Vulnerability Assessment of Arizona's Critical Infrastructure



10th International Conference on Short and Medium Span BridgesQuebec City, Quebec, Canada, July 31 – August 3, 2018HEAT-STRAIGHTENING RIVETED BUILT-UP MEMBERSUrban, Michael1,21 Gannett Fleming, Inc., USA2 murban@Abstract: Heat-straightening steel bridge members is an economical and relatively quick way to repair impact damage. Localized bulges and significant lateral sweep have been successfully removed from girders using the heat-straightening process. When heat-straightening cannot be used, portions of or the entire girder may need to be removed resulting in temporary shoring towers and long-term lane closures for travel lanes both on the bridge and below. Heat-straightening requires only short-term lane closures during the heat-straightening repairs and allows the lanes to be reopened between work days.? Although heat-straightening is a comparatively easy process for rolled and welded plate girders, built-up members can pose problems in successfully bringing the member back to its original shape and may require the entire damaged section be removed and replaced. However, heat-straightening can still be used to restore the shape of portions of the member while only requiring replacement of others. This results in minimization or elimination of temporary shoring towers, less drilling and splice plates, and maintaining the visual appearance of the existing member. This paper presents successful heat-straightening repairs of riveted built up members. Repair calculations, restraining force estimation, steel repairs, and recommendations for similar repairs are discussed.INTRODUCTIONDamage to highway bridge members can occur when over-height vehicles dynamically impact a structure causing significant plastic deformation of the member. Heat-straightening, along with external restraining forces, has proven to be an effective, accurate, and cost-efficient repair technique when applied correctly. The development of appropriate heat-straightening techniques and methods has been extensively research.Heat-straightening a damaged steel bridge member is an economical and relatively quick way to repair impact damage. Localized bulges and significant lateral sweep have been successfully removed from members using the heat-straightening process. When heat-straightening cannot be used, portions or the entire member are required to be removed resulting in temporary shoring towers and long-term lane closures for both travel lanes on the bridge and below. A heat-straightening repair only requires short term lane closures that, in most circumstances, can be removed between work days.Although heat-straightening is a straightforward process for rolled sections and welded plate girders, riveted built-up members from plates, angles, or channels can pose difficulties in successfully bringing the member back to its original shape. For example, when an impact occurs at a location with one or more cover plates riveted to back-to-back angles creating a bottom flange, the heat applied to the plate will not effectively transfer to the adjoining angle. This does not allow the angle to be restored to the original shape using the theories of the heat-straightening process. Because of this, owners may feel heat-straightening is not an option and will choose to have the damaged section removed and replaced. However, heat-straightening can still be used to restore the shape of some of the individual members while only requiring the replacement of, for example, the damaged cover plates. This results in the reduction or omission of shoring towers, fewer splice plates, and less drilling while maintaining a similar visual appearance of the existing member. This paper will discuss successful heat-straightening repairs of riveted built up members.Heat-Straightening BackgroundHeat-straightening is a basic concept which relies on three specific properties of steel. First, once steel passes its yield point, further strain causes permanent deformation. Second, the yield point of steel decreases significantly when the temperature is elevated to around 370°C to 700°C (700°F to 1300°F). Third, steel expands when heated and contracts during cooling. This expansion or contraction occurs in all directions if the steel is unrestrained, but a restraint in one direction can increase displacement in the other unrestrained directions.The same principle can straighten damaged steel bridge members using heating patterns along with restraining forces. There are three main types of heating patterns used during the repair process. All or just one can be used depending on the type and severity of the damage. These include the vee heat, line heat, and spot heat.The vee heat is typically used to remove longitudinal sweep from members. A “V” is drawn in the yield zone and heating begins at the apex. Once the desired temperature is reached, the torch is advanced in a serpentine motion toward the base of the vee. During heating, as metal expands, a hydraulic jack restrains the member from moving laterally toward the base of the vee. As the steel cools, the vee will contract, shortening the side of member elongated by the impact. This process is repeated until sufficient sweep is removed from the member to satisfy straightness tolerances. Typical vee heat depths are three-quarters of the member width with the base of the vee placed on the edge of the member. The width of the base of the vee should be no greater than half of the member width.Another common heating pattern is a line heat. This is used mostly for weak axis bending of plates and localized damage around the impact zone and web bulges. A line is placed on the convex side of the damaged area and subjected to bending moments produced by the restraining force. The line is heated in one pass and during the cooling process, the side of plate that was heated will contract, flattening the convex side of the bend. This is repeated until the damage is sufficiently removed.Spot heats are similar to line heats but are limited to a small area. The spot is heated to the required temperature and then the heat is removed. These types of repairs are good for very thin plates and for removing bulges from webs. Spot heats are typically used in conjunction with line heats for complex yield lines in small areas. The use of the spot heat will prevent a large area of the plate from heating up and concentrate on the location that needs to be straightened.HEAT-STRAIGHTENING CASE STUDIESPrior to the use of welding, riveted built-up members were the common practice for bridge construction. Bridge members such as girders, floorbeams, overhang brackets, truss members, or pier columns were constructed using individual plates or rolled shapes connected with rivets to form a larger member to span greater lengths or provide additional strength. Although this construction practice is no longer typically used, rehabilitation of these bridges is common as they may have been damaged due to large vehicles traveling the roadways.In some cases, impact damage to these older riveted bridges have resulted in the complete replacement of the damaged member or splicing a welded section of similar size to replace the damaged area. Unfortunately, in cases when the bridge is historic or in a highly visible area, this is not an option and requires a more aesthetic repair. Depending on the location of the damage or the vehicular demand above or below the bridge, a full member replacement can become very expensive. The following projects used the heat-straightening process to achieve a similar appearance yet reduced the repair duration and minimized impacts to the traveling public.Wissahickon Ave. BridgeThe Wissahickon Ave. Bridge over Roosevelt Boulevard (S.R. 0001) located in Philadelphia, PA, USA was built in 1968 and had been struck several times resulting in significant localized damage to the fascia girder. The impacts occurred over the southbound lanes of the six lane Roosevelt Boulevard that has a southbound ADT of 64,570. The 37.084 m (121’-8”) single span bridge consists of 11 riveted built-up plate girders supporting four lanes of traffic and two 3.658 m (12’-0”) sidewalks. There are numerous utilities on the bridge including a 406 cm (16”) gas main in the eastern most bay adjacent to the damaged fascia girder as shown in Figure 1. The fascia girders are deeper than the interior girders and only support the raised sidewalk and pedestrian load.Figure 1: Northbound Bridge Cross SectionThe full depth built-up steel diaphragms are spaced 6.248 m (20’-6”) in addition to gas utility supports spaced 2.057 m (6’-9”). The stiffness of the diaphragms and utility supports prevented the fascia girder from undergoing any longitudinal sweep during the impacts thus limiting the repair to removing localized bulges in the bottom flange only. The repair location consisted of three bulges of varying degrees within the localized damage as shown in Figure 2.Figure 2: Impact LocationThe overall dimensions of the damage were 2.438 m (8’-0”) in length and the largest bulge approximately 127 mm (5”) high. The impact was located 12.192 m (40’-0”) from the abutment. Minor bulging in the lower portion of the web was also present and the inside flange angle leg of the fascia was slightly tilted downward. The section at the impact location consisted of back to back 203x203x14.3 (8x8x9/16) angles with two 508 mm x 13 mm (20” x 1/2”) cover plates separated by a 1676 mm x 10 mm (66” x 3/8”) web plate.Due to the heavily traveled roadway underneath in a heavily populated area, long term closures above and below the bridge were not preferred. In addition, the three layers of steel at the impact area required a combination of heat-straightening and steel removal while limiting the disturbance to traffic.Riverton-Belvidere BridgeThe Riverton-Belvidere Bridge is a 199.034 m (653’-0”), 4-span thru-truss bridge, which opened in 1904 that, carries Walter Street across the Delaware River connecting Riverton, PA and Belvidere, NJ, USA. The roadway width is 5.080 m (16’-8”) and carries two lanes between the trusses. The 1.524 m (5’-0”) sidewalk is on the north side of the north truss supported by overhang brackets. The bridge is listed as eligible for the National Register of Historic Places as a double Warren riveted steel truss structure. In 2016, the bridge underwent repairs including the repair of the damaged north truss lower chord members and the south truss end post at the Pennsylvania abutment.Vehicular impact damage to the truss member L0U1 is shown in Figure 3. The member consists of two C180x15 (C7x9.8) channels with a 356 mm x 6 mm (14” x 1/4”) cover plate on top and batten plates below. The localized damage occurred on the inside channel on the flange riveted to the cover plate. The cover plate and channel flange are rotated about the channel web approximately 25 mm (1”) over a length of 0.533 m (1’-9”).Figure 3: L0U1 Repair LocationThe L4L5 lower chord member consists of two 152x102x12.7 (6x4x1/2) angles and one 203 mm x 14 mm (8” x 9/16”) cover plate. The damage to this member was of approximately 6 mm (1/4”) of localized bending of the vertical leg of the angle over 2.134 m (7’-0”). The L5L6 lower chord member consists of two 152x102x12.7 (6x4x1/2) angles and one 203 mm x 10 mm (8” x 3/8”) cover plate. The damage consisted of 13 mm (1/2”) of localized bending of the vertical leg of the angle over 0.610 m (2’-0”). The repair to the damage in the lower chords were not affected by the riveted cover plate therefore, a standard repair with no cover plate removal was performed for these repairs. However, the repair to the end post required the removal of the damaged cover plate. In addition, the repair was needed to be sensitive to the historic nature of the bridge.Walt Whitman Bridge – Gantry AThe Walt Whitman Bridge is a suspension bridge, which opened in 1957, that spans the Delaware River connecting Philadelphia, PA and Gloucester City, NJ, USA. The bridge is part of Interstate 76 and has a total length of 3651.809 m (11,981’-0”) with the main suspension span of 609.600 m (2,000’-0”). The approach spans on each side have overhead sign and signal gantries. The western most gantry on the Pennsylvania side, Gantry A, was damaged due to a vehicular impact. The southern column consists of four 127x127x12.7 (5x5x1/2) angles and two 610 mm x 10 mm (24” x 3/8”) web plates along with secondary members. The damaged occurred to one side of the column and included damaged secondary members. The damage is shown in Figure 4. Figure 4: Gantry A – Impact DamageThe repair to the gantry required the removal of damaged secondary members requiring the temporary support of the gantry. Even with seven total lanes of traffic, long term closures would result in a significant disruption of traffic on a bridge that carries traffic in and out of the city of Philadelphia each day.REPAIR DESIGNEach of these projects present a unique situation, however the basis to the design of the repair is the same; what portion of the member needs to be removed/replaced, what loads can the remaining portion of the member withstand, and what temporary bracing is required to support the member during the repair. Once the heat-straightening repair begins, restraining forces are applied to the member and must be correctly determined. Following the repair, portions of the member that was removed must be replaced to restore the member to the original capacity. Section AnalysisHeat-straightening consists of heating steel in specific patterns utilizing the heating and cooling cycles to rearrange the molecular structure of the steel. To achieve this, the steel must reach a specified temperature. Due to multiple layers of steel and corresponding airspace in between, attempting to heat multiple layers at once to the required temperature would not be successful. Owners have opted to replace the entire damaged area because of the complications involved. However, removing portions of the built-up member, typically damaged cover plates, to expose rolled sections allows the proper heat to be applied for the heat-straightening repair.The design of the repair begins with determining the loading on the damaged member. Identify if short term lane closures can be used to minimize the live load on the member. The damaged fascia girder on the Wissahickon Ave. Bridge under normal conditions does not experience any vehicular live load. For this repair, the sidewalks were closed to remove all pedestrian live load from the girder. The sidewalk closure remained in place throughout the heat-straightening repair. The removal of the two damaged cover plates at the impact location reduced the capacity of the girder by approximately 45%. However, by keeping the bottom angles in place allowed the reduced section to resist the factored dead load only without the need for temporary supports. This prevented long-term shoulder or lane closures typically required to protect the shoring towers. Limiting traffic restrictions to the heavily travelled highway reduced the cost of the repair and benefited the travelling public.The damaged end post truss member on the Riverton-Belvidere bridge was unable to carry the load with the reduced section thus a temporary support was designed to transfer and carry the load through the damaged portion of the truss member. With the temporary compression strut in place the damaged cover plate was removed to expose the damaged channel to be heat-straightened. During the design of the strut, it is important to recognize areas that are being heat-straightened and to leave room for the torches to apply heat to the damaged area.For the damaged gantry on the Walt Whitman Bridge, damaged secondary members were removed from the column. These short members can be easily replaced in kind and are not typically repaired because it is more cost efficient to replace these members. Analyzing the gantry column for wind load required a temporary diagonal brace to be attached to the column and roadway barrier to resist the loading. Because the damage could not be removed in one working day, the temporary braced was placed outside the roadway to allow the adjacent lane to the damaged column to be re-opened at the end of the day.Restraining Force EstimationOne of the most crucial aspects of a heat-straightening repair is determining an adequate restraining force. Unfortunately, localized bulges on flanges are difficult to analyze due to the complexity of the damage yield lines. Simplified methods are required since the heat-straightening repair is considered a cost-effective solution, precluding a three-dimensional analysis of the section to determine the restraining forces. Two approximate methods have been developed and can be used for the determination of the required restraining forces.The restraining force calculation presented in the FHWA Manual for Heat-Straightening Repairs (Avent 1998) is a conservative approach of limiting the restraining force to a percentage of the force required to produce a yield line collapse load. The manual shows a generic yield line analysis to model a typical flange bulge. Caution should always be used when determining restraining forces because over-jacking the member can lead to a sudden fracture. However, if the restraining force is too small the repair can take longer extending lane closures and increasing contractor costs.The moment capacity of a yield line for a plate is defined by Equation 1.[1] Mp = Fy t2 / 4Where Mp is the plastic moment, Fy is the yield stress, and t is the plate thickness. As indicated, the restraining moment, Mj, is to be 50% of the plastic moment shown in Equation 2.[2] Mj < 50% MpThe restraining force can then be determined using Equation 3 based on the external load, Wu (force/area) defined by the yield lines for the localized damage.[3] Pj < Wu a bf / 8Where Pj is the jacking force, a is the length of the bulge, and bf is the flange width. Another method to determine the restraining force is by controlling the deflection in the field. The maximum deflection between supports (e.g. top and bottom flange) must not exceed the calculated deflection limit for the unheated member. There is no defined limit equation for localized damage in the FHWA Manual for bending of the flange about the base of the web (cantilever); however, a limit can be determined for these individual cases based on the limits provided in the manual.Using this approach requires continuous monitoring by the inspector and can be difficult to measure in the field. Without proper control of the forces applied to the damaged girder, over jacking can occur leading to sudden fractures in the girder. Also, without an initial restraining force analysis by the engineer, the contractor does not have a good baseline for what loading would be sufficient.Although the deflection approach is not recommended as the primary restraining force determination, the limits in the specifications can be used as a guideline not to be exceeded during repairs. If the deflection of the girder begins to exceed this specified limit while the restraining force is being applied the pressure should be released and the engineer notified. Depending on the amount of force already applied the engineer may choose to use the calculated restraining force or modify based on additional field measurements.For these projects, another method was developed to address complex impacts with multiple bulges by simplifying the analysis. This method requires fewer field measurements and is based on the geometric properties of the member. As described above, the yield lines of the damage are also used to calculate the restraining force, however, only a single yield line is used. Based on previous research on the instrumentation and finite element modeling of restraining forces applied to localized damage (Connor et al. 2008), the load is distributed to the immediate area below the jack location. Therefore, a reduced yield line length is used for the calculation of the restraining force. The modified approach ignores the transverse yield lines adjacent to the bulge resulting in a single yield line along the base of the web fillet, as shown in Figure 5.Figure 5: Load Distribution LengthThe yield line length, L, is equal to six times the moment arm, a, plus the width of the loading block. Using the Wissahickon Ave. Bridge repair as an example, the outstanding leg length, OL, is equal to the angle leg minus the angle fillet, k, as shown in Figure 6. The loading block was assumed to be placed at three-quarters of this distance resulting in the moment arm, a, as shown in Figure 5. The calculated restraining force for the Wissahickon Ave. Bridge repair was 50.7 kN (5.7 tons). Comparatively, the FHWA manual would have produced a restraining force of 64.9 kN (7.3 tons). Figure 6: Moment ArmA similar approach was used for the Riverton-Belvidere Bridge. For the Walt Whitman Gantry, since the entire angle was bent (global damage), the restraining force was based on the bending of the entire angle. This is consistent with the approach presented in the FHWA manual for the calculation of restraining forces for global damage.Steel RepairsOnce the heat-straightening repair is completed, any portions of the member removed are to be replaced to restore the section capacity of the member. In the case of low clearance bridges, it is ideal to place the retrofit plates on the top of the angle to avoid reducing the vertical clearance. The retrofit plate must extend beyond the cut line in the cover plate to allow the retrofit to fully develop. To reduce this length, larger bolts can be used or placing new bolts between the existing rivets if the existing rivet pitch is long enough. In instances where the appearance of the member is to be consistent with the original, ASTM A325 TC bolts with a round head can be used to mimic the rivet appearance.HEAT-STRAIGHTENING REPAIRFor both the Wissahickon Ave. Bridge and the Riverton-Belvidere Bridge the localized damage required line heats placed along the yield lines. Prior to applying the restraining forces, a visual inspection was conducted on the impacted area to verify no cracks were present. The heating temperature was limited to 600°C (1100°F) and was monitored with temperature indicating crayons or infrared thermometers. No accelerated cooling was used for these repairs and subsequent heating cycles began once the steel cooled to 120°C (250°F) or less.Figure 7 shows the setup of the restraining force jack placed on the localized damage on the Wissahickon Ave. Bridge. The jack was placed near the top flange to prevent any accidental heating of the cylinder. A steel tube was sized for the restraining force with steel blocks welded to each end. Note the cover plates removed from the section. All components of the restraining apparatus were secured and tied off to prevent falling if the pressure fell during the cooling cycle.Figure 7: Restraining Force Setup – Wissahickon Ave. BridgeThe yield lines were marked on the steel with soapstone to show the line to be heated. Multiple line heats were used for each heating cycle along the top of the flange, the bottom of the flange, and the back of the web. Two workers were available to apply each of the heats to speed up the heating cycle. After the steel cooled, the yield lines were re-evaluated and redrawn on the steel. The soapstone marks on the steel are also used to follow the progress of the repair. If the yield lines extend beyond the previous mark, then the restraining force location and any boundary conditions should be re-evaluated to verify that the load is being applied properly. Figure 8 shows the restraining force setup for the Riverton-Belvidere Bridge on the damaged end post channel. The heat-straightening process was similar to what was described above.Figure 8: Restraining Force Setup – Riverton-Belvidere BridgeRECOMMENDATIONSSection AnalysisIt is important to determine what loading is being applied to the damage section and what, if any, live loading can be removed from the bridge. Temporary shoulder and lane closures are typically required during the heating cycles. Depending on the designed repair, traffic may be reopened between each work period. Accurately identify the grade of steel to ensure the required steel repairs can be efficiently designed. In most cases, the rivet holes in the damaged area will be deformed during the heat-straightening repair. Reaming the existing hole to a larger diameter will likely be required and doing so can provide additional strength to the retrofit by using larger bolts.The ability to place bolts between existing rivets with large pitch distances can reduce the development length of the retrofit plate. Another benefit of reducing the retrofit plate length is limiting the number of lane closures beneath the bridge. Larger splice lengths can potentially extend into adjacent travel lanes. Lane closures can be time consuming, increase costs, and impact the traveling public.Steel RepairsReplacing the removed cross-sectional area on top of the damaged flange, in cases of low clearance bridges, can allow for flexibility in retrofit plate width and thickness. Retrofit plates placed on top of the flange avoids reducing the vertical clearance where development length or thicker plates are required. If a flat retrofit plate does not have enough capacity, an additional angle can be placed on top of the existing flange angle. Plates can be welded to fill the required space between the two angles. The additional bolts in the vertical leg of the angle can also transfer the load. High-Strength bolts can replace existing rivets to minimize the development length of the steel plate. Round head TC bolts can be used to match the riveted appearance.Tears in the flange should be drilled at the tip to terminate the tear and then be tapered out as necessary. A 2.5:1 taper is sufficient for deeper gouges provided the cross section has enough remaining capacity (Dexter 2013).Temporary SupportsTemporary supports can be expensive and may require long term lane closures. If the impacted overhead bridge has a low ADT and can accommodate a long-term closure, then this may eliminate the need for temporary supports on the roadway below. If the live load can be removed from the analysis, the remaining member may provide enough capacity to support the dead load during the repair. Temporary supports should be placed away from the heating area and if temporary lane closures are removed at the end of the work day, the temporary supports should not interfere with the roadway once the closure is removed.CONCLUSIONRiveted built-up members can be difficult to repair due to multiple layers of steel affecting the heat transfer to each individual element of the overall damaged member. With careful planning and design, a portion of these damaged members can be removed to allow the remaining portion to be heat-straightened and subsequently repaired with retrofit plates. Temporary supports may be eliminated using short term lane closures, however if temporary braces are required these may be placed outside of the roadway allowing the temporary lane closure to be removed following a work day. Heat-straightening is an effective, accurate, and cost-efficient repair technique which can give owners another option to repairing damaged riveted built-up bridge members.AcknowledgementsThe author would like to thank Michael Cann, P.E. of Gannett Fleming, Inc. for providing the engineering design and Gary Brown of Jupiter Painting Contracting Co., Inc. for successfully completing the heat-straightening repairs. These projects were conducted for PennDOT District 6-0, Delaware River Joint Toll Bridge Commission, and Delaware River Port Authority.ReferencesAvent, R.R. and Mukai, D.J. 1998. Heat-Straightening Repairs of Damaged Steel Bridges. FHWA Report IF-99-004, Federal Highway Administration, USA.Connor, R.J., Urban, M.J., and Kaufmann, E.J. 2008. Heat-Straightening Repair of Damaged Steel Bridge Girders: Fatigue and Fracture Performance. NCHRP Report 604, Transportation Research Board, USA.Dexter, R.J. and Ocel, J.M. 2013. Manual for Repair and Retrofit of Fatigue Cracks in Steel Bridges. FHWA Report IF-13-020, Federal Highway Administration, USA. ................
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