Office Buildings. Skyscrapers. Structural Engineering for Skyscrapers

New York City passed a new planning law in 1982 to regulate skyscraper construction. Its provisions represented an attempt to come to grips with dense traffic, 3 million commuters daily, and town-planning aspects such as maintenance of street spaces, expansion of public sidewalks and subway entrances, pedestrian traffic, availability of daylight and micro-climates - (3).

Structural engineering for skyscrapers. Structural systems and vertical-access elements are of decisive importance when designing skyscrapers. The ratio of usable floor area to construction costs worsens as building height increases. Structural areas and circulation spaces occupy more of the building. Dividing skyscrapers into sections with 'sky lobbies', served by express elevators where passengers can change to local elevators, minimises the space required for shafts and reduces travel time.

Economic efficiency depends on the 'sway factor', i.e. the ratio of the maximum allowable horizontal deformation at the top to the total height of the building (max. 1:600). Horizontal forces (wind) are much more important than vertical loads when making calculations for very tall buildings. Ninety percent of horizontal deformations result from shifting of the frame ('shear sway'), while 10% come from the leaning of the building as a whole.

Frame construction with special wind bracing is impracticable beyond ten storeys. Conventional framework systems result in uneconomical dimensions above the 20th storey. Reinforced concrete framework structures are limited to ten storeys without bracing walls and 20-30 storeys with them. Higher buildings require concrete pipe or double-pipe construction.

Factors determining whether a building is economic include use of materials, appropriate design and efficient structural engineering methods - (2). The John Hancock Center, Chicago, 1965 – (1), was the result of an economical structural approach by Skidmore, Owings & Merrill. The visible structural components were part of the design concept. Use of the pipe principle significantly reduced the use of steel. Its efficiency of operation is due to its multiple uses: floors 1-5 have shops, floors 6-12 are parking spaces, floors 13-41 are flexible-use offices, floors 42-45 have technical facilities and a sky lobby, floors 46-93 are residences, floors 94-96 are for visitors and restaurants, and floors 97 and 98 house television transmission equipment.

New York's Department of City Planning has issued a brochure that contains examples of how statutory requirements attempt to guarantee sufficient daylight and circulation space in spite of the increasing volume of construction.

(1) and (2) Curved surfaces reduced wind load by up to 25%, and also saved 10% in structural steel. (3) and (4) Office tower taking the geometry of its plan from the triangular shape of the site on which it is built. (5) and (6) Part of site transferred to public use in return for a planning gain increasing the number of storeys. (7) and (8) Recessed fagade in the arc of a circle creating a new plaza. The rotunda is an enclosed atrium.

Section of high-rise office building and training centre, including high-rise accommodation for trainees. Centre includes secretarial department, classrooms, computer suite, sales offices, service areas, and underground level with outdoor parking places for cars. Administration high-rise has office space, technical facilities and access to archives and environmental control (cooling and re-cooling plant).