Page 122 - Design of Reinforced Masonry Structures

3.16 CHAPTER THREE

18 are usually not readily available; they also are too large for use in practical masonry
construction applications.
Properties of reinforcing bars and wire of different sizes used in masonry construction
are listed in the appendix. Table A.8 lists the cross-sectional properties of various sizes
of reinforcing bars. Table A.9 gives the areas of a group of reinforcing bars. Properties
of steel reinforcing wire are listed in Table A.10. These tables are referenced in examples
throughout this book.
Examples of deformed bars are shown in Fig. 3.5a. Reinforcing bars are classified accord-
*
ing to the minimum yield stress of steel they are made from. They are designated by the word
Grade, (often abbreviated as Gr.) followed by a two-digit number expressing minimum yield
stress of steel in kips per square inch (usually expressed as ksi). The industry produces four
grades of reinforcing bars: Grade 40, Grade 50, Grade 60, and Grade 75. A Grade 40 bar
indicates a steel reinforcing bar with yield strength of 40 ksi, and so on. The ASTM specifica-
tions for billet-steel, rail-steel, axle-steel, and low-alloy reinforcing bars (A615, A616, A617,
and A706, respectively) require identification marks to be rolled into the surface of the bar to
denote the producer’s mill designation, bar size, type of steel, and minimum yield designa-
tion, as shown in Fig. 3.5b. Grade 60 bars show these marks in the following order:
1. Producing m (usually the first initial of the mill’s name)
2. Bar size number (Nos. 3 through 18)
3. Type of steel
4. Minimum yield strength designation

The minimum yield strength designation is used for Grade 60 bars only and may be
either one single longitudinal line (grade line) or the number 60 (grade mark). A grade line
is smaller and is located between the two main ribs, which are on the opposite sides of all
bars produced in the United States. A grade line must be continued through at least five
deformation spaces, and it may be placed on the side of the bar opposite the bar marks. A
grade mark is the fourth mark on the bar. Grade mark numbers may be placed within sepa-
rate consecutive deformation spaces to read vertically or horizontally. Bar identification
marks may also be oriented to read horizontally—at 90° to those shown in Fig. 3.4b. Grade
40 and 50 bars are required to have only the first three identification marks (no minimum
yield designation).
The type of reinforcing bars used in practice has changed over the past decade. Although
both Grades 60 and 40 exhibit well-defined yield point and exhibit classic elastic-plastic
†
strain behavior (Grade 75 steel does not ), present practice is to specify/use Grade 60
steel as primary reinforcement steel. Grade 40 is most ductile, followed by Grades 50, 60,
and 75. Formerly, Grade 40 steel used to be more economical and also readily available.
Presently (2009), Grade 60 bars are more economical and used by the design professionals
for buildings and bridges. Grade 40-billet steel is available only in the smaller bars—Nos. 3

through 6. A major advantage of using Grade 60 reinforcement is that a smaller number of
bars can be used to achieve the same strength as a larger number of Grade 40 bars. Having
a smaller number of bars also helps alleviate the reinforcement congestion problem caused
by a larger number of Grade 40 bars placed in the cells of masonry units, which can impede
free flow of grout.
It should be recognized that codes and specifications may preclude the use of higher
strength reinforcement specifically. Designers should be careful in selecting/specifying the

*The term “yield stress” refers to either yield point, the well-defined deviation from perfect elasticity (linear
stress-strain relationship), or yield strength, the value obtained by a specified offset strain for a material that does not
have a well-defined yield point, for example, Grade 75 steel.
†Grade 75 steel exhibits elastic-plastic behavior but does not exhibit a well-defined yield point.