Materials Design Project - Case Study Example

MATERIALS DESIGN PROJECT Name: Instructor: Institution: City: Date: Table of Contents Introduction 3 Steel Arches and the Bottom Chord 3 The Arches 4 The Bottom Chord 6 The Vertical Hangers 7 The Steel Supports and Type of Bearings 8 Corrosion Prevention 8 Materials Specification 9 Bibliography 9 Introduction The given project entails a 100m span steel arch bridge construction whose substructure is made of reinforced concrete abutments. The bridge design is expected to provide for a 6-lane carriage way with a width of 24m. The bridge is also supposed to have a reinforced concrete abutment on either bank of the river, 28 meters in length and 24 meters in width. In the same design, the steel arch bridge consists of a large angle alongside flat and box sections whose principal function is to fabricate all structural components of the bridge. The bridge is subsequently expected to support a high loading impact and hence must consist of rail connections. High strength bolts will be used in this case to anchor the bolts and the rail connections. Having previously produced a report on the requirements of concrete for the abutments in the 100m span steel arch bridge project, the current report emphasizes on the detailed analysis of the steel arches, vertical suspensions, pinned and roller supports. In addition, the report also produces a steel specification on the said project and the techniques for preventing the corrosion of the steel. Steel Arches and the Bottom Chord Considering the fact that this steel arch bridge design should provide for a 6-lane carriage way, it is prudent that the type of steel used is not only strong but also of considerably larger cross-section. The arch, in particular, should have a cross-section that is at least twice that of the vertical suspensions. The choice of the correct cross-section will largely depend on the type and grade of the steel material used. The shape (e.g. square or round) of the steel material used will also determine the appropriate cross-section for the arch and the bottom chord to be used in the design of the specified steel arch bridge (Barker & Puckett, 2013). The suitable type of steel used for structural projects such as the steel arch bridge construction is the High Strength Low Alloy Steels (HSLA) as specified by the Australian Standard AS/NZS 3679.1 standards in the Structural Steel – Hot Rolled Bars and Sections (Jarmai & Farkas, 2013). Unlike the conventional black mild Steel whose minimum specifications are much lower, the High Strength Low Alloy Steel (HSLA) has even more superior strength qualities hence suitable for this kind of project. The HSLA Steel to be used is available in three main different shapes i.e. rounds, squares, and flats (Beohar, 2005). The Arches The flat steel may not necessarily be suitable for the arches of this kind of structural project especially considering that the steel arch bridge will be expected to withstand heavy loading due to its 6-carriage way design. In addition, flat steel is not as strong as its rounds and square counterparts whose cross-sections are much higher than that of the flat steel. While both the round and the square High Strength Low Alloy (HSLA) Steels may be used for the design of the steel arch bridge in question, the round steel would be more suitable due to its considerably lower mass per cross-section size of the steel material used. According to Beohar (2005), for the same strength, the round HSLA Steel used usually has a smaller mass than its square counterparts. This reasoning is informed by the fact that the weight to be supported by this steel arch bridge is expected to be heavy hence the need for a strong but lighter design. The lighter the steel material used, the lighter the overall design and hence the lower the overall weight on the bridge upon completion (Jarmai & Farkas, 2013). Having narrowed the choice of the steel material and specifications to be used for this steel arch bridge, it is now important to consider the suitable cross-sectional dimensions for the steel arch and the bottom chord. The round High Strength Low Alloy (HSLA) Steel material is usually available in diameters ranging from 10mm all the way to 90mm depending on the intended design and nature of the structural project at hand (Fu, 2013). Each of these diameter specifications has its own mass per meter approximation. A standard 10mm-diameter HSLA Steel, for instance, has a mass of 0.616kg/m whereas a standard 90mm-diameter HSLA Steel has a mass of 50kg/m (Fu, 2013; Barker & Puckett, 2013). For 100m span steel arch bridge, therefore, the approximate mass for the 10mm diameter and 90mm diameter would be calculated as follows: The above calculations only show the masses for the steel arches only; the masses for the vertical hangers and the bottom chords as well as the steel supports are not included yet. In order to avoid overloading by the steel arch bridge itself, therefore, it would be advisable to settle for a cross-sectional dimension of steel of a diameter of 45mm. According to Fu (2013), the mass per meter for a 45mm-diameter High Strength Low Alloy (HSLA) Steel is 12.5kg. With this cross-section, the calculations for the mass of the arch would be given by: For such a bridge, having steel arches with a total mass of 2.5 tonnes will not be as heavily loaded, but still strong enough to support the expected weight from the 6 carriage loads. The Bottom Chord The considerations to be taken into account in the choice of the correct specification of the steel to be used include the extent of the strength of the concrete abutments. While the grade of steel to be used i.e. High Strength Low Alloy (HSLA) Steel remains the same as that for the arches, there will be some slight changes in the shape of the cross-section and the cross-section itself. Important to mention is that HSLA Steel remains the material of choice for this entire steel arch bridge project. Having used concrete whose characteristic strength is 40Mpa and a 60 degree Celsius maximum internal temperature alongside a 100mm mix slump for the abutments, it is assumed the bridge has enough support. For that reason, the steel used for the bottom chord should have a cross-section that is smaller than that of the steel arches. Additionally, it should not be necessarily as thick. It follows, therefore, that a flat shaped High Strength Low Alloy (HSLA) Steel would be suitable and economical as well without compromising the strength of the bridge. For this particular design specification, a cross-section of 25mm diameter would be suitable as well. With a mass of 0.83kg/m for the HSLA flat steel (Parke, Nigel & Institution of Civil Engineers (Great Britain), 2008), the approximate mass of the bottom chords would be calculated as follows: This mass is in sync with the specified bridge in comparison with the round steel and square steel especially for the bottom chord where support has already been provided through the stable substructures. The Vertical Hangers As was mentioned earlier in this report, the type of steel to be used all through this steel arch bridge construction project will be the High Strength Low Alloy (HSLA) S335 Steel. This follows, thus, that the suitable steel for the vertical hangers will also be the same HSLA S335 Steel as the one used for the arches and bottom chords. As in any steel arch bridge construction, the size of the steel material used for the arches will be slightly larger than that of the steel used for the vertical suspensions. In this case, the most suitable cross-section for the vertical hangers will be 35mm in diameter with a square shape unlike the ones used for the arches and the bottom chords. Denny (2010) posits that a square HSLA S335 Steel with a diameter of 35mm has a mass of 9.4kg/m. This implies a total approximate mass of: In terms of the yield strength, the suitable steel grade for this project would be the S335 whose thickness ranges between 40mm and 63mm. This implies a minimum yield strength of 335N/m2. According to Parke, Nigel & Institution of Civil Engineers (Great Britain) (2008), S335 is usually suitable with bridge construction projects because of two main reasons. First, it is readily available and hence its supply is somewhat guaranteed. Secondly, its yield strength and fatigue are perfectly balanced unlike that for the black mild steel and cast iron whose results show an imbalance between these two parameters. The degree to which a material can strain to fatigue i.e. the ductility for the S335 steel, the black mild steel, and the cast iron all increase with a reduction in the yield strength. The Steel Supports and Type of Bearings The two steel supports will be made of High Strength Low Alloy Steels (HSLA) since this will be the primary material of the main arch. Within its structure, HSLA steels has been found to contain a low percentage of micro alloying elements (below 0.15% in total) and it is specifically designed to offer improved mechanical properties. Typically, High Strength Low Alloy Steels (HSLA), contain 0.07% to 0.12% carbon, up to 2%% manganese and small additions of niobium, vanadium and titanium in various combinations. HSLA – V has numerous industrial applications in structural engineering, especially in the construction of bridges. Nevertheless, a universal testing machine will be sued to test the High Strength Low Alloy Steels (HSLA) before they are finally used up in the project. The bearings in any structural design project have the role of ensuring that forces are effectively balanced between the superstructure and the substructure with minimal or no relative movement at all. For steel arch bridges, it is usually recommended that the number of bearings used in the design should be minimized. This is attributed to the fact that the more the number of bearings, the bigger the responsibility and cost on maintenance (Denny, 2010). While there are a number of types of bearings that may be used, the current steel arch bridge would do well if the pot bearings were used. This is because for this type of bearings, there is negligible or no relative movement in the x, y, and z directions which is how the steel arch bridge in question is designed to behave. Additionally, the pot bearings produces a deforming relative movement about the x, y, and z directions hence a perfect fit for the current project. Corrosion Prevention While it may effectively destroy the physical appearance of the steel arch bridge, corrosion may not necessarily interfere with the structural posture of the bridge. It is prudent, however, to prevent it from eating into the bridge in the first place. The fact that the current steel arch bridge is located near the coastal region it is exposed to a slot of salt which accelerates corrosion. To prevent corrosion, therefore, paint coatings will be applied on the steel used to form a duplex coating system. Primer will first be applied, and then the intermediate coat (usually done at the vendor premises), and eventually the final coat (usually done at the location of the project). Materials Specification As earlier on stated in section three of this paper, High Strength Low Alloy Steels (HSLA), HSLA –V, will be considered as the primary materials of the main arch. Within its structure, HSLA steels has been found to contain a low percentage of micro alloying elements (below 0.15% in total) and it is specifically designed to offer improved mechanical properties. Typically, High Strength Low Alloy Steels (HSLA), contain 0.07% to 0.12% carbon, up to 2%% manganese and small additions of niobium, vanadium and titanium in various combinations. (Frangopol, 1999). HSLA – V has numerous industrial applications in structural engineering, especially in the construction of bridges. Nevertheless, a universal testing machine will be sued to test the High Strength Low Alloy Steels (HSLA) before they are finally used up in the project. The semiautomatic welding machine will be used to join the various sections of the bridge. In some instances where welding may not quite be convenient, bolted splices will be used to make the joints. Bibliography Barker, R. M., & Puckett, J. A. (2013). Design of highway bridges: An LRFD approach. Beohar, R. R. (2005). Basic civil engineering. New Delhi, India: Laxmi Publications. Denny, M. (2010). Super structures: The science of bridges, buildings, dams, and other feats of engineering. Baltimore, Md: Johns Hopkins University Press. Fu, G. (2013). Bridge Design and Evaluation: LRFD and LRFR. Jármai, K., & Farkas, J. (2013). Design, fabrication and economy of metal structures: International Conference Proceedings 2013, Miskolc, Hungary, April 24-26, 2013. Berlin: Springer. Parke, G., Nigel, H., & Institution of Civil Engineers (Great Britain). (2008). ICE manual of bridge engineering. London: Thomas Telford. Read More