12 Types of Loads Considered for Design of Bridge Structures
Construction Dead Load Recommendation. “The combined density of concrete and reinforcing and prestressing steel shall be assumed to be not less than pcf for normal?weight concrete and for lightweight concrete. Exceptions to the lightweight concrete design weight can be requested for approval from INDOT. The weight of formwork shall be assumed to be not less than,15 psf including . Aug 25, · Find below all important loads to be considered in the design of a Bridge. Dead Load “Gravity loading due to structural parts of bridge” Dead consist of permanent gravity forces due to structural Elements. It is simply calculated as the product of volume and material density. Usually, self-weight is applied in the analysis model using the self-weight option of the analysis software. This .
The primary function of how to check lightning arrestor bridge is to carry traffic loads: heavy trucks, cars, and trains. Engineers must estimate the traffic loading. On short spans, it is possible that the maximum conceivable load will be achieved—that is to say, on spans of less than 30 metres feetfour heavy trucks may cross at the same time, two in each direction.
On longer spans of a thousand metres or more, the maximum conceivable load is such a remote possibility imagine the Golden Gate Bridge with only heavy trucks crossing bumper-to-bumper in each direction at the same time that the cost of designing for it is unreasonable.
Therefore, engineers use probable loads as a basis for design. In order to carry traffic, the structure must have some weight, and on short spans this dead load weight is usually less than the live loads. On longer spans, however, the dead load is greater than live loads, and, as spans get longer, it becomes more important to design forms that minimize dead load. In general, shorter spans are built with beams, hollow boxes, trusses, arches, and continuous versions of the same, while longer spans use cantilevercable-stay, and suspension forms.
As spans get longer, questions of shape, materials, and form become increasingly important. New forms have evolved to provide longer spans with more strength from less material. Dead how to play mkv files on windows vista live weight are essentially vertical loads, whereas forces from nature may be either vertical or horizontal.
Wind causes two important loads, one called static and the other dynamic. Static wind load is the horizontal pressure that tries to push a bridge sideways. Dynamic wind load gives rise to vertical motion, creating oscillations in any direction. Like the breaking of an overused violin string, oscillations are vibrations that can cause a bridge to fail.
If a deck is thin and not properly shaped and supported, it may experience dangerous vertical or torsional twisting movements.
The expansion and contraction of bridge materials by heat and cold have been minimized by the use of expansion joints in the deck along with bearings at the abutments and at the tops of piers.
Bearings allow the bridge to react to varying temperatures without causing detrimental stress to the material. In arches, engineers sometimes design hinges to reduce stresses caused by temperature what did the great basin tribes eat. Modern bridges must also withstand natural disasters such as tropical cyclones and earthquakes.
In general, earthquakes are best withstood by structures that carry as light a dead weight as possible, because the horizontal forces that arise from ground accelerations are proportional to the weight of the structure. This phenomenon is explained by the fundamental Newtonian law of force equals mass times acceleration. For cyclones, it is generally best that the bridge be aerodynamically designed to have little solid material facing the winds, so that they may pass through or around the bridge without setting up dangerous oscillations.
Modern bridges, the focus of this article, began with the introduction of industrially produced iron. They have evolved over the past years as engineers came to better understand the possibilities inherent first in cast ironthen in wrought iron and structural steeland finally in reinforced and prestressed concrete.
These materials have led to bridge designs that broke completely with the designs in wood or stone that characterized bridges before the Industrial Revolution. Industrial strength has been an important factor in the evolution of bridges. Great Britainthe leading industrialized country of the early 19th century, built the most significant bridges of that time. Likewise, innovations arose in the United States from the late 19th century through the midth century and in Japan and Germany in subsequent decades.
Switzerlandwith its highly industrialized society, has also been a fertile ground for advances in bridge building. The first bridges were simply supported beamssuch as flat stones or tree trunks laid across a stream.
For valleys and other wider channels—especially in East Asia and South Americawhere examples can still be found—ropes made of various grasses and vines tied together were hung in suspension for single-file crossing.
Materials were free and abundant, and there were few labour costs, since the work was done by slaves, soldiers, or natives who used the bridges in daily life. These bridges were built with long, thin slabs of stone to make a beam-type deck and with large rocks or blocklike piles of stones for piers. Postbridge in Devon, England, an early medieval clapper bridge, is an oft-visited example of this old type, which was common in much of the world, especially China. Videos Images.
Additional Info. Load Previous Page. Live load and dead load The primary function of a bridge is to carry traffic loads: heavy trucks, cars, and trains. Forces of nature Dead and live weight are essentially vertical loads, whereas forces from nature may be either vertical or horizontal. An expansion joint in a steel plate girder bridge. Load Next Page.
What is Dead Load?
Nov 05, · We are focusing on the loads to be considered in the dining of the bridge according to the BS Dead Load. This is a well-known load for us. It is the self-weight of the structure. In addition to the self-weight of the structure, there are other types of loads such as superimposed load. Weights other than the own weight of the structural elements of the bridge can be considered as superimposed dead . Definition of Dead Load in Construction. Structures are designed to withstand forces that are placed on the structure, whether it is a bridge or a building, by the actual weight of the building materials required to accommodate the construction of the project. Items such as concrete, steel, partitions, flooring, ceilings, mechanical and electrical systems all place a gravity weight load on the entire . The dead load of a bridge depends on various factors like depth of girder or truss, span, number of panels, width of bridge etc. The dead load of trusses may be estimated by the following methods: (a) Hudson Formula: Weight per metre of trusses and bracings = A Newton/metre. Where, A = Maximum net area of the tension chord.
In this article we will discuss about:- 1. Introduction to Bridges 2. Types of Steel Bridges 3. Truss Components 4. Economic Span 5. Bridges are structures meant to support rail road traffic, highway traffic or pedestrian loads across openings or crossings or another set or rail or highway traffic or across any natural or artificial obstacles.
Based on the type of traffic for which they are provided, bridges may be classified into- i Highway bridges ii Railway bridges iii Foot bridges for pedestrian traffic.
We also at times come across combined Highway and Railway bridges. Bridges may be made of timber, masonry, reinforced concrete, prestressed concrete and steel. Timber bridges are generally provided for small spans and sometimes as a temporary bridge.
For permanent bridges or small spans not exceeding 12 m, masonry bridges may be provided. For greater spans, the dead load of masonry becomes large and hence masonry bridges work out to be uneconomical. Reinforced concrete bridges are found to be economical for spans exceeding 12 m. Prestressed concrete bridges have been constructed for spans up to 60 m. Arched concrete bridges have been built for still greater spans. Since steel possesses a high working stress compared to other materials, steel bridges work out to be economical for large spans.
Steel bridges are very common for small as well as long spans in railways. Fabricated components of a steel bridge can be easily transported to the site and assembled, thus considerably reducing the construction time. A bridge forms mainly the super structure spanning the required length and it comprises of the floor system, the trusses or girders system, support arrangement and lateral bracing system.
The floor system provides a satisfactory surface to afford easy movement of traffic over it. The floor system transmits its weight and loads due to vehicular traffic to the supporting trusses or girders. The trusses and girders in turn transmit all loads received by them to the abutments or supporting piers.
Trusses and girders are provided with end shoe device to safely transmit the reactions to the supporting abutments. Such an arrangement also makes provision for slight longitudinal movements due to temperature changes.
A lateral bracing system is provided to the bridge, which not only provides adequate stiffness but also minimizes vibrations. Such bracing system also resists lateral forces transmitted by wind action on the structure as well as the moving vehicles. Bridges may be classified into Deck Types Bridge and through type bridges, according to the manner of transference of live load to the bridge.
In the case of Deck type truss bridges, the floor of the bridge is supported at the top chord joints of the truss. In a Deck type plate girder bridge, the floor is supported on the top flange.
In through type truss bridges, the floor is supported at the lower chord joints of the truss and the top chord is provided with lateral bracing.
In a through type plate girder bridge, the floor is supported at the level of the lower flange and the top flanges are braced laterally.
Sometimes the floor is supported on the bottom chord or near the bottom chord and the top chords are not braced. Such bridges are called half through or semi-through bridges. A brief description of some types of steel bridges is given below:. These are convenient where the span of the beam exceeds 5 in. For small spans I-section beams may be used. For spans more than 8 m, built-up I-sections or plate girders are used. By providing a combination of main plate girders and cross beams, the bridges can be made for spans up to 20 m.
For very large spans deck and through plate girder bridges may be used. Generally railway deck plate girder bridges are designed to carry one track. Two single-track bridges are made side by side resting on piers and abutments forming a double track bridge.
In situations when the clearance below the structure is small it is necessary to provide a through girder bridge. These bridges are found convenient for spans up to 50 m.
The box girder consists of steel plates fabricated to box shape and strengthened by angles and channels. These are the most commonly used bridges and are found satisfactory for spans 10 m to as large as m.
Bridges of spans 50 m to 60 m are most common. Cross beams are connected to trusses either at the level of the top chord or at the level of the bottom chord. Accordingly the bridges are deck bridges or through bridges. The trusses with parallel chords can be modified to make curved chord trusses. If trusses whose depths vary throughout the length from both ends the forces in the chord members are more or less equalized. Web members of curved chord trusses are likely to be subjected to lesser forces than in the case of parallel chord trusses.
In order to reduce the lengths of the loaded chords bottom chords of through trusses and top chords of deck trusses it is convenient to make subdivided loaded chords. These are top and bottom members which act like the flanges of a beam. They resist compressive and tensile forces. The chord members are parallel in a truss of uniform depth. Their profile may however range from uniform depth to variable depth as for example in a bowstring truss.
Variable depth profile offers economy. These consist of vertical and diagonal members. In parallel chord trusses, the diagonals offer the required shear resistance. Verticals also carry shear besides providing additional panel points for introduction of loads. Verticals subjected to compression are called posts and those subjected to tension are called hangers. Counters or counter bracers are a pair of intersecting diagonals in a panel where a single diagonal would be subjected to stress reversal.
These are provided in lattice trusses, sway frames and portals. These are compression members provided at supports of single span trusses. This is the structural unit which provides the direct support for vehicular loads. These are beams set normal to the direction of traffic.
These beams transmit the deck loads to the trusses. These are members connecting the top chords of two trusses and bottom chords of two trusses. The bracing system forms trusses in the plane of the top chord and in the plane of the bottom chord. These provide stability and offer lateral resistance to wind action.
In the case of a long span bridge truss the members of the truss are likely to be subjected to large forces. In such cases, cantilever bridges, continuous bridges, suspension bridges and arched bridges are found suitable.
A cantilever bridge consists of two cantilever trusses supporting between their ends a central simple span truss. Continuous trusses have more than two supports and are statically indeterminate.
In an arch carrying a loading, besides vertical reactions these will also be horizontal thrusts at the supports, which reduce the bending moments. Arches may be three hinged, two hinged or fixed arches. The span of a bridge may be so determined that the total cost of the bridge is a minimum. The span to satisfy this condition is called the economic span. The total cost of the bridge consists of the cost of the substructure and that of the superstructure. Very often it is seen that the cost of the substructure forms nearly 50 per cent of the total cost of the bridge.
The cost of pier will not change appreciably for small change in span. Even the cost of the floor-way is not affected much for small variation in span. But it is seen that the cost of the trusses and bracings is directly proportional to the span of the bridge.
Hence, the cost of the whole bridge is a minimum when the cost of one pier and the cost of trusses and bracings corresponding to one span are equal. The various loads, forces and stresses to be considered for the design of bridges are the following:.
Dead load is the weight of the floor slab, track stringers, ballast, guard rails, bracing system, rails sleepers etc. The dead load of the various components may be taken at the following values:. The dead load of a bridge depends on various factors like depth of girder or truss, span, number of panels, width of bridge etc.
The dead load of trusses may be estimated by the following methods:. Kerbs 0. If the kerb width is less than 0. The main girder, trusses, arches or other members supporting the footways shall be designed for the following live loads per square metre of the footway area, the loaded length of the footway taken in each case being such as to produce the worst effects on the member under consideration.
Each part of the footway shall be capable of carrying a wheel load of 40 kN which shall be deemed to include impact, distributed over a contact area mm in diameter. The working stresses shall be increased by 25 per cent to meet this provision. This provision need not be made where vehicles cannot mount the footway as in the case of a footway separated from the roadway by means of an insurmountable obstacle, such as truss on a main girder.
Railings, Parapets or Guide Posts :. Consideration shall be given to the architectural features of the railing or parapet to obtain proper proportioning of its various members and its harmony with the structure as a whole. Consideration shall be given also to avoiding, as far as is consistent with safety and appearance, obstruction of the view from passing motor cars. They shall be designed to resist a lateral horizontal force and a vertical force each of N per metre run applied simultaneously at the top of the railing or parapet.
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