Read the article provided below about \"George Washington Bridge\" and write a s

Question

Read the article provided below about "George Washington Bridge" and write a summary in case study format ( INTRO, BODY, CONCLUSION ).

Structural Study:

History and Influences

A great engineering debate raged along the Hudson River in the early twentieth century.

Increased population densities and booming interstate commerce created new demands for a high

capacity transportation network between New York and New Jersey. Both bridges and tunnels

were proposed as potential solutions for new Hudson River crossings, although the decisions

leading to their physical location, construction method, and total cost were formidable obstacles.

Othmar Ammann, a Swiss-trained engineer with American bridge building experience,

had a solution. His proposal for a Hudson River crossing was a suspension bridge, the longest in

the world at the time, which would carry vehicular traffic from the cliffs of the Palisades to 179th

Street in New York City. Ammann separated himself from his competitors by eliminating heavy

rail lines from the design criteria and by using a new mathematical theory to justify his recordbreaking

span. The George Washington Bridge was completed in 1931 and carried 5,000,000

vehicles across the Hudson River in its first year of operation. In 2011, the total traffic crossing

the bridge was over 100,000,000 vehicles.

Figure 1: Elevation, plan, and section of George Washington Bridge

Structural Description

The George Washington Bridge crosses the Hudson River with two tall steel towers and a

bridge deck suspended from 4 steel cables (figure 1). Each tower is 576 feet tall and consists of

steel members connected in a truss-like framework. The original design intention was to encase

the steel towers in reinforced concrete and then apply a masonry facade. The towers were

instead left in their bare steel form after funds for the bridge ran low with the Great Depression

of 1929.

Four main suspension cables span 3500 feet between towers. The flexible steel cables

are supported at the top of each tower and anchor in heavy concrete blocks at both ends of the

bridge. Each steel cable is 32 inches in diameter and is made up of 26474 parallel steel wires

(figure 2).

Figure 2: GWB main cable prior to compression to circular cross section.

(http://www.panynj.gov/tbt/gwbhistorygallery.htm)

The bridge deck is 119 feet wide and is connected to the main cables by vertical steel

suspenders. The original design carried six lanes of traffic supported by shallow floor beams.

The vehicular capacity was doubled in 1962 with the completion of a truss-stiffened lower deck.

Design Loads

The dominant loads acting on the bridge are its own self weight, live load from vehicular

traffic, and wind. The bridge deck and the steel cables are the main contributors to self weight

considered in this study. We will approximate the self weight of the bridge with a uniformly

distributed load of 39 kips/ft (figure 3).

Figure 3: Design loads for GWB .

Vehicular traffic, and especially truck traffic, produces the highest live load forces on this

bridge in service. It is improbable that all lanes of the bridge will be loaded from end to end with

heavy trucks at the same time. Therefore, our chosen design live load reflects the expected truck

traffic density, not the maximum number of trucks possible on the span. For this study, we will

assume that trucks produce a uniformly distributed live load of 8 kips/ft (figure 3). Note that

these are the total applied loads and are divided equally between the four main cables.

Since the effects of wind on the bridge are well controlled by Ammann’s design, we will

not consider wind as a design load in this study. Oscillatory deflections due to wind are

prevented by the use of a wide, heavy bridge deck.

Analysis

The main goal of this study will be to understand how the main suspension cables carry

gravity loads. We will begin our analysis by following the flow of gravity forces from the bridge

deck to the suspension cables and finally to the tower foundations. Self weight and live load are

applied at the bridge deck level as vertical loads (figure 3). These forces are then picked up by

the steel suspenders and deposited onto the main suspension cables. The main cables are flexible

and modify their geometry to convert the vertical forces into axial tension along their length

(figure 4). The main cable’s axial tension is converted back to a vertical force at the steel towers,

where it is transferred to the ground at the tower foundations (figure 5).

Now that we have an idea of how the forces flow through the bridge, we can perform a

simple calculation to estimate the axial force in a suspension cable at the middle of the span. We

will find that the force in the cable at midpan depends on the length of the span and the ratio of

the cable sag to the span length. The cable is horizontal at the midspan because of symmetry, and

the value of this force can be determined using a free body diagram of half the bridge, and the

concept of moment equilibrium. (figure 5).

Figure 4: Cable and Force Geometry.

Figure 5: Cable force calculation at midspan.

The cable force at midspan, therefore, is given by the simple equation H = wL2/8d. This can also

be expressed as

.

In examining this equation, we can interpret w as representing the external loads acting on the

bridge, L as representing the overall size of the bridge, and R as representing the shape of the

bridge. Thus, the shape or form of the bridge, given by R, transforms an external force w into a

force in the bridge, H. This is the mathematical expression of the concept that function follows

form.

If the total loads as in Figure 3 are used in the above equation for H the force in each cable is

Individual cable force = Hcable = H

4

. The cable stress is cable = Hcable

cable cross section area

= Hcable

Acable

and the safety factor (FS) is FS = allowable stress

cable

= allowable

cable

Moment equilibrium at A

A

Explanation / Answer

Hi,

PLZ GIVE THUMBS UP.

INTRODUCTION-

George Washington bridge oftenly called as GW bridge was built along the Hudson River in the early twentieth century. Increase in the population densities and booming interstate commerce created new demands for a high capacity transportation network between New York and New Jersey which lead to the development of GW bridge. Othmar Ammann, a Swiss-trained engineer with American bridge building experience proposed for a suspension bridge over the Hudson River crossing which would carry vehicular traffic from the cliffs of the Palisades to 179th Street in New York City. The George Washington Bridge was completed in 1931 and carried 5,000,000 vehicles across the Hudson River in its first year of operation. In 2011, the total traffic crossing the bridge was over 100,000,000 vehicles.

BODY-

Structural Descri[tion-

The GW Bridge crosses the Hudson River with two tall steel towers and a bridge deck suspended from 4 steel cables.Each steel cable is 32 inches in diameter and is made up of 26474 parallel steel wires. Each tower is 576 feet tall and consisted of steel members connected in a truss-like framework. There are four main suspension cables spanning 3500 feet between towers.The flexible steel cables are supported at the top of each tower and anchor in heavy concrete blocks at both ends of the bridge.

Design Loads-

Vehicular traffic, and especially truck traffic, produced the highest live load forces on GW bridge. It was assumed that at one instant of time it is improbable that all lanes of the bridge will be loaded from end to end with heavy trucks at the same time. Therefore, the design live load reflected the expected truck traffic density & not the maximum number of trucks possible on the span.The total applied loads were divided equally between the four main cables.Oscillatory deflections due to wind were prevented by the use of a wide, heavy bridge deck.

Analysis-

The main goal of this study was to understand the way in which main suspension cables carried gravity loads.Analysis was done by considering the flow of gravity forces from the bridge deck to the suspension cables and finally to the tower foundations. Self weight and live load were applied at the bridge deck level as vertical loads. These forces were then picked up by the steel suspenders and finally deposited onto the main suspension cables. The main cables were flexible and modified their geometry to convert the vertical forces into axial tension along their length . Then finally main cable’s axial tension was converted back to a vertical force at the steel towers from where it was transferred to the ground at the tower foundations.

Conclusion-

It was a very high level thought project involving huge amount of fundings and new type of technology to be applied. Inspite of various limitations and setbacks the project was successfully delivered within stipulated time period.A lot of design load specifications were to be foolowed and also care was taken to apply appropriate risk factors in the design.

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