Metal Bridge Construction The imposing metal bridges are structures that are built quickly. However, it is costly and also are subject to corrosion from atmospheric agents, gases and fumes from factories and cities. Thus, maintenance is expensive. Steel is the most important material from the late nineteenth century for the construction of steel bridges. At first its use was limited by its high cost. Years later the material drastically low price. Performing impressive monuments of steel. Numerous metal truss bridges in the country for both vehicles and rail, designed abroad and assembled on site, until the 60s of last century. Subsequently, pre-stressed concrete bridge beams T, replaced the metal truss, benefiting national engineering, since the process of design and construction was carried out by Colombian engineers. One of the biggest advantages of steel are its construction in the workshop and the ease of moving to the site for assembly, enabling you to compete with prestressed concrete bridges in inhospitable places of the national territory, or when the time factor construction is a key variable for the work.
Evolution The evolution of the bridges over history has been marked by the progressive research has led to the invention of the lattice being used to stiffen flex their structural elements. Post-World War II development of technology has allowed the metal bridges are an excellent choice when building a bridge achieving greater strength capacity, reduce weight and moldable material. This development is mainly in Germany and consists of the following: 1. Extending the use of the soul full beam, either version of I-beam, and box girder. In 1950 he built the first box girder bridge as large (L = 140 m) made by Stephenson. Later this type is used in many bridges, without actually compete for large lattice structure works, but the lower manufacturing cost structure and soul filled fewer joints to make, determined its imposition. 2. Improving methods of binding (high-strength bolts, since 1940) and the widespread use of electric welding and then automatically, which had a great development in shipbuilding during World War II. This disappears as the rivet bonding method. 3. The development or enforcement of orthotropic slab as metal bridge deck, which sympathizes with the longitudinal bending element and works monolithically, optimizing the use of steel. Automatic welding was very important to introduce this innovation. 4. The best quality of steel, which reduces the weight of the structure.
The cross section is used with one or two drawer cells and economic slenderness is somewhat greater than in the pre-tensioned bridges, the camber ratio / is between l/25 yl/30 light, but the price of the metal bridges often be greater than the pre-tensioned concrete, which has caused a gradual displacement of these lights to increasing. The rise in metal bridges, has given rise to composite bridges, replacing the steel deck (most expensive part of the bridge) by a concrete slab, but the lights are lower. The metal arch bridge has gone on in the two known morphologies the material, using the full or the lattice soul. The metal arc constructed greater few years ago was completed using a lattice structure so as to dash arc. Lately metal arches are being built with tubular section. The lattice in straight bridges has moved on from century it appeared, but their current frequency is small and still sees U.S. technology and Japan.
Generalities Pridge It is a network structure that facilitates those activities they may encounter difficulty in circumventing a natural obstacle or a lane land or sea. The main functions of a bridge are: Withstand vehicle traffic or other on a cross, which can be a river, a canyon or another line of traffic. • Serve safely. • Be economical. • Be designed so aesthetically harmonize and enrich the beauty of your surroundings. Normally two trusses are placed braced parallel to each other, the transmission of loads from vehicles is made in two types: lower panel (the most common) and of top board, as the gauge of the channel allows.
Armor The truss beam is composed of relatively short and slender elements connected at their ends. The weight load of the floor fixed and movable load transmitted across the bridge by means of the transverse beams board connections directly to the armor elements. In the various triangular configurations created by the design engineer, each element is or in tension or compression, as the pattern of charges, but they are never subjected to charges that tend to flex. This system allows for a reasonable cost and with a minimum expenditure of material-saving metal structures from thirty to over one hundred meters, distances are economically impossible for structures based operating bending as simple beams. There are actually many ways to place the elements of armor.
Metal Bridge
In addition these bridges parallel trusses using a set of transverse beams that carry loads and own weight of the vehicle to the lower nodes of the truss. To feed the transverse beams are also used longitudinal beams on which the plate rests directly reinforced concrete panel which serves as the bridge.
Figure # 1: Bridge with steel trusses The steel bridge built helped achieve important lights. The bridges on light metal beams can be overcome to 45 m (similar to traditional prestressed), while metallic lattice bridges has reached 80 m, and metallic bridges arc has reached 100 m, constituting key lights.
Picture # 1: Bottom view of the transverse and longitudinal beams of a steel truss bridge.
The truss members are joined by plates, welded or bolted as shown in Photograph No. 2.
Picture # 2: View of a truss node
Forms of Active Mass The metal bridges are formed by longitudinal members of limited cross section that resist loads by flexing action. The action of the loads is transverse to the length of the element (beam action) are shown in cross section simultaneously tensile and compression, supplemented with the cutting, usually small, transmitting flexural forces is much less efficient than the axial transmission. The beams may be attached rigidly to vertical elements through the nodes, with improved load capacity, decreasing deflections and increased ability to resist horizontal forces, such as wind or earthquake, forming porches . The grating formed with horizontal straight elements in both directions through rigidly knots active form dough systems that increase the bearing capacity of the beams and lower deflections. When the dough is evenly distributed and individual beams disappear, you have the plates or slabs, which allow more loads with less deflection, within certain ranges of relationship between the lights.
Beams The beams are structural elements that can withstand significant loads with limited height. However, this condition causes deflections are large and require to be controlled by minimum heights. It also requires that the materials used can withstand tensile and compression of almost equal magnitude. To optimize their use, the construction industry has developed so-called "structural wide flange profiles" of structural steel, which, however, are limited by the possibility of buckling in the compression zone of the beam. In beams "lattice", as the section is not continuous, the resultant forces of compression and tension elements are concentrated in the top and bottom, and act on their cross-sectional areas, the moment arm or resistant torque characteristic of
flexion, is practically constant, since there is the triangular distribution efforts. The shear capacity of the beam is provided by the diagonal elements, which in this case act in compression.
Features metal bridges • • • • • • •
Uniformity. - The steel properties are not changed significantly with time. High resistance. - The high strength of steel per unit weight will mean that some of the weight of the structures, it is of great importance in bridge large gaps. Durability. - The structures will last permanently if they are properly maintained. Ductility. - The property of a material to withstand large deformations without failure under high stress. The ductile nature allowed to flow locally avoiding premature failures. Tenacity. - Possess strength and ductility, being the property of a material to absorb energy in large amounts. Elasticity. - It is closer to the design hypothesis because it follows Hooke's law. Cost recovery. - They can be reused as scrap.
Types of support: The batteries corresponding to the part of the substructure which supports the panel superstructure, which are deep or shallow foundation piles through or caissons. Most are in reinforced concrete and wall type, head and beam columns with metal tower. We present the type of fixed and mobile supports identified in the stirrups, finding that most are neoprene plates, roller bearings and rocker supports, illustrated in Figure No. 4
Types of metal bridges The armature works in an analogous manner to the beam. The upper row of elements, called upper chord, is in compression, as does the top flange of the beam. The elements that form the lower chord, as the lower flange of the beam remain energized. Vertical and diagonal elements ranging from one to another cord are in tension or compression depending on configuration and changes as the position of the moving load. The items subject only to tension under any possible load pattern are slim. Other elements are more massive, can be parts and leaving the hollow center which in turn are formed by small triangular elements.
Bridges armor or parabolic polygonal The top chord is polygonal with higher cant point in the center. The lower chord is usually horizontal.
Bridges with rectangular frames The umbilical cord is polygonal horizontal.
Bridges with top board armor It is completely under the dash, which relies on the plates of the top chords. Holding the board by means of plates or pins of their lower chords.
Figure No. 5: Armor top board
Bridges bottom board with armor Girders which are attached above the board by fastening elements.
Figure 6: Elements of a truss bridge bottom board
Bridges lateral frame No bracing joining their top chords.
Bridges armor "N" s " U.S. was patented by Pratt brothers in 1844. This configuration is distinguished by its long diagonal down towards the center of the stage, so only subject to tension. Can
vary by either rectangular or polygonal shape. The polygonal frames "N's" of sections in the order of one hundred meters can not have additional diagonal to achieve umbilical cord, termed subdiagonals.
Picture No. 3: Polygonal N'so Parker Bridge.
Bridges armor "double N" s " In 1847 has underlined, in which the uprights are closer to each other and the diagonal cross at their midpoints to finish in the next panel.
Bridges armor "W's" It was patented in 1848 by two British engineers. This configuration has the diagonals in alternating directions and usually combined with vertical elements or poles. A variation of this system has two diagonals in opposite directions, the armor of "X's", also known as "Eiffel system." The armor "lattice" has three sets of diagonal type "W" superimposed.
Figure 7: Types of frame used in bridges
Rigid frame bridges Combine the plates and bridge abutments plate with beams and the beam brackets, this simple combination units form without connecting joints between the pieces. They are surrounded by steel reinforcement of concrete. From very young, are very useful to separate the levels of road and rail crossings. In these crosses is usually desirable that the level difference is minimal and the bridges of the class under consideration are likely to receive lower on one leg than the other types.
Simple truss bridges The armor of the modern bridges take many forms. The Pratt and Warren trusses, higher or lower pitch, are the most used in steel bridges short lengths. For long-span bridges armor used Parker, curved upper strand, also called Pratt truss, and for long span and simple truss structures are used with shelves subdivided. The trusses for long spans are subdivided into so that the length of the beams is not excessive, as it increases the width of the opening, so must the height of the armature so to avoid excessive bending and for reasons of economy. The metal members of the truss bridges were built in many different ways.
Figure 8: Armor lenticular Saltash Bridge (1859)
Bridges siderail The first bridges were established for humanity girder bridges: logs crossed over rivers and ravines. When the man had beasts of burden was forced to place two or more logs on them together and build a deck or flat floor to allow it to happen. When the save distance was greater than the practical length of trunks beams, resorted to placing lengths of timber on a series of intermediate supports or batteries. The beam is a horizontal structure that can hold between two supports load without creating lateral thrust therein. In this type of compression structure develops in the top and tension in the bottom. Wood and most metals are able to withstand stress both types, like the steel reinforced concrete.
Figure 9: Elements of a side girder bridge, using the example of a truss.
Design Every bridge must be designed to safely support all vehicles that can pass over it, over its lifetime. However, it is possible for the designer to know exactly the structure or vehicle which requested the service life thereof.
To ensure the safety of the structure, it must maintain some control measures and make some predictions about the strength to support current and future loads. In Ecuador there is no bridge design code own specific standards for the analysis and design, but the MOP adopted the American AASHTO design code, the same as fixed loads acting on elements of the bridges. This regulation although it has been well tested in the United States for over 45 years, is developed for road features typical of that environment, with geography and traffic volumes different from other measures of comparison. In view of this, it is necessary to bear in mind that when you apply this code (AASHTO), for the design of bridges in our environment, observe the differences noted before, and based on a realistic basis of the characteristics of our country road , make adjustments to these rules. [1] According to the Structural Department of the Ministry of Transport and Public Works for the design of steel bridges using the LRFD method, the same as used for strength limit states of structures through which no applicable limit state is exceeded when the structure is subject all factored load combinations. To provide the structure so that the required strength is less than or equal to the design strength of each structural component.
Considerations of durability Overview The durability of a steel structure is its ability to support, for the life for which it was designed, the physical and chemical conditions to which it is exposed, and that could eventually lead to degradation due to different effects loads and stresses considered in the structural analysis. A durable structure should be achieved with a strategy to consider all possible factors of degradation and act accordingly on each of the phases of design, implementation and use of the structure. A successful strategy for the durability should be noted that a structure may have different structural elements subjected to various types of environment.
Overview • •
• •
The durability of the metal bridge spans the life of the joints, supports, railings, facilities, etc., Very sensitive to sudden changes in the deformed slope. It is necessary to provide for the operation and maintenance phase, ie to prolong the life of the metal bridge periodic maintenance is required in both the infrastructure and superstructure, ie cleaning areas near the batteries or towers, cleaning works drainage under the road surface, board arrangement, reshaping of hits, painting, signage, etc.. The durability is also dependent on the construction methods and materials used must comply with the minimum standards and specifications. The excessive number of joints that always, be very effective, are water inlet paths to the beams and the head of piers and abutments may damage the
structure, as the bad evacuation of water from the boards by the have difficulty properly generating board drains durability problems.
Consideration of durability in the design phase The proposed steel structure should include measures to ensure that the frame length reaches the preset lifetime, according to aggressive environmental conditions and the type of structure. You'll need to include a strategy of durability. The protection against physical and chemical agents aggressive preventive measures are often the most effective and least expensive. Therefore, the durability is a quality that should be considered during the project, studying the nature and intensity of the foreseeable potential corrosive media and selecting structural forms, materials, and enforcement procedures most appropriate in each case. The selection of the type of environment must take into account the existence of a number of factors that are able to modify the degree of aggressiveness, a priori, be considered as characteristic of the geographical area in which the structure is located. Thus, relatively close locations may have different exposure classes according to altitude, the general orientation of the structure, the nature of the surface (vegetation, rock, etc.), The existence of urban areas, proximity to a river, etc..
Strategy for durability The durability strategy include at least the following aspects: a) General requirements You need to follow a strategy that considers all possible degradation mechanisms with specific measures in terms of aggressiveness to which each element is subjected. b) Selection of structural form The project will define the structural patterns, geometric shapes and details that support the achievement of adequate durability of the structure. The project should facilitate the preparation of surfaces, painting, inspections and maintenance. They endeavor to avoid the use of structural designs leading to an increased susceptibility to corrosion. For this, it is recommended that the forms of the structural elements are simple, avoiding excessive complexity and the implementation methods of the structure are such as not to reduce the effectiveness of the protection systems employed (for shipping damage and manipulation of the elements). Will tend to minimize direct contact between the steel surfaces and the water, preventing the formation of deposits of water, enabling rapid evacuation of the latter and preventing the passage of water over the joint areas. To this end, precautions should be taken as to avoid horizontal surfaces available to promote the accumulation of dirt or water, removing sections open at the top to facilitate such accumulation, and suppression of hollow cavities in which water may be retained and the provision of adequate and generous section for driving and water drainage.
When this structure enclosed areas (interior accessible) or hollow elements (interior inaccessible), must take care that they are effectively protected against corrosion. To do this, it must be avoided that water trapped inside during assembly of the structure should be arranged so as necessary measures for ventilation and drainage (interior accessible), and must be sealed effectively against the ingress of air and moisture , by continuous welding, the inaccessible interior. Avoid potential corrosion in narrow holes, cracks and joints overlapping blind by an effective seal which is generally constituted by continuous welds. Special attention should be paid to the corrosion protection of unions, both bolted (so that the screws, nuts and washers have the same durability as the rest of the structure) and welded (making sure that the weld surface is free of imperfections such as cracks, craters and projections, which are difficult to effectively cover the later painting) and take into account, in the case of provision of reinforcements or execution notches (in souls, reinforcements, etc..) the need to allow adequate surface preparation and paint application (continuously welded intersection between the reinforcement and the reinforced element, having a minimum radius of 50 mm in the grooves while preventing water retention). c) Construction details It is recommended to avoid improper construction details as indicated in the figures that follow, using the appropriate considered therein.
Figure No. 11: Preventing the accumulation of water and dirt
Figure No. 12: Making welds Elements
Figure No. 13: Elimination of surface imperfections in the welds
Figure 14: Design efforts recommended for corrosion protection d) Special protection measures In special cases of aggression, when normal protective measures are not considered sufficient, recourse may be available to special protection systems. The project should consider life extra special protection, and establish proper maintenance thereof. Examples of special protection measures may include: a) application of powder coating materials. b) Products for the chemical treatment of surfaces (eg phosphating solutions). c) Cathodic protection (for example, in case of special risk of galvanic corrosion). Conditions to facilitate inspection and maintenance Wherever possible, should provide access to all elements of the structure and to the supports, gaskets and drainage elements, considering the desirability of specific systems which facilitate inspection and maintenance during the Service. Therefore, and since inclusion in service access systems for unplanned maintenance is a difficult task initially, the project will establish the necessary access systems, which may include fixed bridges, motorized platforms or other aids. The essential criterion is that all accessible surfaces of the structure which must be inspected and maintained to be visible and must be within the maintenance operator by a safe method. The operator must be able to scroll through all parts of the structure to maintain and should have adequate space to work on them. Special attention should be paid to the accessibility of the structure enclosed areas such as metal drawers. Access openings must be large enough to allow safe access, both for operators and for equipment maintenance. Recommended minimum dimensions of 500x700 mm (width x height) in rectangular or oval access, minimum diameter and 600 mm in the circular access. Furthermore, there must be adequate ventilation holes protection system used for maintenance.
Considerations constructive Materials: • Quality certificates of origin of the material in terms of chemical composition and strength. • Tensile tests, chemical analysis. • Check the homogeneity of the material by ultrasound and thickness measurement of some sheets. Quality: • In compliance with the specifications, product quality (control of schedules, materials, manufacturing, packaging and assembly). Assembly: • It consists in the assembly and welding of a main element that comprises platabandas souls, stiffeners, gussets, connection angles, etc. Pre-assembly: • Rectify total length of the bridge camber or camber • Correct defects and inaccuracies by the welding process and material preparation • Confirm the proper assembly and adjustment of field joints, welder's stamp. • Dimensional detailed review As an additional procedure is developed indicative plane where the numbering is assigned the ends of the main elements which are to be connected in the preassembly and assembly, suitable for there is no confusion since only the ends are assembled have the same ID.
Mounting: The assembly operation is the most important part of the whole construction process, include: transport, armed himself with the structure, welding, polishing, control and inspection. The Assembly is the assembly of the various components, so that the structure meets the manner provided in the shop drawings to tolerances. No definitive starts screwing or welding assembly joints until you have verified that the position of each joint elements match the final position. • TRANSPORTATION: structural elements into your final website it is by means of large trucks, trailers, while the domestic transport carried out using cranes, hoists feathers or with the respective safety instructions specified by the company in charge of lifting the structure. Transportation should be done outside of working hours of the welders in order to optimize performance and uptime. • ARMED OR MOUNTING: In the armed cords are constructed temporary welds as a prelude to the final solder joints. • WELDING: Within the processes mentioned this certainly is the most important because welding is a form of union.
•
CONTROL: Can perform tests to verify the quality of the steel prior to the construction, determining the quality (yield strength, tensile strength, tensile and compressive), the quality control during the prefabrication joints and the assembly is checked further that the filling material is correct, the voltages are used or suitable amperage, welding positions, and the thickness are met.
Cleaning and painting: • •
Cleaning of surfaces of steel made in plant with sandblasting. The anticorrosive paint or primer applied in plant (phenolic zonc chromate 3mm thick) and a topcoat applied after assembly (aluminum extrareflectivo).
Manufacture of the structure: • • • •
Classification of raw materials: Prints (for rigidity beams, box, brackets, stiffeners), high strength bolts (for connections), Angles (for braces), axles (for poles of pins). Preparation of material: It consists of tracing, cutting, drilling and identification of each item with its corresponding numbers indicating the position, number of flat work concerned.
Storage and handling depending suitable medium. Environmental: physical diagnosis, biotic and socioeconomic area of influence.
Aesthetic aspects: • • • • • • •
landscape design creativity architecture Harmony with the environment texture color reliability
Essays: Nondestructive testing are performed both in plan and assembly to ensure the quality of the weld which is inspected by: radiography, ultrasonic, liquid penetrant, magnetic particle, evaluate the quality of welds to be inspected.
Economics The economic efficiency of a bridge site and traffic dependent, the ratio of savings by having the bridge compared to cost. The cost of life is composed of materials, labor, machinery, engineering, cost of money, insurance, maintenance, refurbishment, and
ultimately, demolition and disposal of its partners, recycling, and re-location, less the value of scrap and reusability of components. In some cases the appearance of the bridge may be more important than cost efficiency. The metal bridges have two types of constraints: the cost of using imported materials, and the need for extensive maintenance.
Figure No. 15: Elements of a side girder bridge, using the example of a truss.
Advantages and disadvantages of steel bridges Advantages: • CONSTRUCTION: Best for gunpoint, steep slopes, where not allowed to install temporary supports. Possibility of prefabricated members of a structure. Several members for attaching ease through various types of connectors such as welding, screws and rivets. Rapid assembly Large capacity of laminated and lots of sizes and shapes. Resistance to fatigue. • ENVIRONMENTAL: It does not pollute the environment It requires the use of natural resources Minimizing waste affecting the ecological environment. Steel is 100% recyclable. • ECONOMIC: Reduction of dead loads between 40% to 50% reduction in foundation costs. Economic benefit to the region by the reduced period of the work.
Lower costs for capacity expansion. Disadvantages: • Maintenance costs Most metal structures are susceptible to corrosion when exposed to water, air, external agents, climate change and therefore require periodic painting. • CORROSION: Exposure to the environment suffers corrosive action must therefore always with enamel coated anti-corrosive primers. • FIRE PROTECTION COST: Because this aspect is greatly reduced resistance during fires. • BRITTE FRACTURE: May lose ductility under certain conditions causing brittle failure at stress concentration sites. Produce fatigue loads and low temperatures are exacerbating the situation. • Susceptibility buckling be slender and thin elements.