Difference between revisions of "713 - Bridge Rehabilitation, Steel"
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Latest revision as of 14:08, 21 May 2014
Furnishing and fabricating structural steel additions/replacements of existing structures and steel portions of temporary supports shall be according to section 707 of the Standard Specifications for Construction. Shop drawings shall be submitted for approval prior to fabrication.
Temporary supports for girder ends may either be as shown on the plans or an alternate design proposed by the Contractor and approved by the Engineer. three types are typically used on MDOT projects, column type supports from below, hanger or suspension type supports from above, and diaphragm types, where the hydraulic jack is placed directly on the substructure unit.
If temporary hanger rods are used, reject any rods with notches, nicks, welds, or excessive corrosion. Each length of bar used to make hanger rods must be sampled and submitted along with certified mill test reports.
Column supports may either bear on compacted natural ground, substructure concrete, or engineered fill as shown on the project plans. In all cases the material must be compacted to at least 95 percent of the maximum unit weight.
Check the plans to verify if cross bracing of the temporary support columns are required, and if so, verify the cross bracing welds are sufficient.
After supports have been erected but before loading temporary supports, ensure the offsets at each of the following locations, in each direction, and the sum, do not exceed one inch.
- The horizontal offset from the top of the column to the bottom of the column
- The horizontal offset of the hydraulic jack to the column centerline
- The centerline offset of the column perpendicular to the beam web
- The offset of the hydraulic jack to the beam web
Once the hydraulic jacks are loaded, and the existing beam is unloaded, ensure the contractor does not remove the jacks until the new pin and hangers are installed and in operation.
During the first four hours after loading, check hourly for settlement of temporary supports, and after that check daily. Shims must be added to correct any settlement in excess of 1/16 inch.
Ensure the contractor does not place the column based plate directly on finished concrete. Timber mats are typically detailed in the plans. A general example a column type temporary support can be found in Appendix 7.06.04A of the MDOT Bridge Design Manual.
Suspension type supports typically consist of and overhead beam and an underslung beam, connected with high strength hanger rods and heavy hex nuts. Material acceptance of the hanger rods is based on MDOT testing, and certified mill test reports. It is important to ensure an MDOT representative witnesses the removal and submission of samples for tensile and yield testing.
Ensure the overhead and underslung beams are in contact with the top and bottom flanges of the beam to be unloaded, and ensure any vertical stiffeners are centered over the beams. Ensure the heavy hex nuts are adequately tightened prior to jacking operations.
Ensure the hydraulic jacks are in the correct location with respect to the overhead beam, and the centerline of the existing bridge beam. Ensure the hydraulic jacks are equipped with locking rings or other positive locking devices to prevent settlement, or sudden unloading in case of hydraulic system failure.
The Engineer may approve the removal of hydraulic jacks after placement of stable shims.
If sufficient space permits the placement of hydraulic jacks directly on substructure units, ensure the hydraulic jacks are placed directly under load bearing diaphragms.
H-1 grout in accordance with section 702 of the Standard Specifications for Construction must be used under hydraulic jacks to provide a smooth and level bearing surface. Ensure the contractor does not apply load until the grout reaches a compressive strength of 4,000 psi.
If multiple hydraulic jacks are used they must be equally loaded and simultaneously advanced to avoid uneven or eccentric load which could damage the structure. Jacks must remain in place until repair or replacement and operation of structural components although the contractor may disconnect lines and pumps provided the equipment remains on the project.
Ensure measurements of all pin and hangers to be replaced are taken, including pin diameters, distance between pins center to center, and length, width, and thickness of link plates. It is important that existing field dimensions are used to verify dimensions for shop drawings as opposed to existing plan dimensions. Ensure the girder web alignment is also measured by laying a straight edge across the opening at the top and bottom of the girders.
If pin and hanger assemblies are being removed from more than one girder at a time, ensure that work is not being done the same end of adjacent girders or from both ends of the same girder for drop in spans. Ensure suspenders and shims are in place prior to beginning removal. The link plates should be cut first, following the diagonal opening between girder ends, or at the pin with a sheet metal shield behind the link plate to protect the girder. After cutting the link plate the pins should be cut with a sheet metal shield around the pin holes.
If the contractor choses to burn a hole through the ends of the pin for ease of removal, ensure the appropriate metal heat shield is in place to protect the pin plates.
Caution must be exercised by the contractor not to damage the pin plates, pin holes or any other portion of the structural steel that will remain in place. If the girder or pin plates are nicked, gouged, cut, or otherwise damaged during the removal process notify the Engineer and Bridge Field Services. A repair plan will need to be submitted for approval.
If existing end diaphragms prevent installation of new pins, do not allow the contractor to loosen or remove the diaphragm. A hole in the web of the diaphragm may be flame cut, ground smooth and cleaned and coated to allow for easier installation of new pins.
It is important to note that cleaning and coating 3 feet each side of the centerline of the pin and hanger is included in the Hanger Assembly, Rem and Erect pay item, and is not paid for as part of Structural Steel Cleaning and Coating pay items.
Ensure all bolt holes for bearing stiffeners are field drilled and not flame cut. Upon completion of the field drilling, ensure faying surfaces, and other contact surfaces are cleaned, and primed per section 715 of the Standard Specifications for Construction. Ensure the bearing ends of the angles or bent plates used as stiffeners are ground smooth, and fit tightly against the top and bottom flanges of the existing beam. Do not allow the contractor to connect the stiffener to the existing beams via welding unless called for in the plans.
Sealant must be applied around the perimeter of bolted or riveted plates or angles on steel beams as shown on the plans or directed by the Engineer. Ensure the sealant must is applied over the epoxy intermediate coat when dry to the touch. The urethane top coat must be applied over the sealant after both the intermediate coat and sealant is fully cured. All surfaces receiving sealant must be clean and dry at the time of application. Ensure sealant is not be applied when the air or surface temperature is below 40 degrees Fahrenheit. Immediately after the sealant is applied, ensure the sealant is tooled to form a 5/16 inch fillet and force it against the contact surface. Sufficient sealant shall be applied to completely fill all pits and depressions regardless of width and depth in the steel beam.
All heat straightening work must be conducted under the direct on-site supervision of a pre-approved structural steel heat straightening specialist listed on the frequently used special provision 12SP713(A), or a specialist who has been approved by Bridge Field Services based on submittal of written documentation of three years’ experience, on a continuous basis, with successfully heat straightening comparable steel structures.
Heat straightening must not be started before obtaining the Engineers approval of the heat straightening types and methods, and after providing at least 48 hours’ notice to the Engineer.
Heat straightening may not be performed if the area is exposed to precipitation, unless the area is housed and approved by the Engineer. The temperature of the steel must be monitored to ensure it does not exceed 1,200 degrees Fahrenheit. The contractor must provide heat indicating devices and make them available to the inspector.
After each heat straightening cycle the steel must be allowed to cool to 250 degrees Fahrenheit before beginning the next heating cycle. It is not permitted to cool the steel by applying water or any other materials or methods that would accelerate cooling.
The application of heat during heat straightening works by providing restraint during the heating process and applying heat in select locations so that when the steel cools it moves back into place. Mechanically straightening the steel is not permitted.
It is important to finish heat straightening before reattaching any cross frames or diaphragms. Forcing the beams into position to attach cross frames or diaphragms is not permitted.
Ensure that all bends, creases, folds and dents in beam, diaphragm and stiffener elements are eliminated. See section 713 of the Standard Specifications for Construction for the tolerances for heat straightening.
Once heat straightening is complete, ensure the contractor grinds burrs, nicks, gouges and scrapes. Report cracks or tears in the beams, or other structural steel elements to the Engineer. Cracks are to be repaired by welding in accordance with subsection 707.03.D.8 of the Standard Specifications for Construction.
If realigning rockers of more than one girder at a time, do not work on the same ends of adjacent girders. Before removing rockers from girders make sure girder ends are stably supported on shims.
Sole plate welds must be removed by grinding unless shown on the plans and approved by the Engineer. Sole plates must be cleaned and coated along with the other contact surfaces, in accordance with the plans, before welding of the sole plates. Ensure the sole plates are placed in the appropriate location to provide the correct rocker tilt in accordance with the plans.
Beam ends must be cut with a plasma cutting torch to provide adjacent vertical beam ends that are 1 inch apart. The same amount shall be cut from each adjacent beam end, but no more than ½ inch from either beam. Beam ends must be marked prior to cutting and guides or straight edges shall be used to provide control. All cut edges must be ground to remove burrs, nicks, gouges, and scrapes before cleaning and coating in accordance with section 715 of the Standard Specifications for Construction.
When trimming beam ends at pin and hanger assemblies, ensure the pin plates themselves are not trimmed. For wide flange beams, the protruding web and fillet weld may be removed.
Remove structural steel as shown on the plans using mechanical methods, plasma cutting, or air-arc gouging. Ensure that removal does not extend beyond the limits shown on the plans, and ensure that adjacent structural steel is not damaged. Removal or damage of structural steel beyond the limits shown in the plans should immediately be brought to the attention of the Engineer and Bridge Field Services. Preparation of existing structural steel to accept proposed components may include grinding, drilling, and coping holes in accordance with the plans. All field welding, inspection, and testing must be in accordance with section 707 of the Standard Specifications for Construction and cleaning and coating must be in accordance with section 715 of the Standard Specifications for Construction. Ensure that any field welding is done by a certified welder with an approved weld procedure. Any vertical field welding must be done in the upward direction (starting at the bottom and working towards the top). Welding vertically in the downward direction is not permitted.
Bolted repairs are often used to strengthen structural steel that has significant section loss. It is important that prior to erecting structural steel for bolted repairs that the faying surfaces of existing steel have been cleaned, primed, and cured as recommended by the paint manufacturer for slip critical surfaces. High strength bolts for bolted repairs on fascia girders must be oriented with the bolt shank pointing upwards through flanges and inward through webs.