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