TO: All Design Section Staff FROM: J. Kapur DATE: March 19, 2002 SUBJECT: LRFD Shear Design
The shear design of prestressed members shall be based on the general procedure of AASHTO-LRFD Bridge Design Specifications article 184.108.40.206.2 using the modified compression field theory. The shear design of all non-prestressed members shall be based on either, the simplified procedure of AASHTO-LRFD Bridge Design Specifications article 220.127.116.11.1 using shear design parameters of b=2.0 and q=45o,or the Strut-and-Tie model based on the AASHTO LRFD Bridge Design Specifications article 18.104.22.168.
The use of Strut-and-Tie model may be considered for deep beams with span to depth ratio of less than 5, and other situations in which the distance between the centers of applied load and the supporting reaction is less than twice the member thickness. Typical members identified for strut-and-tie modeling are:
· Pile caps and shaft caps
· Thick footings
· Crossbeams and diaphragms
· Post-tensioning anchorages
Application of the simplified shear design procedure using b=2.0 and q=45o for all non-prestressed members is equivalent to the AASHTO Standard Specifications which have been successfully used for many years.
Application of strut-and tie or other methods is recommended for special situations where the simplified shear procedure does not apply. When using strut-and-tie model, the orientation of compression struts and tension ties shall be based on the stress flow in the member. For better understanding of strut-and-tie modeling, the web based programs such as http://cee.ce.uiuc.edu/kuchma/strut&tie/index.htm may be used.
If you have any questions regarding these issues, please contact Bijan Khaleghi at 705-7181.