Performance-Based Tolerances for Fabrication of Steel Bridge Members
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
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.
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.
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.
Searching keywords “tolerance OR fabrication” with “bridge” in the title
resulted in no relevant hits.
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:
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.
Amirikian, A. (1965). Dimensional tolerances and Quality Control in
Welded Steel Construction. Highway Research Record, Issue 85.
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:
Kendrick, A., Assakkaf, A. I. (2004). “The Effect of Fabrication
Tolerances on Fatigue Life of Welded Joints.” Ship Structure Committee,
Following research approach/tasks are anticipated for accomplishing the project
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.
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.
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.
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
5. Prepare an interim report that documents the research performed in Tasks 1
through 4 for review and approval by NCHRP.
6. Perform research as per plan developed in Task 4 and approved in Task 5.
7. Based on the research performed, develop rational tolerance for fabrication
of steel bridge members.
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.
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|
|RNS Developer:||John Fisher, PhD, PE Professor Emeritus Lehigh University 117 ATLSS Drive Bethlehem, PA 18015 email@example.com 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 firstname.lastname@example.org|
|Index Terms:||Tolerances (Engineering), Steel bridges, Bridge members, Fabrication, Performance, |
Bridges and other structures