Another fundamental concept in engineering – bracing – involves added additional elements to a frame in order to increase its ability to withstand lateral loads . There are two main varieties of braced frames – concentric and eccentric.
Concentric bracing consists of diagonal braces located in the plane of the frame. Both ends of the brace join at the end points of other framing members to form a truss, creating a stiff frame.
Concentric bracing may be arranged in several different configurations – such as X, K or one-directional diagonal bracing – and the bracing members may be designed to act in tension or compression or both. Balanced diagonal bracing is the most common for medium-rise structures because it provides the same strength in both directions.
Efficient energy dissipation is difficult to achieve in concentrically braced frames.
Eccentric bracing consists of diagonal braces located in the plane of the frame where one or both ends of the brace do not join at the end points of other framing members. The system essentially combines the features of a moment frame and a concentrically braced frame, while minimising the disadvantages of each system.
The eccentric connection to the frame means an eccentric brace transfers lateral forces via shear either to another brace or to a vertical column. When properly proportioned, eccentric braced frames may exhibit a more ductile characteristic and greater energy dissipation capabilities than a concentric braced frame in the same material.