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.

 

[BridgeSmith]

Not familiar with the program, but if the slab is deflecting down in the middle and the supports are fully rigid against lateral movement, is it generating tension at the supports due to the slab trying to pull the corners in toward the center and the rigid supports not allowing it?

 

[EngDM]

Not familiar with the program, but if the slab is deflecting down in the middle and the supports are fully rigid against lateral movement, is it generating tension at the supports due to the slab trying to pull the corners in toward the center and the rigid supports not allowing it?

I tried releasing all but 1 support for plan translation, but the contour diagrams remain same with hotspots at each corner for top reinforcing.

 

[EngDM]

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.

I think you're right. I removed any M12 stiffness and my corner hotspots disappeared, but then I required top steel in the middle since the deflected shape was more like a saddle, due to no twisting rigidity.

 

[BAretired]

Applying reactions at each of the four corners as a concentrated load is not realistic; it creates an infinite shear stress because the reaction is applied over a zero area. Maybe you should try moving the reactions in at least half the depth of slab, and assume the area of each support to be large enough to satisfy the permissible bearing stress.

 

[EngDM]

Applying reactions at each of the four corners as a concentrated load is not realistic; it creates an infinite shear stress because the reaction is applied over a zero area. Maybe you should try moving the reactions in at least half the depth of slab, and assume the area of each support to be large enough to satisfy the permissible bearing stress.

Does this affect bending though? I have checked punching with the true supports modeled (column/shelf angle), but went to pinned corners to play around with getting the moment to act simply supported.

 

[bookowski]

SAFE does not like a point pin support and it is internally accounting for it with a constraint. This is one of the features of csi products that is intended to be helpful, and probably often is, but can hide incorrect modeling/behavior at times.

Put a column at each corner that is tiny, say .5" x .5" and tall, i.e. something with tiny stiffness. There are other ways to force it into the behavior that you are intending to model.

As another test try a one try a 1 way slab with a series of point pin supports, you'll find the same issue - you'll get negative end moments at your intended simply supported slab.

 

[EngDM]

SAFE does not like a point pin support and it is internally accounting for it with a constraint. This is one of the features of csi products that is intended to be helpful, and probably often is, but can hide incorrect modeling/behavior at times.

Put a column at each corner that is tiny, say .5" x .5" and tall, i.e. something with tiny stiffness. There are other ways to force it into the behavior that you are intending to model.

As another test try a one try a 1 way slab with a series of point pin supports, you'll find the same issue - you'll get negative end moments at your intended simply supported slab.

I will give this a shot. I had already tried it by disabling "automatic rigid support at column".

 

[AbdulFatah234]

That seems to be rational, its the same as the analysis of flat slab.
The column zone makes column strip therefore negative moment is presented.

 

[bookowski]

It is not related to the safe rigid zones around columns or strips, it is about a point restraint OoP on an FE element and how safe internally accounts for this. It is an issue so you need to do a workaround as described in my previous post.