UNBONDED PRE-STRESSED CONNECTIONS Prof. John F. Stanton University of Washington, Seattle, Washington, USA The ROSE Sch

1. UNBONDED PRE-STRESSED CONNECTIONS Prof. John F. StantonUniversity of Washington,Seattle, Washington, USAThe ROSE School, Pavia. June 2009

2. REFERENCE DOCUMENT Design Guidelines for Precast Concrete Seismic Structural Systems John F. Stanton & Suzanne D. Nakaki PRESSS Report No. 01/03-09 Available from PCI (and on Rose School website)

3. UNBONDED PRESTRESSED CONCRETE FRAME: GOALS • Zero residual drift (ZRD) • Frame returns to vertical after earthquake • Damped response • Damping restricts peak drift • Little damage even in a large earthquake • Building can be re-occupied quickly

4. POSSIBLE CONFIGURATIONS:(Continuous beams)

5. POSSIBLE CONFIGURATIONS:(Continuous columns)

6. CONFIGURATIONS • Continuous Columns • Often used by precasters • Continuous Beams • Long Heavy Beams • Columns must be accurately placed for bars to fit

7. REINFORCEMENT CONFIGURATIONS • Unbonded prestressing only • Simple to construct, but no hysteretic damping. • Damping could be supplied by external devices or RC frames in parallel with UBPT frame. • UB prestressing + rebar = “Hybrid Frame” • Internal hysteresis (damping) supplied by rebar • All-in-one package.

8. UBPT Prestressed-only Frame Post-tensioning tendon (unbonded) Post-tensioning anchorage Shown with post-tensioned beams, multi-storey columns. Requires post-tensioning on site. Easy fit-up.

9. UBPT Prestressed-only Frame Pre-stressing tendon Bonded region Unbonded region Shown with pretensioned beams, one-storey columns. Can prestress in plant. Columns must be accurately placed.

10. UBPT Prestressed-only Frame Components Unbonded PT tendon Grout

11. Hybrid Frame Components Bonded Rebar Unbonded PT tendon Rebar locally debonded Grout

12. Deformed Shape

13. Deformed Shape

14. Deformed Shape

15. Deformed Shape

16. Hybrid Frame Components (Courtesy Nakaki Bashaw Group) Column bar splices Pre-tensioned strands Sleeves for column bars (grouted) Grout bed at interface Beam rebar (not continuous) Only the pre-tensioned strands cross the interface

17. Hybrid Frame Components (Courtesy Nakaki Bashaw Group) Unbonded region Mild steel (A706) (grouted) Fiber Reinforced Grout Post Tensioned Tendon (ungrouted)

18. UBPT FRAME CONCEPT • PT tendon provides strength • PT provides elastic restoring force • Damping is minimal (use 5% critical)

19. LINEAR NONLINEAR s s ELASTIC e e s INELASTIC e NON-LINEAR ELASTIC BEHAVIOR Elastic: Load and unload along the same path Inelastic: Load and unload along different paths Linear: Load/unload path is a straight line Nonlinear: Load/unload paths are not straight.

20. Force - Deflection PT only • Returns to vertical • Nonlinear elastic • Low damping Rebar only • Residual drift • Nonlinear inelastic • High damping Hybrid Frame combines both features

21. Hysteresis Loops - Combinations 100/0 75/25 50/50 25/75 0/100

22. DESIGN PROCEDURE • Rational design procedure • Design forces same as cast-in-place concrete SMRF • Explicit evaluation of drift capacity • Design of other parts of frame: per code

23. ASSUMPTIONS • All deformation occurs at the interface, not in the beam. • PT is unbonded the entire length of the frame, except for bonded region at each end. • cgp is at mid beam-height.

24. LIMIT STATES • We need to define limit states at which the moment strength will be computed. • Nominal moment strength: • Corresponds to the design moment. • Used to size the reinforcement at the interface • Maximum probable strength • Corresponds to the Maximum Credible earthquake • Forms the basis for Capacity Design forces for adjacent elements.

25. NOMINAL STRENGTH LIMIT STATE • Smaller of • Onset of strain hardening in mild steel (if any). • Yield of PT steel. • Comparable to nominal strength of reinforced concrete frame

26. MAXIMUM PROBABLE STRENGTH LIMIT STATE • Smaller of : • Strain at peak strength of mild steel. • Yield of PT steel. • System must have at least 3.5% drift capacity at MPS limit state.