Design Analysis of Gatwick Bridge - Case Study Example

Topic: Design Analysis of Gatwick Bridge Gatwick Bridge Design Analysis Part 1. Specification and Maintenance The process of selecting materials was based on high quality and type, to achieve airport requirements for standardization and ensuring safety of aircrafts and passengers and create a distinctive environment. Despite the existence of curved planes and lines at the bridge, the materials used in the construction are planar and straight- initial components that are not only less expensive but also allow maintenance. 1.1. Construction Process In the construction process for bridges, a large number of contributions is required from individuals and organizations in a case where a 2700 tone bridge is involved with external glazing. Simple design processes were involved and it included design, building and moving and then erection but it was not possible without the use of heavy equipment. The building process involved consideration of operational safety and meticulous assessment of risks and back up equipment were put in place. 1.2. Realization of Design The design of the bridge accounted for circulation cores containing stairs, lifts and service routes as well as escalators that provide links to departures and arrival levels. Its working principle was simple and effective by inclusion of supporting beams and it is composed of a continuous frame fixed on piled foundations. The major structure is composed of a 198m long spine beam as well as additional Y-shaped supporting columns. The main component of the spine beam is a triangular plated box that is 2.5m below and above a triangular truss in the shape of inverted pyramids that can be seen by passengers. The curve of the bridge represents a real bending moment diagram with a total depth of 6m to 9.3m, resulting into a shape that emulates an actual bending moment diagram. Cladding of the interior was based on the structural curve. Two Y-shaped columns were placed across the center of the taxiway and space of 128m was created between them to allow future taxiway widening. The shape of the column ensures an increase in effectiveness of the structural span. Strength is provided by A-shaped columns to the deck in the direction of main loads such as winds. 1.3. Analysis of design Lateral stability was achieved by using a truss in the roof plane the edges are connected by a glazing reinforcement bars that complete the structural system while contributing to minimum interference of glazed façade. In the design of nodes where there is connection of the deck to the legs and at the junction Y, a 2D model that incorporates various plates was developed. Each leg is reinforced on six 9m long and 900mm diameter piles. 1.4. Dampers Dynamic analysis indicates that the bridge is able to accommodate a maximum number of 1200 passengers at the same time. Before completion of the bridge, there was lack of certainty with regards to dynamic characteristics of the bridge because additional dynamic stiffness was expected in the design of the sliding connections. As a result, the design of the bridge involved consideration of a number of dynamic properties for lateral modes that are likely to be affected by pedestrian excitation, within this range there was a significant variation in frequency and damping requirements at the modes. There was a need for use of a relatively large tuning for the damping system and the recommended capacity. The system is composed of ten sealed stainless steel tanks below the deck at the middle of the span. Damper effect is created by water in each of these steel tanks and the magnitude of damping is about 8 tones. Each tank has an internal divider plates that enables a constant active mass across a number of frequencies. The use of multiple tanks allows tuning to multiple modes. 1.5. The Building Process Mostly traditional techniques were used and the operation was done at 1.5km away from the airport in a yard specifically equipped with the right infrastructure. During construction of the bridge, the major components of the bridge include a 164m long central deck section. The Y-shaped columns and the last erected section that consisted of a 17m long deck section and were connected to the bridge with the use of cores. The goal was to ensure equal fitting of the five components when brought together when brought into their final position. Fit-up is affected by steel components, the quality of procedures or weather conditions and the method of handling fabricated structures. The main processes involved in construction of the bridge involved yard assembly with central segment being constructed first, segments were then added and connected and supporting stools added. This was followed by full assembly of the bridge including trial fit of back spans. The backspans were then removed and main span was lifted on two beams and weighed and stools were removed and assembly of pylon commenced. Lifting towers were then erected and test lift, bridges and towers were placed on transporters. This was followed by removal of lifting towers and placing them over foundations and transporters were removed. This was followed by lofting of the bridge and assembly of pylons, fixing of pylons to the decks and lowering of bridges and pylons into foundations. Lifting towers were then removed and allowed opening of the taxiway. Backspans were then brought into place and fixed into final position. A detailed survey was conducted on the control points on the deck and legs resulted into a better choice for the right geometry that allowed prediction of movements and final fit-up pieces below the deck were obtained to match the geometry. The deck was then stressed don into girders in the yard prior to commencement of glazing, to minimize motion caused by weights of the glazing, concrete deck or other additional loading that was included. Part 2 2.1. Sketch of a plan showing the general arrangement of structural elements. Figure 1. Plan of general arrangement of the structure The major structural elements of the bridge include the following: a. Struts and Ties Struts are sections of the bridge under compression while ties are parts of the bridge under tension. Struts were obtained by using materials that could withstand compressive materials. In the construction of the bridge, struts include the supporting pillars and top part of the floor of the bridge. Ties were made using cables but their stiffness were made as simple as tensile strengths of the materials. In the design process, maximum tensile and compressive forces of the materials were considered in determining the maximum forces that the bridge could withstand. b. Beams These are components that rest on two or more supports, providing the right stiffness that ensures retention of the shape. In the construction of the bridge, beams were placed on top and bottom parts of the bridge to protect the bridge from tensile and compressive forces. These were built using steel beams. Gatwick Air Bridge was constructed using I beams to ensure protection of the bridge from shear stresses. c. Cables Cables were also used during the construction process for the bridge. This is a tie that is under minimum tension but has a certain amount of flexibility and its length to relative thickness is usually high. Suspension cables were built on site from a number of thin strands that were bound and sealed while dampers were added to reduce the amount of oscillations. d. Towers These are structures that were used to hold suspension cables high as well as provision of clearance underneath. These are vertical struts but the forces acting on it may vary considerably from top to bottom as a result of weight. The other concerns in the use of towers are buckling and flexing during construction. The towers of Gatwick Air Bridge were constructed using steel structures. 2.2. Elevation drawing of general structural arrangement of structural elements Figure 2.Elevation of structural general arrangement Functions of major structural elements. a. Piers These are vertical structures that provide support to everything else. These were constructed mainly using concrete materials as well as steel beams to provide support to the bridge . The piers of Gatwick Air Bridge are tapered to pin joints or ball socket joints. Bearings were placed at the tops of the end piers to prevent lateral movements. The b. Decks These are cross beams that era placed at the joints of truss or between them. Cross beams usually have gaps between them that have to be filled. These are components that are used to connect ties and struts. In the case of ties, the attachments are usually bigger than the member itself. Part 3 Calculations i. Calculation of approximate loading for the structure Girder weight will be determined as follows: 1. Ag = b e a m x - s e c a r e a = 1. 0 8 5 i n Y = u n i t w e i g h t o f b e a m c o n G i r d e r w e i g t = 1 . 0 8 5/ 0. 1 5 = 1. 1 3 k/ Ft/ g i r d e r Weight of the Deck Slab will be obtained as follows 1. D e c k s l a b w e i g h t S l a b w i d t h = w i d t h + 1/2 g i r d = 3. 5 2 1 + 1/2 (9. 6 6 7) = 8. 3 5 Ft S l a b t h i c k n e s S = 8 in D e c k s l a b w e i g h t = 8. 3 5 (8/ 1 2)( 0. 1 5 ) = 0. 8 3 5 k/ Ft/ g i r d e r ii. Use of the loadings to determine the maximum forces in two of the most heavily loaded member The total weight of the deck slab will be obtained by use of the following equation If the bridge is 150 metres long, the total weight will be iii. Stresses in two suitable structural members Compressive stresses at any point in the beams of the bridge will be determined by use of the following eqation In case the weight of the bridge is 150kN and the total cross sectional areas of steel beams is 340,m2, the total compressive force is given by = 441.1764N/m2. Tensile stresses will be obtained in a similar manner to compressive stresses as follows: If the tensile force is 150kN and the total areas of the beams are 245m2, the total tensile force is obtained as follows: Tensile stress = = 612.2N/m2.. iv. Comment on the accuracy compared to original designers These analyses will be important in comparison to original designs because it helps in knowing the maximum tensile and compressive stresses that the bridge can withstand. The maximum load will be determined by including safety factor. Part 4. Changes that will be made in the alternative bridge include the following: I. Towers: The towers will be constructed using concrete towers instead of hollow steel beams. This will provide additional protection from compressive forces on the beam. II. Piers: Vertical piers will be used instead of slanting piers to provide additional resistance to winds and currents. III. Replacement of truss with concrete beams: This is the process where the alternative floor of the bridge will have concrete materials to form the main truss materials instead of steel materials. This will ensure the bridge is able to withstand compressive forces. When the considerations of the alternative design are implemented, the bridge will look as shown in the following elevation and plan. Plan and elevation of alternative design for the bridge Figure 3. Structural arrangements of alternative 3. References International Conference On Suspension, Cable Supported, And Cable Stayed Bridges, & Dayaratnam, P. 2000. International Conference on Suspension, Cable Supported, and Cable Stayed Bridges: November 19-21, 1999, Hyderabad. Hyderabad, Universities Press (India) Ltd. Jagadeesh, T. R., & Jayaram, M. A. 2004. Design of bridge structures. New Delhi, Prentice-Hall of India. Ghosn, M., Moses, F., & Wang, J. 2003. Design of highway bridges for extreme events. Washington, D.C., Transportation Research Board. http://trb.org/publications/nchrp/nchrp%5Frpt%5F489.pdf. Read More