuleEra.net

 

[BubbaJ]

Thanks. I don't have any experience with grillage foundations (that I know of) but I will do a web search to learn more.

Do we need to take any extra precautions with the slab? I have it reinforced with #4 @ 24" o.c. each way.

 

[SlideRuleEra]

Ooops... Just noticed that your are talking about a 6" thick floor (rather than a foot or two). You probably do need to tie the slabs on both side of the 6" beams together. Rather than welding lugs to the beams, why not drill/punch/cut a line of holes thru the beam web and run rebar thru the holes into the slabs on both sides.

About grillage foundation, go to my website and download the 1923 Carnegie Pocket Companion, see page 252.

http://www.SlideRuleEra.net

 

[MotorCity]

sduggan,

I would have two concerns about the situation you describe. First, it sounds like the steel beam would be in direct contact with the ground since it is 6" and is embedded in a 6" slab. There are ways to deal with this, but should be a consideration. Second, I would be concerned with concrete spalling where the steel beam is in contact with the concrete at the slab surface. I'm not sure what shape you're using but its been my experience that people can come up with some pretty unusual ways of twisting and pulling on things to get them back into place as you describe above. You may want to consider some sort of system that is independant of the slab (i.e. a stand alone foundation).

 

[BubbaJ]

I won't hold it against you. A line of holes is much more labor friendly and requires a lot less skill than welding.

I appreciate all the help I have received on Eng-tips; differing opinions and varied experiences spur us all to think and grow in our professions. Too often I have found seasoned engineers reluctant to share their knowledge. It's a sad and frustrating situation.

 

[BubbaJ]

Motorcity,
I have those same concerns, as does the contractor. If we weren't talking about (20)- W6 30' long beams embedded in the slab, I would suggest a separate foundation system for them.$$$$

I think this will come down to "educating" the building owner about the potential risks associated with their scheme and getting them to sign off on it if they wish to pursue it.

 

[aggman]

I agree with SlideRuleEra. What they are talking about doing is fairly common in heavy fabrication shops, etc. I would run the rebar through the beams. I would also place web stiffeners in the beams every so often also. Most of the time the pullin you are talking about is completed using 2 ton come-alongs. You should be able to do some basic calculations to prove the slab even if it requires additional reinforcement (likely). I would be very clear to the owner in writing the assumptions you are making. Most of these people know what they are trying to do and what they are pulling with and how many pull points will be used typically at a time. To me the key is getting the owner to take a little ownership in the design so that everyone knows the design and usage parameters.

 

[SlideRuleEra]

sduggan - About the reinforcing... for an industrial application, such as this one, we use a minimum design of #4 @ 12", each way - no matter how little steel calculations require. Over time, this has proved to be a wise move - as MotorCity and aggman says you really never know what type loading the slab will see.

http://www.SlideRuleEra.net

 

[Ron]

sduggan...I would use a Tee section and put holes and continue the rebar through as SRE suggests. The bottom flange won't provide much except a rust channel, anyway.

If Tee sections not available, use back to back angles.

Frame straightening requires vertical and lateral loads, but the magnitudes are probably within a 10-20 kip range, since a come-along is being used.

 

[lexim]

Sduggan...I would agree with the contributons of SlideRuleEra and just to add a couple of notes here.

1. Is it possible to use other smaller sections, (check this with some calaculations). Consider the Tee section and Equal Angles back to back or a smaller section of Tee.

2. Base on the assumed loads to be calculated do you think increasing the depth of slab by 2" will upset the client, otherwise consider doing this. Corrosion becomes a big issue to tackle when it set in in a corbonated environment.

Aileme

 

[apsix]

I agree with the above ie drill thru the web for the rebar.
To guard against corrosion I would use a 600mm wide thickening at each beam, with a minimum of 75mm concrete below the beam and a layer of welded wire mesh within the thickening.

 

[swearingen]

A different tack, used at some frame shops:

Drive a beam vertically into the ground. A 10' W6 driven 9'+ into good soil and allowed to set up can take a great deal of load in any direction.

Advantages: it won't crack the concrete if an expansion joint is left around it, chains and come-a-longs can be easily connected to it without welding (some pre-drilled holes), it may be cheaper because you could have less total beam length, despite the cost of driving them.

Disadvantages: it will be a discrete system - you won't have a continuous surface to work from, if he requires many points to pull from this option will be more expensive.

One other note: you may be able to sell them on a mixture of the two, say half of the bays have continuous beams and the other have the driven stubs. What will happen is he will favor one or the other and gravitate to that side of the shop for any straightening work and do other maintenance in the other bays.

 

[BubbaJ]

Thanks all, after a meeting with the owner, it turns out what they originally said they would be doing is not what they will be doing. They will use the in-floor beams to prop the trailers for welding during the assembly process.

They are going to pour "cow pies" to set the ends of the beams, so the beam won't be in direct contact with grade. Holes will be drilled at 24" o.c. so the rebar can be threaded thru.

I appreciate all of your insights into the design dilemma. I'm sure it will come in handy again.