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Research Projects (1999-00)

Identifying Number


Project Title

Moment-Rotation Tests of High Performance Steel (HPS) I-Girders


Colorado State University

Project Investigator

Dr. Bryan A. Hartnagel
Department of Civil Engineering
Colorado State University
(970)491-4660 or Fax (970)491-7727

External Project Contact


Project Objective

The proposed program will provide additional information on the behavior of high-performance steel (HPS) by conducting experimental tests on four girders fabricated from HPS over a two-year period.

Objectives of the program are:

  • To investigate the flexural strength and ductility of bridge I-girders fabricated from HPS.
  • To compare the flexural strength and ductility of bridge I-girders made from HPS to that predicted by current AASHTO bridge design provisions.
  • To develop a finite element model of I-girders fabricated from HPS
  • To compare the experimental and analytical test results with analytical results presented in the literature.

Project Abstract

Bridge designers now have a new choice of steel available for consideration when planning a bridge. High performance steel (HPS) grade 70W is currently available for bridge construction. The HPS70W was developed under a cooperative research program between the Federal Highway Administration (FHWA), the U.S. Navy, and the American Iron and Steel Institute (AISI). However, current bridge design provisions limit the flexural strength of girders with yield strength greater than 50 ksi (350 MPa) to the yield moment capacity. The flexural capacity of similar bridge girders designed with yield strength less than or equal to 50 ksi (350 MPa) is equal to the plastic moment capacity if certain restrictions are met. Further, if the designer chooses, an inelastic analysis of the girder is allowed with steel yield strengths less than or equal to 50 ksi (350 MPa). Inelastic analysis and design methods offer larger cost savings compared to the elastic analysis provisions. Even with the disadvantage on the flexural capacity, HPS is still competitive with Grade 50 steels because of material savings obtainable with HPS. If these restrictions could be lifted or even relieved, the use of HPS would provide significant cost savings.

Project Description

The AASHTO Load and Resistance Factor Design (LRFD) Specification considers any girder cross section with a specified minimum yield strength exceeding 50 ksi to be non-compact. This limits the load carrying capacity of the cross section to a maximum of the yield moment. The capacity of girders with a specified minimum yield strength less than or equal to 50 ksi can be as large as the plastic moment, provided certain cross section properties are met. Research has shown that the capacity of girders fabricated from steel with a specified minimum yield strength greater than 50 ksi can sustain loads larger than the yield moment. However, more research is necessary before changes in the design specification can be implemented.

Before restrictions on the use of HPS can be removed, adequate knowledge of the material behavior must be known. This proposal is intended to provide additional information on the behavior of HPS. This proposal will support the experimental testing of four I-girders fabricated from HPS. Funding is also included to further the analytical modeling of HPS by extending existing studies to include parameters that were held constant in previous research by others.

Experimental Work

To accomplish the objectives, listed above in item 5 of the proposal, a series of tests will be conducted on four I-girder specimens. The I-girders will be fabricated from HPS70W steel. Two different compression flange slenderness ratios, (bf/2tf), and two different depth of web in compression to depth of web, (Dcp/D), ratios will be used to make up the four specimens. Compression flange slenderness ratios that are near the ultra-compact limit and near the compact limit specified in the current bridge design specifications. Values for Dcp/D will be 0.5 and 0.75. Each of the four specimens will be tested in three point bending, as were all the other specimens in the literature. The simple beam bending of the test specimens represents the portion of a continuous span girder between the inflection points of moment near the interior support, albeit upside down. This length is approximately 20 percent of the span length on each side of the interior support. The condition of moment at the interior support is similar for girders with any number of continuous spans.

Analytical Work

The analytical portion of the project will develop a finite element model of HPS I-girders. Results from this model will initially be compared to results from previous research for validation. An identical specimen will be used for this comparison. After a reliable model is developed, it will be used to predict the behavior of the experimental I-girders. Experimental results will provide verification of all finite element models from this research and from previous research conducted by others. An additional part of the analytical work will be to extend previous finite element investigations to include the effects of varying parameters held constant in previous research. The following parameter was held constant in the previous study:

The depth of web in compression to the total depth of web (Dcp/D) is assumed to be 0.5. In this study, depth of web in compression to the total depth of web (Dcp/D), will be set to 0.5 and approximately 0.75.

For girders symmetrical about the major bending axis (noncomposite), (Dcp/D) is equal to 0.5. However, for girders unsymmetrical about the major bending axis (composite) the (Dcp/D) ratio is closer to 0.75. This is significant because most of the bridge girders designed today act composite with the concrete deck. This will raise the plastic neutral axis and therefore increase the (Dcp/D) ratio. Previous research found the effects of this parameter to be significant for Grade 50 steel. Material parameters affecting the stress-strain behavior of the HPS will be varied. These parameters include the yield ratio, (Fy/Fu), the ratio of the strain at strain hardening to the yield strain, (est/ey), and the strain hardening modulus (Est). The range of each of these parameters will be the same characterization of HPS stress-strain behavior used in previous studies. Experimental tension test coupons are planned for material taken from the I-girder specimens. They will be used to determine the tensile stress-strain behavior of the HPS. This information will then compared to the parameters used in the analytical study.

Geometric parameters of the cross section will also be varied in the finite element models. The parameters that most effect the moment-rotation behavior of I-girders include the compression flange slenderness, (bfc/2tfc), the web slenderness, (Dcp/2tw), and the cross section aspect ratio, (D/bfc). Two compression flange slenderness values will be used that correspond to ultra-compact flange (bfc/2tfc = 0.288(E/Fy)½) and compact flange (bfc/2tfc = 0.382(E/Fy)½). Significant inelastic bending strains can develop in the compression flange before local flange buckling occurs if the flange is ultra-compact. The compact flange limit is that specified by AASHTO. This slenderness limit is relaxed compared to the ultra-compact limit but still allows the girder to reach the plastic moment capacity, Mp for girders fabricated from material with a specified minimum yield strength less than or equal to 50 ksi.

In summary, the analytical model will extend the variation of parameters studied in previous research conducted by others. It will also be used to predict the behavior of the experimental I-girders.

Task Descriptions

The tasks and approximate completion dates necessary to achieve the objectives of the research are:

  • Task 1 – Determine size and geometry of first two test girder specimens. (B. Hartnagel - November 31, 1999)
  • Task 2 – Order test girder specimens (2). (B. Hartnagel - January 15, 1999)
  • Task 3 – Acquire instrumentation and necessary testing fixtures. (B. Hartnagel - January 15, 1999)
  • Task 4 – Develop and refine finite element model. (B. Hartnagel - ongoing through August 31, 2000)
  • Task 5 – Test both I-girder specimens for moment - rotation behavior and strength. (B. Hartnagel - August 31, 2000)
  • Task 6 – Compare test results, finite element model results and previous research results. (B. Hartnagel - September 30, 2000)
  • Task 7 – Prepare interim report and request funding for second year to test two additional girders. (B. Hartnagel - October 31, 2000)
  • Task 8 – Prepare and submit technical paper on experimental tests and analytical models. (B. Hartnagel - October 31, 2000)

Milestones, Dates

  • Starting Data: November 1, 1999
  • Project Milestones: See Task Descriptions (Item 7)
  • Ending Date: October 31, 2001

Yearly and Total Budget

The attached budget is for the first year of the project. The second year will have a similar budget. For the first year the requested amount is $21,025. It is anticipated that and additional $21,000-$22,000 will be requested for the second year.

Student Involvement (e.g. Thesis, Assistantships, Paid Employment)

One graduate student will complete a Master's thesis or Ph.D. dissertation on the project and several undergraduate students will work on the project. The graduate student will supervise and conduct the experimental tests. They will also be responsible for developing the analytical finite element models. Hourly undergraduate students will help in conducting the experimental tests by instrumenting the test girders and preparing the data acquisition system. This will provide them with an opportunity to learn how to conduct experimental research.

Relationship to Other Research Projects

No other related MPC projects to date.

Technology Transfer Activities

The research will provide information on the behavior of high-performance steel. This information could be used by AASHTO to make changes in the current bridge design specifications regarding the design of HPS. Analytical models will be developed to investigate parameters that were not varied in previous research. Identical analytical models will be compared to previous research results to validate the models.

Experimental tests will verify the analytical models presented in this and other research. Many experimental tests have been conducted on I-girders fabricated from steel with nominal yield strength of 50 ksi (350 MPa) or less, however, this project will be one of only a few experimental tests conducted on I-girders fabricated with 70 ksi (485 MPa) nominal yield strength steel. Therefore, results from this project will be significant in developing design provisions for HPS70W steel.

Potential Benefits of the Project

Current bridge design provisions allow the use of high-performance steel but conservatively restrict the load carrying capacity of HPS. This research could provide information necessary to ease some of the restrictions placed on the use of HPS. These changes could allow more economical bridges to be designed using HPS. Designers are reluctant to use inelastic design provisions for the design of new structures regardless of the specified minimum yield strength. The real benefit of inelastic design provisions would be their use in load rating bridges constructed from any grade of steel. However, before inelastic analysis and design can be used to load rate bridges, the inelastic provisions must be available for the design of bridges. A rating method based on inelastic analysis would be beneficial for rural counties without funds necessary to replace all structurally deficient bridges. If load rating based on inelastic analysis was allowed, some bridges might not be structurally deficient because of the reserve strength inherent in continuous span bridges.

TRB Keywords

Inelastic design, high performance steel, bridges

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