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:

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

Figure 1 - Apply diaphragm forces
For a given load case, display any stress or shell force, as shown in Figure 2:

Figure 2 - Display stress or shell force
Where maximum chord forces are expected, draw or define a section cut, as shown in Figure 3:

Figure 3 - Maximum chord forces
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:

Figure 4 - In-plane moment
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:

Figure 5 - Compression and tension chord forces
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:

Figure 6 - Shear and collector forces
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:

Figure 7 - Shear and collector forces
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
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

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