An interesting phenomenon has developed in the San Francisco Bay area resulting in improved versatility of tilt-up construction. Increasingly intricate, textured architectural panels with large window areas have been making it impossible to erect tall panels without expensive strongbacking. A San Jose, California contractor has applied post-tensioning technology to tilt-up construction to solve the expensive strongbacking problem. The application of post-tensioning to overcome lifting stresses in tilt-up panels is believed to be a new development.
The engineering has been a three-phase process. First, panels are engineered through the services of a tilt-up accessories supplier and bending moment calculations are made. A computer is used to print out the bending moment at any given elevation for any given angle of tilt. From this information the location of inserts can be rearranged to reduce stress. Second, bending moment calculations are reviewed by the post-tensioning subcontractor. The required number of post-tensioning tendons and their locations to overcome excessive stresses are calculated. Tendons are installed and tensioned by the subcontractor's field personnel. Third, a consulting engineer is employed by the tilt-up contractor to confirm the procedure because the contractor assumes the risk for this strongbacking-by-post-tensioning. The final results are reviewed by each party.
This article also discusses construction operations used on a complex of three office buildings in Palo Alto, California which illustrate the procedures.