Education

Research Description

Dr. Nau is interested in structural design, structural dynamics, earthquake engineering, and numerical methods. His research is principally in the area of earthquake engineering. He is currently engaged in several research projects on the seismic response of steel and concrete bridge structures, with support from the Alaska University Transportation Center and the Alaska Department of Transportation. In collaboration with colleagues Mervyn Kowalsky and Rudi Seracino, this research involves large-scale experimentation at the Constructed Facilities Lab. Dr. Nau's research interests have recently expanded to include structural repair using fiber reinforced polymer (FRP) materials. Reinforced concrete columns tested as part of a AUTC/AKDOT project have successfully been repaired to restore strength while preserving ductility.

Publications

Behavior of large diameter carbon fiber anchors

Tasdemir, E., Seracino, R., Kowalsky, M. J., & Nau, J. (2023), CONSTRUCTION AND BUILDING MATERIALS, 394. https://doi.org/10.1016/j.conbuildmat.2023.132174

Tensile Behavior of Large Diameter Carbon Fiber Anchors

Tasdemir, E., Seracino, R., Kowalsky, M., & Nau, J. (2022), 10TH INTERNATIONAL CONFERENCE ON FRP COMPOSITES IN CIVIL ENGINEERING (CICE 2020/2021), Vol. 198, pp. 1199–1207. https://doi.org/10.1007/978-3-030-88166-5_104

Seismic Repair of Circular Reinforced Concrete Bridge Columns by Plastic Hinge Relocation with Grouted Annular Ring

Krish, Z. F., Kowalsky, M. J., & Nau, J. M. (2019), Journal of Earthquake Engineering, 25(12), 1–35. https://doi.org/10.1080/13632469.2019.1688205

Seismic performance of reinforced concrete filled steel tube drilled shafts with inground plastic hinges

Aguirre, D. A., Kowalsky, M. J., Nau, J. M., Gabr, M., & Lucier, G. (2018), Engineering Structures, 165, 106–119. https://doi.org/10.1016/j.engstruct.2018.03.034

Closure to "Modified Plastic-Hinge Method for Circular RC Bridge Columns" by Jason C. Goodnight, Mervyn J. Kowalsky, and James M. Nau

Goodnight, J. C., Kowalsky, M. J., & Nau, J. M. (2017, September), JOURNAL OF STRUCTURAL ENGINEERING, Vol. 143. https://doi.org/10.1061/(asce)st.1943-541x.0001867

Development of a ductile steel bridge substructure system

Fulmer, S. J., Nau, J. M., Kowalsky, M. J., & Marx, E. E. (2016), Journal of Constructional Steel Research, 118, 194–206. https://doi.org/10.1016/j.jcsr.2015.11.012

Equivalent Viscous Damping Model for Short-Period Reinforced Concrete Bridges

Khan, E., Kowalsky, M. J., & Nau, J. M. (2016), Journal of Bridge Engineering, 21(2), 04015047. https://doi.org/10.1061/(asce)be.1943-5592.0000803

Modified Plastic-Hinge Method for Circular RC Bridge Columns

Goodnight, J. C., Kowalsky, M. J., & Nau, J. M. (2016), Journal of Structural Engineering, 142(11), 04016103. https://doi.org/10.1061/(asce)st.1943-541x.0001570

Strain Limit States for Circular RC Bridge Columns

Goodnight, J. C., Kowalsky, M. J., & Nau, J. M. (2016), EARTHQUAKE SPECTRA, 32(3), 1627–1652. https://doi.org/10.1193/030315eqs036m

Effect of Seismic Load History on Deformation Limit States for Longitudinal Bar Buckling in RC Circular Columns

Feng, Y., Kowalsky, M. J., & Nau, J. M. (2015), Journal of Structural Engineering, 141(8), 04014187. https://doi.org/10.1061/(asce)st.1943-541x.0001153

View all publications via NC State Libraries

Grants

Seismic Retrofit of Anchorage Port Access Bypass

US Dept. of Transportation (DOT)(7/23/20 - 12/31/23)

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Seismic Retrofit of Anchorage Port Access Bypass

Sponsor: US Dept. of Transportation (DOT)

Start Date: 7/23/20

End Date: 12/31/23

Abstract

Based upon a seismic analysis performed in 2013, the Anchorage Port Access bridge (Bridge number 0455) requires a seismic retrofit to safely accommodate the design earthquake. The bridge, shown in Figure 1, has several deficiencies including steel column-to-cap beam connections that were determined to perform poorly under seismic loading during the first phase of this study. The objective of this study is to develop retrofit recommendations through large scale testing, modelling, and analysis. During phase one of the work, a global computational model (using lumped plasticity frame elements) was developed to assess bridge deformation demands. As part of that work, hysteretic response of the components was represented through consideration of results of a local FEM model and experimental tests. Also included in that model was the impact of damping model choices, soil-structure interaction, and ground motion directionality. The local FEM model considered the details of the individual connections to assess stress concentrations, and was calibrated through the experimental tests. Lastly, the experiments that were conducted included half scale models of two representative bridge bents that were subjected to reversed cyclic loading. Details of each of these components of the first phase of work are described next.

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Seismic Assessment of Anchorage Port Access Bypass

State of Alaska, Department of Transportation(12/01/17 - 12/31/20)

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Seismic Assessment of Anchorage Port Access Bypass

Sponsor: State of Alaska, Department of Transportation

Start Date: 12/01/17

End Date: 12/31/20

Abstract

This research will aim to investigate a series of steel column to cap connections historically employed for bridge design that are felt to be deficient with regards to seismic performance. Computational models for the global bridge system and local connections will be developed, along with large scale seismic testing of bridge components. The goal of the research is to assess the behavior of such connections and eventually propose retrofit techniques as needed.

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Durability of the Grouted Shear Stud Connection at Low Temperatures

US Dept. of Transportation (DOT)(3/01/16 - 5/15/19)

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Durability of the Grouted Shear Stud Connection at Low Temperatures

Sponsor: US Dept. of Transportation (DOT)

Start Date: 3/01/16

End Date: 5/15/19

Abstract

This research proposal addresses the design and behavior of a new, recently developed connection for steel bridge substructures. This new connection, termed the grouted shear stud connection (GSS), was shown to be an effective and improved design over current practice of directly welding hollow circular steel pipe piles to steel cap beams. In previous studies, the new connection performed well, in both quasi-static and dynamic tests at ambient (indoor) laboratory temperatures. Because this new connection relies upon the integrity of grout, a cementitious material prone to degradation from severe environmental conditions typical in Alaska, the durability of this material and the connection as a whole must be assessed. This proposed project focuses on the durability of the GSS by optimizing the grout properties through material testing, and subsequent tests of weathered and unweathered full-scale connection specimens when subjected to temperatures as low as -40° C. The outcome of this research will be a set of design recommendations to maximize the durability and hence the long term performance of the grouted shear stud connection. While developed for new bridge construction, the GSS connection is readily adapted to retrofit applications in existing steel bridge substructures with deficient pipe column to cap beam connections. The results of this study may therefore be of value in the eventual structural upgrade of Bridge No. 455, the Anchorage Port Access.

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Transverse Seismic Design of Bridges with Longitudinal Keyway Joints

US Dept. of Transportation (DOT)(3/01/16 - 5/15/19)

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Transverse Seismic Design of Bridges with Longitudinal Keyway Joints

Sponsor: US Dept. of Transportation (DOT)

Start Date: 3/01/16

End Date: 5/15/19

Abstract

This research proposal studies the behavior of a common bridge system employed in Alaska that utilizes decked bulb-tee girders connected by a diamond shaped longitudinal grouted keyway joint with intermittent steel shear connectors. While the behavior of the joint under gravity loads is well established, the performance under transverse seismic excitation is largely unknown. Through the use of full scale subassembly tests and non-linear modelling, recommendations on (1) Connection limit states; (2) Improvements to the connection (if needed); and (3) Lateral Modeling approaches will be proposed

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Seismic Repair of Reinforced Concrete Bridge Columns by Plastic Hinge Relocation

US Dept. of Transportation (DOT)(5/15/15 - 9/30/18)

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Seismic Repair of Reinforced Concrete Bridge Columns by Plastic Hinge Relocation

Sponsor: US Dept. of Transportation (DOT)

Start Date: 5/15/15

End Date: 9/30/18

Abstract

Modern seismic design practices for bridge structures involve the use of capacity design principles that locate plastic hinges in columns, while protecting against other modes of failure or locations of damage. For large earthquakes, the formation of plastic hinges in columns can lead to buckling and rupture of longitudinal steel. Traditionally, once buckling occurs, bridge columns are demolished and rebuilt because the cost to replace portions of columns can be prohibitive. Replacement is deemed necessary since the inelastic strain capacity of reinforcing bars is severely diminished once buckling occurs, rendering the structure vulnerable to collapse in future earthquakes. Bridge column repair has been studied for some time with several established techniques for repair for shear and confinement critical columns. To the knowledge of the PIs, there is little data available on repair of columns that are otherwise designed to modern standards. Similarly, there is little data available on developing an understanding of when repair is truly needed, and when repair is truly no longer feasible. A recent pilot study conducted at NCSU demonstrated the feasibility of a repair technique which employed the relocation of the plastic hinge to a previously undamaged location within the column. This was successfully employed for columns that sustained buckled reinforcing bars, and showed promise for columns with fractured bars. In this proposal, the pilot study is expanded by developing a suite of repair techniques aimed at achieving plastic hinge relocation in damaged columns. Techniques may include the use of fiber reinforced polymers, high strength steel, reinforced concrete and structural steel. The focus will be on the concept of ‘hinge relocation for repair’ and will consider variables such as the need for rapid deployment following an event, environmental conditions at the time of repair, and expertise of potential repair workers in Alaska. The research will utilize columns that will be built and damaged as part of another AKDOT research project, thus maximizing resources. Recommendations will consist of analysis and design guidelines, as a function of damage level (i.e. strain limits), for repair design of reinforced concrete (RC) bridge column to footing connections. The recommendations will also be applicable to some RC column to cap connections, although specific tests on that configuration are not part of this phase of work.. In addition, analytical studies will be conducted on other bridge column connection types (e.g., reinforced concrete filled steel pipes to pile cap beams) such that the direction for future experimental work on those connections may proceed.

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Seismic Load Path Effects in RC Bridge Columns and Wall Piers

State of Alaska, Department of Transportation(11/15/14 - 6/30/17)

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Seismic Load Path Effects in RC Bridge Columns and Wall Piers

Sponsor: State of Alaska, Department of Transportation

Start Date: 11/15/14

End Date: 6/30/17

Abstract

Ongoing research at NCSU funded by AKDOT and AUTC has investigated the impact of loading history on the definition of strain limit states, as well as the relationship between strain and displacement. The 2009 AASHTO Seismic provisions are a displacement-based document and as a consequence, accurate estimates of displacement at key performance limit states are essential. This research project is the next logical step in the study on load history where the impact of a 2-dimensional load path is investigated. The research will also include a component of study related load path effects on the stability of ductile wall piers. Presented in this proposal is background information regarding the prior study in load history conducted at NCSU, followed by a discussion of the critical issues identified in the literature regarding multi-directional loading and a proposed task-by-task approach to the research program.

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Displacement-Based Design Seminar and Bridge Research Implementation

US Dept. of Transportation (DOT)(11/30/-1 - 8/31/14)

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Displacement-Based Design Seminar and Bridge Research Implementation

Sponsor: US Dept. of Transportation (DOT)

Start Date: 11/30/-1

End Date: 8/31/14

Abstract

During the first two days of the meetings, instructors Kowalsky, Priestley, Calvi, and Goodnight will present the displacement-based design approaches as it is applied to bridge structures. The procedure, which has undergone continual development since 1993, led to a textbook on the topic in 2007. Since that time, refinement have continued to be developed. During the 2 days of the seminar, the procedure will presented, with the end outcome being the tools needed to apply the DDBD approach to the design of highway bridges, typical of those seen in the state of Alaska. Topics covered include seismic demands, equivalent damping, moment curvature analysis, member response, design for SDOF systems, special design cases, impacts of P-D effects, limit state displacements, soil structure interaction, design verification tools, longitudinal bridge design, transverse bridge design, curved bridges, design of column plastic hinges, overstrength, shear design, bridge isolation systems, cable stay bridges, and wharves. Example problems will be posed to attendees at the end of the first day for overnight solution and presentation of results the following morning. The final two days of the meetings will deal with design implementation of past research conducted at NCSU for AKDOT. This will include sessions on the new grouted shear stud connection for bridge substructures, the impact of load history on bar buckling, new proposed plastic hinge length equations, and behavior of concrete filled pipe piles. In addition, an interactive discussion in planned for the final day on current and future research projects. In all cases, these projects have already found implementation into AKDOT practice, and the objective of these sessions will be to present the outcomes, and allow for question/answer/interaction on proposed design recommendations

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Reinforced Concrete Filled Pipe Piles in Soil

State of Alaska, Department of Transportation(8/15/13 - 12/31/16)

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Reinforced Concrete Filled Pipe Piles in Soil

Sponsor: State of Alaska, Department of Transportation

Start Date: 8/15/13

End Date: 12/31/16

Abstract

A research project on reinforced concrete filled pipe piles concluding in May of 2013 had the following objectives: (1) Develop recommendations for strain limits for use in seismic design at key design limit states as a function of diameter/thickness (D/t) ratio and material properties, (2) Develop an equation (via computation) for the plastic hinge length of ?below ground hinges?, (3) Quantify the impact of reinforcing steel on performance and confirm that strain compatibility can be used for prediction of the force-displacement response. These three objectives have been studied through the use of large scale experimental testing, and the analysis of pile members. In addition, work in the project provided recommendations for equations to estimate equivalent viscous damping, which are required for implementation in a direct displacement-based design approach. This proposal builds upon the work previously conducted through the following tasks: (1) Large scale testing of reinforced concrete filled pipe piles in soil; and (2) FEA and fiber-based SSI analysis. The specific goals of this proposed research project are to examine the impact that soil stiffness has on: (1) Pipe pile strain limit states; (2) Plastic hinge length and integration of curvature for deformations; (3) Proposed analysis methods, and (4) Damping

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Strain Limits for Concrete Filled Steel Tubes in AASHTO Seismic Provisions

US Dept. of Transportation (DOT)(11/30/-1 - 5/31/13)

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Strain Limits for Concrete Filled Steel Tubes in AASHTO Seismic Provisions

Sponsor: US Dept. of Transportation (DOT)

Start Date: 11/30/-1

End Date: 5/31/13

Abstract

The use of concrete filled steel tubes in bridge construction is common in the state of Alaska, as well as in other states and regions of the country. One reason for their use is that construction is simplified because the steel tubes serve the dual function of the foundation, i.e., piles below the ground surface, and the above-ground columns for the cap beam. The majority of past research has dealt with concrete filled steel tubes at very small scale and without internal reinforcement. In Alaska, the preferred system utilizes longitudinal reinforcement in addition to the steel tube itself. While available research has shown that the performance of concrete filled steel tubes is satisfactory, a number of important questions remain unanswered for the particular application commonly used in Alaska. The problems that will be addressed in this research project include the impact of reinforcing steel on the behavior of the pile-column, the accuracy of analysis methods for prediction of force-deformation response of the pile-column system, the impact of the ratio of tube diameter to tube thickness (D/t ratio) on the performance of the pile-column at multiple limit states, and the plastic hinge length for the below-ground hinge developed in the pile-column. These problem areas will be examined through a series of 10 large-scale tests on concrete filled pile-columns, and from the development of a finite element model capable of capturing all of the anticipated modes of failure, including local buckling of the steel tube. Two of these tests will be conducted in the environmental chamber at NC State, in an attempt to capture the effects of low temperatures (-40C) on structural behavior and performance. The research will result in design expressions relating D/t to strains at various limit states, design expressions for plastic hinge lengths for concrete filled pipes with internal reinforcing steel, and modifications to moment-curvature analysis tools, if required, to predict the force-deformation response to cyclic loading. The research findings will be summarized in a concise design manual appropriate for AKDOT use.

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Effect of Load History on Performance Limit States of Bridge Columns

US Dept. of Transportation (DOT)(11/30/-1 - 8/15/12)

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Effect of Load History on Performance Limit States of Bridge Columns

Sponsor: US Dept. of Transportation (DOT)

Start Date: 11/30/-1

End Date: 8/15/12

Abstract

This research project investigates the impact of seismic loading history on the design of reinforced concrete bridge columns typical of those used in the State of Alaska. Currently, structural engineers use concrete and steel strain limit states in seismic design which have minimal experimental or theoretical basis. While the strain limits that are typically used attempt to account for cyclic loading which earthquakes impose, there is insufficient basis for their selection. Furthermore, these strain limits are often converted, through monotonic section analysis, to displacement limits for design purposes. But again, the cyclic nature of earthquake loading can significantly alter the relationship between strain and displacement. An understanding of this relationship between strain and displacement, as a function of loading history, is essential for reliable seismic design. The overall objective of this project is to propose strain limit states that account for regional seismic loading histories in Alaska, and to relate these proposed strain limits to displacement limits. The goals of this project will be met through a combination of analytical and experimental studies. A key requirement of the experimental work is the ability to measure large strains (up to 12%). NCSU has an optical measurement system previously purchased for this purpose which has been shown to be reliable for such measurement of strains in reinforcing steel. The seismic loading histories to which the test specimens will be subjected will be determined from a dataset developed by researchers at the University of Alaska-Anchorage. Results from both frame-type and fiber-based analyses using the ground motions from the dataset will guide the initial selection of specimen design variables. A total of nine tests on essentially full scale circular bridge columns will be performed. The details of the specimens will be determined based the results obtained from the current project, from the analytical results, and from each of the previous tests in this proposed phase two project. Given the limited number of tests, it is important to learn as much as possible from each test before designing and conducting additional tests. This approach has worked successfully in current and previous projects sponsored by the Alaska DOT. These nine test units will be subjected to load histories with varying characteristics, but typical of those experienced in Alaska. The results of this research project will be presented as proposed tensile and compressive strain limits and corresponding displacement limits. The primary benefit of this project will be a better understanding of how seismic load history influences the performance of reinforced concrete bridges in Alaska. Alaska DOT engineers will be provided with tools to refine bridge designs optimized for regional seismicity, ensuring that bridges in Alaska remain safe in major earthquakes and serviceable in smaller earthquakes.

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