U type hoop is an important component of reducing the peeling damage of carbon fiber cloth and concrete base material, ensuring the joint force of carbon fiber cloth and concrete base material, and is a part of the reinforcement body of CFRP fabric.

In fact, anchoring carbon fiber cloth and preventing peeling failure are only one of the U type hoops, and the U type hoop is also responsible for the reinforcement of the inclined section bearing capacity of the flexural member. In view of the difference between normal section and diagonal section, the layout of U hoop in beam is different.

Normal section strengthening

When the beams strengthened with CFRP are bonded with U hoops on both sides, it is basically shown that the beam is reinforced only for the normal section bearing capacity. The U type hoop arrangement of normal section can be divided into two types, which are mainly affected by the bonding length of CFRP sheets. It is important to note that the length of the extension only affects the layout of the U type hoop. No matter whether the length of the extension reaches the end, the carbon fiber cloth attached to the bottom of the beam is required to extend to the end in the reinforcement of the section of the flexural member.

When calculating the elongation of CFRP can not meet the requirements, it is necessary to evenly install U hoop anchorage in the extension length range of not less than three channels. In the three U hoop, one set is called end hoop at the end of the beam, which requires that the width of the U type hoop is not less than 2/3 of the width of the carbon fiber cloth at the bottom of the beam, and is not less than 150mm. In addition to the end hoop, the remaining two are the middle stirrup, and the width of the middle hoop is not less than 1/2 and 100mm of the carbon fiber at the bottom of the beam. In addition, both the hoop and the middle hoop need to ensure that the thickness is not less than half of the carbon fiber at the bottom of the beam.

When calculating the extension length of CFRP is enough, the arrangement of U hoop is relatively simple. At this time, at both ends of the carbon fiber cloth at the bottom of the beam and on both sides of the concentrated load, the U type hoop with moderate width can be set up, and the width of the U type can be carried out with reference to the width of the above extended length.

Oblique cross section reinforcement

When the carbon fiber cloth is reinforced for the inclined section of the flexural member, there will be a difference in the arrangement of the encrypted area according to the variety of the component. This article focuses on the reinforcement of the oblique section without considering the encrypt area. In the reinforcement of oblique section, the U hoop must be pasted vertically in the direction of the axis of construction, and the oblique pasting should not be adopted. To distinguish  normal section from reinforcement, the layout of U hoop in oblique section reinforcement needs to be determined by calculation.

In the calculation process of U type hoop, the bearing capacity of the inclined section should be strengthened first, and then the increasing value of the bearing capacity of the oblique section is calculated according to the bearing capacity of the existing structure. Finally, the width and distance of the U type hoop are determined by increasing the value and the correlation coefficient. It is important to note that the bearing capacity of the inclined section after reinforcement is related to the compressive strength of the original member, and it can not be considered to increase the bearing capacity of the oblique section without restriction by increasing the layout of the U type hoop.

In addition, it should be noted that the net spacing between the two adjacent U hoops (the distance between the edge of the U type hoop) should not be more than 0.7 times the maximum stirrup distance in the GB 50010, and should not be greater than 0.25 times of the beam height to ensure the reinforcement effect. At the same time, when the beam height is greater than 600mm, a longitudinal waist pressure band should be added to the waist of the beam, and self-locking device can also be added when necessary.