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Bridge construction Methods of. A.Abbasi PhD 1389. Introduction. Making The Connection
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Bridge constructionMethods of A.Abbasi PhD 1389
Introduction • Making The Connection • Bridges of all types are vital links in infrastructure development. Beam, arch, suspension, truss, cable stayed, cantilever, truss-arch, and lattice truss, from pedestrian walkways to awe inspiring spans of spectacular gorges, each bridge has unique requirements and challenges during fabrication. The same bridge design may require completely different construction methods simply due to site restrictions and accessibility. One site may have unrestricted areas for stockpiling of materials, equipment staging, and traffic control detours while another site may have limited access and require unrestricted traffic flow during construction. The former may be able to use in place fabrication techniques while the latter would probably require prefabricated methods
Construction types • . So while there is no single construction method for bridge erection, there are several broad categories of fabrication techniques. These are: • Falsework or staging: temporary framing and scaffolding to support structural elements during construction. • Span-by-span: structural elements are fabricated in situ between supporting structures to create each span in a sequential process. • Full span erection: fully prefabricated spans are constructed off site, transported to the bridge construction project, and installed whole between supporting structures. • Balanced cantilever: segments are installed or fabricated in situ on opposing sides of a supporting pier until the span is complete.
Additionally, there may be more than one technique employed on a single project to achieve the most efficient construction process. • To facilitate these techniques at least one of two types of heavy equipment are usually present: • Crane assist: structural elements are placed and temporarily held in position by one or more movable lifting cranes located on the ground, barges, or on the bridge itself as construction progresses. • Gantry or launch girder: typically a horizontal steel framework supporting a track and carriage which hoists and transports structural elements along a bridge span, then travels horizontally to the next span.
Prestressed system • Stress Can Be A Good Thing • Another interesting implementation of modern bridge construction is accounting for pre and post completion loading. Since a bridge does not realize full structural strength until all the elements are connected, individual elements may be shifted out of position and/or pre-stressed prior to final connection. For example suspension and cable stayed bridges typically arc away from the true load line until the weight of the decking has been applied. For bridges using structural concrete a technique of post-tensioning is employed to ensure the structural element experiences primarily compressive loading. This is vital as concrete typically has poor tensile strength. Understanding these concepts is important not only during construction but also during bridge maintenance and inspection. A suspension bridge significantly deviating from its load line may indicate missing elements, or tensile microcracks in a concrete beam may indicate insufficient post-tensioning.
Innovation method • Easier Said Than Done • Of course all of these concepts are easy to describe, but challenging, costly, and time consuming to execute. With increasing demands for faster completion time, fewer construction injuries, lower costs, reduced inconvenience, to move in, move out and stay out, bridge construction engineers are constantly striving for the best possible construction solutions. Connecting the dots using the most efficient means available has resulted in extraordinary examples of bridge construction such as the Mackinaw Bridge, the Millau Viaduct, and the Beipanjiang River Railroad Bridge to name but a few. And with constant innovation in civil engineering there will be more, without a doubt.
Load balancing method • With the cantilevering method, the superstructure of bridges is usually built from one or more piers by means of formwork carriers. Normally the structure advances from a short stub on top of a pier symmetrically in segments of about 3 m to 5 m length to the mid span or to an abutment, respectively (load balancing method). • The prestressing tendons are arranged according to the moment diagram of a cantilever, with a high concentration above the pier. Towards the mid span or the abutment the number of tendons gradually decreases.The use of the cantilevering construction method, for medium and long span concrete bridges, is recommended especially where a scaffolding is difficult or impossible to erect as e.g., over deep valleys, wide rivers, traffic yards or in case of expensive foundation conditions for scaffolds.
Pre-stress • Pre-stressing is a construction methodology which is widely used for bridges and buildings today. The main purpose of pre-stressing is to induce desirable strains and stresses in structure and its components.
HorizonOne 22-storey tower and four 23-storey towers comprising a total of 616 residential
Unbonded wires • Bundles of 7mm dia. wires, dia. 0.6" strands or 5mm dia. carbon fibres are encased in a UV-resistant HDPE duct and provided with fatigue resistant anchorages at both ends. Depending on the stay system, the HDPE sheath is filled with a corrosion inhibiting compound or may remain empty.
Multi span bridge • The incremental launching method is particularly suited for the construction of continuous post-tensioned multi-span bridges. It consists of casting 15 m to 30 m long sections of the bridge superstructure in a stationary formwork behind an abutment to push a completed section forward with jacks along the bridge axis. • The sections are cast contiguously, one after another, and are then stressed together. The superstructure, growing section by section is launched over temporary sliding bearings on the piers until the bridge is completed.
Bridge moment • In order to keep the bending moment low in the superstructure during the extrusion phases, a launching nose made of steel is attached to the front of the bridge deck. The launching nose is dismantled after the superstructure has reached the opposite abutment. However, the spans should not exceed 60 m approx. and the bridge sections must be constant. Furthermore the superstructure of the bridge has to be continuous over the whole length and straight or have a constant curvature in plan and elevation
Lunching truss • Launching truss method has been developed for multi-span bridges over difficult terrain or water where scaffoldings are expensive or not feasible at all. • The launching girder itself is normally a steel structure with rather sophisticated equipment, moving forward on the bridge piers span by span. The method is highly adaptable for a wide range of spans and types of superstructure, and it can handle cast-in place concrete, as well as prefabricated elements. In fact, launching girders are most often used for placing prefab segments, match-cast and stressed together, or complete units spanning from pier to pier.
Heavy lifiting • Heavy lifting require full consideration at an early design stage of structural detailing, construction planning and selection of the related equipment. Such systems cover a wide range of applications ranging from lifting, lowering or shifting heavy structural elements to the installation of large pre-stressing tendons and stay cables.
Process of prestressed • Prestressedconcrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensilesteelcable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete. • Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.
How prestresses is done • Contents • [hide] • 1 Pre-tensioned concrete • 2 Bonded post-tensioned concrete • 3 Unbonded post-tensioned concrete • 4 Applications • 5 Design agencies and regulations • 6 See also • 7 References
Pre-tensioned concrete Stressed ribbon pedestrian bridge, Grants Pass, Oregon, USA
Pre-tensioned • Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction. However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line. Thus, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size. Pre-tensioned elements may be balcony elements, lintels, floor slabs, beams or foundation piles. An innovative bridge construction method using pre-stressing is described in Stressed ribbon bridge.
Bonded post-tensioned concrete • Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element. A set of tendons are fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulicjacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is then grouted to protect the tendons from corrosion. This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobeclay) create problems for the typical perimeter foundation.
Comparison of bond and unbonded methods • All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure. Post-tensioning is also used in the construction of various bridges, both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge.The advantages of this system over unbonded post-tensioning are: • Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents • Tendons can be easily 'weaved' allowing a more efficient design approach • Higher ultimate strength due to bond generated between the strand and concrete • No long term issues with maintaining the integrity of the anchor/dead end
Unbonded post-tensioned concrete • Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). The advantages of this system over bonded post-tensioning are: • The ability to individually adjust cables based on poor field conditions (For example: shifting a group of 4 cables around an opening by placing 2 to either side). • The procedure of post-stress grouting is eliminated. • The ability to de-stress the tendons before attempting repair work.
Prestress step by step • Picture number one (below) shows rolls of post-tensioning (PT) cables with the holding end anchors displayed. The holding end anchors are fastened to rebar placed above and below the cable and buried in the concrete locking that end. Pictures numbered two, three and four shows a series of black pulling end anchors from the rear along the floor edge form. Rebar is placed above and below the cable both in front and behind the face of the pulling end anchor. The above and below placement of the rebar can be seen in picture number three and the placement of the rebar in front and behind can be seen in picture number four. The blue cable seen in picture number four is electrical conduit. Picture number five shows the plastic sheathing stripped from the ends of the post-tensioning cables before placement through the pulling end anchors. Picture number six shows the post-tensioning cables in place for concrete pouring.
The plastic sheathing has been removed from the end of the cable and the cable has been pushed through the black pulling end anchor attached to the inside of the concrete floor side form. The greased cable can be seen protruding from the concrete floor side form. Pictures seven and eight show the post-tensioning cables protruding from the poured concrete floor. After the concrete floor has been poured and has set for about a week, the cable ends will be pulled with a hydraulic jack,
5. Post-tensioning cables stripped for placement in pulling anchors
6.Positioned post-end tensioning cables
7. Post-tensioning cable ends extending from freshly poured concrete
8. Post-tensioning cable ends extending from concrete slab
Applications • Prestressed concrete is the predominating material for floors in high-rise buildings and the entire containment vessels of nuclear reactors. • Unbonded post-tensioning tendons are commonly used in parking garages as barrier cable.[1] Also, due to its ability to be stressed and then de-stressed, it can be used to temporarily repair a damaged building by holding up a damaged wall or floor until permanent repairs can be made. • The advantages of prestressed concrete include crack control and lower construction costs; thinner slabs - especially important in high rise buildings in which floor thickness savings can translate into additional floors for the same (or lower) cost and fewer joints, since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced constructions with the same thickness. Increasing span lengths increases the usable unencumbered floorspace in buildings; diminishing the number of joints leads to lower maintenance costs over the design life of a building, since joints are the major focus of weakness in concrete buildings. • The first prestressed concrete bridge in North America was the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania. It was completed and opened to traffic in 1951.[2] • Prestressing can also be accomplished on circular concrete pipes used for water transmission. High tensile strength steel wire is helically-wrapped around the outside of the pipe under controlled tension and spacing which induces a circumferential compressive stress in the core concrete. This enables the pipe to handle high internal pressures and the effects of external earth and traffic loads.
Design agencies and regulations • In the United States, pre-stressed concrete design and construction is aided by organizations such as Post-Tensioning Institute (PTI) and Precast/Prestressed Concrete Institute (PCI). In Canada the Canadian Precast/prestressed concrete Institute assumes this role for both post-tensioned and pre-tensioned concrete structures. • Europe also has its own associations and institutes. It is important to regard that these organizations are not the authorities of building codes or standards, but rather are to promote the understanding and development of pre-stressed design, codes and best practices.
Types of Concrete Bridges • ARCH BRIDGE • Arch bridges derive their strength from the fact that vertical loads on the arch generate compressive forces in the arch ring, which is constructed of materials well able to withstand these forces. • The compressive forces in the arch ring result in inclined thrusts at the abutments, and it is essential that arch abutments are well founded or buttressed to resist the vertical and horizontal components of these thrusts. If the supports spread apart the arch falls down. The Romans knew all about this. • Traditionally, arch bridges were constructed of stone, brick or mass concrete since these materials are very strong in compression and the arch could be configured so that tensile stresses did not develop. • Modern concrete arch bridges utiliseprestressing or reinforcing to resist the tensile stresses which can develop in slender arch rings. • The shape attracted the attention of many of the early pioneers of concrete construction. In 1930, Freyssinet was responsible for a spectacular arched bridge at Plougastel in France and three years later, Swiss engineer, Robert Maillart created the famously elegant Schwandbach bridge in which slender cross-walls tie the arch to the horizontally curved roadway.
REINFORCED SLAB BRIDGE • For short spans, a solid reinforced concrete slab, generally cast in-situ rather than precast, is the simplest design. It is also cost-effective, since the flat, level soffit means that falsework and formwork are also simple. Reinforcement, too, is uncomplicated. With larger spans, the reinforced slab has to be thicker to carry the extra stresses under load. This extra weight of the slab itself then becomes a problem, which can be solved in one of two ways. The first is to use prestressing techniques and the second is to reduce the deadweight of the slab by including ‘voids’, often expanded polystyrene cylinders. Up to about 25m span, such voided slabs are more economical than prestressed slabs.
BEAM AND SLAB BRIDGES • Beam and slab bridges are probably the most common form of concrete bridge in the UK today, thanks to the success of standard precast prestressed concrete beams developed originally by the Prestressed Concrete Development Group (Cement & Concrete Association) supplemented later by alternative designs by others, culminating in the Y-beam introduced by the Prestressed Concrete Association in the late 1980s. • They have the virtue of simplicity, economy, wide availability of the standard sections, and speed of erection. • The precast beams are placed on the supporting piers or abutments, usually on rubber bearings which are maintenance free. An in-situ reinforced concrete deck slab is then cast on permanent shuttering which spans between the beams. • The precast beams can be joined together at the supports to form continuous beams which are structurally more efficient. However, this is not normally done because the costs involved are not justified by the increased efficiency. • Simply supported concrete beams and slab bridges are now giving way to integral bridges which offer the advantages of less cost and lower maintenance due to the elimination of expansion joints and bearings.
BOX GIRDER BRIDGE • For spans greater than around 45 metres, prestressed concrete box girders are the most common method of concrete bridge construction. The main spans are hollow and the shape of the ‘box’ will vary from bridge to bridge and along the span, being deeper in cross-section at the abutments and piers and shallower at midspan. • Techniques of construction vary according to the actual design and situation of the bridge, there being three main types: • i.e. incrementally launchedspan-by-spanbalanced cantilever • Incrementally launched • As the name suggests, the incrementally launched technique creates the bridge section by section, pushing the structure outwards from the abutment towards the pier. The practical limit on span for the technique is around 75m.
Methods of bridge construction • Span-by-span • The span-by-span method is used for multi-span viaducts, where the individual span can be up to 60m. • These bridges are usually constructed in-situ with the falsework moved forward span by span, but can be built of precast sections, put together as single spans and dropped into place, span by span. • Balanced cantilever • In the early 1950s, the German engineer Ulrich Finsterwalder developed a way of erecting prestressed concrete cantilevers segment by segment with each additional unit being prestressed to those already in position. This avoids the need for falsework and the system has since been developed. • Whether created in-situ or using precast segments, the balanced cantilever is one of the most dramatic ways of building a bridge. Work starts with the construction of the abutments and piers. Then, from each pier, the bridge is constructed in both directions simultaneously. In this way, each pier remains stable – hence ‘balanced’ – until finally the individual structural elements meet and are connected together. In every case, the segments are progressively tied back to the piers by means of prestressing tendons or bars threaded through each unit.
INTEGRAL BRIDGES • One of the difficulties in designing any structure is deciding where to put the joints. These are necessary to allow movement as the structure expands under the heat of the summer sun and contracts during the cold of winter. • Expansion joints in bridges are notoriously prone to leakage. Water laden with road salts can then reach the tops of the piers and the abutments, and this can result in corrosion of all reinforcement. The expansive effects of rust can split concrete apart. • In addition, expansion joints and bearings are an additional cost so more and more bridges are being built without either. Such structures, called ‘integral bridges’, can be constructed with all types of concrete deck. They are constructed with their decks connected directly to the supporting piers and abutments and with no provision in the form of bearings or expansion joints for thermal movement. Thermal movement of the deck is accommodated by flexure of the supporting piers and horizontal movements of the abutments, with elastic compression of the surrounding soil. • Already used for lengths up to 60m, the integral bridge is becoming increasingly popular as engineers and designers find other ways of dealing with thermal movement
CABLE-STAYED BRIDGES • For really large spans, one solution is the cable-stayed bridge. As typified by the Dee Crossing where all elements are concrete, the design consists of supporting towers carrying cables which support the bridge from both sides of the tower. • Most cable-stayed bridges are built using a form of cantilever construction which can be either in-situ or precast.
SUSPENSION BRIDGES • Concrete plays an important part in the construction of a suspension bridge. There will be massive foundations, usually embedded in the ground, that support the weight and cable anchorages. There will also be the abutments, again probably in mass concrete, providing the vital strength and ability to resist the enormous forces, and in addition, the slender superstructures carrying the upper ends of the supporting cables are also generally made from reinforced concrete.
