Steel deck with thick concrete topping

[J1D]

I encountered a special situation. It is a c.i.p. cooling tower basin bottom slab (I posted a thread a few days ago about the cooling tower) supported by wide flange steel beams. The depth of the slab is 12”, in which I use 3” deck. Now I found the typical composite beam section tables do not cover the sections with this thick of concrete topping. Any suggestions to deal with this?

Thanks in advance!

 

[DaveAtkins]

I would design the composite beam by hand. Refer to a steel textbook.

DaveAtkins

 

[J1D]

Look like I have to do it by hand. I found some programing demo on the web for this kind of calculation. The slab thickness and deck rib height are input variables. It uses allowable stress approach to compare the stresses of the steel beam and concrete to the allowable. But it is not clear how to calculate the horizontal shear force for stud design.

 

[whyun]

RAMSBeam allows you to input the concrete height above the top flute of metal deck. I have not check for the validity of calculations using extremely thick concrete topping; however, at least you know the tool is available.

 

[JedClampett]

Is there a good reason you want to use metal deck? It seems in a cooling tower environment, it will most likely rust, no matter how well it's galvanized. It might not make any difference with regard to strength, but it will be unsightly. With 12 inches of concrete, it will have to be shored, negating some of the cost savings related to using deck.
Maybe this is a place for good old fashioned formwork.

 

[J1D]

It is a good reminder. The steel deck was considered to save the form of concrete casting (See my post on June 23 for the description of the cooling tower and structural steel.) It definitely works without deck and beams (just increase the slab thickness). This approch was predetermined by the project team. This is my first time to design a composite slab with such a thick topping and for cooling tower basin.

 

[Lutfi]

Be careful in designing any steel near ort around cooling towers. They tend to be very wet with water that contians harsh chemical. Therefore, it is highly corrosive. If you have to use steel in this application, I suggest that you use hot dip galvanized finish, larger concrete cover and higher strength concrete no less than 5000 psi. I refer you to the ACI 318 and its special requirements for highly corrosive environment.

Regards,


Lutfi

http://www.cdeco.com

 

[structuresguy]

Lutfi: When you say hot dipped steel, I assume you are referring to the structural steel, not the rebar. Is that correct? I ask becuase I have heard of HD galv. rebar, but have never heard of it being used anywhere. I know the bond strength is greatly reduced in HDG rebar vs plain rebar.

 

[Lutfi]

You are correct regarding structural steel and galvanized deck (G90).

Bermuda allows galvanized rebars only. I do not design for galvanized bars. I know some DOTs use epoxy coated bars. I heard that they are not all that successful in fighting corrosion. I do not have first hand knowledge of this.

I agree about reduced bonding due to zinc finish. However, I am sure that you can account for the bond reduction.

Regards,


Lutfi

http://www.cdeco.com

 

[UcfSE]

Any blemish, scratch or other damage to the coating will corrode at an accelerated rate to the point that brittle fracture becomes a danger. Because of this some states states will not allow the use of epoxy-coated bars.

 

[SlideRuleEra]

I have seen both galvanized (hot-dip) and epoxy-coated reinforcing steel used in bridge construction. As UcfSE states, the epoxy-coated bars are susceptible to damage. Galvanized material is too, but is much more tolerant since the steel/zinc metallurgical bond allows what amounts to cathodic protection of the damaged area.

The bond between concrete and galvanized rebar takes more time to come up to strength (compared to plain rebar). However required lap splice length for both galvanized and plain rebar is the same (See ACI-315, Paragraph 2.7.1) see this page 5 of this link

http://www.ce.berkeley.edu/Courses/CE123/Notes/315_99.PDF

Totally agree with Lutfi about the corrosive nature of cooling tower water. Suggest that you take every precaution that he has recommended - for all the steel in your project.

http://www.SlideRuleEra.net

 

[J1D]

Somebody mentioned to me that because the water in the basin is alway warm (108F to 75F) and, during winter time, there would be heavy moisture condensation at at bottom of the deck (to be used as form of concrete slab). In this case, would insulation be necessary underneath the slab?

 

[14159]

You can plug this thickness into many of the typical composite beam design programs but who knows if you'll get garbage results. If I develop a composite beam program, I'm not sure I'd be thinking about 12" thick slabs. You'll have to perform manual calculations to verify the results.

I like the idea of designing the beams as noncomposite. If this unshored or shored? If it's unshored, then your beams will be honkers for the wet concrete weight anyway so I'm not so sure you'll save that much by going composite. Not knowing your structure, this is obviously just a speculation.

Like some of the others, I'd be a bit concerned about corrosion. I've done this several times and went CIP concrete every single time, even for steel jobs.

DBD

 

[14159]

"UcfSE (Structural) 4 Jul 05 17:40
Any blemish, scratch or other damage to the coating will corrode at an accelerated rate to the point that brittle fracture becomes a danger. Because of this some states states will not allow the use of epoxy-coated bars."

The office I just left (going back to school yet again!) stopped using epoxy coated rebar in all parking garages for this very reason.

DBD