Column strengthening

In a multi-storey reinforced concrete frame structure, it was found that the concrete strength of 9 frame columns could not meet the design requirements during construction acceptance, resulting in excessive axial compression ratio, which could not meet the ductility requirements required by the limit value of axial compression ratio. On the one hand, the wrapping of CFRP can increase the axial compressive strength of concrete to a certain extent. On the other hand, the transverse restraint effect of carbon fiber wrap on columns can also significantly improve the ductility of columns under horizontal loads and meet the ductility requirements of axial compression ratio.

By calculating the bearing capacity and ductility, three columns with lower strength are wrapped with three layers of CFRP wrap along the whole column height. For the other 6 columns, 2 layers of carbon fiber wrap will be wrapped along the height of the column.

Analysis of bearing capacity and ductility of CFRP wrap reinforcement

After carbon fiber wrap reinforcement, the shear capacity and ductility of frame columns can be significantly improved.

The original structure is 800 x 800 mm frame column with a height of 3.6m. According to the relevant research results, the shear capacity of CFRP wrap can be improved by pressing calculation.

For the whole wrap scheme of the project, the formula is

The design strength of concrete is C40. According to this calculation, the shear capacity of single column should be 1312KN. For the three columns with lower strength, the lowest concrete strength is 28.3 Mpa, and the shear capacity is 1031 KN without reinforcement. When three layers of CFRP are wrapped, the increase of bearing capacity is 319KN, and the total bearing capacity is 1350KN, which is higher than the design bearing capacity. For the other 6 columns, the minimum concrete strength is 35Mpa, and the shear strength of 1209KN is not reinforced. When two layers of carbon fiber are wrapped, the increase of bearing capacity is 246KN, and the total bearing capacity is 1455KN, which is higher than the design bearing capacity.

According to the test results, when the axial compression ratio is determined, the ductility coefficient of CFRP wrapping columns increases linearly with the shear strength and weak bending coefficient V_s / V_m. When the axial compression ratio is 0.48, there is the following fitting formula.

μ=-1.278+5.233V_s/V_M

It can be seen from the upper formula that the greater the shear strength of carbon fiber, the greater the V_s/V_M and the greater ductility factor.

In addition, according to the literature, when the ductility coefficient remains unchanged, the stirrup characteristic coefficient has the following relationship with the axial compression ratio:

The upper formula shows that when the axial compression ratio is increased, the hoop ratio should be increased appropriately in order to maintain the ductility coefficient unchanged.

According to the conservative estimation, assuming that the axial compression ratio of columns increases by 0.3 due to the decrease of concrete strength, the corresponding stirrup characteristic coefficient increment is 0.066, and the new CFRP stirrup characteristic coefficient is 0.097, which fully meets the requirements.

It can be seen from the above analysis that the amount of CFRP wrap should be determined according to the bearing capacity and ductility index when the RC columns are strengthened with CFRP wrap to improve the seismic capacity.