Requirements of Drop panel in flat slab 1

[bdlc2k]

ACI states that a drop panel dimension should be L/6 in both directions. Is this required if the drop panel is being considered for punching only, not for flexural design (say when a two way slab has shearwalls)? Has anyone ever taken this approach?

 

[hokie66]

If a drop panel is not large enough in plan to meet the prescriptive requirements, I see no reason it cannot just be considered as a column capital to resist punching shear. Not sure what the shearwalls would have to do with it.

 

[dik]

It affects the stiffness; I usually proportion the drop panel so that the depth of it is nearly the same as the slab thickness.

Dik

 

[KootK]

Is this required if the drop panel is being considered for punching only, not for flexural design (say when a two way slab has shearwalls)

Nope.

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.

 

[Celt83]

I'll agree with everyone else in that it does not need to meet the L/6 if only used for punching, but it then needs to meet the criteria for a capital.

You will also need to make sure whatever software your using to design the slab is not using the additional depth at the capital for its cross-section design.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Ingenuity]

...design the slab is not using the additional depth at the capital for its cross-section design.

Except (according to this PTI FAQ author) post-tensioned slabs, where caps, evidently, can be used as additional depth for flexure...AND across the whole slab panel width too. Crazy in my opinion!

Link

 

[hokie66]

Yes, PTI is crazy, in lots of respects.

 

[Celt83]

I can see the logic including it in the pt design as it will influence the cross section centroid and in turn the stresses created by the pt.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Settingsun]

the Q&A also appears to limit the consideration of the drop cap to the location of the cap ('in cross sections including the drop cap'), not across the full panel width.

 

[rapt]

Hokie66

You can't say that. That's my line!

Unfortunately engineers read this from a "respected" institution and follow their advice. It gets even worse when they then use the effective depth of the reinforcement and tendons from the bottom of the drop panel for the "middle strip" reinforcement and tendons (because there are apparently no "middle strips" in PT flat slabs)! Rather than from the bottom of the slab.

And then they do the same with the distribution tendons in a slab parallel to a beam as they say that the beam effective widths do not apply either.

It normally only means they are under-designing by about 20% so why worry.

Steveh49
No they are not saying that. They are saying that sections including the drop cap are essentially treated as T sections with the drop cap as the web and the full slab width as the flange. That is now they do it in their examples.

Engineering logic, statics, equilibrium, load paths and common sense appear to not matter!

 

[Ingenuity]

That is how they do it in their examples.

Just to reinforce this point, two recent textbooks on the subject of PT design, one by Aalami and a second by Bondy & Allred have numeric examples using full slab panel width incorporating drop cap properties.

 

[dik]

rapt: I've seen regular conc reinf slabs with drop panels crack in the middle strip and you could drop a business card through the slab... only explanation is that the slab was rotating about the bottom of the drop panel and this affected the middle strip.

Dik

 

[rapt]

dik,

Your comment is a little too general for me to understand what you mean.

Cracked where?

And how were they analysed?

And what reinforcing pattern was used?

 

[hokie66]

Anytime I hear that a crack has gone right through a slab, I think of direct tension cracking rather than flexural cracking. So cracking due to restraint rather than gravity loading.

 

[rapt]

Hokie,

Tend to agree about restraint for this, now that I understand what Dik meant (must be old age).

What happened to the top and bottom reinforcement in the middle strip that should have limited the crack width to something reasonable?

Or did they put all of the top reinforcement in the column strip over the column and drop panel and provide no top reinforcement in the Middle Strip. Have seen this before with very bad cracking! But the middle strip bottom reinforcement still should hold the crack together at the bottom as it would be at a fairly low tensile strain being so far from the extreme tension face!

That result is not consistent with a properly designed and detailed RC flat slab.

 

[hokie66]

I'm not sure what dik meant about the location of the cracks. C-M or M-M? Perhaps he will explain further.

 

[dik]

Rapt:

Cracked where?

In the middle strip along the column centrelines.

And how were they analysed?

I don't know.

And what reinforcing pattern was used?

Middle strip top reinforcing (which was sort of visible in places appeared light.

Dik

 

[dik]

Hokie: They likely weren't shrinkage cracks... occurred in a few places. From the appearance, it appeared that the slab had hinged/rotated ahout the bottom of the drop panel... The slab was 8" and the drops were 9" and the spans (not measured) appeared to be slightly less than 30', maybe 27 or 28. I've no idea of what the mix design was or strength.

It was one of those 'unusual things' you come across... and my business card could slip through to the floor below.

Dik

 

[KootK]

and provide no top reinforcement in the Middle Strip. Have seen this before with very bad cracking!

Interesting. Somewhere, I have an old ACI doc that advocates butting all of the top reinforcement in the column strip only. And I know of a few marquee firs that do this routinely as a means opening up travel lanes between bays and improving constructabilty. I wasn't aware that it had been causing ant problems. I take it that it's your opinion that middle strip top reinforcing should remain in our designs?


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]

My understanding was that the L/6 requirement was in the mild slab section of the code so that one would comply with the assumption underpinning the use of the associated, prescriptive deflection control provisions for two way slabs with drop panels. As far as I know, there are no codified, prescriptive deflection provisions for prestressed slabs. Gotta run the numbers. Therefore, no need for the L/6 as a codified thing. Which is not to say that an efficient design informed by sound engineering judgement won't still steer you back in that direction anyhow.

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.