Info

Shear V

I. J I. J L_J rn n rn bw = Effective shear width of col.

Figure 5.31. Encased composite column; design shear strength parameters.

Elastic properties of composite elements can be transformed into equivalent properties of steel for stiffness analyses using standard procedures. As with steel or concrete frames, it may be more accurate to include a finite rigid joint size in the frame model, particularly when the composite columns are quite large.

The design of composite ordinary moment-resisting frames is not significantly different from the procedures for structural steel and reinforced concrete moment frames. Encased composite columns should have a minimum ratio of structural steel to gross column area of 4%. The shear strength of these columns generally ignores the contribution of the concrete. However, the contribution of the shear strength of the reinforcing ties based on an effective shear width bf of the section, as noted in Fig. 5.31, is permitted. For filled composite columns, it is conservative to neglect the contribution of the concrete to the shear strength of the column. For conditions where shear strength becomes critical, analytically the composite column may be treated as a reinforced concrete column with the steel tube considered as shear reinforcement. Transfer of forces between the structural steel and reinforced concrete should be made through shear connectors, ignoring the contribution of bond or friction.

The design and detailing of reinforced concrete columns in ordinary composite frames is similar to those of intermediate or special moment frames of reinforced concrete. Conservative detailing based on the special moment frame requirements are recommended for these frames in high seismic zones. This is because there is little research on the use of intermediate detailing of concrete columns in these applications.

In ordinary moment-resisting frames, the connections are typically designed to develop the strength of the connected members. However, in seismic design, it is generally desirable to avoid inelastic action in the frame connections. Both structural steel and concrete's contribution to member strength must be considered in the determination of connection strength, including the strengthening effect of the composite action of a steel beam and a concrete slab in the joint regions.

Transfer of loads between structural steel and reinforced concrete elements of a composite moment-resisting frame should be made only through shear friction and direct bearing. Reliance on bond and adhesion forces should not be considered because of the cyclic nature of the lateral loading. In addition, AISC Seismic (i.e., AISC 341-02) recommends that a 25% reduction in the typical shear-friction capacities be imposed for buildings in areas of high seismicity.

Panel zone strength calculations for composite frames with fully encased steel columns may typically be taken as the sum of the steel and the reinforced-concrete capacities. Reinforcing bar development lengths as required by the ACI 318 Provisions should be provided in the detailing of these joints.

Ordinary composite moment frames are permitted for buildings only in seismic design category (SDC) A or B where there is no height restriction. The values for seismic design factors are R = 3, = 3, and Cd = 2.5. These frames are not permitted for buildings in SDC C, D, E, or F.

5.5.1.2. Special Moment-Resisting Frames

The term "special" refers to systems where the elements and connections are designed and detailed to provide maximum ductility and toughness, implying excellent energy dissipation and seismic performance during severe earthquake shaking. In recognition of this ductility, seismic codes allow a maximum reduction in the design base shear for special moment-resisting systems. Because of the recognized ductility and the limited interference with architectural planning, special moment-resisting frames are most commonly used for resisting lateral forces.

Composite special moment-resisting framing systems are similar in configuration to ordinary moment-resisting frames. As in the steel or concrete systems, more stringent detailing provisions are required to increase the system ductility and toughness of the composite speical moment-resisting frame. The commensurate reduction in design lateral forces is identical to that in steel or concrete special moment frames. The goal of seismic detailing provisions is to confine inelastic hinging to the beams, whereas the columns and connections remain essentially elastic. The design base shear prescribed for this system is similar to the special moment-resisting frame systems of steel or reinforced concrete. Likewise, no limitations have been placed on their usage in high seismic zones.

Composite speical moment-resisting frames according to current (2004) belief are subject to the same potential failure mechanisms as experienced by special moment-resisting frame steel buildings during the Northridge, CA, earthquake of 1994, and the Kobe, Japan, earthquake of 1995. The design approach for composite special moment-resisting frames attempts to provide the maximum possible frame ductility, toughness, and energy-dissipation capacity. This requirement results in more stringent provisions for element and joint detailing. Generally these frames are designed to limit inelastic action to the beams, with the intent of preventing potential yielding in columns and connections.

The design and detailing provisions for composite special moment-resisting frames should incorporate all the corresponding provisions of steel and concrete special moment frames. The design should include the strong column-weak beam concept. For composite columns, transverse reinforcement requirements should be equivalent to those required for reinforced concrete columns in special moment-resisting frames. Special details are invariably required to meet the intent of closed-hoop and cross-tie requirements for composite columns with a structural steel core. An example of a closed-hoop detail for an encased composite column is shown in Fig. 5.32.

Steel and composite beams should be designed to meet the more restrictive bf /2f and d/tw compactness limits and the lateral bracing requirements of steel special moment-resisting frames. The additional restrictions are necessary to increase the resistance to local and lateral torsional buckling, allowing the beam elements to develop their fully plastic flexural capacity. However, steel flanges connected to a concrete slab with shear connectors are exempted from this provision since the lateral torsional and local buckling forces are substantially inhibited by the presence of shear connectors and the concrete slab.

columns in SMRF

Plan Section

Figure 5.32. Composite special moment-resisting frames; composite perimeter frame column-to-frame beam connection detail.

columns in SMRF

Plan Section

Figure 5.32. Composite special moment-resisting frames; composite perimeter frame column-to-frame beam connection detail.

Special composite moment frames are permitted for buildings in SDC A, B, C, D, E, or F without any height restrictions. The values for seismic design factors are: R = 8, = 3, and Cd = 5.5.

5.5.2. Braced Frames

Most braced frame construction is of structural steel, although there have been some examples of concrete-braced frames in taller buildings designed to resist wind loads. Two types of steel-braced frame construction are recognized in building codes: 1) concentric bracing, where the centerlines of the various members that frame into a joint meet at a single point; and 2) a relatively new form of braced frame called eccentric brace. Developed during the 1970s and 1980s, this system attempts to combine the ductility of moment-resisting frames with the high stiffness of concentrically braced frames. It consists of bracing elements that are deliberately offset from the centerline of beam-column joints.

The short portion of beam between braces or between the brace and the column is referred to as the link. The link of an eccentrically braced frame is designed to act as a ductile fuse to dissipate energy during seismic overloads. As a result, the design of brace elements can be performed so as to preclude the possibility of brace buckling. With proper choice of the brace eccentricity, i.e., of the length of the link beams, the stiffness of this system can approach that of a concentrically braced frame. The ability to combine the ductility of moment frames and the stiffness of concentrically braced frames has led to increasing use of the system in areas of high seismicity. The two types of bracing, namely, concentric and eccentric, are also applicable to composite systems.

5.5.2.1. Concentrically Braced Frames

As shown in Fig. 5.33, composite concentrically braced frames may be configured using a number of possible combinations of steel, reinforced concrete, and composite elements.

Encased comp. col (typ)

Encased comp. col (typ)

Steel brace (typ)

~Ww 7mr 7/Wr V-bracing

Inverted V-bracing

Steel brace (typ)

~Ww 7mr 7/Wr V-bracing

Inverted V-bracing

X-bracing Diagonal bracing

Steel beam

Comp. column

Comp. column

Two-story X-braced frame (e)
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