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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:
   
   
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2. For a given load case, display any stress or shell force, as shown in Figure 2:
   
   
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3. Where maximum chord forces are expected, draw or define a section cut, as shown in Figure 3:
   
   
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4. 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:
   
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5. 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:
   
   
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6. 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:
   
   
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7. 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

   2559

   2952 lbs., as shown in Figure 7:
   
   
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8. 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

See Also

• Rigid vs. semi-rigid diaphragm article

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