DIAPHRAGM FORCES

1. Applied diaphragm forces (define diaphragm using semi-rigid option)

Image Removed

How do I obtain diaphragm chords and collectors forces within a floor ?

Answer: The collector forces within a floor diaphragm may be measured through the following process:

1. Define a semi-rigid diaphragm, then apply diaphragm forces, as shown in Figure 1: Image Added
   
   
   
   

   
   

   Figure 1 - Apply diaphragm forces

   
   

2. For a given load case, display any stress or shell force

   for the relevant load case

...

1. For

   , as shown in Figure 2:
   
   
   Image Added

   Figure 2 - Display stress or shell force

   
   

2. Where maximum chord forces are expected,

   Draw

   draw or define a section cut

   where maximum chord forces are expected

...

1. Moments in Z are the in-plane moment in slab make sure angle is zero, refresh as required

Image Removed

1. The value of Z moment divided by slab width is equal to compression and tension chord forces

...

1. Shear and collector forces at slab-shear wall connection , draw or define section cut next to support following wall direction

Image Removed

1. Make sure angle is zero, in this case shear/collector force is equal to F2 or 2559pds as expected

1. , as shown in Figure 3:
   Image Added
   
   
   

   Figure 3 - Maximum chord forces

   
   

2. Moments about the Z-axis represent in-plane moments. Ensure that the resultant-force angle is 90 in order to enforce vertical line, then select Refresh, as shown in Figure 4:
   
   Image Added

   Figure 4 - In-plane moment

   
   

3. Compression and tension chord forces are then given as the value of each Z-axis moment divided by slab width, as shown in Figure 5:

                                                                                                                                   Image Added

   Figure 5 - Compression and tension chord forces

   
   

4. For shear and collector forces located at the connection between the diaphragm and a shear wall, draw or define a section cut next to the support which follows the wall direction, as shown in Figure 6:
   
   
   Image Added

   Figure 6 - Shear and collector forces

   
   

5. Again, ensure that the resultant-force angle is 90, and refresh as necessary. Given this example model, the shear/collector force, which is presented in F1, is 2.95 kips (59x100/2)/1000., as shown in Figure 7:
   
   
     Image Added

   Figure 7 - Shear and collector forces

   
   

6. Local axis of section cut  1, 2 and Z are  similar to a frame element, resultant force angle indicates angle between X global axis and section cut local 1 axis
7. This same force will be obtained by reviewing V2 for the wall piers at the edge of the building. Pier forces in combination with section cuts can be used in more complex models to design/check diaphragms

See Also

• Rigid vs. semi-rigid diaphragm article

• Accidental eccentricity design article