Concrete Building System

Most of the material in the preceding chapters has pertained to the design of reinforced concrete structural elements, eg; slabs, columns, beams, and footings. These elements are combined in various ways to create structural system for buildings and other construction. An important part of the total responsibility of the structural engineer is to select, from many alternatives, the best structural system for the given condition. The wise choice of structural system is fare more important, in its effect on overall economy and serviceability, then refinements is proportioning the individual members. Close cooperation with the architect in the early stage of project is essential in developing a structure that not only meets functional and esthetic requirements but exploits to the fullest the special advantages of reinforced concrete, which include the following.

Versatility of from. Usually placed in the structure in the fluid state, the material is readily adaptable to a wide variety of architectural and functional requirements.

Durability. With proper concrete protection for the steel reinforcement, the structure will have long life, even under highly adverse climatic or environmental conditions.

Fire resistance. With proper protection for the reinforcement, a reinforced concrete structure provides the maximum in fire protection.

Speed of construction. In terms of the entire period, from the date of approval of the concrete drawings to the date of completion, a concrete building can often be completed in less time than a steel structure. Although the field erection of a steel building is more rapid, this phase must necessarily be preceded by prefabrication of all parts in the shop.

Cost. In many cases the first cost of a concrete structure is less then that of a comparable steel structure. In almost every case, maintenance costs are less.

Availability of labor and material. It is always possible to make use of local souses of labor, and in many inaccessible areas, a nearby source of good aggregate can be found, so that only the cement and reinforcement need to be brought in from a remote source.

Floor and Roof System:

The types of concrete floor and roof systems are so numerous as to defy concise classification. In steel construction, the designer usually is limited to using structural shapes that have been standardized in form and size by the relatively few prodcers in the field. In reinforced concrete, on the other hand, the engineer has almost complete control over the form of the structurl parts of building. In addition, many small producers of reinforced concrete structural elements and accessories can compete profitably in this field, since plant and equipment requirement are not excessive. This has resulted in the development of a wide variety of concrete system. Only the more common types can be mentioned in this text.

In general, the commonly used reinforced concrete floor and roof system can be classified as one-way system, in which the main reinforcement in each structural element runs in one direction only, and two-way systems, in which the main reinforcement in at least one of the structurl elements runs in perpendicular directions. Systems of each type can be identified in the following list:

(a)   One-way slab supported by monolithic concrete beams

(b)  One-way slab supported by steel beams (shear concrete are used for composite action in the direction of the beam span)

(c)   One-way slab with cold-formed steel decking as form and reinforcement

(d)  One-way joist floor (also know as ribbed slab)

(e)   Two-way slab supported by edge beams for each panel

(f)    Flat slabs, with column capitals for drop panels or both, but without beams

(g)   Flat plates, without beams and with no drop panels or column capitals

(h)  Two-way joist floors, with or without beams on the column lines

Each of these types will be described briefly in the following sections. In addition to the cast-in-place floor and roof system described in this section, a great variety of precast concrete systems has been devised.

Sources of Prestress force

Prestress can be applied to a concrete member in many ways. Perhaps the most obvious method of precompressing is to use jacks reacting against abutments, as shown in Fig.1a. Such a scheme has been employed for large projects. Many variations are possible, including replacing the jacks with compression struts after the desired stress in the concrete is obtained or using inexpensive jacks that remain in place in the structure, in some cases with a cement grout used as the hydraulic fluid. The principal difficulty associated with such a system is that even a slight movement of the abutments will drastically reduce the prestress force.

In most cases, the same result is more conveniently obtained by tying the jack bases together with wires or cables, as shown in Fig. 1b. These wires or cables may be external, located on each side of the beam; more usually, one end of the prestressing tendon is anchored, and all of the force is applied at the other end. After reaching the desired prestress force, the tendon is wedged against the concrete and jacking equipment is removed for reuse. In this type of prestressing, the entire system is self-contained and is independent of relative displacement of the supports.

Another method of prestressing that is widely used in illustrated by Fig. 1c. The prestressing strands are tensioned between massive abutments in a casting yard prior to placing the concrete in the beam forms. The concrete is placed around the tensioned strands, and after the concrete has attained sufficient strength, the jacking pressure is released. This transfers the prestressing force to the concrete by bond and friction along the strands, chiefly at the outer ends.

it is essential, in all three cases shown in Fig. 1, that the beam be supported in such a way as to permit the member to shorten axially without restraint so that the prestressing force can be transferred to the concrete.

Other means for introducing the desired prestressing force have been attempted on an experimental basis. Thermal prestressing can be achieved by preheating the steel by electrical or other means. Anchored against the ends of the concrete beam while in the extended state, the steel cools and tends to concrete. The use of expanding cement in concrete members has been tried with varying success. The volumetric expansion, restrained by steel strands or by fixed abutments, produces the prestress force.

 Most of the patented system for applying prestress in current use are variations of those shown in Fig. 1b and c. Such systems can generally be classified as pretensioning or post-tensioning systems. In the case of pretensioning, the tendons are stressed before the concrete is placed, as in Fig. c. This system is well suited for mass production, since casting beds can be made several hundred feet long, the entire length cast at once, and individual beams can be fabricated to the desired length in a single casting. Fig 2 show workers using a hydraulic jack to tension strands at the anchorage of a long pretensioning bed. Although each tendon is individually stressed in this case, large capacity jacks are often used to tension all strands simultaneously.