Actual axial stress distribution

Equivalent axial stress

Lateral force

FIGURE 22.43 Equivalent column model for frame tube.

Lateral force

Distribution without shear lag

FIGURE 22.43 Equivalent column model for frame tube.

FIGURE 22.44 (a) Encased composite section. (b) Concrete-filled steel column.

FIGURE 22.44 (a) Encased composite section. (b) Concrete-filled steel column.

three-dimensional cantilever tube rather than an assembly of two-dimensional plane frames. The exterior frame tube significantly enhances the structural efficiency in resisting lateral loads and thus reduces the shear wall requirements. However, in cases where a higher magnitude of lateral stiffness is required (such as for vary tall buildings), internal wall cores and interior columns with floor framing can be added to transform the system into a tube-in-tube system. The concrete core may be strategically located to recapture elevator space and to provide transmission of mechanical ducts from shafts and mechanical

22.4.7 Steel-Concrete Composite Systems

Steel-concrete composite construction has gained wide acceptance as an alternative to pure steel and pure concrete construction. Composite building systems can be broadly categorized into two forms: one utilizes the core braced system by means of interior shear walls and the other utilizes exterior framing to form a tube for lateral load resistance. Combining these two structural forms will enable taller buildings to be constructed. Composite Column

Composite columns have been used for many decades, with steel-encased sections similar that shown in Figure 22.44a being incorporated in multistory buildings in the United States during the late 19th century. The initial application of composite columns was for fire rating requirements of the steel section. Later developments saw the composite action fully utilized for strength and stability. Composite action in columns utilizes the favorable tensile and compressive characteristics of the steel and concrete, respectively. These types of columns are still in use today where steel sections are used as erection columns, with reinforced concrete cast around them. One major benefit of this system has been the ability to achieve higher steel percentages than conventional reinforced concrete structures, and the steel erection column allows rapid construction of steel floor systems in steel-framed buildings.

Concrete-filled steel columns, as illustrated in Figure 22.44b, were developed much later during the last century but are still based on the fundamental principle that steel and concrete are most effective in tension and compression, respectively. The major benefits also include constructibility issues, whereby the steel section acts as a permanent and integral formwork for the concrete. These sections were essentially expensive, as the steel section was designed to be hollow, thus requiring large steel plate thickness. This lack of constructional economy has seen the use of concrete-filled steel columns limited in their application through the world. Furthermore, restrictive cross-section sizes have rendered them unsuitable for application in tall buildings where demand on axial strength is high. Composite Connection

For composite frames resisting gravity load only, the beam-to-column connections behave as pinned before the placement of concrete. During construction, the beam is designed to resist concrete dead load and the construction load (to be treated as temporary live load). At the composite stage, the composite strength and stiffness of the beam should be utilized to resist the full design loads. For gravity frames consisting of bare steel columns and composite beams, there is now sufficient knowledge available for the designer to use composite action in the structural element as well as the semirigid composite joints to increase design choices, leading to more economical solutions (Leon and Ammerman 1990; Eurocode 4 1994; Leon 1994).

Figure 22.45a and b shows the typical beam-to-column connections, one using flushed end-plate bolted to the column flange and the other using bottom angle with double web cleats. Composite action in the joint is developed based on the tensile forces developed in the rebars acting with the balancing compression forces transmitted by the lower portion of the steel section that bear against the column flange to form a couple. Properly designed and detailed composite connections are capable of providing moment resistance up to the hogging resistance of the connecting members.

In designing the connections, slab reinforcements placed within a horizontal distance of six times the slab depth are assumed to be effective in resisting the hogging moment. Reinforcement steels that fall outside this width should not be considered in calculating the resisting moment of the connection (see Figure 22.46). The connections to edge columns should be carefully detailed to ensure adequate anchorage of rebars. Otherwise they shall be designed and detailed as simply supported. In a braced

FIGURE 22.45 Composite beam-to-column connections with: (a) flush end plate and (b) seat and double web angles.
(b) >150
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