During the fabrication of steel bridges errors occur. These errors need to be
recognized and corrected properly and efficiently according to each individual situation.
To arrive at the best possible solutions, engineers need not only knowledge of material
and fabrication specifications but also experience and good understanding of the practical
limitations faced by fabricators. This expertise is scattered and varies among both
individuals and DOTs. A database of corrective actions can provide guidance to bridge
engineers and improve engineers' confidence in non-textbook solutions to unusual but not
unique problems. When a sharable and well reasoned repair database is used by several
state DOTs within a geographical region, the database can lead to standardized solution
procedures that would expedite bridge fabrication and would be expected to reduce
fabrication costs, which could result in a reduction of costs passed back to the DOTs.
The the AASHTO/NSBA Steel Bridge Collaboration Task Group 5 is assembling such a
repair database in the context of a FHWA pooled fund research project at the University of
Kansas. The repair database software, Fabrication error Indexed eXamples and Solutions
(FIXS), examines fabrication errors of steel bridge members when detected in the plant and
recommends corrective action. FIXS provides solutions and examples to steel bridge
fabrication errors with graphical and instructive explanations based on both rule-based
and case-based reasoning. This information: the type of error, the reasoning behind
the trial solution, and the outcome of the solution, will be uploaded to the FIXS database
so that when the same or similar type error occurs in the future, engineers can benefit
from or improve upon previous solutions. The objective of the current project is to make easily available to
multiple DOTs. The anticipated delivery mechanism is the World Wide Web.
This
tutorial document is being produced as part of the FIXS project. The intent is to provide a short written document
introducing bridge engineers to issues necessary for good resolution of fabrication error
situations. This tutorial provides not only recommendations of fixes to consider but also
examples of options to avoid. Resolution of
fabrication errors is an engineering task where similar problem situations are encountered
on a semi-regular basis by a DOT organization but infrequently by an individual engineer.
This tutorial is intended to help inform engineers of ways DOTs and fabricators could make
mutually beneficial changes to prevent the occurrence and/or ease the resolution of common
fabrication errors.
Due to a shifted template or layout error, holes were drilled too close to free edge, adjacent bolts or expected edge of splice plate.
QC/QA inspectors may refer to AASHTO Standard Specifications section 10.24 for design requirements for fastener spacing, edge distances, end distances and clear distances. Also, AASHTO LRFD Specifications section 6.13 provides information concerning splices.
1. Too close to end of flange (AASHTO S 10.24.7.1). Minimal stresses in flange at splice locations. Planing end of flange to reduce damage from fabrication processes for small errors. If the offset is too large to correct with planing, do not consider in capacity. Add additional bolts and lengthen splice plate if necessary.
2. Too close to edge of flange. This presents the worst-case scenario and must be considered case-by-case only. Planing edge to remove minor defects may satisfy AASHTO code requirements.
If there is less than one bolt diameter between edge of hole and edge of flange, install bolts but lengthen splice to add adequate number of extra bolts. If hole breaks edge or edge is under contact area of washer, it may be necessary to add enough bolts to fully develop the splice before it reaches the errant bolt location. It may also be necessary to use wider splice plates to allow bolts in errant locations while satisfying criteria for high strength installation (distribution of clamping force).
1. Too close to end of splice plate. Planing edge of plate for small errors. Lengthen splice plate for larger errors.
2. Too close to edge of flange and splice plate. Refer to Flanges section, part 2.
3. Too close to edge of inside splice plate. Planing edge to remove minor defects may satisfy AASHTO requirements. If clearance permits, widen the plate if the hole is too close to side edge. Lengthen splice plate and ignore the errant bolt if the hole is too close to the end edge of splice plate.
4. Too close to end of flange. Refer to Flanges section, part 1.
1. Too close to free edge. Based on hole position, consider effect on splice capacity if the bolt were ignored and investigate how close the hole is to the edge. As is condition may be satisfactory. If unsure, evaluate pattern and any holes not yet drilled to see if spacing and edge distance on other holes permit as is. The last resort is to increase web splice plate size and add a row of bolts.
2. Hole too close to edge of splice plate. Investigate possibility of planing edge. Consider enlarging splice depth if clearance permits. See above alternates if planing and/or enlarging splice depth is unsatisfactory.
3. Hole too close to adjacent hole. Consider parameters in AISCs Specification for Structural Joints Using ASTM A325 or A490 bolts to determine if bolts installation requirements and contact areas justify using as is. See alternates above (1 & 2) otherwise.
1. Bolt too close to edge and contributes significantly to capacity of bolt group. Investigate possibility of planing for small error. For significant error, check vertical bolt spacing to determine if bolt(s) can be added within splice plate. May need to revise splice.
2. Bolt too close to edge and near neutral axis so it primarily carries only vertical shear. Consider planing edge for small error. For larger errors, check design loads on bolts to determine if the errant bolt can be neglected before revising entire splice.
3. Interior bolt. Check if spacing still satisfies AASHTO requirements to leave as is. If it is too close to adjacent bolt, consider leaving hole unfilled (in web) and adding a hole as close as possible to correct location. If holes overlap (looks like a snowman) in web only, splice plates will cover like a hardened washer. Consider as an oversize hole and check allowable vs. calculated load.
There are several types non-conforming fabrication on stiffeners, the most common of which are mis-located stiffeners, incorrect hole placement, and improper stiffener to flange attachment. Repairs will vary, depending on the non-conformance and/or type of stiffener.
Timing is important when making decisions about repairs. If an error is noticed after the stiffener has been tacked into place, but before it has been completely welded, it might be possible to remove the stiffener and place it in the correct location. After the stiffener has been fully welded, removal of the stiffener is a more difficult solution.
Changes in design requirements in recent years have created additional possibilities for errors with stiffeners. Most states require diaphragm connection plates to be connected to the tension flange. State bridge manuals vary in opinion on welding the connection plate to the tension flange. The fabricator should always thoroughly check the contract documents to determine if welding to the tension flange is acceptable. Deviations in the compression flange attachment generally have simpler solutions than deviations in the tension flange. If a hole is inadvertently drilled in a compression flange, the hole may be either filled with a bolt, or a positive connection, similar to a tension flange connection, may be made, depending on the number and location of the misdrilled holes.
Repairs to misdrilled holes in tension flanges typically require the fabrication of a new positive connection to the flange depending on the location of the holes.
Caution should be exercised about rushing to repair a misdrilled hole. Both the QC and QA inspection personnel should carefully verify several factors before completing any repairs. Some items to be verified include:
a) Tension or compression flange verified with design plans and/or shop drawings
b) Edge distances
c) Clearances
d) Hole Spacing
e) Hole Size
Occasionally, a diaphragm connection plate or stiffener is welded to a tension flange where welding is not allowed or is all together prohibited. Depending on the location and extent of the welding done, inspectors will sometimes require the fabricator to break the weld, and install the bolted connection in accordance with the design plans. If breaking the weld is not acceptable, then an alternative solution may be required. Solutions to these types of problems are still under development.
When bearing stiffeners are incorrectly fit in a location other than shown on the design plans or approved shop drawings by more than 6 (150mm), a possible solution would be to leave the stiffener in place, fill any holes with bolts and add a new stiffener at the proper location depending on an analysis of the structure.. Bearing stiffeners incorrectly fit by only a small amount present several problems. Unrepairable damage to the girder may result if the stiffener is removed. If the stiffener is left in place, it may not be possible to add a stiffener at the proper location due to the clearance needed for welding both sides of the stiffener. Should the spacing between the incorrectly fit and the properly fit stiffener be such that fitting the new stiffener cannot be accomplished, the design engineer should be contacted and a complete analysis of all proposed corrective actions should be conducted prior to completion of any repairs.
Intermediate stiffeners incorrectly fit in locations other than shown on the design plans or approved shop drawings, depending on location, may not present serious problems and may possibly be left in place, subject to the owners approval. The fabricator should contact the owner/engineer for an analysis of girder shear and other design criteria for both stiffeners spaced longer or shorter than detailed. If a stiffener(s) is installed on the exterior face of an exterior girder, the owner has the option to require the removal of the stiffeners for aesthetic reasons and place the stiffeners in the required location. If the location of the stiffener exceeds the maximum spacing of stiffeners from analysis, an additional stiffener may be added to reduce the spacing to meet design requirements.
Diaphragm Connection Plates.
When diaphragm connection plates are incorrectly fit in a location other than shown on the design plans or approved shop drawings by more than 6 (150mm), an acceptable solution would be to leave the plate in place, fill any holes with bolts, and add a new connection plate at the proper location, depending on an analysis of the situation and the ability to access the location for additional fabrication and with the owners approval. When diaphragm connection plates are incorrectly fit by a small amount, the as fabricated condition may be acceptable upon review by the owner. The diaphragm will not be perpendicular to the girder and may or may not fit without modification. Care should be taken not to buckle or create a situation where buckling in the diaphragm may occur. In any case, the gusset plate should always be in full contact with the connection plate at the bolt locations.
If a diaphragm is to be attached to a bearing stiffener, checks should be made to verify fit between the diaphragm and the non-conforming bearing stiffener.
When holes are mis-located in a diaphragm connection plate, a bearing stiffener, or an intermediate stiffener where a diaphragm will be attached, several options exist:
1) If the stiffener has not been fit to the girder, the option to re-fabricate the piece exists.
2) New gusset plates may be fabricated to match holes already drilled if fit or not fit.
3) New diaphragm to match the plates the diaphragm will be connecting to.
4) Ability to provide positive connection to the tension flange for the connection plate or stiffener if required.
For any of the above conditions, the fabricator should carefully explore all options and weigh those options for cost, design requirements and time to repair and should be able to justify their proposal to the owner/engineer. In any case, a custom diaphragm is fabricated to fit in this location, the shop drawings should be revised and submitted reflecting the As Fabricated condition to the owner and the contractor.
As the level of complexity of the structure rises, the required solutions for mis-located stiffeners or connection plates become more difficult. The QC/QA inspectors must contact the owner/design engineer to request a design analysis in the following situations:
1) Diaphragm connection plates that also act as intermediate web stiffeners.
2) Curved structures where the diaphragms must line up to resist the horizontal forces due to curvature.
3) Any situation involving earthquake loads carried by diaphragms or cross frames, or any situation where eccentric loading may cause the diaphragm to buckle.
1. If the gouge meets the tolerance of greater than 3/16(5mm) and less than 7/16(11mm) from the cut edge, welding with shielded metal arc welding (SMAW) is required. If gouge is less than 3/16(5mm) deep from the cut edge see below for other solution.
2. If the gouge is less than 3/16(5mm) from the cut edge, grinding to a slope of one inch (25mm) is acceptable. No welding required.
3. Preparation is required for welding, including: air-arc gouging or grinding to the bottom of the burn. Preheat is required.
4. If welding is required, then magnetic particle testing (MT) and/or ultrasonic testing (UT) is required.
5. For additional information on plate conditioning, the engineer is referred to ASTM A6 section 10.2 and any references contained therein. For those repairs requiring welding, section 10.5 in ASTM A6 should also be reviewed.
1. For any discontinuity one inch (25mm) in length or less, no plate repair is required, and need not be investigated.
2. For any discontinuity over one inch (25mm) and 1/8(3mm) maximum depth, no plate repair is required, but the depth should be investigated.
3. For any discontinuity over one inch (25mm) in length with depth over 1/8(3mm) but not greater than ¼(6mm), removal is required, but welding is not necessary.
4. For any discontinuity over one inch (25mm) in length with depth over ¼(6mm), but not greater than 1 inch (25mm), completely remove and weld area required. Aggregate length of the welding shall not exceed 20% of the length of the plate edge being repaired. ASTM A6 section 10.5 stipulates requirements for repair by welding.
Fabricators commonly inquire about substitution of higher strength material. This is not arbitrarily permitted. The substitution requires shop drawing revision. This requires the QC/QA inspectors to consult with the design engineer for approval. In most cases, approval is given.
Plate defects occur in a variety of forms including burn gouges from cutting, deformations due to handling, mill plate defects, direction of rolling, etc. The QA/QC inspectors should refer to ASTM A6 and AWS D1.5 for acceptable tolerances, repair procedures, or rejection procedures. Approval for repairs in accordance with ASTM A6 and/or AWS D1.5 is permitted with the application of an approved WPS.
1.) Overheating may cause irreversible effects, so a repair may not be possible without material replacement. Physical properties of heated areas must be assessed to determine if material is still acceptable for incorporation in structure. Possible indicators include hardness checks (Brinell, Knoop, Vickers, or Rockwell), which can be done in-situ. Also, yield, ductility or Charpy V-notch tests can be done which require removing material for specimens. Metallographic analysis can determine if microstructure has changed, but this is very sophisticated and site-specific for this situation. Tests should be conducted by a qualified testing agency at the fabricators expense. If hardness testing provides adequate assurance, the material may then be accepted. Otherwise, either the material must be completely replaced or an adequate number of specimens must be extracted and tested to verify properties. If tests are acceptable, the fabricator must develop a method to replace the extracted areas of the material.
2.) Other solutions involve replacement of material; extensive non-destructive evaluation, including wet-particle MT, through thickness and surface wave UT, and hardness testing; stress relief of heated areas (difficult, and unwanted distortion is possible); or addition of reinforcing plates.
3.) For additional information associated with heat treatment of structural materials, the engineer is directed to ASTM A6 section 7. Also, refer to section 19 for information regarding retreatment of material.
1. This type of fix should not be used for concealing errant bolt holes. Fill with HS bolt and washers when possible. Do not attempt weld repair without approved weld procedure.
2. If this type of welding has been noticed, if the position permits, drill out entire weld and install high strength bolt with two hardened washers. Drill bit must be slightly larger than and centered on original hole to remove most of the heat affected zone and potential micro cracks present. Before installing bolt, MT and PT inspection by contractual ASNT Level II. Must be done at fabricators expense.
3. If position (or stress) precludes the above bolted repair, drill out and inspect the welded area as above, then fill the hole either cosmetically or structurally.
4. If the drilled hole is partially or completely covered but stresses and proximity of other connections do not require solid metal (for example a bolted diaphragm angle), fill the hole with a steel pin secured by brazing, epoxy, etc. and grind flush.
5. In the case where a plate will cover the errant hole another method to fill the hole using molten zinc has been a viable fix with certain DOTs. The procedure includes chamfering the top and bottom, clean and coat the inside of the hole with a zinc-rich primer, preheat to 200 F and fill the hole with molten zinc. Grind flush, then clean and coat the area.