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BY ABHIJEET B. BHOSALE

Info & history of Golden Gate Bridge

A suspension bridge spanning across Golden Gate, opening into the san Francisco

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

Built by Joseph Strauss Second longest suspension bridge in US

Principal of long suspension bridgethe deck is always suspended by hangers, each

attached to the main cable, which in turn is anchored into the ground at its ends.

Suspension bridge usually has a slender deck which carries bending only (no compression).

In order to suspend something of uniform weight ‘for An equation was developed to prove the cable deformed into a parabola and the horizontal component force in the cable was a constant value.

H=wl*l/8f

ConstructionChallenges1. The bridge in this location should withstand brutal

winds, tide, and fog. 2. It is also located less than 13km from the epicenter of

the most catastrophic earthquake in history.Components1. The anchorages - the massive blocks that grip the

bridge's supporting cables. 2. The north pier, which supports the tower, was built

easily on a bedrock ledge 6m below the water. 3. But on the southern San Francisco side, Strauss had to

build his pier in the open ocean, 30m below the surface.

4. Steel frame and steel cables

A parallel wire construction tech. is used4. weight-blocks which ballast the cable

anchorage. Provided for stabilization

5. Pier: 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.

6. Suspended cantilever construction

Design Issue & Structure detailsLeon S. Moisseiff was named one of the consulting

engineers proposed a bridge far more efficient and beautiful

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 bridge’s cable design

Structure details: 1. San Francisco (south) Approach Viaduct 2. San Francisco (south) Anchorage Housing and Pylons S1 and S2 3. Fort Point Arch 4. Main Suspension Bridge 5. Marin (north) Approach Viaduct 6. Marin (north) Anchorage Housing and Pylons N1 and

N2Total length of Bridge including approaches: = 2,737 mLength of suspension span including main span and side

spans: = 1,966 mLength of main span portion of suspended structure

(distance between towers): = 1,280 mLength of one side span: = 343 Width of Bridge: = 27 mClearance above mean higher high water: = 67 m

Asthetics Fulfillment of function The bridge fulfilled a high

degree of simplicity, which make the bridge beautiful.

Proportions:conveys a decent impression of balance between its mass and voids, and between light and shadow

Order:The appearance of the bridge looks like a mirror image of two similar towers with cables.

Integrating into the EnvironmentTexture:It has rough finishing for piers and

abutments, which makes sense for bridge design.Colour :This colour brings a big contrast with the

sky and seaComplexity :

Loading The most important types of loadings we need to consider on the bridge are:

1. Dead load,2. Super-imposed dead load,3. Live Traffic,4. Wind,5. Temperature

There are five combinations of load:1. All permanent load + primary live loads (vertical traffic loads)2. Combination 1 + wind, and if erection considered, temporary

erection loads.3. Combination 1 + temperature, and if erection considered,

temporary loads4. All permanent loads + secondary live loads and associated

primary live loads5. All permanent loads + loads due to friction at support.

Wind loads max. wind gust: Vc=VK1S1S2

Clear height of bridge = 67m V = 15m/s K1 = 1.53 S1 = 1.00 S2 = 1.39 Therefore, Vc = 32m/s

Horizontal wind load:Pt = qA1Cþ q = 0.613Vc², A1 = solid horizontal projected area , Cþ

= 1.3 (found from b/d ratio = 3.6) Pt (factored) = 9.6kN/mVertical wind load:Pv = qA3CL

q = 0.613Vc² A3 = plan area = 27*1280 = 34560m²CL = 0.4 (found from b/d ratio = 3.6) Pv(factored) = 10.4kN/m

Temperature effect

Other load factors: shrinkage, creep, stress relaxation, earthquake, earth pressure behind abutments, erection loads and so on.

Bending moments

∆T α E σ

20 12*10-6 200000 48 N/sq.mm

σ I =bd³/12 Y M

48000kN/sq.m

27*7.6³/12 3.8m 1.2MNm

Serviceability

In addition to traffic loading, the Golden Gate Bridge must withstand the following environments: 1. Earthquakes, primary originating on the San

Andreas and Hayward faults.2. Wind of up to 70 miles per hour.3. Strong ocean current.4. Temperature stresses.

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.

To minimize this effect, an antiseismic stop device for girder structures of Golden Gate Bridge, is used.

Potential weakness and improvement of the Golden Gate BridgeIn 1989, the epicenter of the Loma Prieta

earthquake was too strong to damage the Golden Gate Bridge.

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 within the next 30 years.

Improvement of bridge

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