I have a Validation > Model > Diaphragms > “Rigid diaphragms on sloping slabs” error. Why do I get this and how do I resolve it?

Question

I have a Validation > Model > Diaphragms > “Rigid diaphragms on sloping slabs” error. Why do I get this and how do I resolve it?
 

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Answer

A rigid diaphragm on sloping slabs validation error occurs when a slab is defined on a sloped plane and the Diaphragm option is set to Rigid.

Rigid diaphragms have infinite in-plane stiffness and zero out of plane stiffness properties and therefore they neither exhibit membrane deformation nor report the associated forces. When a rigid diaphragm is specified, nodal constraints are automatically applied to all nodes within the diaphragm plane and thus they move together as a single unit. Since they do not move relative to one another no axial force can develop within any member within the plane of the diaphragm. In Tekla Structural Designer a rigid diaphragm is only permitted on horizontal levels. They are not permitted on a sloping plane since it will constrain the nodes on all horizontal levels that it interacts with.

If we hover over one of the problem items a tooltip is displayed “A slab is sloping and is set to have a rigid diaphragm. Rigid diaphragms cannot be defined on sloping slabs.”

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We can double click the “Rigid diaphragms on sloping slabs” warning or an individual item listed under the branch to automatically switch to the Solver view regime and highlight the slabs or slab in the model. Alternatively, you can right click and choose Highlight Items from the context menu.

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Now we have identified the slab with the validation error, we can switch back to the Structure view regime and select the slab to display the slab properties in the Properties window.

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To resolve the validation error you can either set the Diaphragm option to None or Semi-rigid.

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We will not talk further about what these options do, as they are discussed in detail in other articles. One such article is “Truss modelling, axial force and the effect of Diaphragms” which is linked in the right hand panel.

Setting the option to None or Semi-rigid (as screenshot below) removes the validation error.

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