“Mr./Ms. Engineer, your mission, should you choose to accept it, is to retrofit the Richmond-San Rafael ‘roller coaster’ Bridge. The bridge, measuring 22,000 ft, has 4 different steel structural systems along its length. Most spectacular are the two 1070 ft spans over active shipping channels – each made by two 535 ft-long cantilever arms that meet in the middle (figure 2). In the current state of the bridge, the foundation piers can shear in an earthquake, causing collapse, the steel diagonal members on the rigid steel tower can fail, causing collapse, and the brittle old riveted connections can fail, causing collapse. The bridge must be able withstand an 8.3 mag quake on the San Andreas fault. You get 700 million dollars. GO”
Such was the challenge faced by engineers in 1999, tasked by CalTrans with designing a retrofit of the Richmond-San Rafael Bridge. They came up with interesting, creative solutions to make the bridge earthquake safe:
1️⃣ NEW TOWER FRAMES – Compare the new and the old towers in figure 3. New steel frames were installed, called eccentrically braced frames (fig. 4). The frames are eccentric rather than co-centric because the diagonals do not frame directly into one another, but instead are separated by an offset structural fuse. This offset fuse did not exist on the old towers. As well as the fuse, the frames feature a network of diagonal bracing, shock-absorbing viscous dampers, and rotating pins. The frame can rock back and forth while the offset fuses and dampers absorb energy in earthquakes, saving the more fragile cantilever deck structure above. Testing of the frame components was performed at UC San Diego’s CalTrans Lab on the hydraulic shake table called the Seismic Response Modification Device (link).
2️⃣ CONCRETE JACKET – Compare the old piers with the new piers in figure 5. Engineers designed pre-cast concrete jackets, which surround the older concrete piers, to prevent the piers from shearing in strong earthquakes. Most of the jackets were installed underwater by cranes on barges and teams of divers, one of whom is shown in figure 6. The installation of jackets over the piers is similar to the strengthening of foundations for land-based bridges with jackets in seismic retrofits (see post: Retrofitting San Diego’s “People’s Bridge” to be Seismically Safe – Seismic Saturday)
3️⃣ BOLTS – 250,000 old rivets were replaced with new, high-strength steel bolts. 500,000 new bolt holes were drilled and new retrofit plates were added. Bolts are easier to install, don’t require an on-site furnace, and do better in earthquakes due to their toughness. A concern of rivets is that because they cool down so fast after taken out of the furnace, they are essentially quenched, and thus very brittle, whereas bolts can be cooled down slowly and subjected to other forms of heat treatment during manufacturing and thus obtained greater toughness (See Heat Treatment of Bolts…). Going across the bridge, one can see the old and new connections (fig. 7). The new bolts have a hexagonal nut, while the old rivets have a cylindrical dome-like head.
Upon completion in 2004, the project was called “the most complex single retrofit program ever attempted by Caltrans” by the Metropolitan Transportation Commission. That the bridge can now withstand an 8.3 magnitude quake on the San Andreas Fault is a testament to the creativity and ingenuity of the engineers.
References
Richmond-San Rafael Bridge Retrofit Completed. Metropolitan Transportation Commission. September 22 2005. https://mtc.ca.gov/news/richmond-san-rafael-bridge-retrofit-completed
Richmond – San Rafael Bridge Seismic Retrofit. Tutor Perini. https://www.tutorperini.com/projects/bridges-roads/richmond-san-rafael-bridge/
Richmond-San Rafael Bridge Seismic Retrofit. Norcal Structural, Inc. https://norcalstructural.com/project/richmond-san-rafael-bridge-seismic-retrofit/
Heat Treatment of Bolts and Fasteners. Bayou City Bolt. https://www.bayoucitybolt.com/heat-treatment-bolts-fasteners.html#:~:text=Heat%20treatments%20of%20stainless%20bolts,of%20the%20ASTM%20A320%20specification.
Saul Chaplin 