When the bottom surface of the secondary beam of the cast-in-situ structure is lower than the bottom surface of the main beam
In the cast-in-situ structure, the bottom surface of the secondary beam will be lower than the bottom surface of the main beam, and the common practice is to adopt the suspension bar method.
Two U-shaped suspension bars are arranged in the main beam, and the total length of the suspension bar extending into the main beam is La, and the length of the horizontal bending section is not less than 20d, and the general schematic is the intermediate continuous support method, and it is easy to ignore the construction practice of the longitudinal bar at the end support, and the following is the middle support method.
What is the structure of the bottom longitudinal bar when it is supported at the end?
The longitudinal reinforcement of the end support must meet the length of 12d ≥ extending into the support, and the longitudinal reinforcement should also be completely mounted on the suspension bar, not only on a suspension bar.
The practice of using hanging bars, after all, the anchorage of longitudinal bars is not as direct as the direct anchoring into the beam and the force is direct, considering that the bottom longitudinal bars extend to the inner side of the main beam reinforcement and bend upward, and the length of extending into the main beam meets 12d.
How is the area of the bar calculated?
First, the concentrated force transmitted from the secondary beam to the main beam is calculated, and then the required reinforcement area is calculated according to the axial tension.
For example, the concentrated load of the secondary beam transmitted to the main beam is 500KN, the suspension bar adopts two, and the steel bar grade is grade 3 steel, then the total area of the required suspension bar is: 500000/360=1389mm2, the area of a single steel bar is 1389/4=347mm2, and the steel bar diameter of the suspension bar is 22 (the area of a single root is 380mm2).
When the bottom surface of the secondary beam is lower than the bottom surface of the main beam, the reinforcement planting method is added
The practice that the bottom surface of the new secondary beam is lower than the bottom surface of the main beam is similar to the cast-in-place structure, except that the suspension bar needs to be implanted in the main beam, and the hanging bar planting needs to meet the margin requirements.
The concrete on the outside of the girder is chiseled, and the first row of longitudinal bars stretches out to the outside of the girder and bends, and the length of the bending section extending into the girder is 12d. The second row of longitudinal bars does not need to be bent, and can be extended into the support for 12d. The backing reinforcement is temporarily fixed with an anti-spill cap or a wooden wedge.