RC Basement Slab - In Plane Edge Loads Inducing High Tensile Forces

[cainy7]

I am trying to model a RC lift pit that can support mezzanine column loads on top of the walls, however when I add these point loads as an edge load to the top of the panel it induces large tensile forces within the panels as seen in the image attached.  Is this result correct, as this means a lot of additional rebar is required as a result?

All panels are designated as slabs and the bottom slab is ground bearing with springs defined. I assume the tensile loads are induced in the walls because the rebar will be activated when the ground slab deflects downwards due to the point load on the wall, this appears to be supported by the results when the wall bottom edge has fixed supports then the tensile forces are much reduced with only a few stress concentrations occurring at panel corners/ intersections.

On the other hand, if the wall was just mass concrete the column loads would be transferred to the slab without an issue but then there are bending moments out of plane to consider due to soil/ water pressures.

[Simau]

Hi @cainy7 

Model will be more helpful than sreenshots.
Moreover the 2 structures dislayed are completly different.

[cainy7]

Model attached with ground bearing slab present and line loads applied to the top of walls, thanks.

[Simau]

@cainy7 

No model attached.

Zip it before attaching

[cainy7]

Apologies, hopefully it worked this time.

[Simau]

@cainy7 

Your model and results are Ok.
I added uplift Uz+ to the raft and didn't find any uplift, so your linear model is ok.

Model attached

[cainy7]

Thank you for the review, it is appreciated.

Can you explain the presence of large tensile forces within the vertical panels, which predominantly occurs under the imposed load case? I assume this is because the rebar is treated as locked in so tension is generated to reduce displacement of the slab under the applied load. 

I am torn whether to include more rebar purely for the tension due to the imposed loads on top of the wall or to ignore this due to the reasons mentioned previously. On a small structure like this, either option wouldn't be too drastic but I would like to know how to best approach this problem in the future for a larger project say a basement wall supporting columns.

[Simau · Accepted solution]

@cainy7 

Reinforcement has nothing to do with efforts in the panels. Reinforcement in not taken into account when you run Robot

Don't look at efforts given with cases that don't act alone, at least self weighthas to be present in each combination.

On another side, reinforcement calculated is no not so big, as shown in screenshots below.

[cainy7]

Perhaps I just need to learn more about the RC design function and output shell results, because what you have shown is what I would expect and calculate by hand whereas when I tried to use the RC designer function I was getting slightly higher values for the area of steel. 

What would be your opinion at corners just ignore the higher value and ensure proper rebar detailing to prevent issues? Similar for under the points loads, I suppose it would just be wiser to locally reinforce the area with additional steel rather than risk cracking.

I appreciate the help.

[Simau]

When calculating reinforcement, pay attention to select the correct comniations. Don't select load cases that don't act alone.

Reinforcement peaks are inevitable in the FE calculation.