L

Steel spandrel

Figure 5.29. Composite column-to-steel column connection detail: (a) plan; (b) elevation; (c) America Tower, Houston, TX. Photograph of building under construction.

W14 column

W14 column

(b)

Figure 5.29. {Continued).

Figure 5.29. {Continued).

limited experience has demonstrated that properly detailed composite members and connections can perform reliably when subjected to seismic ground motions. Because there is at present limited experience with composite building systems subjected to extreme seismic forces, careful attention to all design aspects is necessary, particularly

Figure 5.30. Structural concept for a super-tall building: (a) plan; (b) schematic elevation; (c) interior view of mega module; (d) exterior view of mega module.

to the detailing of members and connections. It is generally recognized that overall behavior of seismic composite systems will be similar to that for counterpart steel or reinforced concrete systems. For example, it is anticipated that inelastic deformations such as flexural yielding occurs in beams of moment frames, and in braced frames axial yielding and/or bulking occurs in braces. However, in composite systems, differential stiffness between steel and concrete components is more pronounced in the calculations of internal forces and deformations than for structural-steel-only or reinforced-concrete-only systems. This is because stiffness of reinforced concrete elements can vary considerably due to effects of cracking.

The seismic response modification factors such as R, ΓΌ,o, and Cd for composite systems are similar to those for comparable systems of steel and concrete. The current ASCE 7-02, NFPA 5000, and IBC-03 include these factors along with design criteria for composite structures. The UBC 1997 does not explicitly make reference to composite construction, but by invoking Section 1605.2 of the code, one would tacitly presume that composite systems are also permitted. This section permits any system that is based on a rational analysis in accordance with well-established principles of mechanics. The expectation is that, when carefully designed and detailed, the overall inelastic response of composite systems should be similar to comparable steel and reinforced concrete systems.

Before discussing seismic design of composite systems, it is worthwhile to revisit the frequently used phrase "design for wind and seismic forces." A clear understanding of this phrase is crucial for proper implementation of a design. The design is made for the greater of wind or seismic forces according to the applicable building code, often supplemented by site-specific studies accepted by the building official. Whereas wind forces are based on wind exposure category, those due to seismic activity are based on a number of design parameters such as seismic design category (SDC) of the building, seismicity of the region, the mass of the building, and the type of lateral-force-resisting system. The greater of the two sets of forces calculated for wind or seismicity is used for the design of the lateral-force-resisting system. However, in seismic design it is recognized that actual seismic forces can be significantly greater than the code-prescribed values. Thus, seismic design includes not only strength requirements but also material and system limitations and special provision for member proportioning and detailing. The purpose of these additional provisions is to ensure that the members and joints do not snap in a large seismic event, but have the necessary ductility to ride out the forces. Therefore, when designing a building located in a high-seismic zone, even when wind forces govern the design, the detailing and proportioning requirements for seismic resistance must also be satisfied.

Bracing systems for buildings consist of structural components in both vertical and horizontal planes. Vertical bracing is provided by the primary elements of the building such as moment-resisting frames, diagonally braced frames, or shear walls. Horizontal bracing typically includes floor and roof diaphragms. Both the horizontal and the vertical bracing should be properly interconnected in order to transfer all lateral forces from their point of origin through the horizontal bracing to the vertical bracing and into the base of the structure. A complete load path throughout the structure interconnecting all elements of the bracing system is an essential ingredient of a properly designed bracing system.

5.5.1. Moment-Resisting Frames

Seismic code provisions distinguish between "special" and "ordinary" moment frames of both steel and reinforced concrete construction. Special moment frames, which must meet additional detailing requirements to provide ductile inelastic response, are designed for lower force levels than ordinary moment-resisting frames.

Early composite designs focused on combining perimeter steel beams with composite steel and concrete columns, with the lateral force design generally controlled by wind forces. Often the steel column section was used solely for erection purposes, with the concrete section designed to provide the required stiffness and strength. Although other possible combinations exist for providing interior moment-resisting frames, few such buildings have been constructed in high seismic regions in the United States. The practice of compositing interior frames is, however, more popular in Japan. In the United States no code provisions were available prior to 1993 addressing design of composite systems and the attendant ductility requirements in areas of high seismicity. During that year, the Building Seismic Safety Council (BSSC) developed recommendations for seismic design of composite steel and concrete construction. These provisions with their subsequent modifications have served as the basis for the design of seismic lateral-force-resisting systems, which are now included in the ASCE 7-02. The two model codes, IBC-2003 and NFPA 5000, by adaptation of the ASCE 7-02, now provide a firm basis for design and detailing of composite building systems. As many as 18 types of composite systems, listed in Table 5.1, are recognized.

Three potential classes of composite moment-resisting framing systems are identified in the model codes: 1) partially restrained moment-resisting frames; 2) ordinary moment-resisting frames; and 3) special moment-resisting frames. These three systems are similar to the moment-resisting frame systems presently identified for use in steel construction. Only the ordinary and special moment-resisting frames are discussed in this work. The designer is referred to seismic provisions for Structural Steel Buildings, ANSI/AISC 341-02 for additional information.

5.5.1.1. Ordinary Moment Frames

The term "ordinary" refers to systems in which the elements are not designed or detailed to provide the maximum potential ductility during inelastic cyclic response. However, to provide acceptable performance and to reduce the potential ductility demand, the lateral design forces are increased significantly over those required of "special" moment-resisting frame systems. Because of their limited ductility, seismic codes have imposed certain restrictions on the use of ordinary moment frames in areas of high seismicity. Where permitted in areas of low seismicity, these systems are often economical because the expense of providing the ductile elements and connections required for special moment-resisting frames far exceeds the cost of providing for increased lateral loads.

A composite ordinary moment-resisting frame may be developed by combining steel and concrete components in a number of ways. These include steel or composite beams combined with steel, reinforced concrete, or composite columns. The most commonly used system to date in areas of low seismicity has included steel beams and composite columns. The columns may consist of encased or filled composite columns. The connections in composite ordinary moment-resisting frames are generally designed to develop the full moment capacity of the steel beams.

The design requirements for ordinary composite moment-resisting frames and the limit imposed on their use are similar to those specified for steel or concrete ordinary moment-resisting frames. The beams and columns of composite ordinary moment-resisting frames may consist of one of a number of possible combinations of structural steel, reinforced concrete, and composite sections. The analysis, design, and detailing of the frame members is quite similar to that required for steel or concrete moment-resisting frames. Force transfer between the elements of a composite frame is somewhat unique, since in general the connections are designed to be stronger than the beams framing into the joint.

The analytical procedures used in the design of composite moment-resisting frames are identical to those used in the design of structural steel or reinforced concrete frames.

TABLE 5.1 ASCE 7-02 (IBC 2003 & NFPA 5000) Design Coefficients and Factors for Seismic-Force-Resisting Systems of Composite Buildings
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