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 ¬†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.
Bracing Tall Buildings
If the steel frame of the multistory¬†building in Fig. 7.16a is subjected to lateral¬†wind load, it will distort as shown¬†in Fig. 7.16b, if the connections of columns¬†and beams are of the standard¬†type, for which rigidity (resistance to rotation)¬†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‚ÄĒ¬†triangles being inherently rigid, even if¬†all the members forming the triangles¬†are pin-connected.
Braced Bents. Bracing of the type in Fig. 7.16c, 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 Fig. 7.16d, or portal frames, shown in Fig. 7.16e,¬†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 (Fig.¬†7.16∆í). 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.
Figure 7.17 illustrates a number of bracket-type wind-braced connections. The¬†minimum type, represented in Fig. 7.17e, 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 Fig. 7.17b should be considered. This is the type that¬†has become rather conventional in field-bolted construction. Figure 7.17c 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. To increase this distance, thus¬†increasing the moment, an auxiliary beam may be introduced as shown in Fig.¬†7.17d, if it does not create an interference.
Deep wing brackets (Fig. 7.17h and i) are sometimes used for wall beams and¬†spandrels designed to take wind stresses. Such deep brackets are, of course, acceptable¬†for interior beam bracing whenever the brackets do not interfere with¬†required clearances.
Not all beams need to wind-braced in tall buildings. Usually the wind load is¬†concentrated on certain column lines, called bents, and the forces are carried¬†through the bents to the ground. For example, in a wing of a building, it is possible¬†to concentrate the wind load on the outermost bent. To do so may require a stiff¬†floor or diaphragm-like system capable of distributing the wind loads laterally. Onehalf¬†these loads may be transmitted to the outer bent, and one-half to the main¬†building to which the wing connects.
Braced bents are invariably necessary across the narrow dimension of a building.
The question arises as to the amount of bracing required in the long dimension,¬†since wind of equal unit intensity is assumed to act on all exposed faces of structures.
In buildings of square or near square proportions, it is likely that braced bents¬†will be provided in both directions. In buildings having a relatively long dimension,¬†as compared with width, the need for bracing diminishes. In fact, in many instances,¬†wind loads are distributed over so many columns that the inherent rigidity of the¬†whole system is sufficient to preclude the necessity of additional bracing.¬†Column-to-column joints are treated differently for wind loads. Columns are¬†compression members and transmit their loads, from section above to section below,¬†by direct bearing between finished ends. It is not likely, in the average building,¬†for the tensile stresses induced by wind loads ever to exceed the compressive pressure¬†due to dead loads. Consequently, there is no theoretical need for bracing a¬†column joint. Actually, however, column joints are connected together with nominal¬†splice plates for practical considerations‚ÄĒto tie the columns during erection and¬†to obtain vertical alignment.
This does not mean that designers may always ignore the adequacy of column¬†splices. In lightly loaded structures, or in exceptionally tall but narrow buildings,¬†it is possible for the horizontal wind forces to cause a net uplift in the windward¬†column because of the overturning action. The commonly used column splices¬†should then be checked for their capacity to resist the maximum net tensile stresses ¬†caused in the column flanges. This computation and possible heaving up of the¬†splice material may not be thought of as bracing; yet, in principle, the column joint¬†is being ‚Äė‚Äėwind-braced‚Äô‚Äô in a manner similar to the wind-braced floor-beam connections.
Masonary walls enveloping a steel frame, interior masonry walls, and perhaps some¬†stiff partitions can resist a substantial amount of lateral load. Rigid floor systems¬†participate in lateral-force distribution by distributing the shears induced at each¬†floor level to the columns and walls. Yet, it is common design practice to carry¬†wind loads on the steel frame, little or no credit being given to the substantial¬†resistance rendered by the floors and walls. In the past, some engineers deviated¬†from this conversatism by assigning a portion of the wind loads to the floors and¬†walls; nevertheless, the steel frame carried the major share. When walls of glass or¬†thin metallic curtain walls, lightweight floors, and removable partitions are used,¬†this construction imposes on the steel frame almost complete responsibility for¬†transmittal of wind loads to the ground. Consequently, windbracing is critical for¬†tall steel structures.
In tall, slender buildings, such as hotels and apartments with partitions, the¬†cracking of rigid-type partitions is related to the wracking action of the frame¬†caused by excessive deflection. One remedy that may be used for exceptionally¬†slender frames (those most likely to deflect excessively) is to supplement the normal¬†bracing of the steel frame with shear walls. Acting as vertical cantilevers in resisting¬†lateral forces, these walls, often constructed of reinforced concrete, may be arranged¬†much like structural shapes, such as plates, channels, Ts, Is, or Hs. (See also Arts.¬†3.2.4 and 5.12.) Walls needed for fire towers, elevator shafts, divisional walls, etc.,¬†may be extended and reinforced to serve as shear walls, and may relieve the steel¬†frame of cumbersome bracing or avoid uneconomical proportions.
Bracing Industrial-Type Buildings
Bracing of low industrial buildings for horizontal forces presents fewer difficulties¬†than bracing of multistory buildings, because the designer usually is virtually free ¬†to select the most efficient bracing without¬†regard to architectural considerations ¬†or interferences. For this reason,¬†conventional X bracing is widely¬†used‚ÄĒbut not exclusively. Knee braces,¬†struts, and sway frames are used where¬†needed.
Wind forces acting on the frame¬†shown in Fig. 7.18a, with hinged joints¬†at the top and bottom of supporting columns,¬†would cause collapse as indicated¬†in Fig. 7.18b. In practice, the joints¬†would not be hinged. However, a minimum-¬†type connection at the truss connection¬†and a conventional column base¬†with anchor bolts located on the axis¬†transverse to the frame would approximate¬†this theoretical consideration of¬†hinged joints. Therefore, the structure¬†requires bracing capable of preventing¬†collapse or unacceptable deflection.
In the usual case, the connection between¬†truss and columns will be stiffened¬†by means of knee braces (Fig. 7.18c). The rigidity so obtained may be supplemented¬†by providing partial rigidity at the column base by simply locating the¬†anchor bolts in the plane of the bent.
In buildings containing overhead cranes, the knee braced may interfere with¬†crane operation. Then, the interference may be eliminated by fully anchoring the¬†column base so that the column may function as a vertical cantilever (Fig. 7.18d).
The method often used for very heavy industrial buildings is to obtain substantial¬†rigidity at both ends of the column so that the behavior under lateral load will ¬†resemble the condition illustrated in Fig. 7.18e. In both (d) and (e), the footings¬†must be designed for such moments.
A common assumption in wind distribution¬†for the type of light mill building¬†shown in Fig. 7.19 is that the windward¬†columns take a large share of the¬†load acting on the side of the building¬†and deliver the load directly to the¬†ground. The remaining wind load on the¬†side is delivered by the same columns¬†to the roof systems, where the load joins¬†with the wind forces imposed directly¬†on the roof surface. Then, by means of¬†diagonal X bracing, working in conjunction¬†with the struts and top chords¬†of the trusses, the load is carried to the¬†eave struts, thence to the gables and,¬†through diagonal bracing, to the foundations.
Because wind may blow from any direction, the building also must be braced¬†for the wind load on the gables. This bracing becomes less important as the building¬†increases in length and conceivably could be omitted in exceptionally long structures.
The stress path is not unlike that assumed for the transverse wind forces. The¬†load generated on the ends is picked up by the roof system and side framing,¬†delivered to the eave struts, and then transmitted by the diagonals in the end¬†sidewall bays to the foundation.
No distribution rule for bracing is intended in this discussion; bracing can be¬†designed many different ways. Whereas the foregoing method would be sufficient¬†for a small building, a more elaborate treatment may be required for larger structures.
Braced bays, or towers, are usually favored for structures such as that shown in¬†Fig. 7.20. There, a pair of transverse bents are connected together with X bracing¬†in the plane of the columns, plane of truss bottom chords, plane of truss top chords,¬†and by means of struts and sway frames. It is assumed that each such tower can¬†carry the wind load from adjacent bents, the number depending on assumed rigid ¬†ities, size, span, and also on sound judgment. Usually every third or fourth bent¬†should become a braced bay. Participation of bents adjoining the braced bay can¬†be assured by insertion of bracing designated ‚Äė‚Äėintermediate‚Äô‚Äô in Fig. 7.20b. This¬†bracing is of greater importance when knee braces between trusses and columns¬†cannot be used. When maximum lateral stiffness of intermediate bents is desired,¬†it can be obtained by extending the X bracing across the span; this is shown with¬†broken lines in Fig. 7.20b.
Buildings with flat or low-pitched roofs, shown in Fig. 7.12d and e, require little¬†bracing because the trusses are framed into the columns. These columns are designed¬†for the heavy moments induced by wind pressure against the building side.
The bracing that would be provided, at most, would consist of X bracing in the¬†plane of the bottom chords for purpose of alignment during erection and a line or¬†two of sway frames for longitudinal rigidity. Alignment bracing is left in the structure¬†since it affords a secondary system for distributing wind loads.
Bracing Craneway Structures
All building framing affected by overhead cranes should be braced for the thrusts¬†induced by sidesway and longitudinal motions of the cranes. Bracing used for wind¬†or erection may be assumed to sustain the lateral crane loadings. These forces are¬†usually concentrated on one bent. Therefore, normal good practice dictates that¬†adjoining bents share in the distribution. Most effective is a system of X bracing¬†located in the plane of the bottom chords of the roof trusses.
In addition, the bottom chords should be investigated for possible compression,¬†although the chords normally are tension members. A heavily loaded crane is apt¬†to draw the columns together, conceivably exerting a greater compression stress¬†than the tension stress obtainable under dead load alone. This may indicate the need¬†for intermediate bracing of the bottom chord.
Bracing Rigid Frames
Rigid frames of the type shown in Fig. 7.14 have enjoyed popular usage for gymnasiums,¬†auditoriums, mess halls, and with increasing frequency, industrial buildings.
The stiff knees at the junction of the column with the rafter imparts excellent¬†transverse rigidity. Each bent is capable of delivering its share of wind load directly¬†to the footings. Nevertheless, some bracing is advisable, particularly for resisting¬†wind loads against the end of the building. Most designers emphasize the importance¬†of an adequate eave strut; it usually is arranged so as to brace the inside¬†flange (compression) of the frame knee, the connection being located at the midpoint¬†of the transition between column and rafter segments of the frame. Intermediate¬†X bracing in the plane of the rafters usually is omitted.