Bridges Being Strengthened to Withstand Earthquakes
Out of the rubble of the San Fernando earthquake of 1971 came revised building codes and plans for seismic retrofitting of bridges in California. When the destructive 7.1 magnitude Loma Prieta earthquake hit the San Francisco Bay Area in 1989, it caused notable collapses in bridges built to outdated seismic standards and not yet retrofitted. The 6.9 magnitude Northridge Earthquake in 1994 would result in still further construction and retrofit modifications.
Currently, five of seven bridges in the San Francisco Bay area under management of the California Department of Transportation (CALTRANS) are being rebuilt or seismically retrofitted to meet new codes for bridge strength – they are the Golden Gate Bridge, San Francisco/Oakland Bay Bridge, Carquinez Bridge, Richmond/San Rafael Bridge and Benicia-Martinez Bridge. The goal of seismic retrofitting is to prevent collapse and has already been completed on the San Mateo-Hayward Bridge.
Much like after the 1971 San Fernando earthquake in southern California, The Lincoln Electric Company is in San Francisco in northern California again, this time with equipment and consumables being utilized in the seismic retrofits on every bridge. Lincoln Electric has been involved in the training of welding operators working on all the bridges to comply with the stringent AWS codes.
For the construction of bridges in the United States, there are additional welding test requirements as specified in the American Welding Society’s (ASW) Bridge Welding Code D1.5, section 5. Welding operators must be qualified on a specific weld process and on-site inspectors help insure procedures are being followed.
These retrofits are extremely important for the metropolitan San Francisco area. According to an April 23, 2003, article from the Sacramento Bee, scientists predict the chance of a 6.7-magnitude or higher quake occurring by 2032 at 38 to 85 percent. When a 6.0 to 6.6 magnitude quake is factored, the rate raises to 80 to 96 percent chance of a quake by the same date.
Golden Gate Bridge
Named one of the “Seven Wonders of the Modern World,” the Golden Gate Bridge is the most famous bridge being brought up to code. Nearly 70 years old and providing passage for over 125,000 vehicles per day, the Golden Gate is undergoing a complete re-fabrication over several phases. This 1.7-mile suspension bridge is anchored by 24,500 tons of wire cables secured by 100-square-foot concrete pylons. Over the years, this concrete has been punished by the ocean’s harsh salt water, which has caused it to slowly deteriorate.
As part of phase II of the re-fabrication, these concrete pylons are being replaced. The joint general contractor team of Shimmick/Obayashi is stripping out the existing concrete and pouring new concrete into a double-walled welded framework. An outer protective wear plate is being welded in place for added corrosion resistance.
Scott Laumann, Project Superintendant, says they are using Lincoln’s DC-600 welders with LN-7 and LN-25 wire feeders for this high-performance, multi-process, multi-pass welding. The outer plates being welded on to the pylons are 1.5-inches thick, 10-foot high and 32- to 44-feet long. Welding these plates required a larger diameter wire capable of out-of-position work for increased production. An 0.072-inch diameter wire was selected.
“The Lincoln wire feeders handle the larger diameter wire nicely,” says Laumann. “We chose the Lincoln welders specifically to work on this outside plating because they are lighter weight and easier to operate.”
This second phase of the Golden Gate Bridge seismic retrofit has been going on for nearly one year and Laumann expects the improvements to the south viaduct and anchorage, Fort Point Arch and south pylons to take another year to complete. In phase one of the project, the north viaduct was retrofitted. Phase three will retrofit the north anchorage, main span and main towers.
San Francisco/Oakland Bay Bridge
Originally thought of during the Gold Rush, the San Francisco/Oakland Bay Bridge wouldn’t become a reality until 1936. Actually two bridges that meet via a tunnel on Yerba Buena Island, the Bay Bridge is a marvel of engineering that holds two records. The Guinness Book of World Records lists the Yerba Buena Tunnel as the largest diameter bore tunnel in the world, measuring 76 feet wide by 56 feet high. The bridge also employs the world’s deepest bridge pier, sunk 242 feet below water level on the east bay crossing,
The vulnerability of the Bay Bridge was shown during the 1989 Loma Prieta earthquake. Bolts holding a section of the upper deck to the trusses sheared, causing a portion of the deck to fall onto the lower deck.
The Bay Bridge’s skyway section, supporting nearly 270,000 vehicles per day, is the current area of activity of the seismic retrofit operations. In addition to bringing the bridge up to current seismic standards, the bridge is being converted from an “over-under” design where the one direction of traffic flows over the top of its opposite, to a traditional side-by-side design.
XKT Engineering, Inc. is handling the welding and construction of the bridge’s new pilings. According to Gary Mathison, Project Manager, these pilings, fabricated from two- to three-inch thick material, A709 grade 50 steel plate, are just over eight feet in diameter (2,500 mm). Each section of the piling is ten feet high (3048 mm) and the seam is tandem submerged arc welded using Lincoln DC-1000 welders with Lincolnweld® 860 flux, and in most cases, 3/16-inch L-61 wire running through NA-5 wire feeders. The sections are then joined with a circular weld using the same process.
The bottom two-thirds of each piling (220 feet), are completed off site and then transported by barge to the bridge where they are driven into the San Francisco Bay floor. The remaining 110 feet of piling are then welded onto the top on site with the aid of a welding shelter that is raised from the barge to the section being welded.
“The Lincoln equipment has been very dependable. We’ve had a good experience with it,” said Welding Superintendent Jim Carroll. “We’ve also had a lot of excellent help from the Lincoln people out here, Bob Hetzendorfer and Russ Farrar.”
The seismic retrofit of the existing Carquinez Bridge has already been completed, but with 116,000 vehicles crossing each day, CALTRANS commissioned a new suspension bridge be built to updated standards. The new bridge will be of orthotropic design – the first of its kind to be built in California since the 1960s – and will be one of fewer than 100 in existence in North America.
California Erectors has been contracted to weld the bridge’s top deck sections and the tub sections that make up the bridge’s bottom sides and edges. To weld the 3/8-inch, A709 grade 50 steel plate that makes up the deck that will support the asphalt, Project Manager Galen Jaeger said they chose to use Lincoln’s subarc welding equipment based on years of previous experience with the company’s products. “We’ve always used Lincoln,” Jaeger said. “That’s what our first selection was.”
The LT-7 self-propelled mechanized DC dual wire feeders are being used with 1/8-inch wire and 5/32-inch electrodes and 860 flux. The power sources are DC-600s in six-pack racks. The welds being made by the LT-7s were 92-feet long.
On a weld of that length, Jaeger said that travel speed was the most critical variable to control. Lincoln sales representative Bob Hetzendorfer was instrumental in helping California Erectors dial in the travel speeds on the LT-7s Jaeger said.
When welding the tub sections of the bridge, Jaeger said it was necessary to first seal the joints with a root pass by the submerged arc welders. The welds were then finished using a flux-cored process (FCAW-S) with the Lincoln DC-600 power sources, LN-25 wire feeders and 0.068- or 0.072-inch diameter Innershield® NR®-232 flux-cored self-shielded electrode (AWS E71T-8). Wire diameter used depended on the thickness of the steel, which varied from 3/8-inch to ½-inch thick.
“Historically we’ve always had good luck with the LN-25s,” Jaeger said. “They always ran well on this type of a project with just general maintenance.”
One of the most heavily traveled bridges in the state, the Benicia-Martinez Bridge allows over 100,000 vehicles per day to travel between the Sacramento Valley and the Contra Costa, Alameda, and Santa Clara County areas. Built in 1962, it is a deck truss bridge with 18 spans crossing 1.2 miles.
This structural stationary bridge has been under construction for a year and a half on land on both sides of the bay. Currently, construction of the working platform over the bay is taking place. This platform is how construction workers will get around on the bridge once construction of the span begins.
Pile Splicing Superintendent Oscar Catolico said that in building this platform, barges set-up identical to those being used on the San Francisco/Oakland Bay Bridge are being utilized with minor modifications to the welding shelters to accommodate the bridges different designs (Catolico is working on both bridge projects). On the barges, two 40-foot lengths of eight foot diameter pipe are joined with a circumferential weld. These are driven into the bay floor as support members. Scaffolding is then placed on these support members.
“The only real difference between this project and the San Francisco/Oakland Bay Bridge project is that these piling are driven in vertical while the pilings on the Bay Bridge are at an 8-degree angle,” said Catolico
Richmond/San Rafael Bridge
Making a 4.04-mile gap over the north end of the San Francisco Bay passable by car for nearly 56,000 motorists daily, the Richmond/San Rafael Bridge is a total of 5.5 miles in length including its approaches. When it was opened in 1956, this cantilever and truss constructed bridge was one of the largest bridges in the world.
The seismic retrofitting on this bridge is in its beginning phases with all work done to this point being done in preparation for welding. The first phase of welding will be to reinforce the support column bases with structural steel utilizing semi-automatic, flux-cored, self-shielded processes.
The on-site, in-position welding to be utilized will be challenging. General Contractor Tutor-Salida is relying on Welding Contractor Koch Construction’s decision to go with welding equipment from the Lincoln Electric Company to accomplish the task. Koch just purchased eight Vantage® 500 engine drives and four Invertec® V-350 PRO rack-mounted, four-packs with accompanying LN-25 wire feeders. Consumables are .072-inch NR 232, also from Lincoln. Welding work on the Richmond/San Rafael Bridge is expected to ramp up in the summer of 2004.
The objective of the collective years of work being put in on these bridges is to save lives and structural damage when the next quake occurs. The Lincoln Electric Company played a significant role, as a supplier of arc welding products, in the initial construction of San Francisco’s many office buildings and bridges to the codes in place at the time they were originally constructed.
The company participated in much of the seismic retrofitting efforts following the 1971 San Fernando earthquake to bring these structures up to the revised codes of that period. And again, Lincoln is honored to play a role in the retrofitting of these five landmark bridges to bring them up to the latest code modifications. These five structures serve an important role in the infrastructure of the daily life of millions of residents in the San Francisco area, and with these latest modifications, will stand the test of time for generations to come.