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Structural and Durability Effects of Debonding Flexural Reinforcing Bars in Plastic Hinges of Concrete Bridges

Description:

Reinforced concrete 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.

The Canterbury 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.

Experimental and 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 hinges.

The suggested problem is relevant to the work of Transportation Research Board (TRB) AFF50 “Seismic Design and Performance of Bridges”.

Objective:

The 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 seismic regions.

Objectives of the research should include, but not limited to the followings:

  1. Investigate the effects of debonding reinforcing bars in the plastic hinges for better seismic performance and enhanced ductility of reinforced concrete columns during an earthquake

  2. Quantify the required unbonded length through testing of large-scale concrete piers

  3. Develop analytical models to predict the moment-rotation behavior of plastic hinges using debonded rebars

  4. Provide recommendations for limit states and performance-based seismic design of reinforced concrete columns with unbonded length of rebars

  5. Develop guidelines for design of new and retrofitting of concrete bridges with debonded reinforcing bars in the plastic hinge zones

  6. Recommend 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

  7. Develop inspection criteria for bridges incorporating debonded rebars in plastic hinges

The research should answer the following questions:

  1. Do the current codes prescribe accurate plastic hinge length for reinforced concrete bridges?

  2. How debonding of rebars can influence seismic performance of bridges?

  3. What are the appropriate debonding lengths/strain limits for enhancing ductility of bridges?

  4. What are the advantages and disadvantages of debonding the rebars?

  5. Is there a correlation between debonding and residual displacement after an earthquake?

  6. What are the recommendations for detailing and improved constructability for bridges with debonded rebars?

  7. What are the effects of debonding on durability and long-term performance of bridges?

  8. What are the appropriate inspection criteria for bridges with debonded rebars?

Benefits:

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.

Related Research:

As mentioned earlier, there are limited information available on the topic proposed. Some of the relevant references are:

  1. Kawashima, 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.

  2. Mashal, 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.

  3. Mashal, 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 (BNZSEE), 49(3).

  4. Mashal, 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.

  5. 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 Engineering, 143(4).

Implementation:

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
Research Priority:High
RNS Developer:Mustafa Mashal, Ph.D., P.E., Assistant Professor, Idaho State University, (208) 282-4587, mashmust@isu.edu. Petros Siders, Ph.D., Assistant Professor, Texas A&M University, (979) 845-2708, petros.sideris@tamu.edu
Date Posted:03/15/2019
Date Modified:03/18/2019
Index Terms:Durability, Debonding, Reinforcing bars, Flexural strength, Hinges, Concrete bridges, Earthquake resistant design, Bridge design,
Cosponsoring Committees: 
Subjects    
Highways
Design
Bridges and other structures

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