Текст C

HIGH RISE BUILDINGS: FRAMES AND TUBES

A large number of high-rise buildings use shear walls to provide lateral stability. A development of the shear wall is the core structure in which a number of shear walls are placed together, usually enclosing lifts, stairs or services to form tubes or other plan shapes. Compared to a single wall the core or shaft structures have 'flange' as well as 'web' parts to resist horizontal forces. However the overall shear and bending moment patterns in shear wall and core structures follow that of a cantilever. In practice there is a difficulty because of the need to provide openings in the core structure for service runs. With only small openings the core structure will not be affected but larger openings will require strengthening round the openings, especially at sharp corners. Most core structures are of poured concrete construction in which additional reinforcement may be placed as necessary but cores may also be built in reinforced or unreinforced masonry. Where a high-rise concrete frame of cast-in-place construction is used, it is natural to connect the frame with shear walls so that they interact with each other.

A further increase in stiffness against horizontal forces can be provided in steel buildings by using a shear truss and frame with belt trusses. In this system deep horizontal outrigger trusses are connected to the central shear trusses which are vertical, and to a horizontal belt truss in the perimeter of the steel frame.

In order to prevent bending moment being passed over into the perimeter columns, the connection between them and the belt trusses can be a pinned one. There are certain best locations for the belt trusses: if two belt trusses are used they would be best placed somewhere near the half and three-quarter points in the height of the building; if only one belt truss is used, it would be best placed somewhere between these two positions, although in practice the highest belt truss is often placed at the top of the building.

The most effective way of resisting horizontal loads, for any given plan area, is to use a cantilever with a solid section; however hollow tubes are more efficient in that they are almost as stiff but use very much less material than a solid section cantilever. The direct stresses of tension and compression and the shear stresses are those connected with a pure cantilever. Solid sections are not practical forms and hollow sections without openings are generally only feasible for towers and other such uses. However high-rise buildings can still be designed to exploit the inherent efficiency of tube action by an adequate choice of form. The purpose, if tube-type systems are used, is to increase the plan area of the tube as much as possible that is to increase the effective section area of the cantilever, and to make the structural behaviour as close as possible to that of the hollow tube without openings in it. As window openings are necessary for the building, the perimeter structure is weaker and more flexible than that of the pure tube. However buildings do have the compensating advantage of being stiffened by the floors, which act as a series of horizontal diaphragms and prevent deformation of the cross-section of the building throughout its height.

Notes:

direct stress напряжение, усилие (растяжения или сжатия)

outrigger truss выносная ферма

pure adj. простой

sharp adj. острый

shear moment момент сдвига

shear truss ферма жёсткости

solid section сплошное сечение

structural behaviour работа/поведение строительной конструкции

Дата добавления: 2014-11-16