Structural Slab Supported at Corners 1

[EngDM]

I'm playing around with a simple 7'x12' slab in SAFE, with pin supports at the corners and I am finding that SAFE still wants me to provide top steel. If this slab is truly pinned, for instance formed onto some sort of angle or support with minimal moment restraint and moreso just shear studs to keep it from sliding, how come I am getting negative bending? The supports are currently placed directly at the corner, so it's not like the moment is being generated over top of the support.

I'm not sure if maybe since the pin support is placed at the datum, which coincides with top of slab it is taking the slab as being supported by the top? I tried shifting my supports down by my slab thickness but it throws ill conditioned warnings.

 

[Tomfh]

Post the bending moment diagram

 

[EngDM]

Post the bending moment diagram

I'm not using strips in SAFE so it won't generate a bending moment diagram, only a contour map of reinforcing required (one for each direction, top and bottom steel).

 

[Agent Coconut]

I'm playing around with a simple 7'x12' slab in SAFE, with pin supports at the corners and I am finding that SAFE still wants me to provide top steel. If this slab is truly pinned, for instance formed onto some sort of angle or support with minimal moment restraint and moreso just shear studs to keep it from sliding, how come I am getting negative bending? The supports are currently placed directly at the corner, so it's not like the moment is being generated over top of the support.

I'm not sure if maybe since the pin support is placed at the datum, which coincides with top of slab it is taking the slab as being supported by the top? I tried shifting my supports down by my slab thickness but it throws ill conditioned warnings.

Which design code you are using?
If you are using Eurocode (EC2: 9.3.1.2(2) or BS8110(3.12.10.3.2), there is a provision for top steel reinforcement even it is pinned support.

However, we still need to see the bending moment diagram, in your case, contour of bending diagram before anything is concluded.(Either the software consider the provision of top steel reinforcement or there is a analysis error/modelling limitation we are not sure)

 

[EngDM]

East-West steel requirements, pink is no steel


North-south requirements, pink is no steel

Blue lines are my reinforcing steel objects, placed at the bottom of slab.

Each corner is pin supported. I tried doing an edge release but the model threw an error saying ill-conditioned.

Designing to CSA A23.3-19.

 

[DaveAtkins]

Is it possible there is some sort of torsion thing going on near the corners that results in tension at the top of the slab?

 

[EngDM]

Is it possible there is some sort of torsion thing going on near the corners that results in tension at the top of the slab?

Interesting. I will have to look into this, perhaps since the slab is being treated as two-way bending and its acting like a bowl?

Edit: I released slab torsion at the edges and it made no difference to the reinforcing required, other than telling me the model is ill-conditioned.

 

[Celt83]

can you show just an image of your model in 3d with the support turned on, do you have single pin, ux uy uz restraint, defined at each corner or are there short wall segments in each direction not clear from your reinf. heatmap?

If you have true pins at all four corners then you will get a small region of tension near the pin

If you have true pins and they are offset to the top or bottom slab surface then the end rotation will be partially restrained and you will get a larger region of negative moment.

can you not plot a contour plot of moments?

 

[Celt83]

There is also the inclusion of the Twisting Moments, Mxy, via the wood-armer method that would likely also generate top steel at the corners.

 

[EngDM]

can you show just an image of your model in 3d with the support turned on, do you have single pin, ux uy uz restraint, defined at each corner or are there short wall segments in each direction not clear from your reinf. heatmap?

If you have true pins at all four corners then you will get a small region of tension near the pin

If you have true pins and they are offset to the top or bottom slab surface then the end rotation will be partially restrained and you will get a larger region of negative moment.

can you not plot a contour plot of moments?

Here is the render:



Pin supports are at top of slab, I tried offsetting them by the slab thickness to the bottom, but it made the model ill-conditioned.

Here is a contour map of shell moments (minimums) MMin. The corners are all negative and transition to positive towards the teal.


Heres maximum moments MMax, no negatives.


And finally the sign convention for it all. Maybe I am missinterpreting what Mmin and Mmax actually represent, and the fact that Mmin goes negative is irrelevant to negative moment.

 

[PMR06]

Can you post images of M11, M22 and M12? I suspect Celt83 is correct and M12 will show negative twisting moments that are adding into the Mmin sum. I am guessing M11 and M22 will not show negative moments, but M12 will.

Or there could be something else going on...

 

[Celt83]

to me Mmin means largest values below 0 if negative or nearest to 0 if all cases positive. Mmax the opposite. So to me Mmin would indicate your regions of negative bending.

Since your supports are at the top surface they restrain that surface from translating which ends up partially restraining rotation at the slab edge. I think there may also be some influence from the force recovery procedure for the finite element itself that adds some negative bending as well.

 

[EngDM]

to me Mmin means largest values below 0 if negative or nearest to 0 if all cases positive. Mmax the opposite. So to me Mmin would indicate your regions of negative bending.

Since your supports are at the top surface they restrain that surface from translating which ends up partially restraining rotation at the slab edge. I think there may also be some influence from the force recovery procedure for the finite element itself that adds some negative bending as well.

Don't the supports have to be at the top for SAFE/ETABS/SAP? Yes the shell is 152mm thick if I show the extruded shell render but if I insert a joint at -152mm, it doesn't actually support the slab.

 

[Celt83]

not familiar with SAFE it may be an option for the slab element itself to offset so the support is at the bottom or midline, check the manual or reach out to their support staff.

 

[EngDM]

not familiar with SAFE it may be an option for the slab element itself to offset so the support is at the bottom or midline, check the manual or reach out to their support staff.

Yea I found the setting in the shell element to make it have the datum at the bottom, so the slab is supported at the bottom as shown in the screenshot below:



However, my contour diagrams for negative bending still have negative. I've seen other stamped drawings with this exact reinforcing (no top steel) so obviously I am missing something. Or they just did it by hand assuming simply supported and went on with their lives. I mean concrete will perform to how it is reinforced anyways, might just get some microcracks at the top at the corners then.

 

[JoshPlumSE]

I respectfully suggest that this may be due to Mxy (or warping) moments. I tend to think of this as a tendency to warp or distort the plate rather than to bend it. Since I'm more of a steel guy, I tend to related it to the Torsional Warping of I shaped beams.

 

[Celt83]

Yea I found the setting in the shell element to make it have the datum at the bottom, so the slab is supported at the bottom as shown in the screenshot below:

View attachment 3267

However, my contour diagrams for negative bending still have negative. I've seen other stamped drawings with this exact reinforcing (no top steel) so obviously I am missing something. Or they just did it by hand assuming simply supported and went on with their lives. I mean concrete will perform to how it is reinforced anyways, might just get some microcracks at the top at the corners then.

moving the supports to the bottom of the slab creates the same cross section rotation restraint that supports at the top of the slab create, try setting the support elevation to the slab mid depth.

Even at mid depth though you will still get some negative bending moment in the corners I believe the primary source may be twisting moment (Mxy) and some other contribution from the inherent error of being finite elements.

 

[EngDM]

moving the supports to the bottom of the slab creates the same cross section rotation restraint that supports at the top of the slab create, try setting the support elevation to the slab mid depth.

Even at mid depth though you will still get some negative bending moment in the corners I believe the primary source may be twisting moment (Mxy) and some other contribution from the inherent error of being finite elements.

I'm just wondering if this is all safe to ignore, otherwise I am confused on how others got it to work (unless they just did hand calcs and assumed simply supported).

 

[Celt83]

Review ACI 447R-18

Ignore = Never

That said if more deflection and top surface cracking can be tolerated you may be able to adjust the shell element to be twist free releasing the Mxy moments.

 

[EngDM]

Review ACI 447R-18

Ignore = Never

That said if more deflection and top surface cracking can be tolerated you may be able to adjust the shell element to be twist free releasing the Mxy moments.

This just seems like an overly complicated analysis for a simple rectangular slab. Without running this in software I never would have put any top steel.

I tried setting my M12 bending modifier to zero and it got rid of my top steel at the edges, but gave me top steel at the middle lmao.

I've reached out to SAFE so hopefully their technical support can walk me through this.