Slab edge release and deflections 3

[Gus14]

I have a 8 By 11 meters slab supported on beams as shown in the attached file. No meaningful continuity can be made use of along the slab edges.

Live load is 2 Kn/m2 and dead loads are 2.5 Kn/m2 with some 5kn/m lightweight partitions loads applied on the slab in the model. the slab thickness is 340 mm. Slab reinforcement for cracked analysis were 8Y16 per meter bottom reinforcement and 8Y14 per meter top reinforcement.

Your file's link is:

https://files.engineering.com/getfile.aspx?folder=6e8c5afa-842c-4990-a1b6-3051950426aa&file=1.pdf

I have made two models in safe, with releasing the slab edges long term deflection were 30 mm. Without releasing edges, long-term deflection are 20 mm.


My question is,
1. Is using a top layer reinforcement with a development hook be enough ensure that the slab edges are fixed ?

Side note, I also used PROKON slab design module and deflections were 16 mm, even with releasing the slab edges. So I will suggest not relying on PROKON for slabs design.

 

[hetgen]

What do you mean by 'slab edge release'? Do you mean ignoring the torsional rigidity of the perimeter beams?

Where in the slab do you require 8Y14 per meter top steel?

Prokon used a very rough method per ACI to calculate flat slab long-term deflection, so the result will not be the same as CSI SAFE but should be close.

 

[Tomfh]

I don’t understand either. Release what exactly?

 

[BAretired]

With a 340 mm thick concrete floor, your dead load is a whole lot more than 2.5 kPa. And 5 kN/m doesn't seem right for lightweight partitions. Better review your loads, but the long-term deflections seem a little too high to me.

You cannot develop edge fixity with reinforcement unless your edge beams can provide sufficient torsional resistance.

BA

 

[hokie66]

Maybe he means 2.5 kPa superimposed dead load. These computer modelling programs calculate the self weight.

 

[Just Some Nerd]

I don’t understand either. Release what exactly?

I think they mean changing from a rotationally fixed to pinned support at the slab edge, in software you'd have the ability to assign an "Edge Release" when selecting the edge of a slab/shell element and turn off bending moments about that slab edge.

I'd definitely not consider simply providing a hook in the top reinforcement as enough to justify not releasing the slab edge, the connection and the beam itself would need to be capable of resisting the moment. Simply developing the bars doesn't do anything for that.

Maybe he means 2.5 kPa superimposed dead load.

Agreed, I would be surprised if they didn't mean that

 

[Tomfh]

I think they mean changing from a rotationally fixed to pinned support at the slab edge

Yes, but when the slab edges are free, what are they are releasing? The column connections?

 

[Gus14]

Thank you everyone, for replying, I have been overthinking this slab for a while now as I have never relied on beams stiffness before. I think "Just some nerd" answered most of the questions.

Do you mean ignoring the torsional rigidity of the perimeter beams?

Yes, ignoring the beam and columns' torsional stiffness.

Where in the slab do you require 8Y14 per meter top steel?

The reinforcement is along the entire slab length in both directions. I could make it more economical, but whatever.

Prokon used a very rough method per ACI to calculate flat slab long-term deflection, so the result will not be the same as CSI SAFE but should be close.

I disagree, I have made a couple of comparisons between the two, and Prokon has always shown significantly less deflection. I think CSI SAFE is more reasonable.

With a 340 mm thick concrete floor, your dead load is a whole lot more than 2.5 kPa. And 5 kN/m doesn't seem right for lightweight partitions.

Yes, my mistake, I should have mentioned that the superimposed dead is 2.5 kn/m2.

As for the partitions, this is as light as they come around here. ( this is because in manufacturing they don't use coarse aggregate, and therefore can't form voids inside the blocks, so they come out as a solid block of cement and sand )

You cannot develop edge fixity with reinforcement unless your edge beams can provide sufficient torsional resistance.

the connection and the beam itself would need to be capable of resisting the moment.

I did have that in mind. Although the software doesn't require a significant increase in torsional transverse reinforcement in beams because negative moments from the slab seem high only in regions near the columns although I reduced the columns stiffness to 0.7. So will increase the shear reinforcement near the columns as required and check that the sections can take the required torsional resistance.

I think if the overall deflection will be in between ( 30 - 20), a 25 mm will be close to L/360 and hopefully the floor tiles don't break.