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Basic recommendations for earthquake protection_9

Tham khảo tài liệu 'basic recommendations for earthquake protection_9', khoa học xã hội, kinh tế chính trị phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 269 the point of view of building response an even more important measure of frequency content is the response spectrum which is discussed below. 8.2.2 Building Response to Ground Motion When a rigid object is shaken so-called inertia forces act on it which increase according to the acceleration of the object and to its mass. If an absolutely rigid building is firmly tied to the ground and shakes with the ground then the inertia forces are transmitted from the ground into the building the magnitude of the force is proportional to the mass of the building and varies with time in the same manner as the acceleration. This simple model is unfortunately inadequate however because no real building is quite rigid. All buildings deform to some extent as they are shaken and the deformation of the building substantially alters the force distribution. Small massive buildings are relatively stiff but as buildings become taller and lighter they tend to become more flexible. When a flexible building is shaken the force acting on any part of it is still proportional to the mass and acceleration of that part but the distribution of forces within the building depends on the way the building itself deforms. Depending on the mass and flexibility of the building the accelerations within the building may be greater or less than the ground accelerations and thus the forces may also be greater or less than if the building was a rigid body. The consequences of this for building design are of great significance. The property of a building which principally determines its dynamic response to earthquake ground motion is its natural frequency. Because all buildings are flexible they will vibrate when jolted and they will then sway backwards and forwards in a regular way. Taller buildings have lower natural frequencies they sway more slowly than lower buildings. A building 10 storeys high may take about a second to sway backwards and forwards in one