Design strip modification at openings in column strip

[RFreund]

This is for a large opening in the middle of a column strip in a two-way slab. Below I show two different longitudinal design strip layouts. The first just starts and stops the longitudinal design strip on each side of the opening. The second adds design strips on each side.

My questions are:
1.) Are both permissible?
2.) This is a detailing question that is somewhat specific to ACI and RAM Concept - If you are familiar with RAM Concept, would the end of the span be considered cantilever or support? Said differently, do you think it is required for top bars that are in the design strip of the opening to extend all the way to the opening or can they simply extend as far as needed for strength and or typical span detailing requirements?

My thoughts:
The difference between the two designs is that you are most likely going to get more top bars in the 2nd option ("added strip" method) at the narrow strip that algins with the opening. The first method (single strip) seems to average out the demand moment over a wider strip. Using cantilever option at the opening just tells the program to extend the top bars to the edge of the opening.

If this were a new design, I would probably use added strips on each side of the opening and denote the end as cantilever (this seems most conservative). However, I'm really trying to sharpen my pencil to evaluate an existing condition. It seems permissible to use the first option (single design strip). I can't think of any code provisions that this would be violating. Meaning that the column strip width is still being used to determine the amount of reinforcement. Not saying this for sure means it's ok, but basically it comes down to what width you can use to "average out" your demand moment and capacity. The argument to denote the end of the span as a support would be that the latitudinal strip supports the slab. Meaning that the end of the span isn't truly a free cantilever.

Option-1 Single Strip

Option-2 Added Design Strips

Latitudinal Strip For Reference

 

[Celt83]

Under rule sets, strength design open up the Mx plot in this plot adjust the contour range to be from -10 to 10 with a step of 100, this will end up show you the moment inflection contours. I like to try and avoid having strips cross the inflection contours where possible so that the integrated moments have limited sign change. Similarly you can look at the My direction the same way as well.

This can help inform the strip setup around the opening.

Note also that all strips do is tell concept to create a panel cut at a specific location and integrate the FEM results to give an equivalent set of cross-section design forces. The only real impact to the analysis is the strips create well defined boundaries for the FEM mesh to be refined within.

Edit: Should mention that the strips do impact the load history deflections.

 

[Celt83]

you could also do this, strips don't have to run column to column:

Just keep in mind that the way the cost weighting works for the bar detailing that you can sometimes get odd numbers when the reinforcement detailing occurs between the two areas. I'll usually bump the cost weight for length up to 10 as a quick double check to see if any continuous bar sets can be broken up to be individual detailed bars.

 

[RFreund]

Celt - All good comments, thank you.

In my case I'm trying to understand the capacity of an existing slab, so I'm trying to sharpen my pencil as much as possible. Making the design strip wider obvious helps this, so I'm trying to better understand what the "upper limit" of that should be. Can I use the full columns strip (or should I)? I will investigate the moment contours as you suggested.
Thanks again!

 

[KootK]

Judgement heavy territory for sure.

In the EW direction, you're in the middle strip so probably not a big deal, as you know.

In the NS direction, you're in the column strips so a bigger deal, again as you know. For this case:

1) Maybe don't sweat local deflection or crack control.

2) Do sweat whether or not the moment redistribution required to use an aggressively wide strip would strain your rebar so far past yield that it might fracture. Different codes say different things about this. In the land of KootK, I'd probably divide the thing up into 3' wide strips and only get concerned if the peak, elastic moment within any of those strips exceeded the average moment capacity within those strips by more than 15%.

There are limits to a slabs capacity to redistribute moments but, at the same time, the whole concept is bunk if that capacity is not significant. It's part of what makes these systems work. I've seen plenty of folks push the redistribution up to 30%.

 

[Just Some Nerd]

I'd mix Celt's suggestion with option 2. I would also think that strip #3 should be narrowed / split into two to match width of thickenings around the column (can examine the moment plots to confirm this, but the thickened panels will tend to draw more of the moment, have never looked at what happens with a peno in front however)

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Why yes, I do in fact have no idea what I'm talking about

 

[rapt]

Australian code suggests that if the penetration width is > 1/8 of the column strip width, the code defined column/middle strip logic does not apply and you need to use FEM (as you have) to determine the stress distributions around that area and in that span.