RAPT Flat plate middle strip deflection 1

[Structimes]

When using RAPT to design a two-way flat plate slab, I was advised that in order to check the maximum mid-span deflection in a slab bay, that one should model the slab in the primary and secondary directions separately, read the middle strip deflection from both directions and add the two middle-strip deflections together.

This approach seems like it is doubling up on the deflection. Does anyone think it is realistic? I am inclined to take the worst case deflection out of the two directions.

 

[r13]

I concur your approach, which is reasonably conservative.

 

[Trenno]

Have a look in the RAPT help/manual file. It discusses two-way slab deflections.

 

[rapt]

Retired13,

Not sure why you concur as the logic is wrong! If you have used RAPT before, you have been using it wrongly, if not, why would you answer when you do not know the program.

Structimes
AS Trenno said, this is discussed in the RAPT manual. Not sure really why we wrote it judging by the few people who actually read it.

The correct answer is the sum of column strip deflection in one direction and the middle strip deflection in the transverse direction. Essentially, a column strip is a support strip and a middle strip is supported by the support strip. So the midspan deflection is the sum of the deflection of the support strip and the transverse slab supported by the support strip.

Or even more correct the average of this calculation in the 2 directions.

Or even more accurate, keep adjusting the distribution of loads in the 2 directions until the 2 deflections calculated this way for both directions are equal.

 

[Tomfh]

OP,

You don't add middle strip deflections together, you add middle strip deflection to column strip deflection.

Column strip is like a "beam" that supports the transverse middle strip, so you have to add the supporting column strip deflection to the transverse middle strip deflections to get the final deflection of the middle strip.

Then you check the total deflection over the diagonal distance.

As rapt says it is in the manual.

 

[r13]

If that's the way how the program does it, I won't argue with it, as I was not a user. But without computer, the practical way to find approximate mid-span deflection is to use displacement compatibility concept to determine load distribution/carrying factors (Δ_L = Δ_S), then calculate the deflection with the factored load (shared percentage of the total load). The deflection derived from the total load over the controlling (long) span is inaccurate, but conservative, because it ignores restraining effect from the other direction. I agree that deflection at the column strip is in addition to the span deflection calculated though.

I apologize, if I've misled anyone.

 

[Structimes]

Thank you, this has been most helpful