Composite Elements

The primary structural components used in composite building construction consist of the following horizontal and vertical elements:

1. Composite slabs/diaphragms.

2. Composite beams.

3. Composite columns.

4. Composite diagonals.

5. Composite shear walls.

5.1.1. Composite Slabs

In steel buildings, the use of high-strength, light-gauge (16- to 20-gauge) metal deck with concrete topping has become the standard floor-framing method. The metal deck has embossments pressed into the sheet metal to achieve composite action with the concrete topping. Once the concrete hardens, the metal deck acts as the bottom tension reinforcement while the concrete acts as the compression component. The resulting composite slab acts as a diaphragm providing for the horizontal transfer of shear forces to vertical bracing elements. Furthermore, it acts as a stability bracing for the compression flange of steel beams.

In a concrete-filled steel deck, concrete is the stiffest part of the system in the horizontal plane. Therefore the shear stresses are primarily resisted by concrete. Thus, for transmission of force between a steel column and the diaphragm, forces must first transfer to the beam through the beam-to-column connection and then to the concrete fill either through welded studs or through puddle welds to the steel deck and finally through the bond and the embossments of the decking to the concrete fill. Each of these transfers must be adequate for the intended forces. When the concrete fill connects directly to concrete shear walls or steel-encased composite concrete beams, reinforcing dowels can be used for direct shear transfer.

Special construction considerations are necessary for studs welded to steel beams through galvanized steel decking because the zinc used for galvanizing can result in poor-quality welds.

When the published values for diaphragm capacities, based on test data, are less than those required by design, the concrete fill can be increased in thickness and adequately reinforced to resist the entire horizontal diaphragm shear. In this case, the metal deck serves only as a form for concrete placement and as gravity tension reinforcement of the composite floor slab as it spans between adjacent floor beams.

As with any diaphragm, the seismic load path including the chord and collector requirements, can best be identified by visualizing the slab as a horizontal beam in each direction. It is usually possible to utilize members already present in the floor system to serve as chords and collectors. For example, the perimeter beams may be designed for chord forces, so the only issues are ensuring that their splices and connections are adequate to resist the resulting forces.

5.1.2. Composite Frame Beams

The gravity design of composite beams is discussed in Chapter 7. Here the focus is on the design of frame beams subject to lateral loads.

For a medium-rise building in the 20- to 30-story range, the typical frame column spacing is usually 25 to 35 ft (7.6 to 10.67 m) and the floor-to-floor height is 12V2 to 13V2 ft (3.81 to 4.12 m). This geometry results in columns that are much stiffer in bending than the beams. Therefore, to limit the deflection of the frame under lateral loads, it is generally more economical to increase the beam stiffness than the column stiffness. The frame beams are typically designed noncomposite because they are subject to a reversal of curvature under lateral loads. However, the shear connectors provided for the transfer of diaphragm shear from slab to the beams also increase their moment of inertia. Nevertheless, the moment of inertia does not increase for the full length of the beam because its response to lateral loads is by bending in a reversed curvature with resultant tension in the top flange. Since concrete is ineffective in resisting tension, the increase in moment of inertia due to composite action can be counted on only in the positive moment region. Although design rules are not well established, a rational method may be used to take advantage of the increased moment of inertia. Occasionally engineers have used a dual approach in wind design by using bare steel beam properties for strength calculations, and composite properties in the positive regions for drift calculations.

5.1.3. Composite Columns

Two types of composite columns are used in building construction. The first consists of a steel section encased in a reinforced concrete envelope (Fig. 5.1). The second consists of a steel pipe or tube filled with structural concrete, as shown in Fig. 5.2. Conceptually, the behavior of a composite column may be considered similar to that of a reinforced

Load-carrying bars

Load-carrying bars

Note: Bond and adhesion must be ignored in calculating shear transfer Figure 5.1. Concrete-encased composite column; design considerations.

concrete column: The only difference is that in generating the axial load versus moment interaction diagram, the steel section is analytically replaced with an equivalent mild steel reinforcement.

Compositing of building exterior columns by encasing steel sections with concrete is by far the most frequent application. The reasons are entirely economic because form work around interior columns does not lend itself to jump forms. The exterior columns, on the other hand, are relatively easy to form using jump forms because they are open-faced: The form work can be "folded" around the steel columns for placement of concrete, and then unfolded and jumped to the next floor, repeating the cycle without having to dismantle the entire form work. However, in Japanese construction it is a common practice to composite the interior columns as well. Their construction makes extensive use of welding for vertical as well as transverse reinforcement (Fig. 5.3).

In the second type of composite column consisting of a steel pipe or tube filled with concrete, typically neither vertical nor transverse reinforcement is used (see Fig. 5.2).

Figure 5.2. Concrete-filled composite pipe column.

Figure 5.3. Japanese composite construction details: (a) I-beam column intersection; (b,c) composite column with welded ties; (d) general view.

Figure 5.3. Japanese composite construction details: (a) I-beam column intersection; (b,c) composite column with welded ties; (d) general view.

However, shear connectors welded to the inner face of the structural steel section provide for the interaction between the concrete and outer shell. Since the placement of concrete does not require form work, this type of composite column may be used in both the interior and the exterior of buildings.

A method of attaching frame beams to pipe columns is to weld the beams directly to the pipes, as shown in Fig. 5.4.

Figure 5.4.

Composite column-to-steel girder moment connection.

Figure 5.4.

Composite column-to-steel girder moment connection.

Figure 5.5. Bank of China Tower, Hong Kong.

5.1.4. Composite Diagonals

Braced frame buildings are mostly of structural steel construction. However, braced frames using composite diagonals and composite columns have been used in a few buildings since the mid-1980s. The majority of these are multistoried braces working in concert with "super composite columns." An outstanding example is the 76-story Bank of China Tower, in Hong Kong, shown in Fig. 5.5.

5.1.5. Composite Shear Walls

A schematic plan of a composite shear wall system is shown in Fig. 5.6. This is similar to a reinforced concrete shear wall system with the exception that a structural steel frame placed within the walls speeds up the construction process (see Fig. 5.6a).

Generally, in an all-concrete system, the walls are interconnected with concrete beams, commonly referred to as link beams, to increase their bending stiffness. If the link beams are relatively short, the resulting shear forces due to lateral loads may be quite large. This may lead to a brittle fracture of the beam unless the beam is detailed with diagonal reinforcement as mandated in most seismic provisions. The resulting detail often leads to congestion of reinforcement. A method of overcoming the problem is to use

Figure 5.6. Composite shear wall with steel beams: (a) plan; (b) elevation.

structural steel beams as link beams between the shear walls, as shown in Fig. 5.6b. The moment capacity of the steel beam is developed in the wall by welding shear connectors to the top and bottom flanges of the beam, as shown in Fig. 5.7.

For resisting large in-plane shear forces, a full-length steel web plate attached to a concrete shear wall may be used. An example of such a construction is the core wall of the Bank of China Building in Hong Kong. In this building all the lateral forces are transferred to the core at the base. To resist the high shear forces, steel plates are attached to the concrete core through shear studs welded to the steel plates, as shown in Fig. 5.8.

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