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BENDS. Frequently, it is
required that reinforcing bars be
bent into various shapes. There are several
reasons for this. First, let us go back to the reason
for using reinforcing steel in concreteto increase
the tensile and compressive strength of concrete.
You might compare the hidden action within abeam
from live and dead loads to breaking a stick over your
knee. You have seen how the splinters next to your knee
push toward the middle of the stick when you apply
force, while the splinters from the middle to the opposite
side pull away from the middle. This is similar to
what happens inside the beam. For instance, take a simple beam (a beam resting freely on two supports near its ends). The dead load (weight of the beam) causes the beam to bend or sag. Now, from the center of the beam to the bottom, the forces tend to stretch or lengthen the bottom portion of the beam. This part is said to be in tension, and that is where the steel reinforcing bars are needed. As a result of the combination of the concrete and steel, the tensile strength in the beam resists the force of the load and keeps the beam from breaking apart. At the exact center of the beam, between the compressive stress and the tensile stress, there is no stress at allit is neutral.In the case of a continuous beam, it is a little different. The top of the beam may be in compression along part of its length and in tension along another part. This is because a continuous beam rests on more than two supports. Thus, the bending of the beam is NOT all in one direction but is reversed as it goes over intermediate supports.To help the concrete resist these stresses, engineers design the bends of reinforcing steel so that the steel will set into the concrete just where the tensile stresses take place. That is why some reinforcing rods are bent in almost a zigzag pattern. The joining of each bar with the next, the anchoring of the bar ends with concrete, and the anchoring by overlapping two bar ends together are some of the important ways to increase and keep bond strength. Some of the bends you will encounter are shown in figure 7-11.
Figure 7-11.-Typical reinforcement bar bends. When reinforcing bars are bent, caution must be exercised to ensure the bends are not too sharp. If too sharp a bend is put into the bars, they may crack or be weakened. Therefore, certain minimum bend diameters have been established for the different bar sizes and for the various types of hooks. These bending details are shown in figure 7-12. There are many different types of bends, depending on where the rods are to be placed. For example, there are bends on heavy
Figure 7-12.-Standard hook details. beam and girder bars, bends for reinforcement of vertical columns at or near floor levels, stirrup and column ties, slab reinforcement, and bars or wire for column spiral reinforcement. SPLICES. Where splices in reinforcing steel are not dimensioned on the drawings, the bars should be lapped not less than 30 times the bar diameter, nor less than 12 in. The stress in a tension bar can be transmitted through the concrete and into another adjoining bar by a lap splice of proper length. The "lap" is expressed as the number of bar diameters. If using the No. 2 bar, make the lap at least 12 in. |
 
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