Composite Building Systems

Composite building systems may be classified into the following categories:

1. Composite shear wall system.

2. Composite shear wall-frame interacting system.

3. Composite tube sytem.

4. Composite vertically mixed system.

5. Composite mega frames with super columns.

Steel link beam

Steel link beam

Steel Beam Embedded Concrete
Composite Moment Frame
Figure 5.7. Moment transfer between steel beam and concrete wall.
Shear Wall Steel Frame Construction
Figure 5.8. Composite shear walls with steel plates: (a) plan; (b) section.
Composite Shear Wall
Figure 5.9. Core-supported composite building: (a) concrete core; (b) concrete core encompassed with steel frame.

5.2.1. Composite Shear Wall Systems

In this system a central core consisting of concrete shear walls is designed to resist the total lateral load while the remainder of the construction surrounding the core is designed for gravity loads using structural steel. The construction sequence, whether the concrete core or the steel surround goes up first, is often project specific. In one version, concrete core is built first, using jump or slip forms, followed by erection of the steel surround, as shown in Fig. 5.9. Although the structural steel framing may not proceed as fast as in a conventional steel building, the overall construction time is likely to be less because the building's vertical transportion, consisting of stairs and elevators, and the mechanical and electrical services can be installed in the core while erection of steel outside of the core is still in progress. In another version, steel erection columns within the shear walls serve as erection columns, and erection of steel for the entire building proceeds as in a conventional steel building. After the steel erection has reached a predetermined level, concreting of the core takes place using conventional forming techniques. To facilitate jumping of forms from one level to the next through the floor system that is already in place, temporary openings are provided in the floor framing around the shear walls.

The behavior of a core-supported composite shear wall building is no different from that of a concrete building designed to resist all the lateral forces in the core. However, the absence of torsional stiffness due to lack of bracing at the building perimeter must be recognized in design. It is advisable to provide at least some reasonable lateral resistance, for example, one bay of lateral bracing at each building face.

If the entire lateral load, including torsional effects, is resisted by concrete shear walls, the steel surround may be designed as a simple framing for gravity loads only. Since there are no moment connections in the steel frame requiring welding or heavy bolting,

Shear Wall Intersections
Figure 5.10. Beam-to-shear wall connection: (a) embedded plate detail; (b) pocket detail.

the erection proceeds much faster. The only nonstandard connection is between the shear walls and the floor beams. Various techniques have been developed for this connection, chief among them the embedded plate and pocket details shown in Fig. 5.10. The floor construction invariably consists of a composite metal deck with a structural concrete topping. The composite shear wall system has the advantage of keeping the steel fabrication and erection simple. Since columns carry only gravity loads, high-strength steel can be used with the attendant savings.

The construction of the floor within the concrete core can be of cast-in-place concrete or of structural steel consisting of steel decking and concrete topping. The connection between the floor slab and the core walls should provide for the transmission of diaphragm shear forces from the floor system to the core. The weld plate detail shown in Fig. 5.10a is the most popular, particularly in a slip-formed construction. During the slip-form operation, weld plates are set at the required locations, with the outer surface of the plate set flush with the wall surface. The plate is anchored to the wall by shear connectors welded to the plate. The bending capacity of the connection is often supplemented with a bent steel bar welded to the plate at top. Experience in slip-form construction indicates that it is prudent to overdesign these connections to compensate for misalignment. Subsequent to the installation of weld plates, structural tee or shear tab connections with slotted holes are field-welded to the plate. Slotted holes provide for additional tolerance in the erection of floor beams.

Slip forming is a special construction technique that uses a mechanized moving platform system. The process of slip forming is similar to an extrusion process. The difference is that, whereas in an extrusion process the extrusion moves, in a slip-forming process the die moves while the extrusion remains fixed.

An important consideration in the design of core-supported buildings is the resistance (or lack of it) to overturning forces. Generally the vertical load resisted by the core due to gravity effects is limited because the floor area supported by the core is relatively small. For tall buildings, this can result in an unfavorable stability condition due to large tensile forces at the base. A method of counteracting the tensile forces is to apply an external prestressing force to the core. Similarly, an equivalent passive prestressing effect can be achieved by increasing the vertical load resisted by the core by manupulating the layout of floor beams. Extending this idea to its limit results in the concept of a building entirely supported on a single central core. Depending upon the floor area and the number of levels supported, several options present themselves for the support of the floor system from a central core. For example, 1) floors can be hung from the top of the center core; 2) they may be hung from story-deep cantilever trusses located at one or two intermediate levels, such as at the top and midheight of the building; or 3) the floor system can be cantilevered at each level. The second scheme has certain advantages primarily due to reduced length of hangers resulting in fewer floor-leveling problems. The advantages of a core-only supported building are 1) it offers views unobstructed by exterior columns at each floor; 2) the absence of exterior columns provides for the commonly sought column-free entrances at the street level; and 3) the undulations on the building exterior common in today's architecture are easy to accommodate.

Galvanized bridge-strand cables can be used as hangers to support the structural steel framing consisting of composite beams, metal deck, and concrete topping. The floor beams are attached to the hanger with simple supports, whereas at the core, pockets or anchor plates cast into the core walls provide for the support of floor beams.

It is common practice to slip-form the center core with an average concrete growth rate of 6 to 18 in./hr (152 to 457 mm/hr). After completion of the core, the second stage of construction in the hung-floor system is the erection of roof girders and draping of the floor-supporting cables. Erection of floor members between the core and the perimeter cables proceeds in a manner similar to that in typical steel building construction. Placement of steel floor decks and welding of shear studs for composite action is followed by placement of concrete topping. Because elongation of the cable due to cumulative floor loads can be substantial, it is necessary to compensate for this effect, during the design and possibly during the construction.

5.2.2. Shear Wall-Frame Interacting Systems

This system has applications in buildings that do not have a sufficiently large core to resist the entire lateral loads. Supplementing the resistance of the core with steel or composite moment frames located at the building exterior is perhaps the most common method of increasing the lateral stiffness. In North American practice, use of interior composite frames is not popular because the cost of form work, placing of reinforcement, and encasing of steel columns and beams with structural concrete far outstrips the advantages of additional strength and stiffness. Therefore, use of composite construction is typically confined to the exterior components. If the erection of steel members within the composite core precedes concrete encasement, it is usually more cost-effective to use

Typical Interaction Area Plan
Figure 5.11. Typical floor plan of a composite building using shear wall-frame interaction.

steel link beams between the shear walls. A schematic plan of this system is shown in Fig. 5.11.

5.2.3. Tube Systems

A framing system often referred to as a composite tube, used extensively in Louisiana and Texas, makes use of the well-known virtues of a concrete tube system along with the speed of steel construction. As in a concrete or steel tube system, closely spaced columns around the building's perimeter connected to deep spandrels form the backbone of the system. Two versions, both using composite columns are popular: One system uses cast-in-place concrete spandrels and the other structural steel spandrels. A relatively small steel beam is often used in the first system to stabilize the steel columns prior to casting concrete for the spandrels. However, in the design of the concrete spandrel, its strength and stiffness contribution is generally neglected because of its relatively small size. Schematic plan and sections for the two versions of tubular system are shown in Figs. 5.12 and 5.13.

In either of these systems, the speed of construction rivaling that of an all-steel building is achieved by erecting steel columns for the perimeter tube along with interior steel columns. Usually steel is erected some 10 to 12 stories ahead of encasing the perimeter columns with concrete. The key to the success of this type of construction for high-rise buildings lies in the rigidity of closely spaced exterior columns which, together with deep spandrels, results in an exterior facade that behaves more like a bearing wall with punched windows than as a moment frame.

Composite Moment Frame
Figure 5.12. Composite tube with concrete spandrels: (a) typical floor plan; (b) typical cross section through spandrel; (c) detail at perimeter column and spandrel intersection.
Architectural Tube Steel ColumnTubular Steel Pipes Sectional Details
Figure 5.13. Composite tube with steel spandrels: (a) typical floor plan; (b) typical cross section through spandrel; (c) detail at perimeter column and spandrel intersection.

5.2.4. Vertically Mixed Systems

Mixed-use buildings typically provide for two or more types of occupancies. This is often achieved by vertically stacking different amenities in a single building. For example, the lower levels may house parking, midlevels, office floors; and the top levels, residential units. Since different types of occupancies economically favor different types of construction, it is logical to mix construction vertically up the building height. For example, beamless flat ceilings with a relatively short span of about 25 ft are preferred in residential occupancies, whereas large spans of the order of 40 ft (12.2 m) are required in office buildings for optimum lease space. These spans are, however, too large for apartments, condominiums, and hotel suites. Therefore, it is possible to introduce additional columns without adversely affecting the architectural layout. The relatively short spans combined with the requirement of a beamless ceiling points toward concrete construction for the floors dedicated to residential occupancies.

In certain types of buildings, use of concrete for the lower levels and structural steel for the upper levels may provide an optimum solution. As an example, a 26-story building constructed in Houston, TX, is shown in Fig. 5.14. The bracing for the lower 13 floors is

Braced Bay With Diaphragm

Figure 5.14. Vertically mixed system: (a) schematic periframing; (b) schematic bracing concept.

Figure 5.14. Vertically mixed system: (a) schematic periframing; (b) schematic bracing concept.

Figure 5.14. (Continued).

Figure 5.14. (Continued).

provided by a combination of moment frame and shear walls, while a braced steel core and steel moment frame interacting system provides lateral resistance for the upper levels. The steel columns are transferred onto concrete elements by embedding them in concrete for two levels below the transfer level. Shear studs, shop-welded to the embedded steel columns, provide for the transfer of axial loads from steel to concrete.

5.2.5. Mega Frames with Super Columns

An efficient method of resisting lateral loads for buildings in the 60-plus-story range is to position columns farthest from the building center with shear-resisting elements in-between. This idea has given rise to a whole new category of composite systems characterized by their use of super columns interconnected across the building with a shear-resisting web-like framing.

The construction of super columns can take on many forms. One system uses large-diameter pipes or tubes filled with high-strength concrete in the range of 6 to 20 ksi

(41 to 138 MPa). Generally, neither longitudinal nor transverse reinforcement is used within the steel pipe or tube. Another method is to encase the steel column with reinforced concrete using conventional forming techniques.

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