(Days in parentheses reflect contract days)
1. Installation of scaffolding below deck (First scaffolding in place on Day 3)
The below deck scaffolding segments were barged in and cable hoisted into place. Due to the load restrictions imposed by the contract, the installation occurred in phases, with a minimal footprint prior to temporary repairs.
2. Install strong backs and saddle beams (Day 5-7)
A pair of wide flange beams was positioned such that they were anchored to the east cantilever span, supported at the finger joint (fulcrum) and extended out over the floorbeam east of the impact. The idea was to saddle it using rods through core holes in the deck and jack up the beam to relieve some of the vertical force imposed to the north truss in that critical location – to assist in geometry restoration. The strong backs were fabricated by FSS, located in Tampa, and trucked to the bridge on Saturday October 5th. Because the beams were 70’ long, transporting them required an Overweight/Overdimensional Permit and Florida Highway Patrol (FHP) Escort. Orchestrating this event in such a rushed timeframe took considerable coordination between multiple FDOT offices, FSS, and two FHP troops. Upon arrival, positioning the strongbacks was a challenge due to the load restriction on the bridge. Instead of craning them into place, a cable pulley system was devised to roll them into position along high strength dollies.
3. Cut and removal of the damaged chord (Day 6-8)
The damaged chord was cut and removed, one three foot section at a time. The sections were hoisted over the bridge railing, documented, and trucked to the FDOT Maintenance Yard for further investigation.
4. Installation of the temporary lower chord (Day 9-13)
The temporary lower chord consisted of four, 66’ long, 1-3/4” diameter high strength threaded rods (150 KSI) with spherical nuts. Fortress anchors for the rods were bolted to the lower chord beyond the limits of the damaged area, with the use of nearly 100 high strength bolts. The fortresses and rods had to be hand handled and positioned into place using manpower and pulley systems rigged above the deck. This was quite a challenge; given that one half of a single fortress weighed about 2,200lbs.
5. Tensioning of temporary lower chord (Day 13-17)
The jacking process was done incrementally over a five day period with increasing bar loads at each stage and a final collective load of 780 kips. A 100 ton jack was used to tension each of the four bars in-turn. The initial tensioning, on the evening of Monday, October 14th yielded stubborn results. The hope was that the gap created by the severing of the tension chord would start to close, but the measured closure after final jacking that night was approximately 1/16” longitudinally – much less than anticipated. Heads were hanging low by the end of that evening and concern was starting to mount regarding the repair strategy. Lift off forces were taken the next day and it was found that the bars had relaxed quite substantially. Over the next four days, a series of jacking phases followed by bar relaxation resulted in much more encouraging numbers and by the fifth day, the longitudinal and vertical geometry of the bridge had been restored, including a nearly perfect restoration of the floor beams from their swept position back to a normal alignment. Survey points and strain gages placed in various locations on the bridge were monitored closely during each tensioning process.
6. Additional truss member repairs (Continuous)
The lower chord replacement was the primary focus of the job, but there were other damaged elements that needed attention as well. The immediate loss of capacity in the lower chord caused the tension force to transfer to and overload the lower later bracing. The north bracing member immediately failed and tore out (block shear failure) from a gusset plate connection. With the failure of the chord and brace, the impact buckled one of the bottom lower lateral gusset plate. All of these members were replaced. Nearby floorbeams were swept out of alignment. This deformation was corrected during the tensioning process. The impact also caused bowing of a set of the upper chord gusset plates which were stiffened with steel angles. Other miscellaneous needs, such as rivet replacements were addressed.
7. Heat straightening process (Day 15-19)
In order to receive the new chord, the gusset plates at both ends of the connection required heat straightening. This process resembles art more than science as it entails alternate cycles of heating and forming while staying within very strict limits of temperature and force ranges. The heat straightening team (International Straightening Inc.) used an oxy-acetylene torch with porta-power hydraulics and custom made jigs to apply up to 8,000psi without force cold bending of the steel. Heat was applied at up to 1200°F for no more than a minute at a time while keeping constant movement with the torch. The effort took approximately 100 man hours.
8. Installation of stub beam at the west connection (Day 19-22)
Given the extent of the heat straightening required of the gussets and fill plates at the west lower chord connection, and the expectation of a tricky fit-up as a result, FDOT Central Office (CO) recommended to first install a short stub beam to extend eastward from the connection, and then splice the new lower chord to it, as opposed to trying to connect the lower chord itself directly to the panel point.
9. Splice new chord between the new stub beam and the existing chord stub end (Day 22-25)
The replacement lower chord was trucked in from FSS in one complete built up section, but once on-site, it was dismantled and installed piece by piece and then bolted together in final position. These steps were necessary because lifting the entire 8,000lb chord and booming out horizontally nearly 40’ from the eastbound lanes would have overloaded the boom truck and been more difficult to weave through the truss members to set in place.
10. Installation of “cheese fill plate” & cover plate (Day 24-25)
Due to the large plastic distortions of the existing outside impacted gusset plate, the team received direction from CO to use a “double-bolt and cheese filler plate method” to add an outside cover plate to the existing connection for the purpose of strengthening it. This method proved very valuable because not only did it strengthen the connection, but it accomplished this without the need to remove the existing gusset plate which would have required removal of all of the existing rivets at once, thus “unzipping” the entire connection. Instead, each rivet that was removed was immediately replaced with a long bolt and nut. The cheese fill plate was then installed with holes large enough to fit around the nuts and then finally, the cover plate was added and bolted in place with a second set of nuts.
One of the larger challenges of the entire project was how to fabricate the fill plate and the gusset cover plate such that the required 199 bolt hole locations would align with the holes in the existing gusset plate. Essentially – how do you accurately make a template from a 7’x6’ plate that must remain in place on the side of a bridge? The answer was to use the on-site surveyor’s laser scanner which is a $130K 3D infrared high-speed, high resolution piece of equipment with short range accuracy of 0.5mm. To make it happen, Will Watts directed the District 2 Bridge Maintenance crew to build a bracket that would attach to the side of the bridge and extend out to hold the scanner eight feet away so that it could do a full scan of the existing gusset plate. The scan was then transferred to CADD and used by the fabricator to fabricate the plates.
11. De-tension temporary chord and remove fortresses, rods, scaffolding, and construction equipment (Day 26-27)
Once the new chord was in place, the remainder of the effort included de-tensioning the temporary chord, removing the rods and fortresses, installing cover plates on the lower chord webs at the fortress locations, and removing the scaffolding from under the bridge and the construction equipment from atop the bridge.