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Introduction:
No race is without challenges. Reaching new heights means facing new challenges every day. With the rapid advancement of modern science the skyscrapers are going towards new heights. However heights are often reason of increased flexibility and lack of sufficient inherent damping. The two main factors that can cause serious serviceability issues to tall buildings are wind motion and earthquake. So scientists had to find ways to solve this two motions that were creating barrier in achieving new heights. And the run for the search came out with some fruitful outcomes that will serve the mankind for eternity.
Structure to the Safety:
When building a high rise building we must keep in mind that selection of the structural system must include an evaluation of its ability and be able to withstand the wind load and earthquake seismic motion as per capability. Selecting an efficient structural system is crucial and it is the most effective means of controlling structure’s lateral and torsional motions. The means of achieving this goals can be mega frames, space frames, vierendeel frames, belt truss, super column or composite steel structure. Various process serves in various ways. We can use any of these structural composition to deal with the wind and seismic motion as per requirement. Outrigger system mainly engages exterior columns. It improves the core by resisting the part of the overturning moment with the increasing height of the building. The belt truss or bandage system is moreover the modification of the outrigger system. In this system stiff floor diaphragms transfers the load to the exterior columns or share wall through horizontal couple from belt trusses. After that it was the invention of Fazlur Rahman Khan that changed the whole thing to amenities. It was his legendary invention of the Bundle or braced tube system. The concept is to continually extend the columns as a tube and to bundle it to face the wind motion as well as to hold back the seismic reflex. Another efficient structural solution is increasing modal mass through shifting the major frequency axis from the main axis of the building shape and altering the mode shapes.
Fig:Outrigger System Fig: Multi Truss System
Modifying Aerodynamics and Auxiliary Sources:
When it is a situation of handling the wind load all we need is to know the mechanics of aerodynamics. Modification can be done in a buildings cross sectional shape, height and also in its size. One way is modifying its corner geometry and shape. Chamfered corners, horizontal slots and slotted corners can reduce along wind and across wind responses considering with traditional shapes. Another way can be just the rounding of the whole structure. Building level pan areas can be reduced by tapering effects, cutting corners, keeping openings or dropping of corners in various shapes. Another way to modify a building to resist the wind and seismic motion would be using auxiliary means which are not mainly structural parts of a building.
There are four major sources of damping which are structural, aerodynamic, soil and auxiliary. However first three forms make only limited contributions. Besides the damping in the building can’t be engineered like the mass and stiffness properties till it’s complete. So additional damping sources must be added. There are passive dampers with indirect energy dissipation such as tuned mass damper, tuned liquid damper and impact damper. These transmit inertial force to the building frame to reduce its motion. There are also passive dampers with direct energy dissipation like viscoelastic dampers, friction system, viscous damping device and metallic dissipater. These are effective in high frequency and low vibration levels against strong winds and moderate earthquakes. For more severe situations active dampers can be used. These dampers are designed using hydraulic or electro-mechanical actuator system driven by appropriate control algorithm. In this types the actuator generates a secondary mass sensing the motion of the building to resist it towards the other way. These can be mass dampers, stiffness dampers or rather be hybrid.
All we need in this modern world is comfort with safety. The purpose of modern science is not to complex but to ease the complexities. Thus achieving new heights could be tough but we must make it easy by means of excellence and keep us safe towards a new morning.