By: Bill Wilson
When you are bringing a series of bridges back to life, it is OK to move on to the next sister.
It’s happening right now in Pittsburgh following the successful rehab of the Andy Warhol Bridge over the Allegheny River, which is part of an iconic trio called the Three Sisters bridges. The focus is now on the second sibling—the Rachel Carson Bridge—which will feel the work starting in the spring of 2019. The Three Sisters bridges were originally constructed from 1924 to 1928 and are the only trio of identical, side-by-side bridges in the world and the first self-anchored suspension spans constructed in the U.S. The set is listed on the National Register of Historic Places, adding a heavy dose of complexity onto the Andy Warhol Bridge, which earned a No. 9 ranking on the 2018 Roads & Bridges’ Top 10 Bridges list.
“Considering the complexity of this project, I think we rose to the occasion,” Aaron Colorito, lead structural engineer for the Andy Warhol Bridge Rehab Project for Michael Baker International, told Roads & Bridges. Michael Baker was the prime designer on this project, with Brayman Construction Corp. serving as the prime contractor. “You can tell the difference when you look at these three bridges. You can tell Andy Warhol is different. It looks so much better.”
Corrosion made its presence known just about everywhere. According to John Tricini, project manager for Michael Baker International, water from a longitudinal joint along the gutter line was reaching the substructure of the bridge, damaging the steel. The Allegheny County Department of Public Works attempted to put pressure on the bleeding by repeatedly patching the deck when conditions called for it.
“It was a losing battle,” said Tricini. “This thing was really going downhill quick.”
The last time the bridge was completely repainted was in the 1970s. Piece-meal paint jobs took place after that, leaving areas with multiple coats of paint.
Corrosion wreaked havoc with the tie-down system at the ends of the bridge, which were frozen in permanent up-lift. Michael Baker International looked at a number of different alternatives to replace the tie-down assemblies without putting the bridge at risk. The tie-downs were replaced with four steel pins with self-lubricating bronze washers in an attempt to create a seal from water. The original tie-downs had a couple of eyebars that passed down through the bottom of the piers and engaged the weight of the pier to help hold the end of the bridge down. When Michael Baker International took a closer look, it found the existing eyebars were not sufficient to carry current design loads.
“So what we ended up doing was drilling and grouting, and post-tensioning some high-strength rods to actually engage a larger mass of masonry, and use that in addition with the existing eyebar assembly to help counter that uplift we get from the live load,” said Colorito.
The original deck consisted of steel buckle plates forming the structural deck, supporting a non-composite concrete fill that acted as the riding surface. This was a technology that started in the early 1900s and came to an end in the 1940s. Over time, however, the concrete fill started to degrade at the bottom where it meets with the buckle plates and ended up floating over the top, so the bottom 2 in. of the deck ended up turning to dust. The new reinforced concrete deck is fully structural, relegating the buckle plates to serve as stay-in-place formwork. To increase the flexural capacity of the stringers, they were made composite through the use of channel-type shear connectors.
The thickness of the new deck ranges from 8 in. at the curb to 11.5 in. at the crown. After the contractor expressed concern about the means and methods of placing material, it was decided to pour the deck one panel at a time then finish the deck by hand using a vibro-screed. For smoothness, Brayman Construction Corp. added a ¼ in. to the pour and once all the panels of the deck were cast and cured the contractor went back and ground the quarter-inch of material partially or completely away to achieve the right cross slope and profile.
The bridge was analyzed for the first time using a fully 3-D finite element model to examine the effects of unbalanced loading and modern design vehicles on the structure. Analyses of construction and modifications to the bridge were completed in Midas/Civil and then checked by hand using techniques devised in the 1940s. Michael Baker International also performed the first Load and Resistance Factor Design analysis of the bridge, and this advanced analysis uncovered a design error in the original 1920s calculations regarding how bending moments were estimated in the stiffening girder and how reactions were calculated at the bridge tie-downs at the bridge ends. The original calculations did not predict uplift under dead-load conditions, even though uplift exists at the ends of the bridge’s main spans. Therefore, in order to safely sever and replace the existing tie-down assemblies, a temporary tie-down system was detailed during construction to keep the ends of the bridge in place.
Attention to historic detail was even evident during the replacement of thousands of rivets. ASTM F1852 high-strength bolts with button heads to mimic the look of rivets were used. The bolts were installed using electric shear wrenches capable of providing uniform torque values and increasing the efficiency of the bolt installation.
Project: Andy Warhol (7th street) bridge rehabilitation
Location: Pittsburgh, Pa.
Owner: Allegheny County Department of Public Works
Designer: Michael Baker International
Contractor: Brayman Construction Corp.
Cost: $25,544,749.02
Length: 1,061 ft
Completion Date: Aug. 23, 2018
About The Author: Wilson is editorial director of Roads & Bridges.
