Structural frameworks are explored and optimized by fine tuning geometry to increase spans while reducing material use. As a structure's dead weight increases, structural capacity is diminished; Achieving great spans with minimal material is the basis of imaginative structural form. Anthony Hunt, a famous structural engineer who worked with Norman Foster and Richard Rogers also taught structures to architects. He defined structural engineering as maximum result (span) with minimal weight (material). This ideal relationship is particularly important in buildings that require large open spaces: stadiums, auditoriums.
Strategies for large spanning roof structures are often based on truss effect or shell / membrane effect to define form actively increasing spanning capacity. Tall buildings also demand an efficient structural ratio as each stacked floor multiplies weight on foundations and imposes greater rotational and moment forces as the edifice rises much like a long vertical cantilevered beam. Reducing these moment forces lessens the lateral forces on a building's framework. The outrigger frame designed for tall buildings uses a solid core and perimeter column organisation with outrigger beams that connect core and columns at certain levels creating a rigid bond between the center and the perimeter of the building. The outrigger beams can be monolithic or trussed but must rigidly connect center and periphery. This equilibrium force is analogous to how outriggers are used on watercraft to increase lateral stability. Floors where outriggers are positioned are usually less flexible than floors where only cores and columns are present freeing up the floor plate from of any structural obstacles.
The Montreal Stock Exchange Tower (Place Victoria Tower today) designed by architect Luigi Moretti and engineer Pier Luigi Nervi in the 1960s is an elegant example of the outrigger frame principle. Well-known for its four towering external structural pillars, it was once the tallest reinforced concrete tower in the world. Mechanical floors 5, 19 and 32 are crossed by diagonal full floor height reinforced concrete truss beams that stabilise core and perimeter. The full height trusses rigidly connect the central stabilized core with floor slabs and corner columns strengthening the entire structural tube. The reinforced concrete prism is not only stabilized vertically by the outrigger beams, but the criss-crossing beams also buttress against rotational constraints.
Place Victoria Tower - stabilized outrigger core in red
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