rachmat wijaya: 1.2. TYPES OF BASIC LATERAL SYSTEMS

During the initial planning stage of any project, consideration should be made for the type of lateral load resisting system(s) to be used in the building. Three basic types of lateral resisting systems are commonly used: braced frames, rigid frames, and shear walls. The structural engineer should be consulted early in the project to establish the type of system best suited for the specific building footprint, height and available locations. Careful consideration should be given to meet the lateral resistance requirements of the structure as well as the architectural needs of the building. In order to meet these needs the engineer may select one or more types of lateral systems. Each system has its own specific limitations and potential architectural implications.

Braced Frames — General

Three types of braces used in braced frames typically seen in buildings today include the cross brace, Chevron (or inverted V) and eccentric brace. Cross bracing is often analyzed by the structural engineer as having tensiononly members. Chevron bracing is used in a building that requires access through the bracing line. Eccentrically braced frames allow for doorways, arches, corridors and rooms and are commonly used in seismic regions to help dissipate the earthquake energy through the beam/girder between workpoints of the bracing/beam interface. Braced frames are generally more cost-effective when compared to rigid frame systems.

Braced Frames — Cross Bracing

Perhaps the most common type of braced frame is the cross-braced frame. A typical representation of a crossbraced frame is shown in Figures 5 and 6. Figure 5 shows a typical floor framing plan with cross bracing denoted by the dashed-line drawn between the two center columns. The solid lines indicate the floor beams and girders. A typical multi-floor building elevation with cross-braced bays beginning at the foundation level is shown in Figure 6. While only one bay is indicated in Figure 6 as having cross bracing, it must be understood that many bays along a given column line may be necessary to resist the lateral loads imposed on a specific structure. One or more column lines having one or more bays of cross bracing may be necessary as well. It is important to establish early on in the development of any project the location of braced bays. These considerations are typical to all of the braced frames discussed in this publication.

Connections for this type of bracing are concentrated at the beam to column joints. Figure 7 illustrates a typical beam to column joint for a cross-braced frame. For taller buildings, usually over two or three stories, these connections could become large enough to minimize the available space directly adjacent to the column and below the beam. This restricted space may have an effect on the mechanical and plumbing distribution as well as any architectural soffit details. The structural engineer needs to be able to provide this type of information to the architect to avoid potentially costly field revisions during construction.

Bracing members are typically designed as tension only members. With this design approach only half of The members area active when the lateral loads area applied. The adjacent member within the same panel is considered to contribute no compressive strength. Utilizing tension only members makes very efficient use of the structural steel shape and will result in using the smallest members. Without full consideration of a specific bay size and amount and location of the bracing, a generalized range of sizes cannot be determined.

Cross-braced frames are composed of single span, simply connected beams and girders. Columns that are not engaged by the braced frame can be designed as gravity load only column. Tables prepared for this publication in the Materials chapter may be used to select preliminary member sizes.

Braced Frames — Chevron Bracing

Chevron bracing (inverted V bracing) is a modified form of a braced frame which allows for access ways to pass through a braced bay line. Figure 8 shows a typical floor framing plan with the bays using Chevron bracing denoted by the dashed-line drawn from between the two center columns. The solid lines indicate the floor beams and girders. Figure 9 shows a typical multi-floor building elevation using Chevron bracing. This system allows the architect to consider placing doorways and corridors through the bracing lines on a building.

There are two types of connections required for bracing elements. At the floor line the connection will be very similar to that required for cross-braced frames. This type of connection is illustrated in Figure 7. The connection at the floor above requires a gusset plate and field welded or bolted connection between the bracing members and the gusset plate. The depth of the gusset plate connection must be considered in the layout and coordination of mechanical ductwork and utility piping above the doorways and corridors.

As a consequence of the bracing configuration, the bracing members are subjected to gravity compressive loads. Each of the bracing members is considered active in the analysis of the system when lateral loads are applied. As a result, the bracing elements are subjected to both tension and compressive forces.

Beams and girders used in the Chevron-braced frame are designed as two span continuous members. This will almost always result in shallower and lighter members when compared to a simple span member of equal column-to-column length.

Eccentrically Braced Frames

Eccentrically braced frames are very similar to frames with Chevron bracing. In both systems the general configuration is an inverted V shape with a connection between the brace and the column and a connection at the beam/girder at the next level up. However, unlike the Chevron-braced frame which has the brace member workpoints intersecting at the same point on the beam/girder for the brace elements. The condition is shown in Figure 10.

This type of bracing is commonly used in seismic regions requiring a significant amount of ductility or energy absorption characteristics within the structure. The beam/girder element between the workpoints of the bracing member shown is designed to link elements and assists the system in resisting lateral loads caused by seismic activity.

Rigid Frames

Rigid frames are used when the architectural design will not allow a braced frame to be used. This type of lateral resisting system generally does not have the initial cost savings as a braced frame system but may be better suited for specific types of buildings.

Figures 11 and 12 show a floor plan and building line elevation of a rigid frame system. Figure 11 indicates the solid triangle designation typically used to show rigid connections between beam and column as well as girder and column. The building elevation shown in Figure 12 indicates the same solid triangular symbols at the floor line beam to column joints.

Connections between the beam/girder and column typically consist of a shear connection for the gravity loads on the member in combination with a field welded flange to column flange connection. Column stiffener plates may be required based on the forces transferred and column size. This type of joint is illustrated in Figure 13. It must be noted that this type of joint requires all vertical utility ductwork and piping to be free and clear of the column and beam/girder flanges. Coping of the beam/girder flanges to allow passage of piping or other utilities is usually not acceptable and must be brought to the attention of the structural engineer as soon as possible.

Shear Walls

This type of lateral load resisting system engages a vertical element of the building, usually concrete or masonry, to transfer the horizontal forces to the ground by a primary shear behavior. Shear walls are usually longer than they are high and are inherently stiff elements. Careful attention to detailing the joint between the shear wall and floor or roof diaphragm elements may be required. Code-specific spacing of masonry shear walls may also impact the interior layout of the building.