Columns Strengthening

1 Project Overview

The main body of a project has 18 floors above ground, and the height of the main building is 63.45m. The storey height is 5.1m for the first floor, 4.4m for the second to fifth and tenth floors, and 3.3m and 3.0m for the other floors. The project adopts the cast-in-place reinforced concrete frame shear wall structure, and the column size is 6.5×7.2m and 6.5×7.8m. The concrete strength grade is C40 below eight layers and C35 above. The seismic fortification intensity of the project is 8 degrees, the design seismic acceleration is 0.20g, the seismic fortification category is Category C, the seismic group is divided into the second group, the seismic grade frame is Class II, the shear wall is Class 1, and the site is Class II.

2 Questions

During the construction, a 2.5m cantilevered outer corridor was added, and the reinforcement of the four adjacent frame columns was not adjusted accordingly, which left hidden dangers to the project quality and safety. The main problems are as follows:

2.1 The minimum longitudinal steel reinforcement ratio after the 2~3 storey column is reinforced with concrete outside does not meet the requirement of 0.7, and further reinforcement is required.

2.2 The seismic performance of the first-layer outer concrete reinforcement method is not good, mainly because one side is not reinforced, and the reinforced stirrups are not closed in circles, and further reinforcement is required.

3 Reinforcement mechanism of FRP wrapped concrete columns

Since FRP wrap reinforcement is mostly used for reinforcement of flexural members, it is still necessary to understand its reinforcement mechanism when used for reinforcement of biased members.

When using FRP to wrap and reinforce the frame column, the main function of FRP wrap is to restrain the internal concrete. Since the tensile strength and elastic modulus of FRP wrap are high, this restraint effect is obvious. When the concrete is compressed and produces lateral deformation, the FRP wrap can restrain the concrete deformation. Increase the ultimate compressive strain of concrete, thus delaying the crushing of concrete in the compression zone, giving full play to the plastic deformation performance of longitudinal reinforcement, improving the ductility of reinforced concrete frame columns, thereby improving the bearing capacity and seismic performance of frame columns.

4 FRP wrap reinforced design

Under the premise of structural force analysis and calculation, combined with the comparison and selection of frame column reinforcement schemes, the selection of FRP wrap reinforcement method is more appropriate. The FRP wrap reinforcement design is shown in Figure 1-FRP wrap reinforcement construction drawing.

5 Summary

For frame columns with poor seismic performance and insufficient structural reinforcement, FRP wrap reinforcement is used. The reinforcement mechanism is clear, the design is reasonable, and it has been used for many years after reinforcement, and there is no abnormality in the working performance of the components. It shows that the FRP wrap reinforced frame column can not only improve the bearing capacity, but also effectively improve the seismic performance of the components, which is worthy of promotion.