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Types of Bridges

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  TYPES OF BRIDGES  Arch bridge  -  An arch bridge  is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side.   Through arch bridge  -  A through arch bridge , also known as a half-through arch bridge  and through-type arch bridge , is a bridge made from materials such as steel or reinforced concrete in which the base of an arch   structure is below the deck, but the top rises above it, so the deck passes through the arch. Cable-stayed bridge  -  A cable-stayed bridge  has one or more towers  (or  pylons ), from which cables support the   bridge deck. There are two major classes of cable-stayed bridges: harp  and fan . The harp  or  parallel   design, the cables are nearly parallel so that the height of their attachment to the tower is proportional to the distance from the tower to their mounting on the deck. In the fan  design, the cables all connect to or pass over the top of the towers. The fan design is structurally superior with minimum moment applied to the towers but for practical reasons the modified fan is preferred especially where many cables are necessary.    Cantilever bridge  -  A cantilever bridge  is a bridge built using cantilevers, structures that project horizontally into   space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large   cantilever bridges designed to handle road or rail traffic usetrusses built from structural steel, or  box girders built   from prestressed concrete.     Movable bridge  -  A moveable bridge , or movable bridge  (common alternative spelling in  American English), is   a bridge that moves to allow passage (usually) for boats or barges. [1]  In American English, moveable bridge   and drawbridge  are synonymous, and the latter is the common term, but drawbridge can be limited to the   narrower, historical definition used in some other forms of English, in which drawbridge  refers only to a specific type of moveable bridge.   Suspension bridge  -  A suspension bridge  is a type of  bridge in which the deck (the load-bearing portion) is   hung below suspension cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 19th century. [3][4]  Bridges without vertical suspenders have a long history in many mountainous parts of   the world.    Truss bridge  -  A truss bridge  is a bridge whose load-bearing superstructure is composed of a truss, a structure   of connected elements forming triangular units. The connected elements (typically straight) may be stressed from tension, compression, or sometimes both in response to dynamic loads.  HOW TO MAKE A BRIDGE   GETTING STARTED When you’re ready to build a bridge, you’ll usually need to get permission from at least one governmental authority. For example, you may need permission for changing an existing grade, moving earth for a foundation, locating a bridge within a flood plain, and closing any stream or body of water. The agencies that grant permits include Water Control Districts, Departments of Natural Resources, Departments of Environmental Regulation, and other similar regulatory agencies. DETERMINING YOUR BRIDGE LOCATION You will need to consider many factors as you decide where to build a bridge. First, you’ll need to consider the high water elevation at your proposed bridge site. Usually you will want to locate the bridge so that there is a clearance or free board between the bottom of the bridge and the high water elevation or 100-year flood level. These criteria may determine the length of the bridge in some locations. The grades of the approaches and banks of the stream or pond also can affect bridge length. You’ll want to have a relatively flat approach to the bridge if this can be accomplished without major grading. BRIDGE DESIGN CRITERIA The most common design codes and specifications used for bridge design are:    “ Standard Specifications for Highway Bridges ,” adopted by the American Association of State Highway and Transportation Officials (AASHTO). This is a set of design criteria developed for highway bridges. It is intended to govern the design of bridges subject to many repetitions of vehicle loads and to heavy vehicle loads. This code requires a uniform live load of 85 psf.   Following is some of the information you will need to accumulate as you consider building a bridge. 1.   WHAT IS YOUR CONSTRUCTION TIMEFRAME?   o   Are your bridges part of a larger construction project? o   When do you hope to begin construction? o   Have you begun the budgeting process? 2.   WHAT IS GOING TO BE THE USAGE OF YOUR BRIDGE?   o   What is the weight capacity requirement? o   How long will it be? o   Will you need to meet AASHTO highway requirements? o   How many lanes of traffic will the bridge need to accommodate? o   Will it support utilities? 3.   SITE CONSIDERATIONS.  When you are deciding on the exact location and layout for your bridge, some specifics that you will need to take into account are: o   The terrain o   The environment o   Site access (to both sides if you are choosing a free-span structure) o   Is there a better location? (shorter distance, better ground conditions, etc)  4.   POTENTIAL PERMITTING AND ENVIRONMENTAL ISSUES.  These issues are quite important and can greatly impact your project plan. You may need to find out the following before building a bridge: o   Are there environmental restrictions? o   Are there certain materials that will be prohibited in the crossing area? o   How wide is the wetland area? Is there a buffer zone extending past this? o   Are piling permitted throughout the length of the crossing? If not, at what points are they prohibited? o   Are there limitations on the construction methods? Contact your local permitting authority for assistance in answering these important questions. 5.   WHAT IS THE TERRAIN YOUR BRIDGE WILL SPAN?   o   Will your bridge cross water, wetlands, ravine, etc.? o   Is water present? o   How deep will the water be during construction? o   How deep can the water get (i.e. 100 year flood line)? o   Is the salinity of the water salt, fresh, brackish? The water salinity can impact the treatment required on the piling and the hardware used on your project. o   Is the flow of the water fast, stagnant or slow? o   Are any soil reports available?

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Jul 23, 2017

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Jul 23, 2017
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