CHAPTER 2

TYPES AND COMPONENTS OF BRIDGES

2.1 INTRODUCTION

In terms of engineering, bridges are discussed by design or type (beam, arch, truss, cantilever, suspension, or moveable); length (usually expressed in terms of clear or overall span); and materials (stone, wood, cast and wrought iron, and what we use today - concrete and steel). Bridges may be classified by how the forces of tension, compression, bending, torsion and shear are distributed through their structure. Most bridges will employ all of the principal forces to some degree, but only a few will predominate. The separation of forces may be quite clear. In a suspension or cable-stayed span, the elements in tension are distinct in shape and placement. In other cases the forces may be distributed among a large number of members, as in a truss, or not clearly discernible to a casual observer as in a box beam.

CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.1.1 THE DIFFERENCES &SIMILARITIES IN BRIDGE STRUCTURE [11]

A bridge taxonomy showing evolutionary relationships

2.2 TYPES OF BRIDGES [1]

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

Bridges are structural solutions to the obstacles which nature puts in the way of physical communication. They cross-waterways, rivers & valleys and connect cities, countries and continents. Many have become international icon, give sense of place to nations and symbolize human aesthic and technological achievements. The history of bridge construction can be said to be the history of evolution of civil engineering. The bridge can be defined as the “Art of spanning space by artificial construction.”There are many ways of describing a bridge type. The bridges may be classified from simple beam to modern long span suspension and cable stayed or bi-stayed bridges. Bridges are mainly classified in terms of the bridge's superstructure, and superstructures can be classified according to the following characteristics:

1. Material of construction2. Span lengths3. Structural form4. Span types5. Load path characteristics6. Usage7. Position (for moveable bridges)8. Deck type (combination & double deck bridges)

2.2.1 CLASSIFICATION BY MATERIALS OF CONSTRUCTION

Bridges can be identified by the materials from which their superstructures are built such as1. Steel bridges2. Concrete bridges3. Timber bridges4. Aluminum bridges5. Polyester Fibre and fiberglass bridges

2.2.1.1 STEEL BRIDGES

A steel bridge is made entirely of steel; a steel bridge has a superstructure that consists, typically, of a reinforced concrete deck supported on steel stringers, one of the most common combinations of materials used for modern highway bridges. (The other common form for highway bridges is a reinforced concrete deck with pre-stressed concrete beams.) Truss highway and railroad bridges are built mostly from steel, although a few old timber bridges still exist. Other medium-and long-span bridges such as cable and suspension bridges, may be truly all-steel structures. Arch bridges are built from both steel and concrete. In a few cases, in which the old concrete deck is to be rehabilitated, an open- or filled-grid deck made from steel is used to replace the deteriorated concrete deck. In some cases, the deck may consist of a steel plate deck instead of a conventional concrete deck.

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

Figure 2.2 Steel Bridge

2.2.1.2 CONCRETE BRIDGES

Concrete bridges include bridges built from both reinforced concrete and pre-stressed concrete. A reinforced concrete bridge generally has all its super-structural elements, such as deck, stringers, and parapets, built from reinforced concrete. A typical short-span pre-stressed concrete bridge has a reinforced concrete deck supported by pre-stressed concrete beams, such as I-shaped or box beams. For shorter spans, contiguously placed pre-stressed concrete planks (solid or hollow slabs) are used with a wearing surface of reinforced concrete.. For medium-span bridges, pre-stressed concrete box girders are used. Arch bridges are also built from reinforced concrete and, in recent years, cable-stayed bridges have also been built from pre-stressed concrete.

Figure 2.3 Concrete Bridge

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.2.1.3 TIMBER BRIDGES

The first bridges to be built in the United States, before 1840, were timber bridges, mostly covered truss bridges. Today timber bridges are seldom built except in parks and recreational facilities, and in logging areas of forests. Typically, the deck and the supporting members (stringers and floor beams) are all-wood members. With the development of glued-laminated (or glulam) timber, both the deck and the supporting members are now built from glulam members. One of the latest developments in timber decks is the pre-stressing of glulam decks, a practice successfully used in Ontario, Canada, for several short-span bridges in the 40-ft range.

Figure 2.4 Timber Bridge

2.2.1.4 ADVANCED COMPOSITE MATERIALS

Long used by the aerospace, aircraft, and defense industries, advanced composite materials (ACMs) are now being explored for structural applications in bridges for both superstructures and substructures. ACMs are high-strength, light, and corrosion-resistant, and they have desirable fatigue properties. These includes

Aluminum Bridges Polyester Fibre and Fiberglass Bridges

2.2.2 CLASSIFICATION BY SPAN LENGTHS

In bridge engineering, it is customary to identify bridges as short-span, medium-span, and long-span, depending on the span lengths.

i. Bridges in which the load effects are governed by a single actual vehicle on the span can be considered short-span bridges (up to 65 ft, or 20 m, long).

ii. Bridges in which the maximum load effects are governed by a train of moving vehicles can be considered medium-span bridges (65-400 ft, or 20-125 m, long).

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

iii. Bridges in which the maximum load effects are caused by a train of stationary vehicles with minimum headway distances can be considered long-span bridges (longer than 400 ft, or 125 m).

In the absence of any established criteria, a common practice is to classify bridges by span lengths as follows:

Short-span bridges 20 to 125 ftMedium-span bridges 125 to 400 ftLong-span bridges Over 400 ft

2.2.3 CLASSIFICATION BY STRUCTURAL FORM

The bridges are also classified by their structural forms, which is necessary because the method of analysis used depends on structural form adopted. The structural form depends on the span lengths, type of vehicular traffic and the availability of material at the bridge site. The main bridge types are as under.

1. Cantilever bridges2. Arch bridges3. Suspension bridges4. Cable stayed bridges5. Truss bridges6. Girder bridges7. Beam bridges

2.2.3.1 CANTILEVER BRIDGES

Cantilever bridges are built using cantilevers — horizontal beams that are supported on only one end. Most cantilever bridges use two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center. The largest cantilever bridge is the 549 feet (167 m) Quebec Bridge in Quebec, Canada (Fig. 2.5).

Figure 2.5 Cantilever Bridge, Canada

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.2.3.2 ARCH BRIDGES

Arch bridges are arch-shaped and have abutments at each end. The earliest known arch bridges were built by the Greeks and include the Arkadiko Bridge. The weight of the bridge is thrust into the abutments at either side. Dubai in the United Arab Emirates is currently building the largest arch bridge in the world, which is scheduled for completion in 2012.

Figure 2.6 Arch Bridge

2.2.3.3 SUSPENSION BRIDGES

Suspension bridges are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the floor of a lake or river. The longest suspension bridge in the world is the 12,826 feet (3,909 m) Akashi Kaikyo Bridge in Japan (Fig. 1.26).

Figure 2.7 Tanana Suspension Bridge

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.2.3.4 CABLE STAYED BRIDGES

Like suspension bridges, cable-stayed bridges are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately shorter. The first known cable-stayed bridge was designed in 1784 by C.T. Loescher. The longest cable-stayed bridge is the Sutong Bridge over the Yangtze River in China (Fig. 2.8).

Figure 2.8 Sutong Bridge, China

2.2.3.5 TRUSS BRIDGES

Truss bridges are composed of connected elements. They have a solid deck and a lattice of pin-jointed girders for the sides. Early truss bridges were made of wood, and later of wood with iron tensile rods, but modern truss bridges are made completely of metals such as wrought iron and steel or sometimes of reenforced concrete. The Quebec Bridge (Fig. 2.5), shown as a cantilever bridge, is also the world's longest truss bridge.

Figure 2.9 Truss Bridge, Yaquina Bay Bridge Newport , Oregon

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.2.3.6 GIRDER BRIDGE

A girder bridge is perhaps the most common and most basic bridge. A log across a creek is an example of a girder bridge in its simplest form. In modern steel girder bridges, the two most common girders are I-beam girders and box-girders.

Figure 2.10 Girder Bridge

2.2.3.7 BEAM BRIDGES

Beam bridges are horizontal beams supported at each end by piers. The earliest beam bridges were simple logs that sat across streams and similar simple structures. In modern times, beam bridges are large box steel girder bridges. Weight on top of the beam pushes straight down on the piers at either end of the bridge.

2.2.4 CLASSIFICATION BY SPAN TYPES

Bridges can be classified by the type of span used with respect to the support conditions, namely, simple or continuous. Short-span bridges are built as single spans having simple supports. In medium or long span bridges, depending on the site conditions, multiple simple spans can be built to span the distance. Such a bridge would consist of two end abutments and several piers as intermediate supports.

2.2.5 CLASSIFICATION BY LOAD PATH CHARACTERISTICS

The bridges are also classified by the load path (or load distribution) in the bridge superstructure, i.e. one-dimensional or two-dimensional systems. A one-dimensional system is one in which the load is distributed in one direction only, such as a slab bridge, in which bending occurs in only one direction, about a horizontal axis perpendicular to the

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

longitudinal axis of the bridge. However, in a slab-stringer bridge, the bending of the slab takes place in two mutually perpendicular directions; such a system is classified as a two-dimensional system, which is much more efficient and economical than a one-dimensional system.

The superstructure of a bridge can also be constructed as a three-dimensional system, which may be more efficient than one- or two-dimensional systems. Essentially, such a system consists of a slab (reinforced concrete or, preferably, pre-stressed concrete) supported on flat, skeletal, single- or double-layer, steel tetrahedrons (pyramid-shaped trusses), resulting in a three-dimensional plane grid. These tetrahedrons are arranged in a horizontal plane in two parallel grids that are interconnected by vertical or inclined web members. The external loads are distributed among the various members of the tetrahedrons Omni directionally in space.

2.2.6 CLASSIFICATION BY USAGE

A bridge is designed for trains, pedestrian or road traffic, a pipeline or waterway for water transport or barge traffic. An aqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height. A road-rail bridge carries both road and rail traffic.

Bridges are subject to unplanned uses as well. The areas underneath some bridges have become makeshift shelters and homes to homeless people, and the undersides of bridges all around the world are spots of prevalent graffiti. Some bridges attract people attempting suicide, and become known as suicide bridges.

These bridges carry non-vehicular traffic and include airport runway bridges, pipeline bridges and conveyor bridges.

2.2.7 CLASSIFICATION BY POSITION

These bridges are constructed over navigable rivers or waterways and made moveable. The position of bridge structure could be moved to allow the large ships to pass through the water way.

Figure 2.11 Tower Bridge, London

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.2.8 CLASSIFICATION BY DECK TYPE

Some bridges have double deck i.e. one above the other. Some bridges carry combinations of both highway and railroad traffic.

2.2.8.1 DOUBLE DECKER BRIDGES

Double-decker bridges have two levels, such as the San Francisco – Oakland Bay Bridge, with two road levels. Tsing Ma Bridge and Kap Shui Mun Bridge in Hong Kong have six lanes on their upper decks, and on their lower decks there are two lanes and a pair of tracks for MTR metro trains. Some double-decker bridges only use one level for street traffic; the Washington Avenue Bridge in Minneapolis reserves its lower level for automobile traffic and its upper level for pedestrian and bicycle traffic (predominantly students at the University of Minnesota).

.

Figure 2.12 Wells Street Bridge (Double Decker Bridge)

2.3 MORE THAN JUST A BRIDGE

Some bridges carry special installations such as the tower of Nový Most bridge in Bratislava which carries a restaurant. Other suspension bridge towers carry transmission antennas. A bridge can carry overhead power lines as does the Storstrøm Bridge.

Figure 2.13 The West Montrose Covered Bridge, Ontario, Canada

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

Figure 2.14 Ponte estaiada Octavio Frias Sao Paulo

Figure 2.15 Ponte Vecchio, Italy

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.4 COMPONENTS OF BRIDGE [5]

Bridge mainly consist of two main parts

1. Superstructure2. Substructure

Figure 2.16(a) Components of Bridge

Figure 2.16(b) Components of Bridge

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.4.1 SUPERSTRUCTURE

The superstructure comprises all the components of a bridge above the supports. Figure shows a typical superstructure. The basic superstructure components consist of the following:

Figure 2.17 Bridge Superstructure

2.4.1.1 WEARING SURFACE The wearing surface (course) is that portion of the deck cross section which resists traffic wear. In most instances this is a separate layer made of bituminous material. The wearing course usually varies in thickness from 2 to 4 in (51 to 102 mm); however, this thickness can sometimes be larger due to resurfacing of the overpass roadway, which occurs throughout the life cycle of a bridge.

2.4.1.2 DECK

The deck is the physical extension of the roadway across the obstruction to be bridged. In this example, the deck is a reinforced concrete slab. In an orthotropic bridge, the deck is a stiffened steel plate. The main function of the deck is to distribute loads along the bridge cross section or transversely. The deck either rests on or is integrated with a frame or other structural system designed to distribute loads along the length of the bridge or longitudinally.

2.4.1.3 PRIMARY MEMBERS

Primary members distribute loads longitudinally and are usually designed principally to resist flexure. In Figure 2.17, the primary members consist of rolled, wide flange beams. In some

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

instances, the outside or fascia primary members possess a larger depth and may have a cover plate welded to the bottom of them to carry an additional parapet or curb load as well as provide for a more aesthetically pleasing structure. Beam type primary members such as this are also called stringers or girders. These stringers could be steel plate girders (i.e., steel plates welded together to form an I section), pre-stressed concrete, glued laminated timber, or some other type of beam. Rather than have the slab rest directly on the primary member, a small fillet or haunch can be placed between the deck slab and the top flange of the stringer. It is also -possible or the-bridge-superstructure to form in the shape of a box (either rectangular or trapezoidal). Box girder bridges can be constructed out of steel or pre-stressed concrete and are used in situations where large span lengths are required.

2.4.1.4 SECONDARY MEMBERS

Secondary members are bracing between primary members designed to resist cross-sectional deformation of the superstructure frame and help distribute part of the vertical load between stringers. In Figure 2.17 a detailed view of a bridge superstructure shows channel-type diaphragms used between rolled section stringers. The channels are bolted to steel connection plates which are in turn welded to the wide flange stringers shown. Other types of diaphragms are short depth, wide flange beams or crossed, steel angles. Secondary members, composed of crossed frames at the top or bottom flange of a stringer, are used to resist lateral deformation. This type of secondary member is called lateral bracing.

2.4.2 SUBSTRUCTURE

The substructure consists of all elements required to support the superstructure and overpass roadway. The basic substructure components consist of the following:

2.4.2.1 ABUTMENTS

Abutments are earth-retaining structures which support the superstructure and overpass roadway at the beginning and end of a bridge. Like a retaining wall, the abutments resist the longitudinal forces of the earth underneath the overpass roadway. In Figure 2.16(b) the abutments are cantilever-type retaining walls.

2.4.2.2 PIERS

Piers are structures which support the superstructure at intermediate points between the end supports (abutments). Since the structure shown in Figure 1.2 consists of only one span, it logically does not require a pier. Like abutments, piers come in a variety of forms, some of which are illustrated in the sidebar, from an aesthetic standpoint, piers are one of the most visible components of a highway bridge and can make the difference between a visually pleasing structure and an unattractive one. Figure 2.16(b) shows a hammerhead-type pier.

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.4.2.3 BEARINGS

Bearings are mechanical systems which transmit the vertical loads of the superstructure to the substructure. Examples of bearings are mechanical systems made of steel rollers acting on large steel plates or rectangular pads made of neoprene. The use and functionality of bearings vary greatly depending on the size and configuration of the bridge. Bearings allowing both rotation and longitudinal translation are called expansion bearings, and those which allow rotation only are called fixed bearings.

2.4.2.4 PEDESTALS

A pedestal is a short column on an abutment or pier which directly supports a superstructure primary member. As can be seen in Figure 2.18 at the left abutment cutaway, the wide flange stringer is attached to the bearing which in turn is attached to the pedestal. The term bridge seat is also used to refer to the elevation at the top surface of the pedestal.

2.4.2.5 BACKWALL

A backwall, sometimes called the stem, is the primary component of the abutment acting as a retaining structure at each approach. Figure 2.18 shows a backwall integrated with a wingwall in a concrete abutment.

2.4.2.6 WINGWALL

A wingwall is a side wall to the abutment backwall or stem designed to assist in confining earth behind the abutment. On many structures, wingwalls are designed quite conservatively, which leads to a rather large wall on many bridges.

Figure 2.18 Wingwall of a two-span bridge crossing the Interstate

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.4.2.7 FOOTING

As bearings transfer the superstructure loads to the substructure, so in turn do the abutment and pier footings transfer loads from the substructure to the subsoil. A footing, like the one in Figure 2.16(b), under a wall is known as a continuous or wall footing.

2.4.2.8 PILES

When the soil under a footing cannot provide adequate support for the substructure (in terms of bearing capacity, overall stability, or settlement, support is obtained through the use of piles, which extend down from the footing to the proper depth. There are a variety of types of piles ranging from concrete, which is cast in place, to steel H-sections driven to sound rock.

2.4.2.9 SHEETING

In cofferdams or shallow excavation, the vertical planks which are driven into the ground to act as temporary retaining walls permitting excavation are known as sheeting. Steel sheet piles are one of the most common forms of sheeting in use and can even be used as abutments for smaller structures. In Figure 2.16(b) a two-lane, single-span bridge is supported at each end by arch web sheet piling abutments providing an attractive and economical solution for this small structure.

2.4.2.10 APPURTANANCES AND SITE-RELATED FEATURES

An appurtenance is any part of the bridge or bridge site which is not a major structural component yet serves some purpose in the overall functionality of the structure (e.g., guardrail). The bridge site, as an entity, possesses many different components which, in one way or another, integrate with the structure proper. The major appurtenances and site-related features are as follows:

2.4.2.11 EMBANKMENT AND SLOPE PROTECTION

The slope that tapers from the abutment to the underpass (embankment) is covered with a material called slope protection, which should be both aesthetically pleasing and provide for proper drainage and erosion control. Slope protection could be made of dry stone or even block pavement material. Figure 2.16(b) shows an abutment embankment being prepared with select granular fill. This type of slope protection consists of broken rocks which vary in size and shape. The form of slope protection varies greatly from region to region and is mostly dependent on specific environmental concerns and the types of material readily available.

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.4.2.12 UNDERDRAIN

In order to provide for proper drainage of a major substructure element, such as an abutment, it is often necessary to install an Underdrain, which is a drainage system made of perforated pipe or other suitable conduit that transports runoff away from the structure and into appropriate drainage channels (either natural or man-made).

2.4.2.13 APPROACH

The section of overpass roadway which leads up to and away from the bridge abutments is called the approach or approach roadway. In cross section the approach roadway is defined by the American Association of State Highway and Transportation Officials (AASHTO) as the "traveled way plus shoulders”. At times the approach roadway maintains the same or a slightly beefed up cross section of the standard roadway. To compensate for potential differential settlement at the approaches, a reinforced concrete slab or approach slab is sometimes used for a given distance back from the abutment. The approach slab helps minimize impact to the abutment which can result from differential settlement.

2.4.2.14 TRAFFIC BARRIERS

Protective devices used to reduce the severity of an accident when a vehicle leaves the road are called traffic barriers. Traffic barriers can range from a guard rail made of corrugated steel to reinforced concrete parapets.

2.4 .2 .15 MISCELLANEOUS TERMS

Some of the more basic expressions and terms that we will use through out the course of the text are as follows:

2.4.2.15.1 VERTICAL CLEARANCE Vertical clearance is the minimum distance between the structure and the underpass. AASHTO specifies an absolute minimum of 14 ft (4.27 m) and design clearance of 16 ft (4.88 m). The location of the structure (i.e., urbanized area vs. expressway) has a great deal to do with how this is enforced by the governing agency.

2.4.2.15.2 LOAD RATING An analysis of a structure to compute the maximum allowable loads that can be carried across a bridge is called a load rating. The guidelines for load ratings are set forth in AASHTO's Manual for Maintenance Inspection of Bridges. Two ratings are usually prepared: o n e that computes the regular, daily capacity of the structure which is known as the inventory rating and a maximum upper limit or operating rating which is computed for special loading conditions.

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CHAPTER 2: TYPES AND COMPONENTS OF BRIDGES

2.4.2.15.3 DEAD LOADS Permanent loads placed on a structure before the concrete slab hardens are called dead loads. For example, in a slab-on-stringer bridge the stringers, diaphragms, connection plates, and concrete slab itself would be considered to be dead loads.

2.4.2.15.4 SUPERIMPOSED DEAD LOADS Superimposed dead loads are permanent loads placed on the structure after the concrete has hardened (e.g., bridge railing, sidewalks, wearing surface, etc.).

2.4.2.15.5 LIVE LOADS

Temporary loads placed on the structure, such as vehicles, wind, pedestrians, etc., are called live loads.

2.4.2.15.6 SHEETED PIT

A temporary box structure with only four sides (i.e., no top or bottom) which can be used as an earth support system in excavation for substructure foundations is called a sheeted pit. The bracing elements used inside a sheeted pit to keep all four sides rigid are called Wales (which run along the inside walls of the sheet piling) and struts (which run between the walls). When this type of structure is used where the ground level is below water, the sheeted pit is designed to be watertight (as much as possible) and is called a cofferdam.

2.4.2.15.7 STAGED CONSTRUCTION

Construction that occurs in places, usually to permit the flow of traffic through a construction site, is called staged construction. An example would be a bridge replacement project where half of the structure is removed and replaced while traffic continues over the remaining portion of the structure. Once the first half has been removed and reconstructed, traffic is then diverted over to the new side while work begins on the rest of the structure. This is an aspect of rehabilitation design which offers some interesting challenges to engineers.

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