Civil Engineering Reference
In-Depth Information
erected—a very favorable situation for reinforced concrete. Finally, the time element fa-
vors structural steel frames, as they can be erected more quickly than reinforced concrete
ones. The time advantage, however, is not as great as it might seem at first because if the
structure is to have any type of fire rating, the builder will have to cover the steel with
some kind of fireproofing material after it is erected.
To make decisions about using concrete or steel for a bridge will involve several fac-
tors, such as span, foundation conditions, loads, architectural considerations, and others. In
general, concrete is an excellent compression material and normally will be favored for
short-span bridges and for cases where rigidity is required (as, perhaps, for railway bridges).
1.6
COMPATIBILITY OF CONCRETE AND STEEL
Concrete and steel reinforcing work together beautifully in reinforced concrete structures.
The advantages of each material seem to compensate for the disadvantages of the other.
For instance, the great shortcoming of concrete is its lack of tensile strength; but tensile
strength is one of the great advantages of steel. Reinforcing bars have tensile strengths
equal to approximately 100 times that of the usual concretes used.
The two materials bond together very well so there is little chance of slippage be-
tween the two, and thus they will act together as a unit in resisting forces. The excellent
bond obtained is due to the chemical adhesion between the two materials, the natural
roughness of the bars, and the closely spaced rib-shaped deformations rolled on the bar
surfaces.
Reinforcing bars are subject to corrosion, but the concrete surrounding them provides
them with excellent protection. The strength of exposed steel subject to the temperatures
reached in fires of ordinary intensity is nil, but the enclosure of the reinforcement in con-
crete produces very satisfactory fire ratings. Finally, concrete and steel work well together
in relation to temperature changes because their coefficients of thermal expansion are
quite close to each other. For steel the coefficient is 0.0000065 per unit length per degree
Fahrenheit, while it varies for concrete from about 0.000004 to 0.000007 (average value
0.0000055).
1.7
DESIGN CODES
The most important code in the United States for reinforced concrete design is the American
Concrete Institute's
Building Code Requirements for Structural Concrete
(ACI 318-02).
10
This code, which is used primarily for the design of buildings, is followed for the majority
of the numerical examples given in this text. Frequent references are made to this document,
and section numbers are provided. Design requirements for various types of reinforced con-
crete members are presented in the Code along with a “Commentary” on those require-
ments. The Commentary provides explanations, suggestions, and additional information
concerning the design requirements. As a result, users will obtain a better background and
understanding of the Code.
10
American Concrete Institute, 2002,
Building Code Requirements for Structural Concrete
(ACI 318-02),
Farmington Hills, Michigan.
