Design and Construction of the Golden Gate Bridge - Case Study Example

Problems with the Construction of the Golden Gate Bridge The Golden Gate Bridge is one of the modern engineering wonders of the world, with its two main towers suspending huge cables which support the entire 1.7 mile long platform where vehicles cross. In earthquakes the bridge simply flexes up to ten feet and swings twenty-seven feet in high winds in each direction without even disrupting traffic. However, it was a long time between the original survey in 1918 to the final opening of the bridge in 1937. Many experts believed the 6,700 ft (2,042 m) strait could not be bridged. Swirling tides and currents, with icy water 500 ft (150 m) in depth at the center and ferocious winds and blinding fogs were seen as insurmountable obstacles for construction or operation of the bridge. In addition, the design elements were changed several times as engineers tried to find the best solution to bridge the gap from Marin County to San Francisco. Even now, retrofitting is ongoing, as it has been discovered that the bridge is not impervious to earthquakes. (Nolte 2007) It is, in fact, a very lucky set of circumstances that this bridge has shown none of the flaws of other constructions of its time, during which civil engineering and geological studies were just in their infancy. With today’s technology, many things would have been changed, and some changes have already been applied, while others are yet to come as studies begin to help us understand the immense amount of good luck which figured into the construction of the Golden Gate Bridge, since it has endured for decades in the presence of immensely powerful destructive forces and unbelievable varied geological and environmental structures. It was simply one of the best circumstances for bridging what appeared to be an unbridgeable gap. Putting aside the amazing developments in engineering of the time, and the intuitive genius of its design which made this bridge possible, the geological and other environmental factors make this bridge a veritable miracle, since they combine to make it possible. The depth of the channel opening into San Francisco Bay is naturally very deep to permit entrance of deep draft ships, but the shelf upon which the main supporting piers stand are very solid bedrock. In addition, the weather elements made construction quite dangerous, but a safety net saved 19 lives, and would have saved more if a piece of the bridge had not broken the net at the same time as ten men fell to their deaths. The low number of fatalities for this construction was a new record for the time. It is well known that this region lies on a major geological fault system where two major tectonic plates are in constant stress, pushing against each other, sliding and causing massive earthquakes, often in excess of 6 on the Richter scale, and sometimes up to 8 points. After the Loma Prieta earthquake of October 17, 1989, some fifty miles away, it was decided to retrofit the bridge for resistance to earthquake damage, since it was discovered that the piers and other supports, plus certain other parts of the bridge were vulnerable after all. The following illustration from State Mining and Geological Board (2006) shows a detailed view of all the planned retrofits. The measures included adding or strengthening the steel plates in various places, adding stronger anchors to the bedrock, installing isolators, replacing many smaller towers and bracing members plus adding cover plates to them, installing dampers, stiffening the trusses on the main span and strengthening the connections between them, stiffening the lower trusses of the towers and internally reinforcing the foundations of the towers and piers, replacing or adding bracing members, expansions joints and pieces of roadway deck, closing the roadway joints and reinforcing anchorage housings. It is an expensive proposition, but deemed much less costly than replacing the bridge. Phase one on the Marin approach was completed in 2002. Phase two was completed in 2006. Sensors and instrumentation to record seismic activity and enable engineers to know exactly where stress occurs were installed. Retrofitting the north tower and viaduct was done. This posed great difficulty due to the steep slope of the terrain adjacent to the site of the north pier and the ocean on the other side. Work was restricted to four days to accommodate tourist visits, and work was almost always done in high wind or high wave conditions. The deep icy water below was an added hazard, as was daily morning and evening fog that often eliminated visibility and strayed all day long. With only a very small staging area and virtually no storage for either tools or materials, work was done carefully using only the on hand tools and supplies, and the supply chain was carefully planned and managed. The last phase of the retrofit included the placement of 119 sensors to detect seismic movement, as illustrated below. The second and third illustrations show the precise location of each of the 119 sensors which allow close monitoring of the status of the bridge. This will allow repairs when necessary to be focused upon the correct areas, and will prevent collapse caused by neglecting these needs, Most of all, the monitoring can assure adequate warning should the bridge become dangerous and this will prevent tragic accidents. The last illustration shows where the sensors were placed in the viaduct and approaches. The report of Huang and Kao in the State Mining and Geological Board’s Field Trip Guide to the Golden Gate Bridge (2006) details the retrofit completely for those who wish to see it all. It also details the historic and possibly future seismic activity and how these sensors will prevent damage from escaping notice and point to needs for repair. In this way it is hoped that any damage done by wind or earth movement can be repaired before it leads to the collapse of the entire structure. What is truly amazing is that these retrofits were all completed without ever closing the bridge to traffic. The earliest known geological survey of the San Francisco area was done by William Phipps Blake in (1858). He described the huge variation in the geological structures and deposits, Blake identified most of the geological structures as sandstone and shale, with eruptions of trappean and serpentine rock. He said that all of these were of very recent geological age. Point Reyes and the Farallon Islands seem to be mostly granite, as are the surrounding Santa Cruz Mountains. The tight sandstone shows eruptions of lime and quartz in many places, though never together, from possible intrusions through steam vents along the sides of racks and fissures. The sandstone was quarried from several locations for construction in the city. Blake noted that “the extensive metamorphism, and the uplifted condition of all the strata, indicate the proximity of igneous rock.” He noted that these were not exposed. Blake also noted fine alluvial clay in the marshes south of the city and adobe underlying much of the area of San Francisco. Many artesian wells pierce this alluvial layer and water is found at 50 to 150 feet. Blake marveled at the presence of formations and rock from various ages covering eons of time. All of this should have indicated the long history of tectonic movement, and the wide variety of different ages of formation creates a less than settled foundation for any construction, especially a bridge. It is probable that only a suspension bridge can survive these conditions over time. Testing the retrofit parts for the Golden Gate Bridge was problematic, since they could not realistically be tested in situ. Many were tested using scale models of the bridge parts and the retrofit mechanisms. This was actually a very recent development for engineering testing on the sca;le matching this large sized construction. http://walrus.wr.usgs.gov/reports/reprints/Barnard_EOS_87.pdfAiken has been involved in designing testing procedures and the codes for the tests based in California for a couple of decades. He has been involved with retrofitting many old structures in California for better earthquake survival. What is significant about his report (1998) is the great detail he goes to in describing the equipment used in the retrofit and the testing used to prequalify it for the Golden Gate Bridge. He says, “Viscous dampers are an integral part of the seismic retrofit of the suspension spans of the Golden Gate Bridge, included to control the motions of the deck spans relative to the main tower legs. The importance of the dampers to the overall retrofit, and the large size of the devices, led to a thorough damper prequalification testing program.” While wind, fog and icy water are obvious hazards to construction, the presence of the massive Pacific Ocean just outside the inlet to the bay is often not considered by the layman as a huge hazard, since it only presents problems during storms or after quakes. Another nearly invisible factor is the presence of, “Sand wave crests can be traced continuously for up to two kilometers across the mouth of this energetic tidal inlet, where depth-averaged tidal currents through the strait below the Golden Gate Bridge exceed 2.5 meters per second during peak ebb flows. Repeated surveys demonstrated that the sand waves are active and dynamic features that move in response to tidally generated currents.” (Barnard 2006) Important issues for sediment management, “include dredging of the navigation channel, local coastal erosion, sand mining for construction and fill, and sources of sediment for beach nourishment.” These sand waves may also decrease turbulence by retarding the ebb and flow, separating it into eddies around the sides. At the entrance to San Francisco Bay, the channel has been scoured down to bedrock reaching 113 meters. “Tidal currents accelerate through the narrow, erosion-resistant rocky strait spanned by the Golden Gate Bridge, forming one of the deepest natural channels in the world. The flow through this channel is forced by an enormous tidal prism of 2 × 109 cubic meters (528 billion gallons), resulting in tidal currents that typically exceed 2.5 meters per second.” (Barnard 2006) This keeps the narrowest part of the channel very clean, requiring little or no dredging for shipping traffic through the mouth under the bridge. However, this also results in the formation of one of the largest sand wave fields in the world on both sides of the Golden Gate, which decreases the turbulence under the bridge. Most of the sand and gravel of this field comes from the Sierra Nevada, and has been carried to San Francisco Bay via by the San Joaquin and Sacramento rivers or from local erosion of the Marin Headlands. It is a natural and self-regulating system, and requires little adjustment by man. This was simply a very good place to build a bridge. So the Golden Gate Bridge is not only a unique and awesome structure which rates as a modern engineering wonder of the world, but it is inspiring in its serendipitous design in light of the relatively primitive geological scientific surveys. The major geological feature of the area is sandstone and shale, both not usually strong, but these are, in fact, very compacted layers and are, therefore, quite strong. In addition, the bedrock promontories on both sides are very stable, though there may be viscous layers in the formations which actually act as natural dampers. In addition, though the geological factors are found to be very complicated and ranging in age from very recent to spanning eons, the bridge just happens to be in the right place and built with the necessary engineering traits. The construction was fraught with problems of wind, fog, icy water and cold. Still the project set a new low record for the number of fatalities. Since the Loma Prieta earthquake, new studies were done concerning the safety of the bridge in spite of the flexibility and its ability to sway in earth movements or high winds, because it was discovered that some constructions thought to be impervious to earth movement were not. This led to a three stage retrofitting of the bridge for both earthquake proofing in new support, dampers and slides and sensors to locate possible damage after a quake. The most recent studies of the layer of sand dunes under the water just outside the bay points out another factor in the survival of the bridge, since these serves to reduce the turbulence, which is quite considerable as it is. So the Golden Gate Bridge is not just an engineering wonder, it is in line with modern engineering principals for the geology of the region almost by pure accident. ‘ References Aiken, Ian D.. TESTING OF SEISMIC ISOLATORS AND DAMPERS-CONSIDERATIONS AND LIMITATIONS . PROCEEDINGS, STRUCTURAL ENGINEERING WORLD CONGRESS, SAN FRANCISCO, CALIFORNIA, JULY (1998). http://www.siecorp.com/publications/papers/ida_1998.pdf Barnard, Patrick L. .EOS . Giant Sand Waves at the Mouth of San Francisco Bay . PAGES 285, 289 . Vol. 87, No. 29, 18 July 2006 . http://walrus.wr.usgs.gov/reports/reprints/Barnard_EOS_87.pdf Blake, William Phipps . 1858 . Geological Reconnaissance in California . in State Mining and Geology Board . 2006 . Field Trip Guide to the Golden Gate Bridge. www.consrv.ca.gov/.../reports/.../GOLDEN%20GATE%20BRIDGE.pdf Nolte, Carl . 2007. "70 YEARS: Spanning the Golden Gate:New will blend in with the old as part of bridge earthquake retrofit project". San Francisco Chronicle. http://www.sfgate.com/cgibin/article.cgi?file=/c/a/2007/05/28/MNGV7Q2QGI1.DTL. State Mining and Geology Board . 2006 . Field Trip Guide to the Golden Gate Bridge. www.consrv.ca.gov/.../reports/.../GOLDEN%20GATE%20BRIDGE.pdf   Read More