Retrofitting San Diego’s “People’s Bridge” to be Seismically Safe

Figure 1: The 1st Avenue Bridge over Maple Canyon

Shrouded by eucalyptus in Maple Canyon is one of San Diego’s most impressive bridges. This #seismicsaturday we feature the 1st Avenue Bridge.

The 463-foot bridge was built in 1931, and is nicknamed “The People’s Bridge” as it was funded by San Diego’s first public infrastructure tax. The bridge was actually assembled first in Ohio, before pieces were loaded on trains and brought to San Diego. The main span is a massive steel arch (fig. 1). The arch is connected to concrete abutments on either side with slanted pin connections (fig. 2). The pin allows the beams and columns to rotate slightly, allowing the bridge to flex in earthquakes and when heavy trucks drive across without putting repetitive stress on the support.

Figure 2: Massive pin connection at foundation. The pin allows the columns to rotate slightly and flex.

Seismic Retrofit

A 13-million dollar seismic retrofit was completed in 2010. If you look closely, you can spot the differences between old and new:

Connection Sleeves

Above the pinned connections, sleeves have been added around the bridge columns (fig. 3). The sleeves help secure the main columns of the bridge to the steel embedment into the foundation. One can spot these sleeves by noticing that they are secured by bolts with hexagonal nuts, as opposed to the original components which are are all connected by rivets with circular heads.

Figure 3: New retrofit sleeves assed to tbe left and right of the columns. The new sleeves are identifiable by their use of hez bolts rather than rivets (with circular heads).

Tie Rods

To tie the bridge to the side abutments, large threaded rods have been added (fig. 4). The rods are anchored into the concrete abutments on one side and bolted to the deck on the other, keeping the bridge tied to it’s abutment in an earthquake, and preventing the deck from being thrown-off and separating from the concrete abutment. These rods are are analogous to tension ties (fig. 5) used when building wooden decks beside houses.

Figure 4: Threaded rods, embedded on one side to the concrete abutments and bolt in the other side to tbe bridge deck, keep the deck from sliding off the abutments during an earthquake.

Figure 5: Tension tie connecting a wooden deck joist to a house floor joist. This connection would keep the deck tied to the house in tbe case of an earthquake. In this instance, two Simpson Strong Tie DTT2Z are used, connected by a tie rod.

Old and New Nuts Reveal Confining Sleeve

Now is where things get super cool. Look closely at the nuts in fig. 6 and 7. Do you notice a difference?

The first is slightly conical on top while the second is flat. The difference likely means that the bolts were installed at different times, with the second bolt probably installed during the 2010 retrofit. The second bolt looks to be part of a layered metallic sleeve (fig. 8), that encases the the concrete of the original foundation to prevent shearing during an earthquake and deterioration over time.

Figure 8: Metallic sleeve encasing the original foundation, added during the 2010 retrofit, to strengthen the foundation during an earthquake

The People’s Bridge is a wonderful example of how old, corroding structures can be retrofitted to be safe and beautiful once again. It must have taken a lot of creativity on the part of the retrofit bridge engineers, T.Y. Lin International, to add modern reinforcement while retaining the bridge’s rustic appearance.

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