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Stability of High-Rise Buildings


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Stability of High-Rise Buildings

  2. 2.  Introduction to High-Rise Buildings  The Structural System Of High-Rise Buildings  Stability of High-Rise Buildings  Stabilizing the Structure Against Wind Loads TOPICS TO BE DISCUSSED
  3. 3. What is a High-Rise Building?  Emporis Standards defines a high-rise as "A multi-story structure between 35–100 meters tall, or a building of unknown height from 12–39 floors.“
  4. 4.  The International Conference on Fire Safety in High-Rise Buildings defined a high- rise as "any structure where the height can have a serious impact on evacuation.”
  5. 5.  A building is, from a structural engineer’s perspective, to be considered a tall building when, due to its height, the lateral forces suffered by the structure play a significant role in the design.
  6. 6. Famous High-Rise Buildings of Pakistan
  7. 7. The Structural System Of High-Rise Buildings  The structural system of high-rise buildings is designed to cope with the vertical gravity loads and lateral loads caused by wind or seismic activity.
  8. 8.  The foundations of high-rise buildings must sometimes support very heavy gravity loads, and they usually consist of concrete piers or piles that are sunk into the ground.
  9. 9.  The most important factor in the design of high-rise buildings, however, is the building’s need to withstand the lateral forces imposed by winds and potential Earthquakes. Most high-rises have frames made of steel or steel and concrete. Their frames are constructed of columns (vertical-support members) and beams (horizontal-support members). Cross-bracing or shear walls may be used to provide a structural frame with greater lateral rigidity in order to withstand wind stresses.
  10. 10. Stability Of High-Rise Buildings  As height increases, the rigidity and stability requirements become more important, and they are often the dominate factors in designing.  Due to P-Δ effect, in which the eccentricity of gravity load increases to such a magnitude that it brings about to collapse of the columns as the result of axial loads. Therefore, an important stability criterion is to assure that the predicted wind loads will be below the load corresponding to the stability limit.  The second consideration is to limit the lateral deflection to a level that will ensure that architectural finishes and partitions are not damaged.
  11. 11. Factors Affecting The Stability Of High-Rise Buildings The factors that affect the stability of a high-rise structure as a whole are the lateral loads which include:  Seismic forces (Earthquake Loads) at the base of structure.  Wind loads at the top (Higher stories) of the buildings.
  12. 12. STABILIZING THE STRUCTURE AGAINST WIND LOADS  Wind is essentially the large scale horizontal movement of free air. It plays an important role in the designing of tall structures because it exerts loads on building. It is a phenomenon of great complexity because of the many flow situations arising from the interaction of wind with structures.
  13. 13. EFFECT OF WIND WITH INCREASE IN HEIGHT:  Wind is caused by air moving from high pressure to low pressure. Its direction is influenced by the earth’s rotation. Near the earth’s surface, the motion is opposed and the wind speed is reduced by surface friction. At the surface the wind speed reduces to zero and then begins to increase with height, and at some height known as the gradient height, the motion may be considered to be free from earth’s frictional influence and will attain its “gradient velocity.”
  14. 14. WIND EFFECTS ON STRUCTURES  Wind produces two different types of effects on tall buildings: static and dynamic STATIC:  Structurally, static effect is a term of analysis independent of time and Static wind effect primarily causes elastic bending and twisting of structure. DYNAMIC:  But dynamic analysis is an attempt to take into account how the system responds to the change through the period of time. For tall, long span and slender structures a dynamic analysis of the structure is essential. Wind gusts cause fluctuating forces on the structure which induce large dynamic motions, inducing oscillations.