Concrete Slab encased with WT's Design

[Superstar123]

Hello all,

I am a new structural E.I.T and I am trying to design a slab that is encased with WT sections. How do I approach this design? How do I determine the required spacing between the WT sections?

Any help would be greatly appreciated.

Thanks

 

[jayrod12]

Surely this idea came from someone somewhere in your office, perhaps those are the types of questions to ask them? As a new EIT, you aren't expected to know how to do anything, much less everything.

If the concept were to make a sort of composite slab type thing using the WT as the tension steel, and the concrete provide the compression flange, they may be spaced such that they don't share the concrete flange with the adjacent piece. Or if the WT are doing all of the bending strength work, they could be spaced at whatever distance the concrete can span between the WTs.

 

[JoelTXCive]

Do you have sketch of your cross section?

I think you mean the WT sections are going to be embedded in the concrete to form a composite section of some sort.

Also, is this going to be a 1-way or 2-way element. (I think 1-way, but you need to be sure)

 

[SteelPE]

Sounds like a Girder-Slab system to me.

https://www.girder-slab.com/

If that is the case, you don't design it, they do.

 

[Superstar123]

Hi all,

This type of project is new to our office and I was assigned to it before we realized how complicated it was. In other words, no-one in my office has designed something like this before. See the attached sketch for some clarity.

@JoelTXCive, it is a 1-way slab!

 

[BridgeSmith]

There's a reason why no one has done what being proposed in your sketch - it's a more difficult, more expensive, and less predictable way to do a reinforced concrete slab. Why not just use reinforcing bars?

Design-wise it would be basically the same as a conventionally reinforced concrete slab, with the exception of figuring out how much, and to what extent, the web of the tee section is to be included as tension reinforcement. As you move towards the top of the slab, it will take much more tension strain at the bottom of the section (and therefore increasingly large deflections) in order to produce substantial strain and stress in the upper portion of the web.

There's also the not small matter of ensuring the WT section remains locked into the concrete and doesn't 'slip'. Reinforcing bars have deformations to provide a positive mechanical connection to the concrete. Unless you add something, such as shear studs, channels, lugs, etc. to the WT, you'll be relying on the bond strength between the steel and concrete to ensure composite action (thus the "less predictable" part I mentioned earlier).

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JoelTXCive]

I have not done one of those. The side view looks like a continuous beam, so you are going to get negative moments at the supports.

I only bring it up because the majority of the WT steel is at the bottom and there is little to no steel in the upper tension zone. In my mind, that type section is set up for a simply supported span; and not continuous.

I would figure out your max positive and negative moments; and then look and see if that style slab is even appropriate.

 

[jayrod12]

Quickly on Girder-Slab. Where I practice, they make it seem like they design it, however they don't. They'll provide design assist, however the EOR has to detail and show all sizes directly on their drawings. It is not equivalent to delta beam by Peikko in that regard. They tried to pull that bait and switch on me on one project. Construction Manager: "If we change from delta beam to girder slab we can save $200k" Me: "Does those savings take into account the design of the connections, the design of the beams themselves, sealed shop drawings, and can they accommodate the torsion that will be applied on beams A, B & C. Please provide written confirmation that they can do those things and still save the project money." Fun fact, I'm still waiting for that written confirmation and the building is fully standing using delta beams by Peikko.

Now, back to the OPs question, why would you want to build it this way? Who decided that this was the best construction method for this floor system? As said by BridgeSmith, it appears to be more difficult and more expensive than a standard concrete slab.

 

[Superstar123]

We cannot use reinforcing rebar because the shear within the slab is MUCH lower than the shear produced by our load. Vc = 77 kN, Vf = 374 kN.
I have been instructed to create a combined section where I will be taking the shear capacity of the concrete, subtracting the area of the WTS, shear capacity of the Wt's and shear capacity of the Plate and adding them together. This gave me a reasonable shear capacity that passed my check.

Now I am trying to check the bending. Which brings me to my next Question.

I want to check the bending for just the steel members (WT and Plate) to avoid to complication of checking the encased concrete slab. For this I will need to determine the effective width of my WT-Plate section. Can anyone point me in the right direction? I know how to do this for concrete but not steel. Is it the same approach?

 

[Settingsun]

I'd say this is a form of 'filler beam' deck - that's what I've seen it called in bridges. A quick web search turned up some guidelines. They all appear to assume H beams so you'd have to adapt to your WT case. Or switch to H beams.

Any reason the WTs aren't sitting on the plate?

 

[Superstar123]

Thanks steveh49, I will do some research! They are sitting on the plate, we will be stitch welding them. My original sketch was just a concept design, sorry for the confusion.

 

[WinelandV]

Is this a floor for a building? A bridge deck?

Either way, I'd size the concrete portion to span a SHORT distance between the steel members, and use WF beams (instead of WT shapes) underneath the concrete. Design as non-composite and instead of trying to figure out if the top flange is braced, see what size beam you need if it's fully unbraced.

I'm not saying what you're showing can't be done, but there are more straightforward way to accomplish what you want.

 

[BridgeSmith]

Have you considered dispensing with the steel beams, plate, etc. completely, and using a 300mm +/- conventionally reinforced concrete slab?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Superstar123]

This is the roof of a dog house-like structure It is intended to protect objects from falling rocks and be moved by a loader with forks. We have decided to create a grillage pattern out of W6's and fill the pockets with concrete to keep the weight.

Thank you everyone for your help and ideas! Our first approach was much too complicated.