According to ACI 318-05, Section 11.6.1, the formula for calculating the Threshold Torsion for a concrete beam, is the same as formula given in ACI 318-99. Except that Strength Reduction factor, φ (Phi) has been reduce from 0.85 (ACI 318-99) to 0.75 (ACI 318-05).
Looks like the Threshold Torsion, has been reduce by about 13%. Or, am I missing another factor?
Thank you,
Farzin S. Rahbar, SE
David C. Weiss Structural Engineer & Associates, Inc.
(818) 227-8040 Ex. 13 Fax: (818) 227-8041