It's good news that the Golden Gate Bridge is finally going to get a retrofit. It has been a well-kept secret that San Francisco's most famous landmark is old, brittle and vulnerable.
in the photo: the arch structure of the Golden Gate Bridge over Fort Point on the San Francisco side
The last time I walked out onto the bridge was to watch the Queen Mary 2 come into San Francisco Bay. I remember looking at the rivets and the arch bridge which covers Fort Point. I realized that the Golden Gate Bridge is a house of cards waiting to crumble.
That arch bridge has long been known to be the weakest point of the Golden Gate Bridge. The arch bridge is the section that spans Fort Point. The arch rests on steel pins. In a major earthquake the arch would be jolted up and off the pins, causing it to completely collapse. The result might be that the suspension portion of the bridge might still stand, but the San Francisco approach would almost certainly be cut off by the collapsed arch bridge section.
Seventy years ago steel was a lot different that steel today. The Golden Gate Bridge is built mainly of conventional plain carbon steel, which is iron and carbon alloyed with manganese, silicon and copper. Back in the 1930's they used increased carbon content to make steel harder and stronger, but it also made the steel more brittle, not a good attribute in a major earthquake.
In 1992, the Carderock Division, Naval Surface Warfare Center partnered with AISI and the Federal Highway Association (FHWA) to develop new and improved steel alternatives for bridges. The team brought together a cadre of professionals in steel production, bridge design, bridge fabrication and welding, as well as specialists from the U.S. government and academia.
The result was a new type of steel, known as high-performance steel (or HPS), which provided up to 18% cost savings and up to 28% weight savings when compared with traditional steel bridge design materials. The new steel is also more flexible, has greater strength and will last much longer than conventional steel. HPS went from concept to application in just five years, and is now being used in 42 states.
The fracture toughness of high-performance steel (HPS) is much higher than the conventional bridge steels. With higher fracture toughness, high performance steels have much higher crack tolerance than conventional grade steels.
When the Golden Gate Bridge was built in the 1930's, steel fabricators used the hot-rivet method. The GGB was fabricated by inserting rivets into holes pre-drilled in the steel; the rivets are then peened using an air hammer on one side and a bucking bar (an inertial mass) on the head end. As these cool slowly, they are left in an annealed (soft) condition, while the plate, having been hot rolled and quenched during manufacture, remains relatively hard. Under extreme stress the hard plates can shear the soft rivets, resulting in failure of the joint. A major earthquake would create that kind of extreme stress. The Golden Gate Bridge could very likely collapse in a major earthquake.
The rivets driven when the bridge was built in the 1930s will be replaced by newer, stronger HPS steel bolts that are twice as strong as the originals. The bridge's piers will have high-performance steel tendons wrapped around them which will significantly increase the ability of the base piers to withstand violent shocks and earth movement.
When the GGB was built is was a modern engineering wonder of the world. Even today is is often regarded as the best bridge ever built. Still, this engineering masterpiece is constructed of old materials and it is in need of retrofit ... and it needs the work done quickly.
Now ... if we can just hold-off that big earthquake!
Resource and Story Tools:
The San Francisco representatives on the GGB Board are:
(click on any name for a biography)
John Moylan, Board President
Tom Ammiano
Bevan Dufty
Dick Grosboll
Sabrina Hernandez
Jake McGoldrick
Lynne Newhouse-Segal
Janet Reilly
Gerardo Sandoval
Specifications and Details of Project:
Phase 1: Done in 2002
North approach viaduct: Installed isolators, replaced and added some bracing members, replaced towers, added cover plates, strengthened foundation, replaced expansion joints, closed roadway joints
Phase 2: Done in 2007
South viaduct: Installed isolators, replaced towers and bracing members, added cover plates, strengthened foundation, replaced expansion joints, closed deck joints
Pylon S2: Strengthened with steel plates, internally and externally, and anchored to bedrock
South anchorage housing: Strengthened by reinforcing internally, replaced west wall and strengthened east wall, strengthened foundation
Pylon S1: Strengthened with steel plates, internally and externally, and anchored to bedrock
Fort Point arch: Installed energy dissipation devices and expansion joints, added bracing, strengthened members and modified bearings
Phase 3A: Fall 2007-2011
Pylon N1: Strengthen by reinforcing internally, anchor to bedrock
North anchorage housing: Strengthen by reinforcing internally, replace roadway deck
Phase 3B: Early 2009-early 2012
Side span: Replace expansion joints, install dampers
Concrete fender: Replace portions of curb
Stiffening trusses: Replace some lateral bracing, strengthen connections
Tower: Install stiffeners, strengthen connections
Pier: Strengthen with prestressed steel tendons
Tower saddle: Strengthen and immobilize saddle/ cable connection
Suspension span: Install dampers at towers and pylons, strengthen stiffening trusses
More information: Golden Gate Bridge, Highway and Transportation District, San Francisco Chronicle
0 comments:
Post a Comment