With the belt trusses connected to the tube will allow the gravity load to distribute equally the columns. As the building is asymmetrical the gravity load is different on each tube. Willis Tower is made of system of columns that are spaced 15 inch on center with each of nine tubes share two faces columns. This made the bundled tube system very efficient (Khan FR and WP Moore, 1980) The bundled tube system decreased the shear lag which made the structure lighter, allowing 33 pounds of structural steel per square feet. This technology reduces the lateral stiffness and avoids gravity settlement on the tower. The tubes are connected at different levels with truss belts. On the 50th, 66th, 90th floors the numbers of tubes reduce by two or more. The steel structural system used to construct the Willis Tower, 108 stories at height of 442 meter, is bundled tube structure made of 9 tubes each 75 feet by 75feet. Today Middle East has the tallest tower by far with Burj kahlifa standing at 828m. Asia was the winner having constructed the Shanghai Financial Centre and Taipei 101 (height of 508m). A century later other continents were changeling their new technology and finance to construct the tallest building. In 1973 New York Trade Centre was surpassed by the Sears Tower in Chicago at 442m. In 1930 the rivalry was between downtown’s Bank of Manhattan Company building and Cities services building as tallest building. The method of the 'tube' structural was first used on Sears Tower, after a few decades the same method has been used to construction the tallest building in the world standing today Burj Khalifa.

The economic efficient structural system has helped to design variety of shapes and scale for skyscrapers. In 1960s, a structural engineer Farzlar Khan designed a structural system that is innovative idea called 'tube' which are framed tube, trussed tube and bundle tube. However today reinforced concrete is the main material used for the construction of the sky scraper, damping vibration and giving the structure mass make the reinforced concrete a reliable material to be used. In 1900 the technology of steel frame developed in various structures in Chicago and New York where advancing to constructing skyscraper from only steel frame.

When designing a super-tall structure the fundamental aspects that have to be considered are the safety of the habitat around, as sky scraper are built in urban areas, the structure has to be able to support its weight, also to resist the weather condition and earthquake in the area. The three structures have been built in various environment and climates, this will mean their design has to be unique to cope with different environmental conditions. The construction projects that have been chosen for this assignment are all rated as super-tall buildings in world from different locations Middle East (Dubai), Far East (Taipei) and Northern America (Chicago). Willis Tower Sears Tower Construction Essay

Sears tower