Engineering the Future Chapter 12 Tower in the Sky

The Tallest Tower Burj Dubai Dubai is going through period of unprecedented growth New laws/regulations are making it easy and inexpensive to do business there Building to be an emblem of the city’s prestige, raising the stature of Dubai as a global tourist destination Dead load is nearly half-million tons (1 billion pounds!!!)

Rising Higher Can only build as high as current technology allows Middle Ages – stone walls – building taller required lower walls to be thicker; stone towers have few windows because they weaken the structure Industrial Revolution – designing with metal frames Steel frame carries load, not the outer walls so the outer walls can have plenty of windows

A Matter of Geometry Shape of frame can minimize compression or tension that individual beams and columns bear Shear – a force that results in deformation of an object in which its parallel planes remain parallel but move past one another A tall structure may start to bend as shear increases (Bending when one side in tension and the other compression)… can topple if too much bending

Engineers want to minimize effects of bending Do so by adding diagonal members – forming a truss Truss – triangular arrangement of structural members that increases a structure’s rigidity, prevents joints from sliding apart when a shearing force pushes the side of a frame Empire State Building – tallest building 1931 – Skeleton is a 3- D grid of columns and beams, which restricted indoor space. Building with a central core, like a spine that is a wide column held together with trusses or walls Often durable reinforced concrete for columns and core walls

Using concrete Cement, crushed rock or sand, and water Performs well under compression Modern concrete also uses fly ash from coal-fired power plants, ground up blast furnace sag from steel manufacturing, and silica dust from computer chip manufacturing instead of some of the cement These waste products make it stronger while contributing to recycling Reinforce with steel bars to improve tension

Design of Burj Dubai Central core and perimeter columns are supported from deep underground Underground portions of columns called piles – each weighs about 3000 tons Central core resembles hub of wheel with 3 spokes, spreading out into 3 wings of structure so weight is distributed over the 3 wings Each wing has its own walls along the corridors and perimeter columns – each designed to be stable structure in and of itself

Confusing the Wind Wind can reach 160 miles per hour at the top of the structure Weight of building helps protect it Wind acts on building’s surface in 2 ways: Pushes directly on surface to produce a force called “drag” Swirls around edges creating small “vortices” When these develop at opposite sides of a building they can actually push the building from side to side - purposely design so building is flexible (also good for earthquakes)

Wind Period – the time it takes a full cycle of a building to sway from one side to the other and back again (11 seconds for Burj tower) If vortices begin pulling structure side to side in rhythm with the period, the building will begin to sway a greater and greater distance from vertical Compensating: vary the width of the building and shape of the floors – each change will have vortices occurring at different speeds (changed 24 times in Burj Dubai!) – won’t get in a rhythm Tuning a building – analyze storms and wind speeds to determine safest period for a building – do this by moving the weight higher or lower (higher the weight, the longer the building’s period)