Structural and Durability Effects of Debonding Flexural Reinforcing Bars in Plastic Hinges of Concrete Bridges
columns subjected to large earthquakes and aftershocks can go through many
cycles of inelastic deformation and larger displacement. The earthquake forces
will place higher demand in the plastic hinge zones. Previous studies have
shown that the plastic hinge and strain penetration lengths can influence the
ductility and fatigue performance of reinforced concrete columns greatly,
especially when it comes to accommodating larger drifts and more cycles of
inelastic deformations, respectively.
earthquake sequence in New Zealand showed that most reinforced concrete
structures, detailed according to recent building codes, did not develop full
plastic hinge length as it was outlined in the building codes. This resulted in
a small number of cracks concentrated in the critical regions such as
column-to-footing connection. This strains the rebars over a short length, and
therefore compromises the ultimate displacement ductility as well as increases
the risk of low-cycle fatigue failure during an earthquake or any subsequent
aftershocks. There are still many questions to be answered as to why most of
the reinforced concrete buildings designed according to modern building codes,
performed poorly during the Christchurch earthquake. In particular, the effects
of leaving an unbonded (taped) length of the longitudinal rebars in reinforced
concrete columns and quantifying the unbonded length for better ductility and
performance-based earthquake design have not been extensively investigated.
Previous limited studies by Kawashima et al. (2001) and Mashal et al. (2014,
2016, 2019), and Nikoukalam and Sideris (2016), showed that debonding the
rebars in the plastic hinge zone can increase the ultimate drift capacity of
the column. It can also help with the fatigue performance, and therefore
enhancing the ultimate ductility and seismic performance. However, these
studies were limited in scope and were more focused on connections between
precast concrete elements. It did not include a rigorous study on quantifying
the required unbonded length, and lacked numerical modeling using finite
element analysis. At the same time, not much work has been done on the topic of
debonding flexural rebars in the context of performance-based design.
Furthermore, the effects of debonding on residual deformation following an
earthquake are still unknown.
numerical investigations are required to fill the aforementioned gaps in the
current state of knowledge on seismic behavior of reinforced concrete columns
with debonded rebars in plastic hinges. This is not limited to new bridges, but
also for retrofitting of existing concrete bridges. For an experimental work,
preliminary modeling using finite element analysis will be needed to guide
selection of test parameters such as unbonded length, strain concentration
levels, effects from size of the rebars, post-cracking stiffness etc.
Large-scale testing needs to be carried out to provide accurate effects and
observations from debonding of rebars in plastic hinges.
In addition to the
experimental and analytical work, the research should include development of
design guidelines, construction detailing, durability considerations, and
inspection criteria for bridges with debonded reinforcing bars in plastic
The suggested problem
is relevant to the work of Transportation Research Board (TRB) AFF50 “Seismic
Design and Performance of Bridges”.
scope of this project is to use simple and practical techniques such as
debonding of rebars over a certain length, to enhance ductility of reinforced
concrete columns during an earthquake. Furthermore, the debonding aims to lower
the strain levels in the rebars; thus reducing the chances of low-cycle fatigue
failure during the earthquake. Debonding of rebars is a cost-effective and
practical design approach that can be implemented in bridges located in high
of the research should include, but not limited to the followings:
effects of debonding reinforcing bars in the plastic hinges for better seismic
performance and enhanced ductility of reinforced concrete columns during an
required unbonded length through testing of large-scale concrete piers
analytical models to predict the moment-rotation behavior of plastic hinges
using debonded rebars
recommendations for limit states and performance-based seismic design of
reinforced concrete columns with unbonded length of rebars
guidelines for design of new and retrofitting of concrete bridges with debonded
reinforcing bars in the plastic hinge zones
debonding details that will not compromise durability and long-term performance
of bridges. Any adverse effects of debonding should be explored not only from
the structural perspectives, but also durability and potential for corrosion
inspection criteria for bridges incorporating debonded rebars in plastic hinges
research should answer the following questions:
Do the current
codes prescribe accurate plastic hinge length for reinforced concrete bridges?
How debonding of
rebars can influence seismic performance of bridges?
What are the
appropriate debonding lengths/strain limits for enhancing ductility of bridges?
What are the
advantages and disadvantages of debonding the rebars?
Is there a
correlation between debonding and residual displacement after an earthquake?
What are the
recommendations for detailing and improved constructability for bridges with
What are the
effects of debonding on durability and long-term performance of bridges?
What are the
appropriate inspection criteria for bridges with debonded rebars?
Several state DOTs in the United States such as
Caltrans, Oregon DOT, Washington DOT, Alaska DOT, Utah DOT, and Idaho
Transportation Department have bridges located in moderate-to-high seismic zones.
Most of the pre 1980s bridges lack proper seismic detailing in plastic hinges
and require retrofitting. Similarly, for construction of new bridges, the
plastic hinge detail has to correlate with observed damage following an
earthquake. The research proposed here could help in enhancing ductility and
seismic performance of both existing and new bridges located in
moderate-to-high seismic zones. It uses simple and practical approach of debonding
rebars over certain lengths to achieve better seismic performance for the
bridge structure. Results from the research have the potential to refine bridge
design manuals and specifications for seismic resistant design for the
aforementioned DOTs. The research could also help to refine the AASHTO LRFD
Bridge Design Specifications. If the research is not funded, the current
specifications regarding seismic design (e.g. plastic hinge lengths, ultimate
ductility, low-cycle fatigue performance etc.) may not correlate with observed
damage to bridge structures following an earthquake. This could create
confusion, uncertainty, and lack of trust in bridge design
manuals/specifications among the bridge practitioners. The 2011 Christchurch
earthquake and 2016 Kaikoura earthquake in New Zealand demonstrated the needs
for refining and improving seismic detailing of bridges so the expected seismic
performance correlate with observed earthquake damage.
mentioned earlier, there are limited information available on the topic
proposed. Some of the relevant references are:
K., Hosoiri, K., Shoji, G., and Sakai, J. (2001). “Effects of Un-Bonding of
Main Reinforcements at Plastic Hinge Region on Enhanced Ductility of Reinforced
Concrete Bridge Columns. Structural and Earthquake Engineering.” Proceedings of
Japan Society of Civil Engineering, 689 (I-57), 45-64.
M., White, S., and Palermo, A. (2014). “Experimental Testing of Emulative
Connections for Accelerated Bridge Construction in Seismic Areas” Proceedings,
9th Austroads Bridge Conference 2014, Sydney, Australia.
M., White, S., and Palermo, A. (2016). “Quasi-Static Cyclic Testing of
Emulative Cast-In-Place Connections for Accelerated Bridge Construction in
Seismic Regions” Bulletin of New Zealand Society for Earthquake Engineering
M., and Palermo, A. (2019). “Emulative Seismic Resistant Technology for
Accelerated Bridge Construction” Elsevier Journal of Soils Dynamics and
Earthquake Engineering, Special Issue on Earthquake Resilient Buildings, 120.
Nikoukalam, M. T., and Sideris, P. (2016). “Experimental
Performance Assessment of Nearly Full-Scale Reinforced Concrete Columns with
Partially Debonded Longitudinal Reinforcement”. ASCE Journal of Structural
DOTs interested in the implementation of the research
results would need confidence in adequate performance of the plastic hinge
detail with debonded rebars from the perspectives of structural strength, construction
practicality, long-term durability and post-earthquake/regular maintenance
inspections. Simplified formulas quantifying the required debonded length and
standard details showing the debonding arrangement have to be developed and
included in bridge manuals. Activities such as technical workshops/webinars
through TRB AFF50, AASHTO, and other platforms will be important to build
confidence among the bridge practitioners for implementation of the research
results. Further research programs such as validation using shake table testing
of the scaled bridge, publication and peer-review of the research results could
also help in a wider implementation of the research by state DOTs.
|Sponsoring Committee:||AFF50, Seismic Design and Performance of Bridges
|Research Period:||12 - 24 months|
|RNS Developer:||Mustafa Mashal, Ph.D., P.E., Assistant Professor, Idaho State University, (208) 282-4587, email@example.com. Petros Siders, Ph.D., Assistant Professor, Texas A&M University, (979) 845-2708, firstname.lastname@example.org|
|Index Terms:||Durability, Debonding, Reinforcing bars, Flexural strength, Hinges, Concrete bridges, Earthquake resistant design, Bridge design, |
Bridges and other structures