Drop Panel vs Shear Cap vs Column Capital 1

[Struct-007]

What is the difference between drop panel and shear cap and column capital in Two-way slab system?

 

[r13]

Drop panel is a thickened slab around column to gain shear capacity, and often to avoid the need for shear reinforcement. Column capital is a massive inverted truncate cone for circular columns, or truncate pyramid for rectangular, and square columns. I am not 100% certain, but seems column capital is used for structures with gravity load is the primary concern, and shear demand is moderate. I've never heard of shear cap. Do you mean pile cap?

 

[Struct-007]

Thanks r13!

Shear Cap is defined in ACI 318-14 (Page 40) as a "projection below the slab used to increase the slab shear strength".

I am not sure what implications each of these elements have on design. For example, when calculating the clear span (ln), do we measure the distance from edge of (shear cap / column capital / drop panel) to the adjacent one? And is the reinforcement of shear cap and column capital the same? etc.. I need to do more reading in ACI about the differences between these elements.

 

[hokie66]

Struct007

They are to deal with punching shear, not flexure. The first thing most of us do in designing a flat slab or flat plate is to consider punching shear.

 

[Struct-007]

Thanks hokie66.. but I believe column capital will affect the clear span between columns (ln) and thus will reduce the moment demand. However, I am not sure if (ln) will be also reduced when using drop panels or shear caps.. It seems each element has its own implications.

 

[dik]

...also significant flexure. An important feature of drop panels is that it attracts -ve moment, reducing the +ve moment for most of the slab and has a greater 'd' to accommodate the increased -ve moment. It also significantly reduces deflections so a thinner slab can be used. This increases the -ve moment a bit and further reduces deflections. Although not required by code, I typically use a drop panel of a similar depth as the slab, with a depth equal to the slab form thickness + a dimensioned lumber depth... for eg. 3/4" formply + 7-1/4" (2x8)... for an 8" depth of drop panel.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

I am not sure if (ln) will be also reduced when using drop panels or shear caps..

l[sub]n[/sub] will not be changed by the drop panel, or column capital, but the critical shear will have to be checked on two locations - d/2 from column faces and from perimeter of drop panel and capital.

 

[JoshPlumSE]

From ACI 318-2014:

Section 2.3: Terminology:
column capital - enlargement of the top of a concrete column located directly below the slab or drop panel that is cast monolithically with the column.

drop panel - projection below the slab used to reduce the amount of negative reinforcement over a column or the minimum required slab thickness, and to increase the slab shear strength.

shear cap - projection below the slab used to increase the slab shear strength.​

This term (as I understand it) was introduced in the 2008 edition of the ACI 318 code. The idea is that a "shear cap" is really closer to a column capital in that its sole purpose is punching shear and that it's dimensions are much smaller, closer to what you'd see for a column capital... Just cast with the slab instead of the column.

I don't really like the distinction in the terms very much.

Note: I first encountered this when I worked for RISA and they developed the RISAFloor - Elevated Slab Design module. I was not allowed to be involved in the project very much (political reasons), but they chose to use the term "shear cap" everywhere because that program did NOT take into account any "drop panel" specific requirements related to dimensions or reduction negative reinforcement. If I remember how it was implemented, however, I believe there could be some inconsistencies in the program related to this. Anyone who's using that feature of the program should probably take a really, really close look at what it's actually doing. I'm not sure that I have confidence that it was implemented correct.

 

[dik]

I've not use column caps or shear caps... but, I've seen them in place. They are not common in these environs, nothing recent.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[wcfrobert]

All of them have the primary purpose of resisting punching shear.
[ul]
[li]Drop panels have the added benefit of additional depth for your negative flexural steel.[/li]
[li]Column capital have the added benefit of reducing the clear span of slab & unbraced length of column.[/li]
[li]I've never heard of shear caps until today. Seems like redundant terminology to me.[/li]
[/ul]

A good alternative for punching shear is stud-rails. They are easily to install and cheaper than the other options. Availability might be an issue depending on where you live.

Where I am from, contractors shy away from drop panel and definitely column capitals (hard to form, labor is too expensive).

 

[r13]

it seems to me, drop panel and column capital are often encountered in underground parking structures, and concrete water tanks. They are rarely used these days due to high labor cost as mentioned above.

 

[dik]

haven't used them lately (haven't had a project where they could be used), but have used them for a lot of high quality office buildings (< 20 stories) where the space under the slab, but above the drops has been effectively used for mechanical and electrical equipment... had one project where they mocked the M&E to determine the minimum floor to floor height.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Celt83]

Drop Panels are extremely common in my locale for mild slabs
Shear Caps, again in my locale, are less common and typically only used in flat plate Post-Tensioned Slabs when Stud-Rails don't work.

Column capitals - we rarely use

Stud-Rails are gaining popularity but typically are much more expensive than placing a drop panel, so we limit their use to our PT jobs to maintain floor to ceiling clear heights.

My Personal Open Source Structural Applications:

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

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[dik]

I've never used stud-rails...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Celt83]

I've not been keen on using them and this article recently linked to in another post doesn't leave me with any more confidence regarding their performance: Link

My Personal Open Source Structural Applications:

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

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

I'm using drop panels in a project right now. Didn't make me a very popular person on the design team, but everyone was screaming about the amount of steel and concrete. Drop panels were the only way to reduce both in any meaningful way.

 

[dik]

and you can end up with a really good floor, to boot.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[rapt]

Agree with the definitions above.

Shear Caps are an American invention and are nominally 4' (1.2m) square and 2 - 4" deep (50 - 100mm) and were purely meant to increase the punching shear capacity by increasing both the effective shear depth and the perimeter which is related to depth.

But they are not wide enough to provide a realistic increase in flexural capacity at the column relative to the overall slab panel width so should be ignored for this. The width of the drop cap forming the compression face is not wide enough compared to the overall column strip width and all of the column strip reinforcement cannot be assumed to have the effective depth of the drop cap. Unfortunately some of the later fashionable texts on PT slabs in USA by Aalami and Bondy/Aldred are suggesting they can be considered to act like drop panels for flexure.

Apparently builders in USA were willing to price drop cap formwork as flat plate formwork, whiile drop panel formwork was much more expensive.

Column capitals have gone out of fashion as too hard to form. They simply increase the shear perimeter and move the flexural moment critical section further into the span based on the location of the support face.

Drop panels as discussed above are the most efficient form of flat slab, but again some USA textbooks on PT slab design with drop panels are very misleading allowing the effective depth of the drop panel to apply to all tendons and reinforcement over the full panel width (as they are now suggesting with Drop Caps above).

 

[JoshPlumSE]

Column capitals have gone out of fashion as too hard to form. They simply increase the shear perimeter and move the flexural moment critical section further into the span based on the location of the support face.

My impression is that the majority of the time column capitals are used today are for architectural reasons. If the columns are exposed then they "look better" with some form of a capital on top (and sometimes on the bottom).

 

[rapt]

Josh,

Most times I was involved with them, they were requested by the structural engineer to improve punching shear capacity.

To form a sloped column capital maintaining the column shape is not that easy for a form worker, especially for circular columns but even for rectangular, so often they were a square shape of at the top of the column (constant width over the depth of the capital), so architects actually hated them.