Allowable Stresses in Concrete
The allowable stresses at release are calculated as per AASHTO Standard (LFD) Art. 9.15.2.1. For allowable stresses after losses, there are three stages at which the stresses should be checked as per AASHTO Standard (LFD) Art. 9.15.2.2 (a), (b) and (c). These stages are as follows:
- Final 1: The compressive stresses under all load combinations except as stated in Final 2 and Final 3, shall not exceed 0.60 f'c.
- Final 2: The compressive stresses due to effective prestress plus permanent (dead) loads shall not exceed 0.40 f'c.
- Final 3: The compressive stress due to live loads plus one-half of the sum of compressive stresses due to prestress and permanent (dead) loads shall not exceed 0.40 f'c.
Precast/Prestressed Girder flags any stresses exceeding the allowable compressive stresses for Service 1/Final 2 and Service 1/Final 3. Under Service 1/Final 1, it flags both the exceeded compressive and tensile stresses. For simple spans, only the stresses under positive moment envelope are checked, whereas, for multi-span bridges, the stresses are checked for both the positive and negative moment envelopes.