topic Re: Diaphram definition (large displacements and rotations) in Robot Structural Analysis Forum

Diaphram definition (large displacements and rotations)

omarsQSKVQ — Fri, 11 Jul 2025 08:12:00 GMT

I have a building consists of two full area (352*462 ft) mezzanine floor
the system is a steel one way decking, released main beams and joists and the decking supposed to transfer the lateral loads to the braced frames for the 4 outer elevations (all supports are pinned)
as per the attached snapshot also i attached a simple model that i face same problem in define this concept
i tried the rigid link , panel calculation model (membrane) also i tried shell with a custom definition with 0 bending and shear stiffness i really get bored so please help.

Re: Diaphram definition (large displacements and rotations)

Simau — Fri, 11 Jul 2025 14:12:30 GMT

Hi @omarsQSKVQ

Just add some horizontal bracings

Model attached

Re: Diaphram definition (large displacements and rotations)

omarsQSKVQ — Fri, 11 Jul 2025 17:01:04 GMT

Do you think i need to add horizontal bracing "Is it a must? "
or the composite slab work as a diaphram to transfer the load to the vertical bracing
the attached snapshot from Design Examples (AMERICAN INSTITUTE OF STEEL CONSTRUCTION)
is it the same system ?

Re: Diaphram definition (large displacements and rotations)

Simau — Fri, 11 Jul 2025 18:21:47 GMT

Another solution is to use an orthotropic slab of thickness equivalent in weight to your one way deck.
Here, i assumed your composite deck have a SW of 1.5 kN/m² then equivalent to a 6 cm thickness orhotropic deck.

Model attached

Re: Diaphram definition (large displacements and rotations)

Refaat — Sat, 12 Jul 2025 12:23:11 GMT

Hi @omarsQSKVQ

1. Pinned -Pinned release ( only Ry ) .
2. Remove the click from (Disregard density) since the self-weight is excluded from mass participation.
3. Delete duplicate case (6) from the converted cases
4. The modified file is attached.

Re: Diaphram definition (large displacements and rotations)

omarsQSKVQ — Sat, 12 Jul 2025 13:05:00 GMT

I can't thank you enough for your help.
For the seismic mass participation definition, Is it okay to define it as per the attached snapshot?
and one more question, I don't need to define panel thickness aright?
I think whole the trouble because of the wrong releases definition.
really thanks again.

Re: Diaphram definition (large displacements and rotations)

Refaat — Sat, 12 Jul 2025 13:59:01 GMT

Hi @omarsQSKVQ

Is it okay to define it as per the attached snapshot?

Yes

I don't need to define panel thickness aright?

Possible if diaphragm is used as well the slab weight considered as uniform load . Define the slab thickness is more preferable.

I think whole the trouble because of the wrong releases definition.

Certainly

Re: Diaphram definition (large displacements and rotations)

Simau — Sun, 13 Jul 2025 11:57:33 GMT

Hi @Refaat

Maybe the releases hypothesis was a company requirement who did not want to bother with MZ bending moment in the connections.