Diaphragm - Difference between Shell and Joint Diaphragms

[Rifat]

As salamu alikum,

What is the difference between of shell diaphragm & Joint diaphragm? Can anyone tell me the criteria for appropriate situation for different diaphragm application?

 

Thanks in advance

[Simple Structures]

The Question is a little vague, and also difficult one at the same time.

I am not an expert on finite element modelling but here are general thoughts to move the discussion along, based on my basic understanding.

Firstly, here is my interpretation of question you have asked - You are referring to modelling the diaphragm (floor) and frame (columns and walls) for your computer analysis program input?  

1. In a concrete frame building, one uses the floor slabs to act as diaphragms (deep beam analogy approach, where beam-depth is assumed width of slab, we then have tension and compression flanges/ chords etc) to transfer horizontal wind or notional force or earthquake loading to the core walls or shear walls, which in turn take it not the foundation and the ground. In this case, we detail the edges of slab with additional 'deep beam' reinforcement, and also design the connection between the slab and vertical elements (walls and columns) to transfer the load. Openings in the slab, floor plate shape all play a part in the way diaphragm will act. Also, a slab diaphragm can be designed by hand calculations using 'deep beam theory' approach!

2. Under horizontal wind or seismic loading, there will be a vertical element (column, wall) drift or sideways movement, and also side-way deflection of the slab, at mid span of slab 'length'' (along edge of slab).

3. Refer to the User Manual or commentary of the software you are using, look at its definition and input requirement of the two types of diaphragm discussed in your question. Some seismic codes also define - shell, rigid, flexible diaphragms etc. Rules and definition for each are given in the relevant codes. Span-to-depth ratios in the horizontal plane come into play here.

4. In an analysis I can make the diaphragm rigid (ignoring deformation) or model the in-plane stiffness as a shell diaphragm. In this case one will get slightly different results from the computer output, based on what assumptions we made in modelling stiffness. If no stiffness is inputted, then no output of force will be possible.

5. Therefore, putting it simply, the slab (diaphragm) is inputted in many analysis software as shell elements with defined nodes, and with defined parameters. Then where the frame (columns and walls) meet the slab elements, joint nodes are created, and these are checked and input with the relevant degrees of freedom and/or joint releases. Forces and moments can then be taken at these shell and joint nodes. 

6. During modelling the diaphragm and walls are attached to the same mesh nodes, whilst diaphragm and column are attached to the same joint.

7. Finally, look at the Design Codes definition of the same & also your software User Manual, as a starting point to getting an answer to your query.

I am not sure if I have answered the question asked by you, but hope I have at least moved the topic along, for other engineers to contribute further?

 

 

 

 

[G_Farooq]

Shell and Joint Diaphragm both serve the same purpose but assigning diaphragms to shell objects is preferable as it will automatically constrain internally created joints.  Just make sure that if you are using Joint diaphragm, your floor is manually meshed, fully, because any joints created by additional meshing will not have the constraint. While as explained above, for shell diaphragms, auto mesh can be used.