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Performance-Based Tolerances for Fabrication of Steel Bridge Members

Description:

Tolerances are required in fabrication of steel bridges so that designers can be assured the actual structure does not deviate too far from the design, and fabricators have some leeway during fabrication. Such leeway is needed because bridges are heavy weldments comprised of steel plates that themselves have allowable waviness and variation in thickness and because distortions from welding, cutting, and other heat-associated operations can be predicted and accounted for. AASHTO publishes tolerances for steel bridges mostly in three locations; 1) the AASHTO LFRD Bridge Construction Specifications, 2) the AASHTO/AWS D1.5/D1.5M Bridge Welding Code and to a lesser extent, 3) the AASHTO LRFD Bridge Design Specifications. For instance, the Bridge Welding Code contains dimensional tolerances for weldments including warpage and tilt of flanges, web out-of-flatness, camber, and acceptable levels of undercut. The LFRD Bridge Construction Specifications mostly reference back to the Bridge Welding Code but do have specific requirements such as allowable pre-welding gaps between rib and deck plates for orthotropic steel decks. Similar tolerances exist in the LRFD Bridge Design Specifications for orthotropic decks.

Published tolerances are generally based on best practices or workmanship criteria, not necessarily based in performance of the structure. Further, designers sometimes arbitrarily impose more restrictive tolerances resulting in added costs that have no bearing on the performance of the member. There is an urgent need to review all tolerance criteria published in AASHTO specifications for use in the fabrication of steel bridges and determine if they are needed for structural performance or are merely workmanship an or aesthetic requirements. Rational criteria should be used to define new tolerances if the existing are proven too restrictive based on structural performance requirements, and additional restrictions provided if aesthetics or workmanship must govern. The research should identify how fabrication practices used to achieve the specified tolerances, including cutting, fit up, distortion control, welding, and restraints, affect structural performance. Additionally, the cost associated with restrictive tolerances should be documented.

This research problem statement contributes directly to the Committee on Bridges and Structures 2014 Strategic Plan Prioritized Objective 4: Maintain and Enhance the AASHTO Specifications, and to the T-17 effort to develop the AASHTO Metals Fabrication Specification.

Objective:

The objective of this research is to develop performance-based dimensional tolerances for fabrication and erection of steel bridge decks and superstructures, both in shop and field applications. Supplemental tolerances based on visual aesthetics shall also be provided along with estimated costs to achieve them.

Related Research:

Literature search on TRID was performed using the following search criteria:

·

Searching the keywords “bridge tolerance” yielded only one possible relevant hit. The on-going NCHRP 12-99 project entitled “Recommended Guidelines for Prefabricated Bridge Elements and Systems Tolerances and Dynamic Effects of Bridge Moves,” however this project is addressing tolerances between prefabricated elements, the concern of this RNS is the tolerances within the prefabricated element itself.

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Searching keywords “tolerance OR fabrication” with “bridge” in the title resulted in no relevant hits.

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Searching keywords “bridge OR tolerance” with “performance” in the title resulted in no relevant hits.

·

Searching just keywords “tolerance OR fabrication” itself resulted in two hits relevant to steel bridges:

o

Thimmhardy, E. G., Korol, R. M. (1988). Geometric Imperfections and Tolerances for Steel Box Girder Bridges. Canadian Journal of Civil Engineering. Volume 15, Issue 3.

o

Amirikian, A. (1965). Dimensional tolerances and Quality Control in Welded Steel Construction. Highway Research Record, Issue 85.

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The National Technical Reports Library was searched and no additional references could be found pertaining to steel bridges, however numerous references could be found pertaining to steel ship structure fabrication. This indicated that likely other industries beyond bridges have some published literature that can be used for the project. One example would be:

o

Kendrick, A., Assakkaf, A. I. (2004). “The Effect of Fabrication Tolerances on Fatigue Life of Welded Joints.” Ship Structure Committee, Washington D.C.

Tasks:

Following research approach/tasks are anticipated for accomplishing the project objectives.

Phase I

Task 1. Collect and review relevant domestic and foreign literature, research findings, information and existing specifications regarding tolerance requirements for fabrication of steel bridge members and their influence on member performance. This information may be obtained from published and unpublished reports, and contacts with transportation agencies and other public and private organizations.

Task 2. Synthesize the literature review to identify (1) the key parameters that affect the performance of fabricated steel members, (2) the tolerance requirements for steel member fabrication, and (3) the relevant sections of the existing fabrication specifications that need to be revised for addressing the arbitrariness in the specifications.

Task 3. Assess the relevance of the fabrication parameters and tolerances identified in Task 2 and identify those most relevant to fabrication of structural steel bridge members for further consideration in this research.

Task 4. Based on the findings of Task 3, develop a proposed plan that includes both computational and experimental investigations, to be executed in Phase II, for determining rational tolerances for fabrication of structural steel members for highway bridges.

Task 5. Prepare an interim report that documents the research performed in Tasks 1 through 4 for review and approval by NCHRP.

Phase II

Task 6. Perform research as per plan developed in Task 4 and approved in Task 5.

Task 7. Based on the research performed, develop rational tolerance for fabrication of steel bridge members.

Task 8. Develop recommendations for revisions to the relevant specifications for rational fabrication tolerances for steel bridge members.

Task 9. Prepare a final report that documents the entire research effort.

Implementation:

The full implementation of this research is to develop code language that would be adopted by AASHTO CBS in the AASHTO Metals Fabrication Specifications that are currently under development by T-17 Technical Committee on Metals Fabrication. As part of this specification development, it is anticipated that presentations will be given to T-17, the National Steel Bridge Alliance Task Group 2, Fabrication, and to the Joint AASHTO-AWS D1.5 Bridge Welding Subcommittee in order to disseminate the findings and to develop support for the anticipated code provisions.

Sponsoring Committee:AKC70, Fabrication and Inspection of Metal Structures
Research Period:24 - 36 months
Research Priority:High
RNS Developer:John Fisher, PhD, PE Professor Emeritus Lehigh University 117 ATLSS Drive Bethlehem, PA 18015 jwf2@lehigh.edu Sougata Roy, PhD Associate Research Professor, Department of Civil and Environmental Engineering Rutgers, the State University of New Jersey 100 Brett Road Piscataway, NJ 08854 Justin Ocel, PhD, PE Structural Steel Research Program Manager Federal Highway Administration 6300 Georgetown Pike McLean, VA 22101 justin.ocel@dot.gov
Date Posted:03/31/2020
Date Modified:04/28/2020
Index Terms:Tolerances (Engineering), Steel bridges, Bridge members, Fabrication, Performance,
Cosponsoring Committees: 
Subjects    
Highways
Construction
Bridges and other structures

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