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# Value for Rf in UCS-79

## Value for Rf in UCS-79

(OP)
Can someone tell me how Compress comes up with the value for Rf in the UCS-79(d)calculation?  I assumed it was .17/D in the past but now realize that is incorrect.
Thanks,
Replies continue below

### RE: Value for Rf in UCS-79

COMPRESS reports the forming strain for heads calculated in accordance with UCS-79(d)(5).

The radius Ro used is infinity, as specified in the Code for a flat plate. The quantity Rf/Ro is thus taken to be zero (0).

Radius Rf is taken to be that of the knuckle portion of the head where the strain will be the greatest. Rf is taken to be equal to 0.17 * D, where D is the centerline diameter of the head at the tangent line. The factor 0.17 for the knuckle radius is taken from UG-32(d).

The thickness used in the calculation will be that of the head straight flange (if one is present) because it is assumed that this represents the plate thickness before forming. If no straight flange is present then the specified minimum thickness of the head is used.

For example, a 60" ID head, 0.5" min thk, with 0.625" straight flange, will find Rf as: 60 * 0.17 + 0.625 / 2 = 10.5125". Or without a straight flange:  60 * 0.17 + 0.5 / 2 = 10.450".

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