Design of bracing to resist forces induced by wind, seismic disturbances, and moving loads, such as those caused by cranes, is not unlike, in principle, design of members that support vertical dead and live loads. These lateral forces are readily calculable. They are collected at points of application and then distributed through the structural system and delivered to the ground. Wind loads, for example, are collected at each floor level and distributed to the columns that are selected to participate in the system. Such loads are cumulative; that is, columns resisting wind shears must support at any floor level all the wind loads on the floors above the one in consideration for frame bracing .
Bracing Tall Buildings
If the steel frame bracing of the multistory building inbelow Figure a is subjected tolateral wind load, it will distort as shown in below Figure b, if the connections of columns and beams are of the standard type, for which rigidity (resistancetorotation) is nil. One can visualize this readily by assuming each joint is connected with a single pin. Naturally, the simplest method to prevent this distortion is to insert diagonal members— triangles being inherently rigid, even if all the members forming the triangles are pin-connected.
BracedBents. Bracing of the type in above Figure C, called X bracing, is both efficient and economical. Unfortunately, X bracing is usually impracticable because of interference with doors, windows, and clearance between floor and ceiling. Usually, for office buildings large column-free areas are required. This offers flexibility of space use, with movable partitions. But about the only place for X bracing in this type of building is in the elevator shaft, fire tower, or wherever a windowless wall is required. As a result, additional bracing must be supplied by other methods. On the other hand, X bracing is used extensively for bracing industrial buildings of the shed or mill type
Moment-Resisting Frames. Designers have a choice of several alternatives to X bracing. Knee braces, shown in above Figured, or portal frames, shown in above Figure e, may be used in outer walls, where they are likely to interfere only with windows. For buildings with window walls, the bracing often used is the bracket type . It simply develops the end connection for the calculated wind moment. Connections vary in type, depending on size of members, magnitude of wind moment, and compactness needed to comply with floor-to-ceiling clearances.
below Figure illustrates a number of bracket-type wind-braced connections. The minimum type, represented in below Figure e, consists of angles top and bottom: They are ample for moderate-height buildings. Usually the outstanding leg (against the column) is of a size that permits only one gage line. A second line of fasteners would not be effective because of the eccentricity. When greater moment resistance is needed, the type shown in b should be considered. This is the type that has become rather conventional in field-bolted construction. below Figure c illustrates the maximum size with beam stubs having flange widths that permit additional gage lines, as shown. It is thus possible on larger wide-flange columns to obtain 16 fasteners in the stub-to-column connection.
The resisting moment of a given connection varies with the distance between centroids of the top and bottom connection piece in frame bracing. To increase this distance, thus increasing the moment, an auxiliary beam may be introduced , if it does not create an interference
References:
- BUILDING DESIGN AND CONSTRUCTION HANDBOOK for Frederick S. Merritt & Jonathan T. Ricketts