ENGM052 – LONG SPAN BRIDGES – JESUS RODRIGUEZ RODRIGUEZ UN6235031 February 25, 2014 ENGM052 – LONG SPAN BRIDGES – COURSEWORK 1 1) Main elements of a typical Suspension Bridge The load path of suspension cables serves to understand the main elements of the bridge: load is transferred by the deck to the hangers, which take the load up to the main cables, which are held up by the towers and finally tied into the anchorages. Load Path and Main elements (Source: Lecture1 notes) ANCHORAGES Anchorages are responsible for resisting the tension from the main cables, i.e., they tie the cables into the ground. There are different ways for resisting this force, but in its simplest form anchorages are large concrete structures that work by gravity action. In effect, the concrete mass weight resists the vertical component of the cable force, while the horizontal component is resisted by friction with the ground or passive resistance of soil in front of the anchorage. An alternative for this, where good rock foundation is present, is to anchor the cables into a frame positioned in a chamber built inside the rock mass. Another possible solution, if deck is stiff enough, is to anchor the cables directly into the deck, in a self-anchored type structure. Gravity Anchorage Mechanism (Source: own elaboration) 1 ENGM052 – LONG SPAN BRIDGES – JESUS RODRIGUEZ RODRIGUEZ UN6235031 February 25, 2014 TOWERS The towers are vertical elements that support the cables, i.e., they hold up the cables acting as a prop taking up the compression forces generated. Therefore, cables pass over the top of the towers along cable saddles. Usually it is composed of two vertical legs braced by means of crossbeams to prevent buckling. The final component is the foundation of the tower at the base, and usually when built in water conditions large caissons are provided. Another secondary function of the towers is aesthetically, as they govern the visual impact of the bridge. They can be built in different shapes and materials (A-type, H-type, etc.). Tower Elements (Source: own elaboration) MAIN CABLES They are the main load carrying element of the bridge. In modern bridges they are made with high strength steel wires, either in ropes (for smaller spans) or parallel wire strands. They are composed of a number of individual groups of wires, which are compacted, wrapped and coated into a large one cable (diameters around 0.6-1m). Cable installation techniques include aerial spinning of individual wires through a wheel, and pulling of prefabricated strands into place. HANGERS Hangers are the elements that suspend the deck, taking the load up from the deck to the main cables. They are tied to the main cables by means of steel bands or grips and also pinned to the deck. They are installed in pairs, either vertical or inclined. The former is more common and simple and the latter can improve the stiffness of the system and the response against wind loading. 2 ENGM052 – LONG SPAN BRIDGES – JESUS RODRIGUEZ RODRIGUEZ UN6235031 February 25, 2014 Vertical and Inclined Hangers (Source: own elaboration) DECK The suspended deck function is three-fold:    Acts as a stiffening beam for the whole bridge system, providing torsional and bending rigidity. Serves as the carriageway for traffic, transferring the load transversely to the hangers. Withstands longitudinal bending between hangers. Usually built in steel to minimize weight, the three more common types of decks are: truss deck, beam-slab, or box girder. Of the three, box girder gives the better performance as it has a high strength/weight ratio with great torsional rigidity and a streamlined form more suitable to aerodynamic stability. Deck types (Source: own elaboration) 2) Main elements of a typical Cable Stayed Bridge The main difference of cable stayed bridges with respect to suspension bridges is the elimination of the anchorages, as the cables carrying the loads generate a large horizontal load that is resisted by using the deck as a strut. The stay cables are tied to the pylon, which withstands large compression forces and bending moments. Transfer load mechanism (Source: own elaboration) 3 ENGM052 – LONG SPAN BRIDGES – JESUS RODRIGUEZ RODRIGUEZ UN6235031 February 25, 2014 PYLONS The pylons anchor the stays, so large compression forces are generated on them. Also, due to non symmetrical loading and transverse loading, pylons are subject to large bending forces. In addition to anchor the stays, the pylons provide the overall stability of the bridge. Three main types of pylon arrangement:    I-shape: single leg arrangement, only suitable if one plane of stays and box girder deck is provided for torsional stability A-frame: used with either single or double plane of stays, as it provides good torsional and transverse stiffness H-frame: only suited for twin plane of stays. Different Pylon Arrangements (Source: own elaboration) STAY CABLES The stays are tension members that support the deck and hang it from the pylons. They are anchored both at the pylon and the deck. Stay cables can be formed by parallel wires, spiral wires or locked coils, where all of them are made up of individual galvanized wires. The stay layout can be categorized as follows:    Single stays: requires stiff heavy decks Widely spaced stays Multi-stays: uses thin light decks and is the most common type. It can be subdivided into the following stay arrangements: o Fan: cable forces concentrated at pylon top. The most efficient but it is a problem for the congestion of anchorages o Harp: less congested but less efficient o Semi-fan: a combination of the two above 4 ENGM052 – LONG SPAN BRIDGES – JESUS RODRIGUEZ RODRIGUEZ UN6235031 February 25, 2014 Stays layout (Source: Lecture2 notes) All of the stays arrangements described above can be provided in a single plane of stays or in a twin stays arrangement, depending on both the pylon and deck type chosen. DECK The deck provides the longitudinal stiffness and carries the load to the stays. The deck has to withstand large horizontal forces, and it is usually in tension at midspan where the stays tend to pull away, while in compression at the pylons. In addition to act as a longitudinal beam, it distributes stay forces into deck members. As mentioned for suspension bridges, typical deck configurations include: box girder, truss deck, edge beams; either in concrete, steel or composite. One relevant aspect of deck behavior is articulation at pylon, which includes: unsupported (freely hanging), supported off bearings, and built into the pylon as a crossbeam. Deck articulation (Source: own elaboration) 3) Factors affecting the conceptual design of the principal elements of Cable Stayed and Suspension Bridges Following, some of the most relevant aspects of design are described. 5 ENGM052 – LONG SPAN BRIDGES – JESUS RODRIGUEZ RODRIGUEZ UN6235031 February 25, 2014 SPAN: the span is chosen to suit the location, with the objective of minimize disruption or obstruction below the bridge. Usually this is the case for long deep valleys or a main navigation channel. The navigation clearance needed both horizontally and vertically will govern the span chosen. TRAFFIC: traffic requirements will determine the total width needed to allocate the lanes for the expected daily traffic. Plus, if rail traffic is present dynamic load effects will be important, and traffic could be organized into a two-tier arrangement. ENVIRONMENT: harsh climates can affect the possibilities for bridge construction methods in a number of different ways. Plus, in very corrosive or aggressive environments the choice of materials will strongly impact its durability. GROUND: ground conditions are important to determine the location and type of foundations and anchorages. Plus, in deep water with poor foundation material the use of caissons may be enforced. AESTHETICS: the design of this kind of bridges is strongly influenced by aesthetics reasons and architects play a leading role in the choice of different layouts and configurations. The choice of pylons and towers shape, along with cables and stays configuration, will have an enormous effect on the visual perception of these bridges, as well as landscape integration of the whole. CONSTRUCTION: the assessment of the most suitable construction method for each situation will determine the most advisable solution for the elements of the bridge. For example, if prefabrication of towers is deemed to be the most cost effective solution, steel box units preassembled and delivered to site will be the most suitable option for the tower legs. Moreover, the position of the towers will be provided considering ease of construction, and will be kept out of water if possible. ECONOMY: as in all civil engineering structures, economy will dictate the possibilities for bridge design. However, if money is not of great concern, sate-of-the-art and innovative structures could be provided, although not always the most structurally efficient solution (e.g. Alamillo Bridge, Spain). LANDMARK: more often than not, the choice of this kind of bridges and its configuration is governed by the achievement of a reference point for the location of the bridge. Both suspension and cable-stayed bridges are usually landmark structures that help to promote and develop the possibilities of the location where they exist. COMPATIBILITY OF DESIGN: when considering cable-stayed bridges, the choice of different stays, pylons and deck configuration will need to be compatible with each other. For example, H-shape pylons cannot exist with single plane of stays, and single plane of stays cannot be specified if truss deck bridge is the option preferred. 6