DelDOT Bridge 3-352 in Laurel, DE
DelDOT’s BR 3-152 is located on US-13 Alternate (Central Avenue)in downtown Laurel, Sussex County, Delaware. The bridge spans Broad Creek, a tributary of the Nanticoke River. Eastern Highway Specialists, Inc. (EHS) was the low bidder and subsequently awarded the construction contract for DelDOT’s BR 3-152 for the contract value of $3,529,543. The construction involved converting the existing single leaf bascule span (drawbridge) into a fixed two-span structure with concrete girders while maintaining the original 1923 Scherzer Rolling Liftbridge aesthetics and its place on the National Register of Historic Places. The Scherzer Rolling Liftbridge was invented in Chicago, Illinois in 1893 to solve a railroad right-of-way acquisition issue that would not allow enough land for the typical railroad “swing-bridge” which rotates horizontally to allow river traffic to pass. BR 3-152 has a “younger sibling” Scherzer Liftbridge in Milford, DE over the Mispillion River that is nearly identical in design but is a slightly shorter span and a few years newer (1929).
EHS seeks out historic bridge projects including steel trusses and covered bridges, as the management team enjoys the technical challenges that these projects present. BR 3-152 did not disappoint with the technical challenges that it presented and were ultimately overcome to complete the project. These included jacking to perform counterweight retrofits, forming the new pier concrete underwater and completing a UHPC closure pour between Spans 1 and 2. Central Avenue was closed to traffic in May 2018 in order to allow construction to take place in a single phase. As the town of Laurel’s residents patiently watched the progress of the bridge for eighteen months, it was ecstatically re-opened to traffic in October 2019. As with any projects that EHS completes, Safety and Quality are held at the highest priority. BR3-152 exemplified and upheld our safety brand “Everyone Home Safe” with no recordable or lost time injuries for the over 13,000 Manhours that were worked. The bridge was also one of two finalists for our internal Quality Award for 2019 where our employees are recognized for the quality work performed for the year and demonstrates the “Quality is our mark” motto that we strive to achieve.
Alternate Counterweight Jacking Plan:
One of our first construction tasks at BR3-152 was to make permanent modifications to the counterweight rack system. In order to do this the contract plans called for jacking the counterweight and its “gear” off of the “track” which is also the top of the main girders. This would have involved disassembly of portions of the gear and track and then reassembly once work was completed. The work also required assembly of custom “jack boxes” and jacking loads that exceeded 160 kips.
Original Jacking Scheme Presented in the Contract Plans
EHS quickly determined that this scheme, although possible, was not the simplest and most economical way to jack the counterweight. We looked to simple mechanics to come up with an alternate solution. By using the existing bascule girders (1 and 2) as lever arms and rotating them below their usual elevation around the pivot point at the pier we were able to achieve the 1/8” jacking of the counterweight without using any jacks.
The main challenge was determining how much “lowering” was required at Girder 1 and 2 to achieve “lifting” of the counterweight. Due to the skew at the south abutment Girders 1 and 2 are different lengths (56-7” and 72’-3” respectively) and thus required different values of lowering. We also could achieve the “jacked” state of the counterweight and not have to remove and reinstall any portions of the gear or corresponding track. Due to the long lever arms the forces that were required were reduced to approximately 30 kips versus the original 160 kips that would have been required.
Chart A & B were created to determine the amount of lowering that would be required at Point A to lift the counterweight 1/8” (0.125”) at Point B
The lowering was achieved by installing temporary steel that EHS had on hand from previous jacking and steel repair projects. We attached the temporary steel to the existing concrete at the south abutment and connected the temporary steel to Girders 1 & 2 with C-clamps and utilized four 7/8” diameter threaded rods with heavy hex nuts to perform the lowering. We lowered Point A in 1⁄4” increments while surveying Point B. As part of this process we had to demo approximately 4” of the top of the South Abutment to accommodate the lowering of the bascule girders. This concrete was later covered by the new South Abutment concrete that was poured closer to the pier location.
Underwater Pier Conversion:
The existing pier(s) of BR 3-152 were independent large columns that were the fulcrum for the bascule girders for the portion of the bridge that was originally “moveable”. In our contract, the plans called for the existing steel floor beams, stringers and open steel grid deck to be removed and replaced with two spans of concrete girders and a concrete bridge deck. The first major structural modification to achieve the project goal was to create a single reinforced concrete pier without dismantling the current piers that the bascule girders rested on. The plans called for two rows of seven micro-piles to be installed between the existing piers and then a reinforcing cage and concrete to be placed between the existing piers, essentially creating a single pier to support the concrete girders.
Rather than installing a cofferdam in the creek, which would have been difficult due to the existing structure having to remain in place, EHS elected to utilize divers and grout bags to seal the bottom of the formwork required for the pier concrete. We chose this method due to the existing rip-rap that was in place providing scour protection between the existing piers. While the EHS crew set and held forms in place, the dive crew placed and filled grout bags to seal off the interface between the rip-rap and the form panel. This allowed the form to be pumped out for rebar placement and the eventual pouring of the concrete.
EHS Crew During Pier Concrete Pour
Completed Pier Concrete
Completing the Link Slab:
Originally the contract drawings called for an 8’ long link slab to be poured on top of the pier connecting the deck concrete of Span 1 and 2 with the Pier concrete. The Link Slab concrete that was specified was a 4,500 psi fiber reinforced concrete that did not contain any traditional coarse aggregate. As there was very little history of the long term performance of this mix design, DelDOT elected to modify the Link Slab details and utilize Ultra High Performance Concrete (UHPC). UHPC had been successfully utilized across the country as a closure pour material and has a far superior ultimate compressive strength of over 20,000 psi. DelDOT had utilized UHPC on a few prior projects and asked EHS to incorporate it into the BR 3-152 project. EHS was happy to accommodate the design change as it would give us an opportunity to work with UHPC and further expand our technical skillset.
UHPC is a blend of cement, sand, ground quartz and silica fume that when field mixed with water, high range water reducer and 0.008” diameter steel fibers creates a fluid mixture that bonds extremely well to existing concrete surfaces that have been properly prepared. The high ultimate strength allows for much smaller pours, on BR 3-152 it reduced the link slab from 8’ long to only 2’ in length.
EHS Crew Placing the Initial Batch of UHPC in the link slab joint
Essential Personal Protective Equipment (PPE) for mixing the UHPC materials
UHPC requires a formwork “lid” to be installed due to its highly fluid state.
A final pour into a bucket with a hole cut in the bottom provides enough head pressure to force out any air left in the forms.
Setting Span 2 Concrete Girders
Span 1 and Span 2 Concrete Girders in Place
EHS Crew Pouring Bridge Deck Concrete