Transfer Slab - Combined One Way and Punching Shear Reinforcement 3

[ooox]

I'm designing a transfer slab with no drop panels and I'm finding I need both one way shear and punching shear reinforcement which is something I never encountered before. I normally provide drop panels to avoid needing punching shear reinforcement altogether and so I just have one way/beam shear reinforcement in the column strips in a few isolated areas. We have a number of challenges on this project with head clearances and long below slab drainage runs which has meant drop panels were discounted early in the design process. Increasing the slab thickness is not really an option I'd like to consider as the slab is performing adequately in flexure and long-term deflections after the attachment of partitions are well with the L/480 limits. Increasing the concrete strength from 6 to 8ksi will only increase the shear strength by 15% which doesn't help either.

My thoughts are:

1. Increase column sizes or provide a column capital to avoid need punching shear reinforcement and then design for one way shear reinforcement only. This will result in some large column sizes which could impact parking spaces. I'm also not sure if column capitals are commonly done anymore (I'm in the N.E. U.S.).
2. Design/detail the slab for punching shear and account for this reinforcement when calculating the Vs for one way shear reinforcement.

Just gauging what others have done in this situation.

 

[Celt83]

How thick is your slab currently?

Are you sure the one-way shear reinforcement is actually required, some full slab modelling software doesn't check V at D away from supports but instead checks it at the face which can yield shear bars, another thing that may occur is odd inflection points which result in a very small "d" depth which with some investigation can sometimes be worked around.

1. we use capitals quite a bit, also in the NE US
2. no, can't double count the reinforcement.

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

Open Source Structural GitHub Group:

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

 

[ooox]

Thanks Celt,

My slab is 16" thick. I've ran some hand calculations to check the one way shear and it is required. The software is showing the shear reinforcement where you would expect it, and is consistent with my calculations.

You can't double count the shear reinforcement - is this stated in the ACI somewhere? I guess I can see the logic - v[sub]s[/sub] for punching shear is calculated slightly differently to V[sub]s[/sub] for one way shear, and is averaged over the area of the critical section.

 

[Celt83]

16" is a pretty decent thickness, are you taking advantage of the advanced shear formulas or just looking at the 2*sqrt(F'c)*b*d formula.
Which software package are you using?

My Personal Open Source Structural Applications:

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

Open Source Structural GitHub Group:

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

 

[ooox]

I'm using RAM Concept which can get a little funky sometimes when it comes to shear reinforcement. At the moment I'm using the simplified shear formula but I'm intend to work up a spreadsheet to use the detailed method so I can justify the larger shear capacity. I'm also not considering V[sub]u[/sub] > 0.5*phi*V[sub]c[/sub]to see if I need to provide min shear reinf. as my 'beam type' is integral with the slab.

 

[Celt83]

Gotcha

For Ram Concept:
1. Are you considering torsion in your design strip (you should be), some of the methods like wood-armor can produce segments with a reversal of moment to be in equilibrium and the section design gets impacted by the below?
2. Concept does design in stages so occasionally you can wind up in a design step where program designed steel doesn't exist yet so that can mess with the shear check. Try running the model then setting all the program designed bars to be user bars and then run again this forces their existence thru all the design stages which can sometimes eliminate the shear reinforcement.

My Personal Open Source Structural Applications:

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

Open Source Structural GitHub Group:

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

 

[ooox]

Yes, I'm using the Wood-Armer method for twisting moments in the design strips, that's my default for all RAM models.

I will try setting the program designed rebar to user deinfed bars. I've never done that before, but can see how this could this could help.

Thanks Celt, you've been very helpful.

 

[r13]

Seems you are trying to come up extra strength to support own weight. Do you really need 16" slab? A rare event. Also, I don't understand the one way action, when you designing a two way salb.

 

[KootK]

In my opinion, one and two way shear reinforcing demand are not additive where they would overlap.

 

[SethGuthrie]

Here are some Punching Shear tips to refer to:

https://communities.bentley.com/pro...i/12218/ram-concept-punching-shear-checks-faq

In Ram Concept we do not mix and match the one-way shear reinforcement and the punching shear checks. We only use stud rails for the latter. In reality a shear cage could serve both purposes, but that's beyond the scope of what Ram Concept will design.

 

[Celt83]

Funny enough I'm working on a transfer slab now and just ran into the same issue here are some screen clip to illustrate what I mean.

Program designed top steel:

Program required shear reinf.:

Audit of the cross section, note Wood-Armer causes moment reversal in some design envelopes and as detailed the top steel does not get into this section so "d" is taken from the bottom mat to the compression face, 1.8+/-": Edit: Actual if you read the audit you'll note it says effective depth is 0 so switches to using column effective depth.

Making the top bar a user bar and extending the length, need to document this in the contract doc since it won't follow standard detailing rules now:

The Shear bars go away:

and the cross section audit still have the moment reversal but now the top bar is long enough to be in the design section and d is now 10.13":

My Personal Open Source Structural Applications:

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

Open Source Structural GitHub Group:

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

 

[SethGuthrie]

I sometimes wish there was a manual override for "d" myself. Ram Concept always calculates it based on the designed rebar and strain. More on the topic of one-way shear reinforcement here:

https://communities.bentley.com/Pro...Wiki/ram-concept-shear-reinforcement-faq.aspx

 

[KootK]

A software vendor has little choice but to handle the issue below with kid gloves on. Luckily, I'm not similarly constrained.

Folks who ignore a failing one way shear check need to revisit mechanics of materials 101. Although we often treat shear capacity and bending capacity separately, they are in fact very much coupled. If you don't have your one way shear capacity, you also don't have your designed bending capacity. And all the punching shear capacity in the world doesn't fix that. The only reason to ever not check one way shear is because you're highly confident that it won't govern, not because you think that it doesn't matter.

 

[r13]

When engineers design flat slabs by hand, they often ignore the one-way shear checks.

Not quite true. After obtain analysis results, you shall always exam the stress diagrams. A quick glance on stress contours will red flag locations with excessive shear force compared to the fixed capacity. Since shear reinforcement in the span is highly undesirable, you shall consider taking adjustment steps to eliminate the problem. If unavoidable, you shall look at the shear diagram for the entire span/strip to arrange the shear reinforcement, and design the moment transfer reinforcement separately.