[英语学习]土木工程专业英语-part 1

土木工程专业英语,天津大学 尹越,Part 1:Earthquake Engineering and Structure Dynamics,Text and Reading Material,Unit 6 Dynamic Analysis on StructuresUndamped SystemUnit 20Earthquakes and Earthquake-Resistant StructuresA New Kind of Earthquake-Resistant Building,Related Book or Journal,Anil K. Chopra, Dynamics of Structures Theory and applications to Earthquake Engineering, 清华大学出版社，2005，5 Earthquake Engineering and Structure Dynamics, Journal of International Association of Earthquake Engineering (IAEE), John Wiley & Sons, Inc.,Text Dynamic Analysis on Structures,It is not always possible to obtain rigorous mathematical solutions for engineering problems.In fact, analytical solutions can be obtained only for certain simplified situations. dynamic/static,Text Dynamic Analysis on Structures,For problems involving complex material properties, loading, and boundary conditions, the engineer introduces assumptions and idealizations deemed necessary to make the problem mathematically manageable but still capable of providing sufficiently approximate solutions and satisfactory results from the point of view of safety and economy.,Text Dynamic Analysis on Structures,material propertydensity, elastic (Youngs) modulus, poissons ratio, yield stress, shear modulus loadingdead load, live load, wind load, load distribution, concentrate load, line load, uniform load section propertyarea, moment of inertia, section modulus, radius of gyration, first moment of area,Text Dynamic Analysis on Structures,The link between the real physical system and the mathematically feasible solution is provided by the mathematical model which is the symbolic designation for the substitute idealized system including all the assumptions imposed on the physical problem.,Text Dynamic Analysis on Structures,Degree of FreedomIn structural dynamics the number of independent coordinate necessary to specify the configuration or the position of a system at any time is referred to as the number of degrees of freedom. SDOF/MDOF system,Text Dynamic Analysis on Structures,In general, a continuous structure has an infinite number of degrees of freedom.Nevertheless, the process of idealization or selection of an appropriate mathematical model permits the reduction in the number of degrees of freedom to a discrete number and in some cases to just a single degree of freedom.,Text Dynamic Analysis on Structures,Fig. 6. 1 shows some examples of structures which may be represented for dynamic analysis as one-degree-of-freedom system, that is, structures modeled as systems with a single displacement coordinate. Fig.=figure, Eq.=equation, table,Text Dynamic Analysis on Structures,These one-degree-of-freedom systems may be described conveniently by the mathematical model shown in Fig. 6. 2 which has the following elements: (1) a mass element m representing the mass and inertial characteristics of the structures; (2) a spring element k representing the elastic restoring force and potential energy capacity of the structure;,Text Dynamic Analysis on Structures,(3) a damping element c representing the frictional characteristic and energy losses of the structure; (4) an excitation force F(t) representing the external forces acting on the structure system.The force F(t) is written this way to indicate that it is a function of time. Internal force: axial force, bending moment, shear,Text Dynamic Analysis on Structures,In adopting the mathematical model in Fig. 6. 2, it is assumed that each element in the system represents a single property; that is, the mass m represents only the property of inertial and not elasticity or energy dissipation, whereas the spring k represents exclusively elasticity and not inertia or energy dissipation. Finally, the damper c only dissipates energy.,Text Dynamic Analysis on Structures,The reader certainly realizes that such “pure” elements do not exist in our physical world and that mathematical models are only conceptual idealizations of real structures. As such, mathematical models may provide complete and accurate knowledge of the behavior of the model itself, but only limited or approximate information on the behavior of the real physical system.,Text Dynamic Analysis on Structures,Nevertheless, from a practical point of view, the information acquired from the analysis may very well be sufficient for an adequate understanding of the dynamic behavior of the physical system, including design and safety requirements.,Text Dynamic Analysis on Structures,Dalemberts PrincipleAn approach to obtain the equation of the motion is to make use of DAlemberts Principle which states that a system may be set in a state of dynamic equilibrium by adding to the external force which is commonly known as the inertial force.,Text Dynamic Analysis on Structures,Fig. 6.3 shows the free body diagram with inclusion of the inertial force . This force is equal to the mass multiplied by the acceleration, and should always be directed negatively with respect to the corresponding coordinate. displacement, velocity, acceleration negative, positive,Text Dynamic Analysis on Structures,The application of DAlemberts Principle allows us to use equations of equilibrium in obtaining the equation of motion. For example, in Fig. 6.3, the summation of forces in the y direction gives directly: , which is the equation of motion for one-degree-of-freedom system.,Text Dynamic Analysis on Structures,The use of DAlemberts Principle in this case appears to be trivial. This will not be the case for a more complex problem, in which the application of DAlemberts Principle, in conjunction with the Principle of Virtual Work, constitutes a powerful tool of analysis.,Text Dynamic Analysis on Structures,As will be explained latter, the Principle of Virtual Work is directly applicable to any system in equilibrium. It follows then that this principle may also be applicable to the solution of dynamic problems, providing that DAlemberts Principle is used to establish the dynamic equilibrium of the system.,Text Earthquakes and Earthquake-Resistant Structures,Destructive earthquakes occur as a result of sudden release of energy stored in the earths crust in weak zones known as geologic faults. Upon the release of the energy, a rupture develops that may extend for several hundred kilometers along the fault.kN: kilo-Newton, kg: kilogram, km: kilometer,Text Earthquakes and Earthquake-Resistant Structures,The rupture may occur in a sufficiently deep location from the ground surface so that it may not appear on the surface. When the opposite sides of the ruptured zone move longitudinally against each other, the earthquake motion is called “strike-slip”; whereas, when the dislocation of the sides occurs across the width of the ruptured zone, the motion is “dip-slip”. longitudinal, lateral, vertical, horizontal,Text Earthquakes and Earthquake-Resistant Structures,Earthquakes are often measured by their magnitudes.The magnitude is a measure of energy release and is defined with the Richter scale.Magnitudes of 4 or more are believed to be significant to the engineering design.The magnitude is a constant value that describes how large an earthquake is.,Text Earthquakes and Earthquake-Resistant Structures,In engineering design, however, the earthquake intensity, rather than the magnitude, is used.The intensity is a measure of earthquakes destructiveness.The intensity depends primarily on the magnitude and the distance between the epicenter and the location where the intensity is evaluated.,Text Earthquakes and Earthquake-Resistant Structures,The common measure for the intensity is the Modified Mercalli Intensity Scale.However, in engineering design, the ground acceleration is often used as the intensity and applied to the structure as the base excitation intensity level. Empirical relations, known as attenuation equation, relate the intensity at a desired location to the magnitude and distance.,Text Earthquakes and Earthquake-Resistant Structures,Earthquake damage to a structure depends mainly on the response of the structure to the dynamic forces arising from the ground shaking.The response of the structure depends on many factors including the ground motion peak value as well as its frequency and duration. shaking table test,Text Earthquakes and Earthquake-Resistant Structures,Also, the natural frequency of the building, soil condition at the site of the building, construction practice, and overall design of the building influence the response to the earthquake loads.Natural frequency of the building, in turn, depends on its geometry and the types of construction material used.,Text Earthquakes and Earthquake-Resistant Structures,Because of the variety of factors involved in the response of a building to the earthquake loads, it is difficult to generalize the type of damage that may occur to the building.Rather, the potential earthquake damage has to be investigated on a case-by-case basis.,Text Earthquakes and Earthquake-Resistant Structures,Damage to buildings varies to a great extent with the buildings ability to dissipate energy and dampen the vibration during an earthquake.As described early, the natural frequency of a building and the frequency and the duration of the ground shaking also play important roles in the damage.,Text Earthquakes and Earthquake-Resistant Structures,The buildings geometry and the structural material influence its ductility, damping ability, and natural frequency of vibration.Thus, depending on the type of building, the damage can range from only a few cracks to major cracks and/or total collapse of the structure.,Text Earthquakes and Earthquake-Resistant Structures,Buildings with high natural frequency of vibration (i.e. short period of vibration) exhibit much stiffer behavior during an earthquake.This is especially true for low-rise buildings (such as residential buildings, shopping centers, etc.) which often possess a natural period of vibration of 0.006 to 0.25 sec. i.e.= that is to say, in other words etc.= etcetera,Text Earthquakes and Earthquake-Resistant Structures,The stiffer behavior may cause the building to suffer more damage during an earthquake.In such a case, however, if the ductility (i.e. the ability of the structure to deform as the load applies) is increased, the building becomes less stiff and thus less vulnerable to potential earthquake damage.,Text Earthquakes and Earthquake-Resistant Structures,When the natural period of vibration of the building is short, the vibration tends to quickly reach the stationary stage.As a result, the damage to the building will primarily depend on the duration of the ground shaking. With longer duration, the building will be more likely to suffer damage.,Text Earthquakes and Earthquake-Resistant Structures,Buildings with higher ductility exhibit longer periods of vibration. As a result, they are less likely to suffer damage.This is especially true if the natural period of vibration of the building is much longer than the period of the ground vibration. In terms of individual structural members, a member with more ductility deforms more and thus delays reaching the critical failure stage.,Text Earthquakes and Earthquake-Resistant Structures,This is the type of behavior in metal structures and, to certain extent, in timber structures. However, concrete and masonry structures are less ductile and need to be de