One way shear in slab 2

[aenger]

Hi guys.

I am doing one-way slabs and hope you can clean up my doubts about the one way shear in slab.
As they are one-way slabs I treat them as wide beams.
Normally it works fine but this time after carrying out the calculations the slabs fail in shear because of there are some huge point loads near the support.
So my question is how do you guys deal with it? I mean, Normally we don't arrange fitments in slab, so it seems that increasing the thickness of the slab is the only way now if I don't change the structure system?

Thanks

 

[dik]

Can you thicken the slab locally to reduce the shear stress in the concrete? You should treat it as a slab and not a beam...

Dik

 

[slickdeals]

You should consider it as a one-way slab with an effective width. Some guidance on calculating this effective width is given in the attachment.

You will likely have to thicken the slab to make it work without shear reinforcement. However, if the forces are much higher, you might have to treat this thickened slab as a beam and provide shear reinforcement as necessary.

 

[dik]

slick... can you cite the source? Is it from a British standard?

thanks, Dik

 

[Ingenuity]

slick... can you cite the source?

BS8110...and AS3600 has a similar effective width calculation.

 

[dik]

Ingenuity:

thanks... Dik

 

[KootK]

- it is my understanding that the British Standard stuff is appropriate for flexure but not for shear. The commentary for that section would seem to suggest that as would a rational examination of certain loading scenarios. It always needs to be remembered that, for situations like this, there's no such thing as an "effective width" for load distribution. It's arcane but more accurate to say that there is an "effective width for any particular response parameter". What is the effective width for flexure will usually not match the effective width for shear etc.

- to my knowledge, the attached PPT presentation is the state of the art on this. I have the corresponding ACI paper from 2013. It's great but there really isn't anything that you'd need from the paper that isn't also in the PPT.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Can't get the attachment to take on the slide show PDF. Try this: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IDS]

the attached PPT presentation

I see no ppt.

I already had the linked ACI paper, but thanks for the reminder; well worth a re-read.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[aenger]

Thanks for your replies.
By wide beams I meant dividing one-way slab into strips with effective widths.
The client doesn't want to increase the slab thickness.
What I don't quite understand is can we use this effective width to calculate the shear capacity for slabs subjecting to point loads?
And what does one-way shear reinforcement in slabs look like if the shear reinforcement is required? I have seen the shear studs being used in slabs for punching shear but never seen beam-type shear reinforcement in slabs.

 

[dik]

It may not be a matter of wanting...

Dik

 

[KootK]

What is the target thickness for the slab? Stud rails are one option for thin slabs. I'll put shear rebar in slabs at least 16" deep.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

What I don't quite understand is can we use this effective width to calculate the shear capacity for slabs subjecting to point loads?

I believe that we've already answered this question for you above. A particular effective width may work but an arbitrary effective width probably won't. You've mentioned that you're treating the slab as a bunch of one way beams but you haven't told us anything about how you've subdivided your slab into those beams. It matters.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WillisV]

KootK can you link the PPT mentioned?

 

[KootK]

KootK can you link the PPT mentioned?

I did that above WillisV.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MrHershey]

The main issue with the one-way effective width is it assumes that one-way slabs with concentrated loads behave like beams and don't get strength contributions in two directions. That's certainly a conservative assumption, but can be overly so as it ignores all strength contributions in the other direction. Data in the paper KootK referenced backs that up. Using a 45-degree effective width with ACI 318 (red line, which is mine) results in conservative results, but often extremely conservative. Would note the 45 degree is pretty close to what British Standard's width would be near the support.

The paper indicates that Regan's method (blue line, also mine) actually most closely matches test results (normalized as green line). It's actually pretty astonishing how closely it mirrors test results. This method was developed in London in the early '80s and incorporates both one-way and two-way action. Or more correctly, it recognizes that concentrated loads are inherently a two-way problem but doesn't treat all sides equal like one would in a standard two-way problem. I've used it many times for point loads close to supports. It's a longer process than taking just an effective width, but doesn't take too long. If you throw it in a quick spreadsheet so values are easy to manipulate, it's not really any longer of a process than ACI.

 

[dik]

Chased down the publication:

in English and Dutch (I believe).

Dik

 

[dik]

and the pdf of the powerpoint display:


Dik