golden gate outdoor exhibition - curee

Golden Gate Outdoor Exhibition

Descriptions and Summary Illustrations of exhibits

September 18, 2010Bob Reitherman, CUREE

* See Plan of Exhibits site plan document of same date. BBold face numbers indicate numerals keyed to numerals on thelarge centerpiece model.

ExhibitNumber* Exhibit Description Illustration

1

Full-size Footprint of One Tower LegOutline of full-size footprint of one leg of one towerembedded in the paving; provides a sense of scalefor the visitor (the nearer South Tower being 0.4miles and the North Tower 1.2 miles away).Companion (#2) panel explains the outline. A“discovery” or surprise for the visitor and a photoopportunity.

1-b

Bridge TowersExplains how the entire weight of the bridge deck,with vehicles, pedestrians, bikes, etc., is carried up tothe main cables and up over the towers. Where thecables go over the towers they hand off that load.The towers must carry all the weight down to theirfoundations. The size of the “footprint” at thefoundation is related to the tremendous weight thetowers support.

2

Seismic Retrofit of Lattice TrussThe original bracing members (struts) in the archstructure and adjacent approach spans were made ofhundreds of small pieces of steel riveted together. Aseismic retrofit has replaced many of these latticetruss struts with modern one-piece steel tubes – butwith triangles carefully laser-cut out so thereplacement struts look like the originals.


3

South Tower FoghornWhen it’s foggy, visitors may not be able to see theBridge, but in that case they get to hear the foghorns.An explanation of what they’re hearing: SouthTower (single booming blast) and Midspan (doublehigher-pitched horn), each with its own rhythm/rate.Fog also tells the story of the Golden Gate’sinfluence on weather in the Bay Area.

4

Midspan FoghornSame exhibit as #4, but there are two separate diskswith numerals at this tabletop exhibit to direct thevisitor’s gaze to the two different locations on thelarge centerpiece model, orienting them as to what“South Tower” and “Midspan” mean.

[see #3]

5

Suspender Rope Strength “Chairlift”The weight of the deck and what it carries issupported by the 500 vertical lines that extendacross the Bridge, the steel suspender ropes. Theexhibit translates the weight of the visitor(s) into amultiple of the capacity of the piece of suspenderrope that holds them up (a factor in the range of3,000 to 4,000).

6

Deck Torsional ResistanceCompleted in 1937, the design of the Golden GateBridge did not benefit from knowledge about thecollapse of the Tacoma Narrows Bridge in 1940,which twisted itself to collapse in a moderate breeze.After a windstorm on December 1, 1951, the GoldenGate Bridge was retrofitted in 1954 with horizontaldiagonal bracing between the bottom edges of thevertical deck trusses. This makes the deck muchharder to twist, which is felt by the visitor in thiscomparison of pre- and post-retrofit bridge decks.


7

Tower Height/Cable TensionThe twin cables would slope more steeply if thetowers were taller. When the slope is steep, more ofthe tension (pulling) force in the cables is efficientlydirected upward, rather than horizontally. But tallertowers (the Bridge’s were the tallest in the worldwhen built) mean more structural and constructionproblems and more cost – a classic engineeringtradeoff.

8

Fort Point ArchThe fort beneath the Bridge was obsolete within afew years after its 1861 completion, due to morepowerful cannon-fire that could penetrate brick walls5 feet thick. But the Golden Gate Bridge engineerswent to the trouble of spanning over it, rather thandemolishing it, using a long-span arch, historicpreservation ahead of its time for the 1930s. Asuspension bridge works like an arch turned upsidedown, and vice-versa.

9

History MuralThe history of the Golden Gate Bridge has manythemes: financial and political challenges to developit, engineering and construction challenges to build it,the Depression-era workers who worked on it, itseffect on the regional economy, its use in numerousmovies and television shows.


10

Highlights, Facts & FiguresIllustrated extracts from the popular (6th ed.) book ofthe same name by Mary Currie, GGBHTD PublicAffairs Director. The Bridge District hasaccumulated a great deal of experience answeringvisitor questions, and this exhibit deals with some ofthe most frequently recurring ones.

11

Seacoast Fortifications MapFrom the exhibition site, it is a short walk, bike ride,or drive to numerous seacoast fortifications in theGolden Gate National Recreation Area. This map-based display points the way to these attractions,with a brief overview of the eras of defensive worksthat span from Castillo de San Joaquin in 1794 tothe age of missiles guarding against nuclear attack.

12

Concrete Fortification Wall Cross-sectionThe cut through the Battery Lancaster wall providesthe view and circulation to the north, and also revealsa cross-section of a 15-foot-thick defensive wall,behind which a cannon was placed in 1898. By then,cannon fire from ships had become more powerful,making such defensive works necessary.

13

Aesthetics of the BridgeWithin the limits of engineering necessity, bridgedesigners have aesthetic latitude in making decisionssuch as: architectural detailing (which is Art Deco onthe Golden Gate Bridge), proportion of main spanto side spans, height of tower to span ratio, shape oftower, form of strut across top of tower.


14

How a Suspension Bridge WorksEngineers call it a load path – the path taken by theloads (e.g. the weight of the structure and what itcarries, the wind forces on it) as they flow throughthe structure to reach the foundation. A suspensionbridge load path is basically simple, but is notapparent to non-engineers. The vertical suspenderropes pull the weight of the deck up to the maincables. The main cables carry it to the tops of thetowers. The weight goes down the two towers to thefoundation. The ends of the cables must beanchored to resist the tension (pulling) force thatacts horizontally, inwardly, and vertically, upwardly.

15

What’s a Rivet?Millions of rivets, rather than bolts, were primarilyused to connect the thousands of pieces of steeltogether to make the Golden Gate Bridge. Thisexhibit explains how a rivet initially looks like a boltwith a head on one end. It is heated red-hot, insertedthrough the holes the pieces of steel to beconnected, then pounded to form a mushroom-headon the shaft, permanently clamping the piecestogether. As the steel cooled, it shrank and theconnection tightened up.

16

Be an Ironworker Photo Op.This exhibit is simply for fun. It serves the needs ofvisitors to take snap shots commemorating their visit.Visitors stands behind a realistic looking backdropand poke their heads through an opening, makingthem look like an iron worker up high on the Bridge.


17

Maneuvering RingsVisitors currently do not know what the large steel“doorknockers” in Batter Lancaster are. They wereused to maneuver the large cannon in that area, whenropes or chains were attached to them and soldierscould tug the cannon into position.

18

Centerpiece Large Model of BridgeThe 90-foot-long 1:80 scale detailed model, made ofstainless steel, has four functions: (1) a table ofcontents for satellite exhibits (numbers on the largemodel are keyed to nearby exhibits); (2) a 3Dstructural diagram of the Golden Gate Bridge,differentiating structural from nonstructuralcomponents; (3) a big “draw” to beckon visitors tothe outdoor exhibition area; (4) a beautiful sculpturein its own right.

19

Geology of Bridge FoundationsTectonic forces acting on the area around theGolden Gate created a complex variety of rocks.Serpentine forms the bedrock under the anchorageand South Tower. The bedrock at the north side ofthe Gate is chert, made up of the fossilized skeletalremains of billions of radiolarian protozoa. Thedifference in bedrock required foundation designs tobe customized each foundation location’s properties,including load testing prior to construction.


20

Braille Model of BridgeA bronze tactile-readable tabletop model of theBridge with Braille captions. Not described here areways to make the exhibits accessible to people ofvarying capabilities, including language (see #29).

21

The Ways the Bridge VibratesThe Bridge has many modes of vibration, or ways inwhich it can move in response to wind orearthquakes. This mechanical hands-on exhibit letsvisitors “excite” (shake) the model in varying ways tosee how it responds. The main span between thetowers can sway sideways, back and forth like aswing, or it can take on a snake-like pattern of verticalor horizontal displacement depending on the rate atwhich it is moved. In the try-out of the prototype atthe Bridge, it also served the useful purpose ofproviding visitors with photo opportunities andactivities when the Gate was socked in with thick fog.

22

Tracking Daily Movements of the BridgeIn a normal day, the Bridge deck rises up a few feetat night when it gets colder and the steel in thecables shrinks and there is less traffic weight on it. Itgoes down in the daytime with rise in temperatureand traffic load. It sways to the east when the windcomes off the ocean from the west. This exhibitdisplays actual measured movements of the bridge,which can be “played” forward and backward.


23

Maximum Bridge Deflection, LeftA small structure can’t move much or it would break.(If a house swayed one inch / 2.5 cm to the side in anearthquake, there would be damage). A very largestructure can move (displace) a large amountbecause the amount of bending per unit length issmall, yet it still adds up to large overall amount. Thisexhibit simply posts (literally with posts) the sidewaysmovement at midspan the Bridge is designed toaccommodate: 27.7-inch (8.4 m) to left (west) andthe same amount to the right (east).

24

Maximum Bridge Deflection, CenterSame as #23 except this has a different number toidentify the different location of this exhibit on thesite plan.

[see #23]

25

Maximum Bridge Deflection, RightSame as #23 except this has a different number toidentify the different location of this exhibit on thesite plan.

[see #23]


26

LIFETILES Animation of ConstructionUsing the artistic technology developed by RufusButler Seder, this glass tile mural animates as thevisitor walks past it, displaying first the Golden Gatebefore the Bridge was built, then the stages ofconstruction (foundations and towers, main cables,vertical suspender cables, deck).

27

Current Work on the BridgeThis “exhibit du jour” is a placeholder for changingexhibits related to current work on the Bridge – andthere is always work underway on the Bridge. Withor without “docents” such as an ironworker, exhibitswould explain to the public what is being done at thetime they are visiting the Bridge.

28

Color of Bridge, Painting, CorrosionThe International Orange color of the Golden GateBridge is very much part of its landmark signature.The choice of that color involved interestingaesthetic studies done when the Bridge was underconstruction. There were alternative paint schemesproposed, such as the yellow-and-black stripe designintended to make it more visible for aircraft. The jobof removing rust and painting the Bridge is a constantone, and involves steeplejack or mountain climbingability on the part of the painters to access hard toreach areas high in the structure. High-performanceprimers and paint are also required to be durable inthe wet and salty environment.


29

Welcome, Orientation (south entrance)A display at both the south and north entrances tothe Battery Lancaster gallery provide multi-lingualorientation for the visitor. Cell phone interpretationof the exhibition and individual exhibits is provided ina number of languages. Smart phone access to theGGBHTD website provides another means ofaccessing information. Braille is included on thedisplay.

30

Welcome, Orientation (north entrance)

[see #29]

31

Pointers to Bay Area SightsNon-obtrusive sighting pointers to the localgeographic locales of interest, such as AlcatrazIsland, Angel Island, Raccoon Strait (which marksthe ancient river course). Miniature tabletop displaymounted on guardrail.


32

Feel Wind PressureThe Golden Gate is a very breezy place, the onlygap in the coast range for hundreds of miles. Cool,dense, high pressure air over the cold ocean pushesits way into the Bay and California’s interior, whilethe land heats up and the air over it becomeswarmer, less dense, and lower in pressure. Windforce varies as the square of the velocity, a fact thevisitor can intuitively sense by pushing on this exhibit.They feel how much harder it is to push with a forceequivalent to a wind of various speeds. A visitor feelshow hard it is to push back against a wind speed ofabout 60 mph / 100 kph, for example.

33

Deck AerodynamicsWind blowing across (transversely to) the deck of asuspension bridge can make it begin to twist – a veryundesirable type of structural response. This exhibitmounts two similar scale-model bridge decks next toeach other. One is aerodynamically superior to theother in subtle ways that make it stable in the breeze.The other noticeably twists. The Golden GateBridge has been extensively studied analytically andin wind tunnel testing to evaluate its windperformance.

34

Weather StationEverybody talks about the weather – especially atthe Golden Gate, where the weather is often quitedifferent than the visitor experienced only a fewmiles away. This exhibit displays live weather data atthe Bridge and at an inland location. Time and dateare displayed, because it enhances the value of asnapshot as a memento of visiting the Bridge on aspecific day.


35

Original and Retrofit GuardrailThe wind blowing across the deck of the Bridge islike the air flowing over the wing of an airplane. Slightchanges in geometry can have a surprisingly largeeffect. The original guard rail on the Bridge presentsa rather solid barrier to the wind, because of thewidth and close spacing of the vertical stiles.Retrofitting the guard rail to have slimmer verticallines would improve the aerodynamics of the Bridge.

36

Ferry Vessel Weight Compared toSuspender Rope StrengthThe Golden Gate Bridge, Highway andTransportation District operates a fleet of buses andferry vessels as well as the Bridge. For ferrypassengers, this exhibit visually communicates thestrength of the Bridge. Each of the 500 verticallines across the Bridge – the suspender ropes – isstrong enough to hold up an entire fully-loaded ferry.

golden gate outdoor exhibition - curee

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