تواصل_تطوير المحاضرة رقم 194 دكتور / هشام ندا المدير الدولي لقسم الكباري بشركة خطيب وعلمي عنوان المحاضرة " Prestressed columns; application and behaviour" يوم الإثنين 23 يناير 2023 الثامنة مساء توقيت القاهرة التاسعة مساء توقيت مكة المكرمة و الحضور عبر تطبيق زووم من خلال الرابط https://us02web.zoom.us/meeting/register/tZ0tcOiqpj4qGd2gW6_QEuVYtJrcgIv-tkW2 علما ان هناك بث مباشر للمحاضرة على القنوات الخاصة بجمعية المهندسين المصريين ونأمل أن نوفق في تقديم ما ينفع المهندس ومهمة الهندسة في عالمنا العربي والله الموفق للتواصل مع إدارة المبادرة عبر قناة التليجرام https://t.me/EEAKSA ومتابعة المبادرة والبث المباشر عبر نوافذنا المختلفة رابط اللينكدان والمكتبة الالكترونية https://www.linkedin.com/company/eeaksa-egyptian-engineers-association/ رابط قناة التويتر https://twitter.com/eeaksa رابط قناة الفيسبوك https://www.facebook.com/EEAKSA رابط قناة اليوتيوب https://www.youtube.com/user/EEAchannal رابط التسجيل العام للمحاضرات https://forms.gle/vVmw7L187tiATRPw9 ملحوظة : توجد شهادات حضور مجانية لمن يسجل فى رابط التقيم اخر المحاضرة.

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

11. Applications
Self-Centering Columns .

12. Performance Based Design

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.

16. Force-Based Design

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.

22. 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