Structural Engineering Principles Coursework Example | Topics and Well Written Essays

Structures and designs Name Institution Instructor Subject Date Table of Contents Table of Contents 2 1 Structural engineering principle 3 2 What is second moment of area? 8 2.1 Bending moment 9 2.2 Shear force 10 3 Compressive and tensile force diagrams for truss systems 11 4 Differences between Limit State Design and Permissible Stress Design 13 5 What types of loads need to be considered and why are load factors used? Give common examples of load combinations used for design. 14 5.1Load combinations used for design. 15 6 Why are material factors used and why do they vary for different materials 16 7 Comparison of Steel framed, concrete framed and timber framed construction. 16 7.1Concrete framed construction vs. Timber Frame construction 18 7.2Concrete framed construction vs. Steel framed construction 18 7.3Timber Frame construction v/s steel framed construction 19 1 Structural engineering principle The basic principles in structural design are of the same significance and any compromise may lead to structural hazards. Of major importance, are the principles relevant to human safety and mostly those concerned with lateral bracing. Some of these principles include; Bending Moment This is a very crucial principle for strong design of a given structure. It is the resultant turning moment that acts on a beam as a result of all the forces that acts on one side of a particular point. As noted by Mendes and Bruno (2009), moment is basically the principle of the construction lever. Moment can be considered as a combination of distance and force. Moment= force × distance Bridge truss and box beam take advantage of this by placing strongest material on one side, outside the edges far from the central axis. Equilibrium This is a vital aspect in ensuring that bodies stay stationary at a given point. Absence of equilibrium can cause object supposed to stay in one place to dislodge into space. The force acting on the structure must be the same and equal at the same time as noted by Allen and Joseph (2009).. Equilibrium must resist small extraneous disturbances and influences in order to ensure that the whole structure is Stable. External Forces Equilibrium A hanging object remains in that position because the force of gravity pulling it downwards equals the upward tensile force provided by the hanging rod. Figure 1Internal Forces Equilibrium In a structure, each part needs equilibrium for the internal force that acts on it. Lack of this, the structure cannot stay in one place. In the figure above the hanging rod is at equilibrium under external force W. However, every part in the hanging rod must have internal equilibrium of force W that acts at each end of the rod. Conditions of Equilibrium Figure 2 conditions for equilibrium The figure above shows forces which are equal and opposite while the moment must also be equal and opposite in direction. By considering moment about a certain point which is on the left side, moment about a point can be found. Triangle of Forces In triangulated structures like roofs trusses, forces follow a certain direction and are influenced by angles, which are between them. Figure 3 Triangle of forces Figure 4 Response of materials when they are subjected to forces A body responds differently when subjected to force. Force in equilibrium will remain stationery but it may get deformed. Deformation of the body may not be visible to an eye, but are accompanied by different internal forces (Mendes and Bruno, 2009). Relationship between internal forces and deformation depends on applied force, for example compression or tension. Behaviour under Tension Strain and stress Stress This is force divided by the cross sectional area. Where P= the acting perpendicular force A= area where the pressure is acting Where = change in distance = original length Young modulus is the relationship between stress and strain where = stress =strain = Young modulus Behaviour under Bending If a given body undergoes a tensile force it elongates and if subjected to compression it shortens. Behaviour under Shear Shear force acts vertically when the bench is at a right angle. This forces cause stresses, which are parallel to the beam. Response of materials to environment Material expands when subjected to temperature change, while the porous material swell and shrink with varying moisture content. These two factors depend on the environment. Effect of Restraints E = elastic modulus of a material A= cross sectional area L= length E= cast iron, units 10 000 kgf/mm2 Stability and Robustness This principle states that a building must be safe and serviceable. Stress should not exceed certain levels deformation and should be less than what can be tolerated. 2 What is second moment of area? The second moment of area is a cross section property used in predicting beam resistance to deflection and bending around an axis on a cross sectional plane called area moment of inertia (Ambrose and Harry, 2002). Beam deflection under load depends on the load and the geometry in the cross section of the beam. It’s obtained by summing up the product of the elemental areas and the square of the length. This means that Where I= second moment of area or the moment of inertia the elemental perpendicular distance distance dA= the elemental area 2.1 Bending moment This is the resultant turning moment that acts on a beam as a result of all the forces that acts on one side of a particular point a stipulated by Allen and Joseph (2009). Bending moment on the end side is always equal to zero. Calculation of the bending moment is broken into two. Figure 5 diagram showing distribution of forces R1 is reaction at the left end of the beam while Rr is the reaction at the right end of the structure beam From x=-0 and X=L/2 From x=L/2 and X=L Bending moment in the formula at any point can be found using below formula Bending moment’s diagrams entails plotting bending moment against various point position along the beam. The generalized diagram for bending moment for the beam is as shown. Figure 6 bending moment diagram 2.2 Shear force This is a force on the beam that tends to transverse to the given beam that would make it to shear across a given section. Shear force on the beam end equals vertical forces that support the reaction (Mendes and Bruno, 2009). Shear force f(x) at point x can be calculated by the equation. X= distance from the beam left end. The shear force diagram plots shear force against point position along the beam. Figure 7 Shear force diagram. 3 Compressive and tensile force diagrams for truss systems A truss is a structure that is built with bars pin connected at the end forming stable framework. Mostly truss are composed of different triangular elements with bars at upper chord which is under tension force (Mendes and Bruno, 2009). Trusses are mostly used in bridges, electric power, span roof and space structure. Truss analysis is used to determine tension and compression force. Figure 8 distributions of forces in a truss Figure 9 Calculation for tensile and compressive forces = angle between F1,2 and F1,3 The equilibrium in y and x directions would mean that F1,2. cos F1,3=0 and F1,2.sin +(5/2).W=0 tan and . Therefore, sin= and Cos =1/2 Hence F1,2= -(5/.W whereas F1,3= (5/2 / ).W . As noted by Mendes and Bruno (2009), the negative signs is an indication that forces along 1,2 are in compression while the positive sign indicates that forces along 1, 3 are in tension. When the structure experiences push force, the bar is in compression designated as negative sign (-ve). By using an equilibrium equation ∑ Fy = 0, and ∑ Fx = 0 to solve unknown forces. All forces are joined together when forces are in tension that is, (+ve) part. Two unknown’s values can be determined by the two equations 4 Differences between Limit State Design and Permissible Stress Design Limit state design (LSD) is method of design that is mostly used in structural engineering. This is a structural condition beyond which the relevant design criterion is not fulfilled as noted by Ambrose and Harry (2002). LSD aims at sustaining all actions that are likely to occur in the process of designing a structure. On the other hand, permissible stress design is a philosophy design that is used by civil engineers. Designers ensure that structure stresses due to service load do not exceed elastic limit. The limit is determined by making sure that structures stresses remains within the required limit using factor of safety. Many engineers today use limit state design in structural engineering work. 5 What types of loads need to be considered and why are load factors used? Give common examples of load combinations used for design. Loads types Dead Load, D This includes weight of items attached to a structure and are mostly meant to remain as built location all the life of a structure. Columns, beams, floor slabs, roof, exterior walls, and mechanical equipments are all considered as dead loads. Computation of dead loads can be done accurately with a high degree of accuracy and confidence. Live Load, L This load comprises anything that can be moved inside or outside the structure. This includes furniture, people, equipment and other items that are similar. Roof Live Load, L This load is associated with different loads that roof structure will use during the construction and during maintenance. Roof live load has short duration and is much smaller than the normal live loads. Snow Load, S These loads mainly occur in cold climates and have varying duration. Magnitude of snow loads depend mostly on terrain, local weather patterns and latitude. Rain and Ice, R Predictability of these loads is just like snow loads. Wind Load, W This load is very dynamic and used where static approximations are used. Earthquake/Seismic Load, E Dynamic events are the major causes of earthquakes. Static equivalent methods can be used in estimating forces on a structure. 5.1Load combinations used for design. Many structures use the load listed above during their lifetime. There is a challenge on how these loads can be combined reasonably. Direct combination of these loads is not likely, that is, one does not expect full live load occurring at the same time in the presence of snow load in the event of design of wind storm. Choice of this is based on design philosophy being used.. Example of load combination includes Resistance and Load Factor Design Allowable Strength Design 6 Why are material factors used and why do they vary for different materials Material factors are used for technical appraisal, economic factors and technical competence. Material factors are usually chosen based on different considerations; this includes material factors that control probability of material failure. Material factors vary between material resistance and for some other properties like weld-ability and strength. In specifying material factors the following should be considered requirements Selection and evaluation of the candidate material Cost of the material shear strength 7 Comparison of Steel framed, concrete framed and timber framed construction. Many structures are either framed with steel, concrete or timber. Each of this has advantages and disadvantages and choice of material depend on the project nature. All this materials are strong and simple to use Figure10 steel framed construction (Construction field, 2011) Figure 11Concrete framed construction (Moore and Mehrain, 1990) Figure 12 Timber framed construction (Construction works, 2011) 7.1Concrete framed construction vs. Timber Frame construction Costs In terms of cost, the timber framed construction is relatively cheaper and does not require much expertise. Concrete and steel are very expensive and require much skill. Form It is possible to mould concrete into different shapes. This allows intricately textured surface and smooth curves which is too tedious if timber is being used. Durability Concrete is a stable constructing material. It can resist compression and shear forces (Ambrose and Harry, 2002). Timber framed structures are flexible and resistant to earthquakes. Vulnerabilities Timber-framed structures are very vulnerable to wood-boring infestation. Concrete are impermeable but can sustain damages if they swells because of moisture. 7.2Concrete framed construction vs. Steel framed construction Thermal activity Both steel and concrete can sustain high temperatures. In addition, steel is prone to melt when exposed to high temperatures Safety Concrete is safer in case of terrorist or fire attack. Concrete is resistant to explosion. Moreover, Steel ductility and strength together with engineering design make it safe in seismic zones Cost The cost of concrete is steady despite, substantial increase in building material prices. On the other hand, the price of steel fluctuates depending on its availability. Material availability Cases of winter may influence the availability of cement making the shortage of cement. Weight Steel frame construction is light as compared to concrete frame at the same time offering good amount of strength. 7.3Timber Frame construction v/s steel framed construction Timber structures are strong and are able to withstand extreme temperatures. Timber continues to be a common mode of framing in different parts of the world. Timber comes from environmental sources and thus, often a green material than steel. Steel frames are used mostly in building apartments as it is faster to assemble than timber. As noted in Construction field (2011), steel framed construction is non combustible as compared to the timber based construction. References Allen, E. and Joseph I., 2009. Fundamentals of building construction: Materials and Methods. Hoboken, NJ: Wiley. Ambrose, E. and Harry, P., 2002. Simplified mechanics & strength of materials. New York, NY: Wiley. Construction field, 2011. Steel frame construction. Available at: http://constructionfield.net/steel-frame-construction-process/. [Accessed on 7th march 2012]. Construction works, 2011. Construction works. Available at: http://constructionworkss.com/2011/12/29/timber-frame-construction/. [Accessed 7th march 2012]. Mendes, G. and Bruno, L., 2009. Strength of materials. New York: Nova Science. Moore, D and Mehrain, M., 1990. National geophysical data centre. Available at: http://www.ngdc.noaa.gov/hazardimages/picture/show/294. [Accessed on 7th March 2012] Van, E., 2000. Construction materials for civil engineering. Kenwyn: Juta. Read More

Structural Engineering Principles - Coursework Example

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