The choice of whether a structure should be built of reinforced concrete, steel, masonry, or timber depends on the availability of materials and on a number of value decisions.
|Fig. 1: Reinforced concrete building elements|
1. Economy. Frequently, the foremost consideration is the overall cost of the structure. This is, of course, a function of the costs of the materials and of the labor and time necessary to erect the structure. Concrete floor systems tend to be thinner than structural steel systems because the girders and beams or joists all fit within the same depth, as shown in the second floor in Fig. 1, or the floors are flat plates or flat slabs, as shown in Fig. 2. This produces an overall reduction in the height of a building compared to a steel building, which leads to
(a) lower wind loads because there is less area exposed to wind and
(b) savings in cladding and mechanical and electrical risers.
Frequently, however, the overall cost is affected as much or more by the overall construction time, because the contractor and the owner must allocate money to carry out the construction and will not receive a return on their investment until the building is ready for occupancy. As a result, financial savings due to rapid construction may more than offset increased material and forming costs. The materials for reinforced concrete structures are widely available and can be produced as they are needed in the construction, whereas structural steel must be ordered and partially paid for in advance to schedule the job in a steel-fabricating yard. Any measures the designer can take to standardize the design and forming will generally pay off in reduced overall costs. For example, column sizes may be kept the same for several floors to save money in form costs, while changing the concrete strength or the percentage of reinforcement allows for changes in column loads.
|Fig. 2: Reinforced concrete building elements|
2. Suitability of material for architectural and structural function. A reinforced concrete system frequently allows the designer to combine the architectural and structural functions. Concrete has the advantage that it is placed in a plastic condition and is given the desired shape and texture by means of the forms and the finishing techniques. This allows such elements as flat plates or other types of slabs to serve as load-bearing elements while providing the finished floor and ceiling surfaces. Similarly, reinforced concrete walls can provide architecturally attractive surfaces in addition to having the ability to resist gravity, wind, or seismic loads. Finally, the choice of size or shape is governed by the designer and not by the availability of standard manufactured members.
3. Fire resistance. The structure in a building must withstand the effects of a fire and remain standing while the building is being evacuated and the fire extinguished. A concrete building inherently has a 1- to 3-hour fire rating without special fireproofing or other details. Structural steel or timber buildings must be fireproofed to attain similar fire ratings.
4. Rigidity. The occupants of a building may be disturbed if their building oscillates in the wind or if the floors vibrate as people walk by. Due to the greater stiffness and mass of a concrete structure, vibrations are seldom a problem.
5. Low maintenance. Concrete members inherently require less maintenance than do structural steel or timber members. This is particularly true if dense, air-entrained concrete has been used for surfaces exposed to the atmosphere and if care has been taken in the design to provide adequate drainage from the structure.
6. Availability of materials. Sand, gravel or crushed rock, water, cement, and concrete mixing facilities are very widely available, and reinforcing steel can be transported to most construction sites more easily than can structural steel. As a result, reinforced concrete is frequently the preferred construction material in remote areas.
On the other hand, there are a number of factors that may cause one to select a material other than reinforced concrete. These include:
1. Low tensile strength. As stated earlier, the tensile strength of concrete is much lower than its compressive strength (about ); hence, concrete is subject to cracking when subjected to tensile stresses. In structural uses, the cracking is restrained by using reinforcement, as shown in Fig. 3, to carry tensile forces and limit crack widths to within acceptable values. Unless care is taken in design and construction, however, these cracks may be unsightly or may allow penetration of water and other potentially harmful contaminants.
|Fig. 3: Stresses in a RCC beam|
2. Forms and shoring. The construction of a cast-in-place structure involves three steps not encountered in the construction of steel or timber structures. These are
(a) the construction of the forms,
(a) the construction of the forms,
(b) the removal of these forms, and
(c) the propping or shoring of the new concrete to support its weight until its strength is adequate. Each of these steps involves labor and/or materials that are not necessary with other forms of construction.
3. Relatively low strength per unit of weight or volume. The compressive strength of concrete is roughly 10 percent that of steel, while its unit density is roughly 30 percent that of steel. As a result, a concrete structure requires a larger volume and a greater weight of material than does a comparable steel structure. As a result, steel is often selected for long-span structures.
4. Time-dependent volume changes. Both concrete and steel undergo approximately the same amount of thermal expansion and contraction. Because there is less mass of steel to be heated or cooled, and because steel is a better conductor than concrete, a steel structure is generally affected by temperature changes to a greater extent than is a concrete structure. On the other hand, concrete undergoes drying shrinkage, which, if restrained, may cause deflections or cracking. Furthermore, deflections in a concrete floor will tend to increase with time, possibly doubling, due to creep of the concrete under sustained compression stress.