Lateral ForceResisting Systems

There are several systems that can be used effectively for providing resistance to seismic lateral forces. Some of the more common systems are shown in Fig. 2.6. All of the systems rely on a complete, three-dimensional space frame; a coordinated system of moment frames, shear walls, or braced frames with horizontal diaphragms; or a combination of the systems.

1. In buildings where a space frame resists the earthquake forces, the columns and beams act in bending. During a large earthquake, story-to-story deflection (story drift) may be accommodated within the structural system without causing failure of columns or beams. However, the drift may be sufficient to

Lateral Forces Architecture

Shear walls

Figure 2.5. Elevation irregularities: (a) abrupt change in geometry; (b) large difference in floor masses; (c) large difference in story stiffnesses.

Shear walls

Figure 2.5. Elevation irregularities: (a) abrupt change in geometry; (b) large difference in floor masses; (c) large difference in story stiffnesses.

damage elements that are rigidly tied to the structural system such as brittle partitions, stairways, plumbing, exterior walls, and other elements that extend between floors. Therefore, buildings can have substantial interior and exterior nonstructural damage and still be structurally safe. Although there are excellent theoretical and economic reasons for resisting seismic forces by frame action, for particular buildings, this system may be a poor economic risk unless special damage-control measures are taken.

2. A shear wall (or braced frame) building is normally more rigid than a framed structure. With low design stress limits in shear walls, deflection due to shear forces is relatively small. Shear wall construction is an economical method of bracing buildings to limit damage, and this type of construction is normally economically feasible up to about 15 stories. Notable exceptions to the

Moment Resisting Frame System

Figure 2.6. Lateral-force-resisting systems: (a) steel moment-resisting frame; (b) reinforced concrete moment-resisting frame; (c) braced steel frame; (d) reinforced concrete shear walls; (e) steel frame building with cast-in-place concrete shear walls; (f) steel frame building with in-filled walls of nonreinforced masonry.

Figure 2.6. Lateral-force-resisting systems: (a) steel moment-resisting frame; (b) reinforced concrete moment-resisting frame; (c) braced steel frame; (d) reinforced concrete shear walls; (e) steel frame building with cast-in-place concrete shear walls; (f) steel frame building with in-filled walls of nonreinforced masonry.

excellent performance of shear walls occur when the height-to-width ratio becomes great enough to make overturning a problem and when there are excessive openings in the shear walls. Also, if the soil beneath its footings is relatively soft, the entire shear wall may rotate, causing localized damage around the wall.

3. The structural systems just mentioned may be used singly or in combination with each other. When frames and shear walls interact, the system is called a dual system if the frame alone can resist 25% of the lateral load. Otherwise, it is referred to as a combined system. The type of structural system and the details related to the ductility and energy-absorbing capacity of its components will establish the minimum ^-value, a seismic coefficient defined later, used for calculating the total base shear.

The design engineer must be aware that a building does not merely consist of a summation of parts such as walls, columns, trusses, and similar components, but is a completely integrated system or unit that has its own properties with respect to lateral force response. The designer must follow the flow of forces through the structure into the ground and make sure that every connection along the path of stress is adequate to maintain the integrity of the system. It is necessary to visualize the response of the complete structure and to keep in mind that the real forces involved are not static but dynamic, are usually erratic and repetitive, and can cause deformations well beyond those determined from the elastic design.

Load Path Lateral
Figure 2.7. Diaphragm drag and chord reinforcement for north-south seismic loads.

2.2.8. Diaphragms

Earthquake loads at any level of a building will be distributed to the vertical structural elements through the floor and roof diaphragms. The roof/floor deck or slab responds to loads like a deep beam. The deck or slab is the web of the beam carrying the shear, and the perimeter spandrel or wall is the flange of the beam resisting bending. Three factors are important in diaphragm design:

1. The diaphragm must be adequate to resist both the bending and shear stresses and be tied together to act as one unit.

2. The collectors and drag members (see Fig. 2.7) must be adequate to transfer loads from the diaphragm into the lateral-load-resisting vertical elements.

3. Openings or reentrant corners in the diaphragm must be properly placed and adequately reinforced.

Inappropriate location or large-size openings (stair or elevator cores, atriums, skylights) create problems similar to those related to cutting a hole in the web of a beam. This reduces the ability of the diaphragm to transfer the forces and may cause failure (Fig. 2.8).

2.2.9. Ductility

Ductility is the capacity of building materials, systems, or structures to absorb energy by deforming into the inelastic range. The capability of a structure to absorb energy, with acceptable deformations and without failure, is a very desirable characteristic in any

Diaphragm Earthquake
Figure 2.8. Diaphragm web failure due to large opening.

earthquake-resistant design. Concrete, a brittle material, must be properly reinforced with steel to provide the ductility necessary to resist seismic forces. In concrete columns, for example, the combined effects of flexure (due to frame action) and compression (due to the action of the overturning moment of the structure as a whole) produce a common mode of failure; buckling of the vertical steel and spalling of the concrete cover near the floor levels. Columns must, therefore, be detailed with proper spiral reinforcing or hoops to have greater reserve strength and ductility.

Ductility is measured by the hysteretic behavior of critical components such as a column-beam assembly of a moment frame. It is obtained by cyclic testing of momentrotation (or force-deflection) behavior of the assembly. The slope of the curves shown in Figs. 2.9a and b represents the stiffness of the structure, and the enclosed areas the dissipated energy. The areas may be full and fat, or lean and pinched. Structural assemblies with curves enclosing a large area representing large dissipated energy are regarded as superior systems for resisting seismic loading.

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Responses

  • omar
    How to work out torsion for cantilever rigid diaphragm?
    1 year ago

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