Suspended Concrete slab with no beams

[Matthew S]

I'm new to these forums and I had a quick question. I'm designing a suspended slab for a client and they want no visible beams on the exterior. two sides of the slab will connect with the house and the other two sides will be free. the two free sides will be held up with two HSS 4x4x1/4 columns. see the attached image. I designed this in RAM Elements and it just gives a required steel area to combat the forces. I can not find a design guide with this type of slab anywhere and was wondering if anyone could help. basically this will be a flat plate slab. I need help with the shear forces that will act on the columns (I also have a 16"x16" steel plate on top of the HSS to spread out the shear forces and act as a 16x16 column. Any help would be appreciated.

 

[structSU10]

As you have it its a two way concrete slab - see ACI for analysis / design procedures. you have to consider bending, shear, and punching shear at the columns. How will you attach it back to the existing building? will it rely on shear friction or have a bearing wall?

Does this have to be cast in place concrete? I would think you can do a wood deck and have the girder on the outside be the same depth as the joists, then apply a ceiling so its all one thickness.

 

[Matthew S]

@structSU10 The slab will rest on the exterior walls. I'm normally don't work with suspended concrete slabs and I'm more of a steel guy. The client wants a concrete slab instead of a traditional wood deck. I just don't know how to calculate the punching shear on an exterior column in a flat plate slab.

 

[structSU10]

Do you have access to ACI 318-14? The two way slab section goes over the calculation. It also has a couple hand methods with column strips and middle strips to design the slab.

 

[Matthew S]

I have the ACI 318-14 and the methods in the code are for 3 continuous spans each way. I only have 2 spans on the north side and 1 span on the west side. The code is confusing me and I'm having a hard time with this. Maybe i'm missing something obvious.

 

[r13]

I might do one way design as shown sketch below. The main direction runs N-S, with an embed beam at north edge, and supported on the wall on the south. In the E-W direction beyond the embed beam, only T/S steel are required.

 

[Settingsun]

I don't know RAM but it sounds as though you can get the reactions at the posts from RAM. Is that what you're asking, ie what are the forces rather than what's the capacity?

 

[Matthew S]

@retired13 I normally do a beam and treat it as a one way slab. The contractor would like a flat plate to make it easier to construct. So are you saying design a beam into the slab? Then just treat it as a one way slab?

 

[r13]

Yes. You need to stiffen the north edge to accommodate one way slab action in N-S. You might have to adjust the width of the embed beam, or thicken the slab, if the load is excessive, that resulting in congestion of rebars within the narrow band. For the middle strip, I'll assume pinned at the north end beam, so not to cause excessive torsion on the beam.

 

[JLNJ]

It looks to me like will probably work. No need to stiffen the north band. I don't think there will be any torsion as there is nothing to resist it.

I'm not sure I agree that your 16" square plate will be effective. If it's more flexible than the concrete, all the punching force will go through the concrete at d/2 from the face of the HSS. Maybe you embed the plate in the slab for a sleek look.

 

[Matthew S]

@retired13 Ok i designed a beam that is 12 inches wide and 7 inches deep. I put 3 #4 bars evenly spaced and the moment capacity is roughly 16 kip-ft and the required moment is 10.6 kip-ft. I think it will work and now Ill just select some reinforcing from the CRSI manual for T/S. Am I thinking about this right? I've never designed a beam inside a slab so this is all new to me. My boss is going to go over all my calcs but i wanted to make sure i had something presentable before i put it on his desk.

 

[r13]

7" slab is thinner than I preferred for double layer reinforcement (min 8"). But, yes. I utilize embed beam in many applications. Since the beam is one way element, make sure shear is adequate. You might need shear reinforcement though. You shall check ACI318 for minimum reinforcement for slab, and apply to E-W direction (T&B). You didn't mention the design of column and middle strips, how the resulting reinforcement looks?

Also, what type of loads on the slab (don't forget concrete self weight)? Did you apply load factor accordingly? The reinforcing steel for the edge beam seems little too light.
You should discuss with your boss about this design approach for his acceptance before dive into it.

 

[Matthew S]

I was using 7 inches from the code. I'll bump it up to 8 inches for the double layer reinforcement. I'm checking the shear now and for the middle and column strips I'm using #4 bars @ 12" O.C. each way and that satisfies the minimum reinforcement.

@JLNJ I figured the plate would act like a column and i could base my d/2 off that dimension. I was going to put the plate in the slab but there is a brick exterior going around the column and it will cover the plate and hide it.

 

[Matthew S]

retired13 I included the slab self weight and a 40 psf live load with the correct load combo. and for the edge beam i meant to say 3 #5 bars top and bottom.

 

[r13]

Note that the column and middle strip shall be designed as one way beam, they may or may not require reinforcement more than the minimum, but you have calculate the bending moment, and deliver the loads/shear to the end supports. For column strip with column and wall support at ends, you can assume fixed end beam. For the column strip with embed beam and wall support, assume fixed at wall, and pinned at the embed beam. Do the same for the middle strip.

Did you follow ACI in assign strip width, and proportion the slab load accordingly? Just a check and reminder.

 

[Matthew S]

I designed the strips as a one way beam exactly how you just described and that's how i came up with the #4 bars each way. I followed the ACI design for the the strip widths. Thank you so much for your help and the quick response!

 

[r13]

Glad to hear you got handle of it. Good luck.