Structural Slab Supported at Corners

[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.