Figure 7-4.-Tension in steel bars.
Then, when it seems the bar will snap like a rubber band it recovers strength (due to work hardening). Additional pull is required (fig. 7-4, view C) to produce additional stretch and final failure (known as the ULTIMATE STRENGTH) at about 55,000 psi for mild steel.
The job of bending reinforcing bars is interesting if you understand why bending is necessary. There are several masons. Let us go back to the reason for using reinforcing steel in concrete - the tensile strength and compressive strength of concrete. You might compare the hidden action within a beam from live and dead loads to the breaking of a piece of wood with your knee. You have seen how the splinters next to your knee push toward the middle of the piece of wood 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 pad 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 all-it is neutral.
In the case of a continuous beam, it is a little different. The top of the beam maybe 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. It 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 the reason you may have to bend some reinforcing rods in almost a zigzag pattern. The joining of each bar with the next, the anchoring of the bar ends within 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 be required to make in reinforcing bars are shown in figure 7-5.
The drawings for a job provide all the information necessary for cutting and bending reinforcing bars. Reinforcing steel can be cut to size with shears or with an oxygas cutting torch. The cutting torch can be used in the field.
Before bending the reinforcing bars, you should check and sort them at the jobsite. Only after you check the bars can you be sure that you have all you need for the job. Follow the construction drawings when you sort the bars so that they will be in the proper order to be bent and placed in the concrete forms. After you have divided the different sizes into piles, label each pile so that you and your crew can find them easily.
For the job of bending, a number of types of benders can be used. Stirrups and column ties are normally less than No. 4 bar, and you can bend themContinue Reading