This is for a large opening in the middle of a column strip in a two-way slab. This appears to have been covered before and there is a good YouTube tutorial on it, but they don't discuss openings in the middle of the column strip. 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/thoughs are:
1.) Are both permissible?
2.) Would the end of the span be considered cantilever or support?
My thoughts:
The difference between the two designs is that you are most likely going to get more top bars in the "added strip" method. 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. However, if I'm really trying to sharpen my pencil or 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. 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 Strips Each Side
Latitudinal Strips For Reference
Certainly, a single full-width design strip is going to average the forces and probably produce less steel. That's common for PT slabs in the US. A single strip with middle strips breaks it into three parts and probably results in more steel. Adding more strips continues to refine the demand based on local stresses and may result in even more steel. Your comment about treating the slab as a cantilever can be nullified if you just turn off span detailing for that strip.
In this case, since the angles are significant, and there are drop panels aligned to the global axis, I would personally tend to use straight staggered strips rather than largely skewed strips.
Seth - Thanks for the response. Good idea on the staggered strips. I'm trying to decide if it is reasonable to avoid creating the additional "side strips" shown on Option-2 as this is an evaluation of an existing slab. Adding the side strips in option-2 creates more local reinforcement that isn't there. But if I keep the full design strip, then I have enough reinforcement (because I pick up more bars in my design strip without the moment increasing much). This sorta feels like 'cheating', but if I look at ACI, they allow a column strip to be 0.25*(L1,L2). So, it seems like using the single width strip is "allowed".