Cable Stayed Pedestrian Bridge Design - Coursework Example

Cable Stayed Pedestrian Bridge Design Introduction of Surrey is one of the leading universities of United Kingdom lying in the south east of the country. It is situated in the town of Guilford, Surrey. This university makes use of a pedestrian bridge for entering in the premises of the university. This bridge is of extreme significance as it joins university area to the rest of the town and helps in keeping good balance of socio-economic life in university. The already constructed bridge is more than 100 years old and in a very bad condition to carry heavy transportation to and from the university. This old bridge carries gas pipelines along with phone cables which are needed to be accommodated in new design as well. Also this bridge provides a secondary path over the railway lines, thus providing an easy and safe transportation of pedestrians across the railway lines. Figure 1: Map showing the construction site for the new bridge Purpose of building a footbridge in this area is to shorten the path for pedestrians, to approach university. Cable Stayed Bridge Idea of stayed bridge was first introduced in 16th century which was then furnished to produce an engineered cable stayed bridge after Second World War. This engineered bridge was constructed in Europe to provide pedestrians with the shortest path to their destination. Cable stayed bridges are considered to be like ordinary suspension bridge as their physical appearance resemble a lot. These bridges differ from suspension bridges in a way that the suspension bridges are having two towers but cable stayed bridges make use of only a single tower to hold all the load of deck and traffic moving on the deck. In cable stayed bridges, the single tower is responsible of holding all the compressional forces acting on the bridge and tension is distributed by making use of stays. Such bridges are having specialized orthotropic decks which are furnished with continuous girders and stays for support. The analysis of such bridges is carried out by making use of linear elastic analysis. This technique is used because of the triangulated force approach used in the construction of stayed bridge. In real time applications, bridge is subjected to a number of torsional and shear forces that are being applied through the environment like, air currents, deformation of structure, load distributions etc. but currently we are not concerned with these parameters and considering only the live and dead loads. A typical suspension bridge force vector is shown below: Figure 2: Force vector diagram The purpose of conducting this research is to come up with the best bridge design which is economically as well as socially feasible. Major concern is to construct a project which can be conveniently handled by the crew of county works while erecting the bridge and economically least expensive when the concern is to transport the raw material to the construction site. Therefore, it has been decided to take steel as the construction material for primary infrastructure, whereas, the deck will be of wooden plans with a width of 3 feet. The final structure will consist of a single tower with stays connected inn fan like shape to provide better support. Here is the model of the bridge which will be constructed to replace the old bridge connecting University of Surrey to the town. Figure 3: Auto Cad drawing of the proposed bridge design Length of cable stayed bridges may vary between 500 feet to3000 feet approximately and are better solution as compared to suspension bridges which are much expensive in terms of construction and material consumption. The construction of this type of bridge is supported by a specific pattern that is needed to be followed while construction. Types of Cables (stays) Stays play a vital role in keeping the bridge in position and are critically designed to meet the design criteria different types of stays can be used depending upon their strength and design demands. Figure 4: Typical stay strand Mostly used stays are seven wired twisted strands which make up a diameter of about 3-7mm. Different types of cables used are: Helically wound galvanized strand Strands of parallel wire cables Parallel wire strands Locked coil strand Among all these types, most reliable is the parallel wire strand with highest modulus of elasticity approximately equal to 1860MPa and young’s modulus, E=190,000MPa. Tower Types Figure 5: Types of towers Above is the view of different types of tower shapes most commonly used in the construction of pedestrian footbridges. In our design A shape tower is the recommended one. Deck Analysis Deck is the vital part of the bridge design. It is the part which is needed to be strong enough to carry the load of all traffic crossing the bridge. Therefore, corrugated composite metal sheet of 20 gage is used as a base of deck. Above it 4 inch thick concrete layer was set to give strength to the bridge to cope with rush of students and vibrations imparted by the railway track. Figure 6: Design of deck for footbridge Properties of deck material are listed below: normal weight concrete A36 steel: fc’ = 3600 psi fc’ = 36 ksi E = 3500 ksi E = 29,000 ksi Width of deck was taken to be approximately 1 foot, so the area of the deck was calculated to be 0.570 square inch per foot. It is being assumed that the whole bridge is of composite steel therefore, this area is calculated as a part of ideal rectangular beam. Below is the descriptive stress strain diagram f simple beam carrying the applied load (dead and live). Figure 7: Stress strain diagram The beam is considered as 1 foot wide to provide better strength to the bridge, and 3.2 inch deep. Now first step of analysis is to determine the depth of beam. Area of beam was determined to be: As = 0.570 in.2/ft. Thickness of steel: 0.0475 in. Therefore, Depth of beam was determined to be 0.559. After calculating the dimensions of beam we can calculate the stresses being applied on the beam per unit length: According to euro code 3, the load contributions for steel and concrete are listed below: Concrete loading 38.20 psf Steel loading 1.94 psf Total deck load 40.14psf Assumed live load 100psf Now applying the load factors to these values yield following loading results: The above calculated value of load factor was then used to calculate the ultimate moment of the beam as follows: This inequality determines that the suggested design of deck is stable enough to carry the applied live loads along with the dead loads of the deck. This strength of deck is sufficient enough to carry maximum live load on the bridge. Calculation for the deflection of the beam In order to calculate the deflection of the concrete beam, first we need to calculate the moment of inertia of the beam by converting all loads to equivalent steel’s moment: Figure 8: Transformation of moment of inertia Determining the centroid and area of cross section of beam: As As Moment of inertia of ideal beam can be calculated as A summary of the calculations is shown in Table 1-B: Table 1-B: Moment of Inertia of Beam Portion of Beam Area, in2. Centroid about y-axis, in. I, in.4 Transformed Concrete 4.58 1.64 1.18 Steel 0.570 0.0238 4.00  = 5.15 Ay/A = 1.46 in.  = 5.18 Moment of inertia of combined material is now determined as a single beam material of composite steel. We can now complete the analysis by determining the reaction forces of the applied loading on the beam. Deflection of beam can then be calculated as: Dead load: Live load: Total Load: The total force in the compression and tension zones of the beam is: Having known ‘1’, and having found ‘a’, the location of the neutral axis of the beam can be determined using the following equation: Finally, using the neutral axis, the strain of the reinforcing steel at the point of failure for the concrete can be calculated by the similar triangle method. This yields the following ratio: Read More