Proceedings of Bridge Engineering 2 Conference 2007 27 April 2007, University of Bath, Bath, UK THE STUDY ON SAN FRANCISCO GOLDEN GATE BRIDGE Kei Fung Sameul, Kwan University of Bath, Architecture and Civil Engineering Department Abstract: In this paper, I am going to analyse the San Francisco Golden Gate Bridge in terms of the considerations of construction method, structure, aesthetics, loadings, serviceability, strength, the effect of earthquake, wind, temperature, and creep, durability, intentional damage, possible future changes and construction improvement. Adequate amount of calculations are involved in order to the feasibility of the bridge. Keywords: suspension bridge, parallel wire construction, suspended cantilever construction, deco design, Fritz Leonhardt’s 10 rules, wind tunnel test, compressive membrane action, blast-resistant structural systems, seismic retrofit 1 General 1.1 Information and History of Golden Gate Bridge Golden Gate Bridge is a suspension bridge spanning across Golden Gate, the opening into the San Francisco Bay from the Pacific Ocean, shown in Fig. 1.1. It is classified as suspension bridge because it has a fairly flat deck which is suspended by the hangers attached to the main catenary cable. Ref. [1] It provides a link to the city of San Francisco by connecting the northern tip of the San Francisco Peninsula to Marin County as part of US Highway 101 and California State Highway 1. to the congestion of ferry happened frequently on the bay. Bridge-builder and engineering Joseph Strauss became convinced that a bridge was necessarily constructed across the bay. The engineering challenge in this project was very difficult; the Golden Gate Bridge area has winds up to 96 km per hour, and strong ocean current sweep through a rugged canyon below surface. Construction started in 1993. The construction budget at the time of approval was $30.1 million. Actual construction costs turned out to be $36.7 million, resulting in a cost overrun of 22%. The Golden Gate Bridge was the longest and largest Suspension bridge in the world by the time of 1927 when it was completed and started opening to traffic. It has become an internationally landmark and recognized symbol of both San Francisco and the United States. Nowadays, it is still the second longest suspension bridge in the United States. 1.2 Principal of Long-Span Suspension Bridge Figure 1.1: Golden Gate Bridge Before the Golden Gate Bridge was built, the only way to travel across San Francisco Bay was by ferry. Due For a suspension bridge, the deck is always suspended by hangers, each attached to the main cable, which in turn is anchored into the ground at its ends. The deck provides some stiffness to the system so that concentrated loads on the deck are spread to several hangers. Suspension bridge usually has a slender deck which carries bending only (no compression). Static design of this kind of bridge is straightforward. Of more concern normally is the lateral wind loading. In order to suspend something of uniform weight ‘for example: the deck’ off a cable, we have to decide the shape which the cable would choose to take. An equation Eq. (1.1) was developed to prove the cable deformed into a parabola and the horizontal component force in the cable was a constant value. H=wl2/8f Equation 1.1 Where w is the weight per unit length, l is the plan length of the cable and f is the dip of the cable at midspan. 2 Constructions Today, people call it the "most spectacular bridge in the world." However, a century ago, building the Golden Gate Bridge was seemed like an impossible task. Ref. [2] The bridge in this location should withstand brutal winds, tide, and fog. It is also located less than 13km from the epicenter of the most catastrophic earthquake in history. Joseph Strauss was the only one engineer who was willing to gamble that his bridge could withstand such destructive power. More than one million tons of concrete is used to build the anchorages - the massive blocks that grip the bridge's supporting cables. The north pier, which supports the tower, was built easily on a bedrock ledge 6m below the water. But on the southern San Francisco side, Strauss had to build his pier in the open ocean, 30m below the surface. He built a huge water-tight cofferdam and pumped in hundreds of tons of concrete. By 1935, the towers were complete, and cable-spinning began. Steel frame and steel cables are also used in the Golden Gate Bridge. The fabricated steel used in the construction of the Golden Gate Bridge was manufactured by Bethlehem Steel in plants in Trenton, New Jersey and Sparrows Point, Maryland and in plants in three Pennsylvania towns: Bethlehem, Pottstown, and Steelton. The steel was loaded, in sections, onto rail cars, taken to Philadelphia and shipped through the Panama Canal to San Francisco. The shipment of the steel was timed to coincide with the construction of the bridge. Ref. [3] Cable spinning began in October 1935. To create the cables, Roebling developed a method called parallel wire construction. The innovative technique enabled a cable of any length and thickness to be formed by binding together thin wires. It promised to give engineers the freedom to build a bridge of infinite length. Ref. [4] It is understood that the Golden Gate Bridge is constructed under suspended construction, which involves hanging bridge segments or elements from cables falls under this category of bridge construction. It is expected the deck was suspended from the main cable in large segments. In Fig. 2.1, the larger picture shows one of the two weight-blocks which ballast the cable anchorage. On top, there is a crane. These structures are concrete shells loaded with material heavy enough to resist three times the anticipated strain. Under them are eye-bars which reach deeply into concrete anchorage for a relentless grip on the southern shore. Forward from the anchorage and weight-blocks is the cable housing. It will shelter the cable strands from sun and rain where they spread fanwide from the pylons, each to its eye-bar. At the far end of the housing and just short of old Fort Winfield Scott is the beginning of Pylon S-1. This structure and Pylon S-2, just beyond the fort and nearer completion, will guide the cables and aid to support the southern end of the bridge’s floor, and a study steel arch will be wedge between them. These are the sole connecting links between land and the tower to be raised upon the pier shown on left hand corner in Figure 2.1. The inside photograph is of the south piers and its protective fender. The trestle leading from alongside the fort to the fender was built for construction and to supply concrete mixes ‘on the run’ by truck with an approximate capacity of 4 cubic meters per load. Everything except the steel and some heavy pieces of equipment was supplied across the trestle. The steel, much of it in fabricated pieces, need to be lightered. Figure 2.1 The pier, made up of 147,600 tons of concrete, was built in 37m of open water. This was achieved by the concrete fender that in itself is a marvel of construction; it is 108m long and 56m wide at the centre line of the bridge. The fender is composed of 152,600 tons of concrete. It was built to assist the construction of the pier and to protect it from the sweep of ocean current. Sea water comes in through surge-holds to the space between the pier and fender to counter-balance pressure on the fender. The windlass-like machines that flank the trestle’s junction with the fender are the hoisting engines. Each is equipped with 324 m of cable to ply the tackle of cranes that climb over the tower on a traveller truss to keep pace with each new height. Tracks around wooden platform on the fender support the ‘whirley’, the movable crane near the hoisting engine. The long-arm crane in the foreground will lift steel off lighters. Its boom can support 80 tons safely. The process of suspended cantilever construction is also expected to be the construction methods took part in the project. The construction of the deck started from each of the two towers, shown in Fig. 2.2.In order to reduce the enormous hogging moment acting over the pier during cantilever construction, we often use temporary cable stays to help support the cantilever. Since suspension bridges have permanent towers and cables available for suspended cantilever construction, the construction of Golden Gate Bridge is very cost-effective. Concrete is cast in on the top of the deck on site. However, in-situ concrete has disadvantages; it is often inferior compared with pre-cast concrete. Figure 2.2: the construction of deck Ref. [5] The construction time line of Golden Gate Bridge is shown in Table 2.1: Table 2.1 Leon S. Moisseiff was named one of the consulting engineers. He studied Strauss' original plans, calling for a hybrid cantilever and suspension structure across the strait. This plan was regarded as ugly, a far cry from the elegant, understated lines that define the great bridge today. Moisseiff proposed a bridge far more efficient and beautiful then the original design and theorized that a long-span suspension bridge could cross the strait Ref. [7] The Golden Gate Bridge’s design was very complex which made up of five types of structure not typical of most highway system bridge. In addition to the suspension bridge the approaches include a steel arch bridge, two concrete anchorages, two steel truss viaducts and three concrete pylons. The Golden Gate followed this design below the roadbed, but modified it above the deck to big open rectangles without cross-members, framing the blue sky and producing a lighter look. The towers were indented as they rose in an art deco design which further lightened the bridge’s appearance. Vertical “fluting” on their surfaces augmented this upward thrust The bridge’s cable design, spun, wire by wire, by three carriages that moved back and forth between anchorages, was similar to the Brooklyn Bridge It is arguably the most beautiful suspension bridge ever built; its colour, textures, complexities, simplicity and proportions obey all of Leonhardt’s subsequent recommendations. 3.2 Structure Detail Golden Gate Bridge is a suspended and a gravityanchored structure. Basically, it consists of six main structures: 1. 2. 3. 4. 5. 6. San Francisco (south) Approach Viaduct San Francisco (south) Anchorage Housing and Pylons S1 and S2 Fort Point Arch Main Suspension Bridge Marin (north) Approach Viaduct Marin (north) Anchorage Housing and Pylons N1 and N2 In Addition, 4700 tons of truss members were used as additional bracing member in the deck to resist wind loading within the bridge as well as preventing the oscillations. Ref. [6] The most conspicuous precaution was the safety net used during the construction of the Golden Gate Bridge, suspended under the bridge from end to end. The net recorded to save the lives of 19 men. 3 Design Issue and Structure Detail 3.1 Design Issue The design of the Golden Gate Bridge is unique. There is not another bridge to use as a model. The nowfamiliar art deco design and International Red colour were chosen. 3.2 Length, Width, Height, Weight Total length of Bridge including approaches: = 2,737 m Length of suspension span including main span and side spans: = 1,966 m Length of main span portion of suspended structure (distance between towers): = 1,280 m Length of one side span: = 343 Width of Bridge: = 27 m Width of roadway between curbs: = 19 m Width of sidewalk: = 3 m 4.2 Proportions Clearance above mean higher high water: = 67 m The Golden Gate Bridge conveys a decent impression of balance between its mass and voids, and between light and shadow. Moreover, it’s geometric balance between depths and spans, lengths and spans are also excellent, it is shown in Fig. 4.1. Total weight of each anchorage: = 54,400,000 kg Original combined weight of Bridge, anchorages, and approaches: = 811,500,000 kg Ref. [8] The Bridge has two main cables which pass over the tops of the two main towers and are secured at either end in giant anchorages. These main cables rest on top of the towers in huge steelcastings called saddles. The quantities of concrete and steel used in the Golden Gate Bridge are shown in Table 3.1. Table 3.1 Concrete Quantities Cu. m. San Francisco Pier and Fender 99,400 Marin Pier 18,000 Figure 4.1 139,160 4.3 Order Anchorages, Pylons, and Cable Housing Approaches 21,800 Paving 19,115 Structural Steel Quantities Kg. Main Towers 40,280,000 Suspended Structure 21,772,000 The order in the lines and edges of the Golden Gate Bridge are regarded as a good example of a suspension bridge. There are no additional edges and struts to arouse peoples’ mental disquiet. The appearance of the bridge looks like a mirror image of two similar towers with cables. It is a useful aesthetic trick to make a bridge in good order. Anchorages 3,991,000 4.4 Refinement Approaches 9,250,000 There are many refinements which can be used to produce an aesthetic bridge. For Golden Gate Bridge, the Americans use smaller spans as one nears the abutments. It keeps the aspect ratios of the ‘rectangles’ between ground, piers and deck constant. Moreover, there are only two columns across the width of the bridge deck. It can prevent the oblique angles of view from creating an opaque barrier in the bridge. 4 Aesthetics Ref. [9] The Golden Gate Bridge was painted with orange vermilion, deemed ‘International Orange’. The U.S. Navy initially designs the bridge to be painted with black and yellow stripes to assure even greater visibility for passing ships. However, rejecting the use of carbon black and steel grey, Consulting Architect Irving Morrow selected the distinctive orange colour because it blends well with the span's natural setting as it is a warm colour consistent with the warm colours of the land masses in the setting as distinct from the cool colours of the sky and sea. Moreover, it can also provide with enhanced visibility in fog for passing ships. The bridge is widely considered one of the most beautiful examples of bridge engineering, both as a structural design challenge and for its aesthetic appeal. I am going to analyse the aesthetics of Golden Gate Bridge base upon the Fritz Leonhardt’s 10 rules of a beautiful bridge: 4.1 Fulfilment of function The Golden Gate Bridge clearly reveal how its structure works, as well as impact a feeling of stablity; the towers in between holding up a main cable, which is attached with numerous smaller clables to support the decks. The bridge also fulfilled a high degree of simplicity, which make the bridge beautiful. 4.5 Integrating into the Environment It is always considered that a suspension bridge is one of the most suitable bridges across a wide span of water. It is achieved by the Golden Gate Bridge in between the San Francisco Peninsula and the Marin County. To integrate into the environment, be aware that it always depends on what effect a bridge designer is trying to achieve, certain bridges may not be appropriate in other particular places. However, it is simple brilliant for Golden Gate Bridge in this area. 4.6 Texture It has rough finishing for piers and abutments, which makes sense for bridge design. 4.7 Colour Colour in orange vermilion is nearly used for the entire bridge. Normally it is not easy to create an aesthetics bridge with this colour, but it is achieved in Golden Gate Bridge. This colour brings a big contrast with the sky and sea; the warm colour for the bridge and the cool colour for the sky and sea. Table 5.2: Calculation for Dead and Superimposed Dead Load (factored) 4.8 Character The Golden Gate Bridge is classified to have a high degree of ‘character’. People can easily figure out how the cables, piers and towers work in the bridge. It is a good example for other bridge. 4.9 Complexity The complexity of the bridge is shown in Fig. 4.2. The bridge follows the ‘keep it simple’ rule for a bridge. Only cables and anchorages are used to support the decks. However, it is still able to maintain a certain amount of complexity in a bridge in order to visually stimulate the people. In this way, the Golden Gate Bridge is successful. 4.10 Nature The Golden Gate Bridge successfully incorporates the nature into the design by having piers blending into the sea and placing decks and fort point arch each side of two islands. 5 Loading All bridges are designed according to limit state philosophy. We must check the Golden Gate Bridge at the Ultimate Limit State (ULS), to prevent collapse, and the Serviceability Limit State (SLS), to ensure the bridge is serviceable. 5.1 Load Types and Combinations The most important types of loadings we need to consider on the bridge are: 1. 2. 3. 4. 5. Dead load, Super-imposed dead load, Live Traffic, Wind, Temperature γf3 2. 3. 4. 5. 5.3 Traffic and Pedestrians In order to calculate the traffic loading on the span between two cable hangers (supports) on one side of the bridge, the carriageway width and number of notional lanes have to be known, then HA and HB loading can be worked out. Table 5.3: calculations on KEL, HA and HB loading Width of bridge = 27m No. of notional lanes = 6 Separation of hangers (supports) = 20m HA (unfactored) = 30kN/m Total HA (factored) on 6 lanes = (30+30/3*4)*1.1*1.3 = 100kN/m KEL (unfactored) = 120kN KEL (factored) = 120*1.1*1.3 = 171.6kN HB loading is considered to have 30 units HB (unfactored) on each axle = 30*2.5*4 = 300kN HB (factored) on each axle = 300*1.1*1.3 = 429kN These loads are used in different combinations, in order to find the worst case of loading. To find the wind load, first, the height of the deck above the ground needs to be known. Then the maximum wind gust (Vc) can be derived from Eq. (5.1): γfl 1.10 1.10 1.05 1.75 1.10 1.10 1.10 1.30 1.10 1.00 There are five combinations of load: 1. The weight of suspended structure = 21,772,000kg The weight of concrete paving = 19,119kg Total length of longest span = 1280m Therefore, Dead Load (factored) = (21,772,000+19,115)*10/1280 *1.1*1.05= 187kN/m Assume Superimposed Dead Load (factored) = 38.5kN/m 5.4 Wind Load Table 5.1: partial factors for different load cases Load Case at (ULS) Dead Superimposed dead Traffic Wind Temperature 5.2 Dead and Super-imposed Dead Load All permanent load + primary live loads (vertical traffic loads) Combination 1 + wind, and if erection considered, temporary erection loads. Combination 1 + temperature, and if erection considered, temporary loads All permanent loads + secondary live loads and associated primary live loads All permanent loads + loads due to friction at support. Vc=VK1S1S2 Equation 5.1 Table 5.4: calculation of Vc Clear height of bridge = 67m V = 15m/s K1 = 1.53 S1 = 1.00 S2 = 1.39 Therefore, Vc = 32m/s The Horizontal wind load, Pt in N, acting at the centroid of the part of the bridge under consideration is given by Eq. (5.2): Pt = qA1Cþ Equation 5.2 Table 5.5: calculation of Pt q = 0.613Vc² A1 = solid horizontal projected area = 7.6*1280 = 9278m² Cþ = 1.3 (found from b/d ratio = 3.6) Pt (factored) = 9.6kN/m Moreover, an important action by wind is uplift or a vertical downward force. The nominal force is giving in Eq. (5.3): Pv = qA3CL Equation 5.3 Table 5.6: calculation of Pv q = 0.613Vc² A3 = plan area = 27*1280 = 34560m² CL = 0.4 (found from b/d ratio = 3.6) Pv(factored) = 10.4kN/m Using the results calculated in Chapter 5.2 and 5.3, putting all these dead, superimposed dead and traffic load on the span between two cable hangers (supports) on one side of the bridge, the reaction force calculated on the hangers is 4200kN. The maximum bending moment takes place in the mid-span in this case. Using moment equilibrium equation, the maximum bending moment on the bridge is found to be 18000kNm. Then we calculate the maximum bending moment that the Golden Gate Bridge can resist, which is determined by the second moment of area ( I value ) of the cross-section of the deck, the design strength of the material ( σ), and the distance from neutral axis ( y ), which is giving in Eq (5.5): M= σI / y Equation 5.5 5.5 Temperature Effects Table 5.9: calculation of bending moment There are two temperature effects in bridge: 1. 2. Overall temperature increase or decrease, Variation in temperature between top and bottom surface. To determine the amount of stress induced to the bridge by temperature difference, we can calculate from Eq. (5.4): σ = ∆TαE Equation 5.4 Where ∆T is temperature difference, α is the coefficient of thermal expansion for steel and concrete which is taken as 12*10ֿ6/。C, E is Young Modulus of steel. Table 5.7: calculation of σ ∆T 20 α 12*10ֿ6 E 200,000 σ 48N/mm² The Bending Moment caused by temperature effect is giving in Eq. (5.5). σ 275,000kN/m² I =bd³/12 27*7.6³/12 Y 3.8m M 71MNm To conclude, since this bending moment is larger than the maximum bending moment exerted by loading. The size of the deck used in the Golden Gate Bridge is actually feasible. However, since the majority of deck is consist of steel truss, the assumption of I is much higher than actual one, and also affects the position of neutral axis, and so does Y, so the M calculated is too high. 6 Serviceability and Strength of Golden Gate Bridge The Golden Gate Bridge is flexible and strong. The bridge's designers carefully calculated the graceful dip of the suspension cables between the two towers to carry the needed weight. The cables had to be flexible enough to bend up to 27 feet laterally, in the Gate's formidable winds, and strong enough to support the structure of the bridge. The planned cables would be so long and strong that they would need to be fabricated in place. 6.1 Serviceability Table 5.8: bending moment by temperature difference σ (axial) 48,000kN/m² I =bd³/12 27*7.6³/12 Y 3.8m M 1.2MNm 5.6 Other Load Factor There are also many other ways in which a bridge may be load. For example, such as shrinkage, creep, stress relaxation, earthquake, earth pressure behind abutments, erection loads and so on. However they are not being taken into account in my calculation. 5.7 Feasibility To check the feasibility of the Golden Gate Bridge, load combination 1 and the ULS partial factor are used, because it is reasonable to say that traffic load will bring the worst loading case in the bridge. Wind load is not considered because it exhibits uplift force which counteracts with the dead and superimposed dead load. Temperature effect is not taken into account as well because the assumption of I value made is very inaccurate. Golden Gate Bridge carries 6 traffic lanes with 27m in width. It is also open to pedestrians and bicycle. The Golden Gate Bridge represents a vital transportation link to the San Francisco Bay Area, serving more than 40 million vehicles a year. The highest volume of traffic was recorded with 162,414 vehicles due to the failure of the Oakland Bay Bridge on October 27, 1989, after the Loma Prieta Earthquake jarred the Bay Area. In addition to traffic loading, the Golden Gate Bridge must withstand the following environments: 1. 2. 3. 4. Earthquakes, primary originating on the San Andreas and Hayward faults. Wind of up to 70 miles per hour. Strong ocean current. Temperature stresses. 6.1.1 Earthquake The Bridge has performed well in all earthquakes to date, including the 1989 Loma Prieta Earthquake which was measured with a magnitude with 7.1; again the bridge suffered no damages because of the new retrofit design standards for existing structures. The earthquake-resistant foundation and isolation bearing on the approaches resist the earthquake well. Both the San Francisco and Marin approaches to the Bridge were retrofitted to increase earthquake resistance in 1980. The Golden Gate Bridge should enable itself to survive an earthquake of 8.3 on the Richter scale. More information about earthquake and its effect on the bridge is discussed in Chapter 10. 6.1.2 Wind The Golden Gate Bridge has been closed for 5 times due to poor weather condition (wind). The most notorious of these incidents was in 1951, when 112km/h gusts caused such turbulence that the deck swayed 4.3m in either direction and the deck whipped up and down erratically. However, the bridge remained undamaged. It is contributed to the use of 4,700 tons of steel open truss system which was stiff enough and break up the wind that oscillation would kept minimum. Ref. [10] Wind engineering have undergone windtunnel test for the Golden Gate Bridge, section study was performed to refine and improve the aerodynamic of the cross section. Over 50 configurations were investigated in order to increase the critical flutter wind speed from 96km/h to over 105 km/h. Flutter occurs when the interaction of a bluff section and the wind create a motion. It is very sensitive to the solidity ration of the parapet in the test. The depth of the deck in Golden Gate Bridge is 7.6m thick, which is too thick and result in catching too much wind force. Therefore, a dynamically-shaped deck should be used to replace the original deck in order to alleviate the wind forces rather than try to carry these potential huge forces and thus improve the bridge’s performance under wind load. 6.1.3 Temperature Effect Temperature fluctuations are an important consideration during bridge design, there are two temperature effects; overall temperature increase or decrease, and variation in temperature between top and bottom surface. When the temperature varies, there is a temperature difference between the top and bottom surface on the deck, temperature induces stress into the deck, and so does the strain. Hence, it will cause the bridge lift up. If the piers are stiff, it means the piers will have huge stress resultants to resist. In the Golden Gate Bridge, the piers used is consist of two separate columns, it is a very clever design which can reduce longitudinal temperature stresses; These piers are very stiff in bending, but very flexible to move laterally at tops. This prevents high longitudinal stresses being developed in the deck as well as reducing moments and shears. 6.2 Strength The strength of a Golden Gate Bridge’s suspended structure is derived from the parabolic form of the sagging high-strength cable. This parabolic form is designed so that its shape closely follows the exact form of the moment diagram. This creates a highly efficient structure. The sagging cable performs best under symmetric loading conditions because the cable may deform significantly as it attempts to adjust to an eccentric loading. As the cable adjusts to this load it shifts the rest of the structure. This adjustment causes secondary stresses in the horizontal surface and additional deformation. The parabolic curve of the cable is also susceptible to developing harmonics from eccentric or lateral loads such as wind. These increased harmonics can create significant movement in a structure, sometimes enough to cause dramatic failure, as in the case of the Tacoma Narrows bridge. Rather extensive calculations must be made to determine the natural frequency of a suspension structure and to test the stiffness of its horizontal surface in order to prevent the structure from developing destructive harmonics. It is expected the real strength of the Golden Gate Bridge is higher than the estimated value. It exhibit hidden reserves of strength due to: 1. 2. 3. 4. 5. Average strength of materials; Compressive membrane action; Work-hardening of steel reinforcement; compressive steel presence; Presence of surfacing. Compressive membrane action is far and away the single most important reason why bridges exhibit greater capacity then expected. Since the concrete bridge decks were cast directly onto abutments. This rough bearing creates huge membrane effects within the deck, increasing the theoretical yield-line analysis by 3 or 4 fold typically. Moreover, during yield-line assessment, we assume the steel yields, and the steel yield strength fy = 230MPa, it is conservative. Because having yield, the steel bar can stretch up to about 8%strain, steel workhardening occurs and can reach an ultimate strength of over 300MPa, and thus there is substantial safety margin within the steel strength itself. 7 Creep Effect Although the Golden Gate Bridge is regarded as a steel frame and steel cable structure, large amount of concrete is used on the bridge; including the anchorages, paving, pylons, piers, approaches and so on. Therefore effect of creep in concrete should be considered carefully in terms of the bridge structure and material properties. Creep of concrete can also be another load effect on the Golden Gate Bridge. Creep is the term used to describe the tendency of a material to move or to deform permanently to relieve stresses. It occurs as a result of long term exposure to levels of stress that are below the yield or ultimate strength of the material. The rate of this damage is a function of the material properties and the exposure time, exposure temperature and the applied load. Creep can make concrete no longer perform its function. However, moderate creep is sometimes welcomed because it relieves tensile stresses that may otherwise result in cracking. Eq. (7.1) is a general creep equation. Equation 7.1 where C is a constant dependent on the material and the particular creep mechanism, m and b are exponents dependent on the creep mechanism, Q is the activation energy of the creep mechanism, σ is the applied stress, d is the grain size of the material, k is Boltzmann's constant, and T is the temperature. To minimize this effect, an antiseismic stop device for girder structures of Golden Gate Bridge, it can absorb any little shift of the girder structure occurring as a result of the rotation or the translation of the rest axis caused by creep. This longitudinal sliding of the system is to be such as to allow also the slow deformations under static conditions to occur because of creep without giving rise to remarkable reactions. Ref. [11] It is also proved that substituting small amout of concrete cement with natural pozzolan helps to lower the water content of concrete. Therefore, the creep of concrete can be significantly reduced. 8 Durability and Maintenance Bridges need to be maintained from time they are in service. The Golden Gate Bridge is subject to a very corrosive environment, including fog and salt spray. Therefore, the Golden Gate Bridge is painted every day by a crew of maintenance workers to prevent deterioration of the structural components. The high commitment to the maintenance has saved the bridge from corrosion and rust and prolonged the bridge’s life. To elongate the durability of the bridge, maintenance of structure is always necessary. However, most of the bridges were not designed to be maintained, the Golden Gate Bridge is one of them. The people who designed the original bridge paid less attention to maintenance of structure thus the durability of the bridge is affected. The maintenance of the Golden Gate Bridge has been very difficult since the day it began operation. For example, the bridge was stiffened with lateral steel bracing installed beneath the deck to increase the torsional resistance, traveling scaffold was used which was awkward and difficult to operate. Several men lost their lives due to the faulty scaffolding. We should also understand that the material used in retrofitting are different than those used in the original construction. The Golden Gate Bridge is expected to have a durable life time because it was well constructed, its structure is fantastic and lots of retrofits have be made including its foundation, deck, pylon, anchorage and so on. 9 Intentional Damages In this chapter, I will focus on the terrorist attacks and people committing suicide on the Golden Gate Bridge. 9.1.1 Terrorist Attacks Recent terrorist attacks, such as the on the Alfred P.Murrah Federal Building in 1995 and the World Trade Centre in 2001, are clear examples of the fact that those civil engineering structure are drawn to the attention of terrorists, the destruction of these has become one of the objectives of terrorist attacks. Although no attack has been made on bridges up to now, terrorist treats received by the state of California say that the Golden Gate Bridge is definitely being considered as a potential targets by terrorist organizations. It is important that if the Golden Gate Bridge were to fail as a result of a terrorist attack and impede ship traffic into the San Francisco Bay, the effects could be catastrophic for the regional and national economy. It is always expected that the terrorists might seek the destruction of bridge structures consists of detonating an explosive device. The explosion induces pressures of significant magnitude on structural member. Since these ‘blast loads’, are typically not accounted in design process, it can cause significant damage to the structure which in turn result in collapse of bridge. Therefore the developed structural systems capable of providing adequate level of protection against this type of blast load are necessary. 9.1.2 Development of Blast-resistance System The blast-resistant structural systems should still perform satisfactorily under other loadings acting on the bridge. There are some important similarities between seismic and blast effects; both are rarely events. Due to economic considerations, the energy imposed on structural members by these events is dissipated through inelastic deformations, rather than elastically absorbed. A multi-hazard bridge pier concept Ref. [12] can be used in the Golden Gate Bridge, which is capable of providing protection against both collapse under both seismic and blast loading. A fully composite concretefilled steel tube (CFST) continuous column into the footing was deemed to be the other solution which can comply well with the multi- hazard bridge pier concept. CFST columns exhibit good energy-dissipation capabilities. The foundation beam consists of concreteembedded C-channels linked to the column through steel plates. This connection concept is illustrated in Fig. 9.1. Figure 9.1: Details of column-to-foundation beam connection 9.2 Suicides The Golden Gate Bridge is a notorious site for suicide. The official counted that the number of suicide between 1995 and 2003 approached one thousand, there was an average of one suicide jump every two weeks. Although suicides would not cause any damages to the bridge structurally, it affects the image of Golden Gate Bridge. Methods have been discussed to reduce the number of suicides. One idea introduced has been to close the bridge to pedestrians at night. Cyclists are still permitted across at night, but they have to be buzzed in and out through the remotely controlled security gates. Ref. [13] The construction of suicide deterrent system is promoted on the bridge, although it has been thwarted by engineering difficulties, high costs, and public opposition. 10 Potential weakness and improvement of the Golden Gate Bridge 10.1 Potential weakness In 1989, the epicenter of the Loma Prieta earthquake was too strong to damage the Golden Gate Bridge. The earthquake was a catalyst for the extensive seismic retrofit the San Francisco landmark. Due to the bridge’s outstanding design and the large amount of structural improvements used, the bridge is estimated to have a long life time. After the construction of the bridge was completed, the bridge has undergone installation of wind bracing within the truss, replacement of the vertical support cables, and replacement of the original concrete deck with an orthotropic steel deck. After the earthquake, a restrainer retrofit project was necessary in order to increase its earthquake resistance, as scientific organizations say that there is a 62% probability of at least one magnitude 6.7 or greater quake capable of causing widespread damage, impacting the San Francisco Bay region within the next 30 years. They also estimate it could take less than 60 seconds to destroy if an earthquake's epicenter hits close to the bridge. Even a weaker earthquake could cause unrecoverable damage that would close the bridge To deal with this, the retrofit supercomputers are being used to simulate an earthquake's effect on each part of the bridge, and a comprehensive vulnerability study of the bridge is needed. The north and south approaches were determined to be vulnerable to collapse under a major event because of the high support towers, which result in great ‘rocking’ force. The signature span was also exposed to the possibility of significant damage. The connections from the tower saddle to the main cable could sever, large longitudinal displacements could result in adjacent spans striking the towers, the Fort Point arch could become unseated, and the comparatively underreinforced south pylons flanking the arch span could sustain extreme damage. 1. 2. 3. 4. Strengthening the existing foundations Total replacement of the four supporting steel towers and strengthening of Bent N11, shown in Fig. 10.1. Replacement and addition of top and bottom lateral bracing and strengthening vertical truss members and truss connections The structural system has also been modified to minimize effects of ground motions on the structure by the following: connecting five, simply-supported truss spans into a continuous truss, shown in Fig. 10.2 installing seismic expansion joints at the north and south ends of the viaduct truss, and installing isolator bearings Fig. 10.3 atop the new steel support towers at the Pylon N2 support and at Bent N11. Figure 10.1: The North approach retrofit includes replacing the support towers 10.2 Improvement of the bridge The seismic retrofit measures applied to the Bridge structures consist of various methods of structural upgrades and include both the strengthening of structural components and the modification of structural response of the structures so they can better respond to strong motions without damage. Ref. [14] Three construction phases were established as follows: • • • Figure 10.2: Five independent spans were tied together to create a continuous truss Phase 1 would retrofit the Marin (north) Approach Viaduct Phase 2 would retrofit the San Francisco (south) Approach Viaduct, San Francisco (south) Anchorage Housing, Fort Point Arch, and Pylons S1 and S2 Phase 3 would Main Suspension Bridge and Marin (north) Anchorage Housing 10.2.1 First phase Figure 10.3: installation of isolator bearings The major strengthening measures implemented on the Marin (north) Approach Viaduct included the following: 10.2.2 Second phase It is the most complex part of this project in terms of construction and design. This phase encompasses structural retrofit of many different types of structures of the south approach, including the south approach viaduct, anchorage housing, Fort Point arch, and south pylons. Retrofit measures developed for each of these structures reflect their different behavior under seismic ground motions and their interaction at points of interface. The steel support towers and bottom lateral bracing of the south approach viaduct will be entirely replaced, seismic isolation bearings and joints will be installed at the roadway level. A massive internal shear walls are constructed of the south anchorage housing. External and internal steel plating will be added to south pylons walls. Addition of a new external concrete is cover on the external surfaces of the pylons. To reduce longitudinal and transverse forces between the Fort Point arch and the pylons, a battery of energydissipating devices (EDDs) will be installed. Prototype testing is conducted on scale-model EDDs to work out the effectiveness and reliability of dissipating energy by generating friction between plates of dissimilar metals under simulated seismic loading. The EDDs will help controlling a limited movement of the arch, which will reduce the member stresses that are induced within the truss itself. 10.2.3 Phase 3 The third phase of the Golden Gate Bridge Seismic Retrofit Construction Project has been separated into two sub phases as follows: 1. Phase 3A: Retrofit of the North Anchorage Housing and Pylon N1, see Fig. 10.4 2. Phase 3B: Retrofit of the Main Suspension Span, Main Towers, South Tower Pier and Fender Phase 3 involves retrofitting the suspension portion of the bridge, which comprises a 1,280 m main span and two 343 m side spans. The signature span towers, which rise 227 m above mean sea level, are made up of multicellular built-up members constructed of riveted steel plates and angles and have a combined weight of approximately 40,300 Mg. Phase 3 retrofit measures include replacing some of the top lateral bracing and connection strengthening within the stiffening truss, installing viscous dampers to "cushion" the towers from potential adjacent span impact, and adding stiffeners and strengthening connections within the towers. Horizontal steel tendon prestressing at the bases of the piers, expansion joint replacements, concrete fender repairs, and the strengthening and immobilization of the connections between the tower saddle and the main cable are included in additional measures. The viscous dampers act similarly as the EDDs, which absorb seismic energy and therefore reduce stress within the tower and truss members. They also prevent the pylons from experiencing additional forces. The viability of the viscous damper design was verified by assembling and physically testing scale models. Computer modeling verified that, in every extreme event, controlled rocking of the suspension span towers on their bases after the retrofit will conform to the design criteria. Such limited rocking will not cause the tower legs to buckle, because of the strengthening of outer cells within the towers at both the pier and roadway levels only. Figure 10.4: It includes the strengthening of concrete foundation Reference [1]. General information of Golden Gate Bridge. http://en.wikipedia.org/wiki/Golden_Gate_Bridge [2]. Construction history of Golden Gate Bridge, http://www.pbs.org/wgbh/buildingbig/wonder/struct ure/golden_gate.html [3]. Parallel wife construction of cable, http://www.madehow.com/Volume-5/SuspensionBridge.html [4]. Construction of suspended bridge, http://www.madehow.com/Volume-5/SuspensionBridge.html [5]. Construction timeline, http://goldengatebridge.org/research/ConstructionTi meline.php [6]. The use and idea of safety net, http://www.californiahistorian.com/articles/goldengate-bridge.html [7]. Design history, http://www.californiahistorian.com/articles/goldengate-bridge.html [8]. Structure detail of Golden Gate Bridge, http://goldengatebridge.org/research/factsGGBDesi gn.php [9]. Anesthetic of Golden Gate Bridge, http://en.wikipedia.org/wiki/Golden_Gate_Bridge [10]. Wind tunnel test and aerodynamic design, http://www.tfhrc.gov/pubrds/winter96/p96w46.htm [11]. The use Pozzolan cement to reduce creep effect, http://www.chamorro.com/community/pagan/Azma r_Natural_Pozzolan.pdf [12]. Blast resistant bridge piers, http://www.structuremag.org/archives/2007/March %202007/C-StructuralDesign-BlastResistantMar07.pdf [13]. Suicide barrier, http://www.sfgate.com/cgibin/article.cgi?f=/c/a/2005/11/04/MNG9UFI71E1.D TL [14]. Comprehensive seismic retrofit of Golden Gate Bridge, http://www.pubs.asce.org/ceonline/1100feat.html
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