Bracing is considered as an efficient and economical method to laterally stiffen the frame structures against the wind loads. A braced bent includes the columns and girders with the primary purpose of supporting the gravity loading, and diagonal bracing members that are connected so that total set of members form a vertical cantilever truss to resist the horizontal forces. Bracing is considered efficient as the diagonals tend to work in axial stress and therefore need minimum member sizes in providing the stiffness and strength against horizontal shear.With the increase in trend of constructing tall buildings,there must be a cost effective structural form of bracing system that needs be used in tall buildings against the lateral loads.A regular shape tall building can be analyzed for wind loads acting along the minor axis of bending of column and acting along the major axis of bending of column.
Similarly,when wind loads along the minor axis the building is braced in minor direction of bending and when the wind loads along the major axis the building is braced in minor direction of bending. Moreover, various options of bracing provision in different bays of the building at same level have also been identified.
Bracing can be categorized into the following types;
This type of bracing isgenerally used when the availability of the opening spaces in a bay of frame are required. Diagonal bracing is usually obstructive in nature because it blocks the location of opening which ultimately affects the esthetic of the building elevation. It also sometimes hinders the passage for use. Diagonal bracing can be either single or double diagonal . If there is no architectural limitation, diagonal bracings are considered to be the most efficient in resisting the lateral forces due to wind as these form a fully triangular vertical truss. The beams and columns are actually designed to take up the gravity loads only.
The full diagonal bracing is not preferably used in areas where a passage is required. In such cases, k – bracings are used over diagonal bracing because there is a room to provide opening for doors and windows.
Besides K-bracing, there is another type of bracing in which door and window openings can be allowed which is known as eccentric bracing . Such type of bracing arrangement cause the bending of the horizontal members of the web of braced bent.
Generally these types of braced bents tend to resist the lateral forces due to the bending action of beams and columns. These provide less lateral stiffness hence less efficient as compared to diagonal bracingUnder the action of gravity loads, columns shorten axially due to the compressive loads. As a result the diagonals are subjected to compression and beam will undergo axial tension due to the tying action . In situations where diagonals are not connected at the end of the beam, the diagonal members will not carry any force because no restraint is provided by the beams to develop force. Therefore, such bracing will not take part in resisting the gravity loads.