تواصل_تطوير المحاضرة رقم 194 دكتور / هشام ندا المدير الدولي لقسم الكباري بشركة خطيب وعلمي عنوان المحاضرة " Prestressed columns; application and behaviour"
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The document discusses the concept of prestressing in concrete structures, highlighting its advantages in enhancing load resistance, controlling cracking, and improving earthquake performance. It examines the behavior of prestressed columns, particularly the impact of axial load levels on residual displacement during seismic events. The findings suggest that increasing axial load elevates both lateral load capacity and cracking moments, which positively influences residual displacement control.
تواصل_تطوير المحاضرة رقم 194 دكتور / هشام ندا المدير الدولي لقسم الكباري بشركة خطيب وعلمي عنوان المحاضرة " Prestressed columns; application and behaviour"
- 2. INTRODUCTION
- 3. Prestressing
- 4. Prestressing Concept in Nature The high risk of fiber buckling reveals that the compressive strength of wood is only about a quarter to a half as high as its tensile strength. Relieving part of the compression will lead to higher lateral load resisting system.
- 5. Prestressing Concept in Everyday Life Pre-Compression Pre-Tension
- 6. Prestressing Concept in Concrete • Unlike trees, concrete holds very poor tensile behavior that is almost one tenth fc ’ • Prestressing Pre-compresses the sections’ fiber in tension to balance the final load • Curvature of cable adds upward internal pressure that balances the load as a plus to decompression. • Prestressing controls the cracking by reducing the effect of flexure. • Prestressing enhances the shear stresses by applying compression forces and upward forces
- 7. Columns General Behavior • Columns are mainly elements to resist axial compression load. • Moment comes through the framing with the slab system, which is small relative to the compressive load. • Prestressing adds additional compressive load to the system. • Moment is reversal at the event of an earthquake which makes eccentric prestressing useless. Prestressing technique applies compressive stresses to the concrete, which , does not help columns in their final operating condition especially with the reversal nature of lateral load !
- 8. Why? The question becomes; Why would someone prestress a column?!
- 9. Applications • Slender Columns: Prestressing introduces additional advantages to concrete columns. It transforms a cracked section into a non-cracked one, thus enabling the entire concrete section to resist bending moments. This becomes significant when bending predominates. • Precast columns: Always subjected to transportation and erection stresses, prestressing supplies a higher resistance to cracking during handling.
- 10. Applications – ABC Construction
- 13. Earthquake Loading on Structures • Earthquakes loading on structures has different nature than other loading which requires different handling. • Unlike other loadings, the waves at foundation level are transmitted to the building center of mass through the lateral load resisting system. • The masses shake in different frequencies which generate forces required to be safely transmitted to ground using the lateral load resisting systems. • The generated loads are coupled with the structural properties (Mass/Stiffness)
- 14. Force-Based Design Center of mass wave input Force Demand at mass level Columns Design to resist the forces increase in columns stiffness LIMIT THE FORCE LEVEL TO SYSTEM FLEXURAL CAPACITY
- 15. Performance Based Design Concept • The PBD focuses on saving lives through allowing damage to structural elements at predetermined locations. • Such elements are sacrificial element that dissipate the energy input of the earthquake through plastic hinge formation, hence, suffer high level of damage.
- 17. Associated Damage • Bridges suffer high level of damage due to this concept in the form of; Extensive cracking Cover spalling Ductility reduction in steel reinforcement. Permanent Deformation • The cost of repair of the structures is usually high and in many cases the damage is beyond repair and it has to be demolished.
- 18. Residual Displacement • Following the Kobe earthquake in Japan more than 100 reinforced concrete bridge columns were demolished because of residual drift indices exceeding 1.75%. • Japanese design criteria, 2002, have changed to explicitly require designers to limit permanent residual drifts to less than 1%.
- 19. Ductile Failure Load-Disp. Curve 1st Yield Max. Capacity Load Reversal Residual Disp. Deformed Shape Cracking Pattern Cracking
- 20. Prestressed Columns as Self-Centering (Restoring) Systems
- 21. Self-Centering Systems • Past two decades witnessed the emerge of self-centering systems while maintaining the energy dissipation concept • Many methods were developed in order to prevent the excessive damage to columns where plastic hinges form and return the structure to its original position. Rocking Systems, Using Self-Centering Material (SMA), and, Prestressed Columns.
- 23. Prestressed Columns Dynamic Behavior • Unbonded prestressed columns are widely used as self-restoring systems by adding prestressing cables through the column core. • It is believed that Cables have significant restoring behavior of pulling back the column, and hence the bridge, closer to its original position. • It is believed that the reduction of residual displacement is due to the pulling force of the cables or it is due to the application of additional axial force imposed by the prestressing? RC Vs Prestressed Columns
- 24. Case Study The research by Sun Zhiguo et al., (2015), “Experimental and numerical investigations on the seismic behavior of bridge piers with vertical unbonded prestressing strands” contained excellent experimental research for this study that has different prestressing levels and shapes. It has been chosen as a case study for the project.
- 25. Analysis Results n = 0.1 , 8#12 n = 0.2 , 8#12 n = 0.2 , 8#8 n = 0.15 , 8#12 n = 0.25 , 8#12 n = 0.2 , 8#12
- 26. Results Facts • The analytical models’ results have very good agreement with the experimental results. • The elimination of the prestressing cables and substituting their effect with axial load did not change the behavior of the column in terms of strength, stiffness and more importantly the residual displacement control. • The higher the axial force level is, the less the residual displacement. Combined analytical results
- 27. Why? The question becomes, how the axial force level affects the residual displacement?
- 28. Qzcebe and Saatcioglu’s Model • Residual displacements are controlled by pinching effect. • Pinching point location is a horizontal level at first cracking load. • In loading reversal, the curve changes the slope (k) once it hits the pinching level Vcr. • Unloading stiffness (k) is controlled by D, Dy and k2 where unloading stiffness above the pinching point is given by Equation 1 and by Equation 2. Qzcebe and Saatcioblu Model
- 29. Axial Force Level Increase Effect • Higher axial load elevates the cracking load, Vcr1 versus Vcr2 without considerable change in k2. • The unloading slope value, k, is not affected much. • The curve changes direction from the displacement peak at higher load level with gain in residual displacement; d1 • As the load path reversal hits the cracking level at a higher value, Vcr2 , another gain in plastic displacement residual, d2, is attained. • Final result is the total residual displacement gain that is equal to d1 plus d2. schematic load-displacement curves of two columns with different axial load levels
- 30. Conclusions • Prestressing as a technique to achieve self-centered columns is caused by the increase in axial load and not due to the prestressing pulling (restoring) force. • Increasing the axial load level elevates both the lateral load capacity and cracking moment (Pinching level). Both has positive effect in residual displacement control. • Several means could be used in order to achieve higher cracking level. • The increase in axial force level could be achieved by many means that might be cheaper and simpler in detailing. Prestressing Reducing the number of columns per bent. Reduced columns diameter. Using longer spans to obtain higher axial load column levels. Using engineered Cementous composite, ECC.
- 31. THANK YOU