Bridge Design: Truss Systems - Coursework Example

Bridge Design—Truss Systems Introduction Truss is a fabricated structure, which has triangular units that serve as load bearing structure for the bridge design. Using materials in an efficient manner, truss bridges are economical and have the longest history. Accordingly, the many types of truss bridge like cantilever bridge, truss-arch and transporter bridge have come up over the period, using different types of truss designs. The truss design has applications from subjects like static and physics, as laws of motion determine the particular requirement according to the bridge design. The design assumes that the members of the truss will come into action when any tension or compression is applied to the structure. Depending on the buckling pressure and other static parameters, different units of the particular truss act under compression, while others do the same when the structure is under tension. The history of truss bridges started in United States using wood as the truss members due to abundant availability of the same in various regions. While timber wood members were used for taking the compression, the iron rods served as members for taking the tension. Started from 1820s, United States witnessed building of such truss bridges that included lettice truss bridge and other iron truss bridges. However, wrought iron replaced the timber in such trusses during 1870 to 1930. Thereafter the steel truss bridges that had greater resistance to rust started coming up, throughout America and other places in the world. While there are many types of truss systems available for the specific bridge design, this paper shall discuss Fink, Howe, Pratt and Warren types. Fink truss system This type of truss is used mostly in most of residential buildings. The boards and steel bars come together at the roof top end to form an intersection, thus providing the required support to the rafters and rigidity to the roof. A V-shaped web supports the interior of the structure, as the same is formed as a single bar with arms going upwards to meet at an intersection at the top. While the Fink trusses can form up to 80 feet, a single truss can be made up to the length of 33 feet. A double truss of this type can go up to 54 feet. This type of truss has found wide use in railroad bridges, as the design was preferred for building bridges in Baltimore and Ohio, Western and Norfolk railroad used such truss systems in their bridges. History In the year 1852, Albert Fink, a railroad engineer designed the Fink truss system for a bridge. Thereafter, West Virginias saw the building of a bridge with this type of truss over the Monongahela River. The bridge had largest span at the time in North America. Fink trusses have the advantage of meeting diverse design requirements, as they are able to match the shape and size parameters, due to the flexibility in the ratio of strength to weight, within these trusses. The image of a bridge with this type of truss system along with a sketch of Fink truss system structure is shown in the enclosed Appendix, as Fig. 1 and 2. Strength of Fink trusses These trusses allow a load of up to 20 Lbs per square feet, particularly for the residential buildings. However, the recommended board size for achieving the best size will be 2 by 6 feet, as it will take care of any lifting or sagging under severe whether conditions. Rooflines are best built with Fink type trusses, as these provide necessary structural support to them in an excellent manner. However, these types of trusses require fabrication and assembly at the site only. They cannot be carried to the site in a finished shape. (Fink Truss, 2015.) Howe Truss system William Howe designed this truss in the year 1840, using mostly the wood for truss members. Being suitable for longer span bridge construction, these types of trusses became more useful than Pratt truss systems with wide use of Howe systems in most of the railroad bridges during that period. Wood being available in plenty at many places in North-west USA, many such bridges were built in those regions using Howe truss systems. Forces distribution Fig 3 & 4 in the Appendix show the position of forces under different load conditions while using a Howe truss. While the load is being applied to the entire top portion of the bridge, the forces react in a different manner when the load is concentrated in the center. However, in both cases the total load is always ‘100’, allowing the calculations to assume the members being a percentage of total loads. The forces are greater on the internal members of the truss when load is concentrated, while they are comparatively lesser when the load is spread along the top line. The figure 5 in enclosed appendix gives the image of a Howe truss bridge built in 1954 at Calhoun County Alabama. Howe trusses use more of wood than Pratt trusses. Therefore, they were preferred earlier when iron, steel was expensive, and unavailable in many areas, where wood was locally available in abundance from forests, particularly in states like Pennsylvania. Howe Trusses used beams made from wood for diagonal units, as they remained in compression, while iron and steel was used for vertical members bearing tension. Diagonal members being longer, the system required more wood than iron. In certain cases, both tension and compression members can be made from wood when using the Howe truss system.(Boon, 2011) Pratt truss Caleb and Thomas Pratt designed this type of truss in the year 1944. These trusses were mostly used for building railway bridges as they could bear heavy load over the long span of bridges. Based on many variations of this truss design different bridges in Baltimore, Pennsylvania and Parker have been built over the period. Spread of forces Fig 6 and 7 in the attached appendix gives the details of forces as displayed in the Pratt truss when the load is centered and when it is spread. While Fig 6 shows the action of forces when the load is localized at a central place, the Fig 7 gives details about the same when the load is spread. Accordingly, the calculations for different members of the truss towards their load bearing capacity can be done keeping in view that total load for both the drawings is always equal to hundred. In Pratt design trusses, the two ends of the forces diagram remain same in both localized centered load and spread load conditions. While the bottom board does not change much in the two sketches, there are quite important changes in the layout of internal truss members, when considering the two different condition of display for the forces applying on the truss. In the diagram for centered load, it is evident that the amount of forces applied to the internal members of the bridge structure increase largely, while the top layer of such bridge has increased forces, in case of the centered localized loaded bridge. The display of forces and subsequently the loading of the members of truss has much significance to the performance of the bridge. It is better to have the load spread across the total span of the bridge for the purpose of safety and better performance. However, the circumstances and bridge design must allow for such type of truss design to be incorporated in the model bridge. Pratt truss is preferred due to its solid design and ease of construction. However, for longer spans of railroad bridges, some designers may go for Howe trusses. Fig 8 in the enclosed appendix shows the image of a real bridge with Pratt truss design. This image belongs to the “motel 6 Bridge” located in Hall County Nebraska. (Boon, 2011) Warren Trusses James Warren patented this type of truss bridge in the year 1844, which made the bridge construction with such design most popular across the world. It uses triangular design for the spread of forces along the truss members. Although Neville truss also uses triangular design, there is a marked difference between the two designs. While warren trusses use equilateral triangles for load spread on the bridge, Neville truss uses isosceles triangles for the same purpose. Accordingly, the forces minimize to compression and tension, with the Warren truss design, allowing the forces to switch from compression to tension. This can be experienced often when the moving load like that of car or train passes the bridge. The switching takes place particularly near the bridge center. Display of Forces Fig 9 and 10 gives the sketch of forces playing with different load conditions -localized on the center and spread out loads. In this design, all the forces are larger when middle of the bridge has total concentrated load, resulting in the top and bottom planes of the bridge to remain under the large huge forces. Hence, for the purpose of safety, forces must be calculated as spread out across the bridge top for carrying more weight. However, the Warren truss design often requires the bridge to have centralized and localized forces, instead of having the spread out design. The calculation of the forces and load must be done based on this fact for most of the truss members. Warren trusses can be made easily with lap joints and the layout of members comes in between the bottom and top cords of the bridge. The image of a Warren truss bridge is given in the Fig 11 in the enclosed appendix. (Boon, 2014) Work cited Boon, G, Warren Truss, (2014), garrettsbridges.com, retrieved from: http://www.garrettsbridges.com/design/warren-truss/ Boon, G, Pratt Truss, (2011), garrettsbridges.com, retrieved from: http://www.garrettsbridges.com/design/pratt-truss/ Boon, G, Howe Truss, (2011), garrettsbridges.com, retrieved from: http://www.garrettsbridges.com/design/howe-truss/ Fink Truss Design, (2015), Rooftrussdesign, retrieved from: http://rooftrussblog.com/fink-truss-design/ Read More