Click the Project Parameters button to activate the Project Parameters screen, as shown below; it defaults to the Limiting Stress tab.

You can modify the limiting stress values by typing over the default values or have the program use the default stress factor values to calculate the limiting stresses by selecting the Use Factor To Calculate check box. Notice that when this check box is selected, the text boxes become grayed out, returning to the default values. For more information about stress limits for concrete, see LRFD Art. 5.9.4.

AASHTO LRFD (Article 5.9.4) and AASHTO Standard Specifications for Highway Bridges (Article 9.15.2) specify stress limits for concrete; specifically, pre-tensioned and post-tensioned concrete components. Tensile and compressive stress limits are specified for fully prestressed components for two stages of the Service Limit State -- before and after prestress losses.

Tensile stress limit keeps the bridge component from cracking in service, thus minimizing the increased stresses and fatigue of the prestressing strands at the cracks. Tensile limit (6'c) is lower than modulus of rupture (7.5'c) to safeguard against fatigue in tension. Fatigue of strands is not a problem in uncracked concrete.

Traditional compressive stress limits (0.45f'c and 0.6f'c) protect concrete from excessive creep deformations and fatigue (0.4f'c limit). Although the above stress limits are only specified for fully prestressed components and not for reinforced concrete components of composite members, the practice varies across the United States.

Precast/Prestressed Girder, by default, only checks the stress limits for the top and bottom fibers of precast, prestressed concrete girders. Optionally, the user may elect to also check the stresses at the top fiber of the cast-in-place, reinforced concrete deck.