Abstract
In the seismic damage of bridges,it is common for the cracking damage of the plastic hinge area of the pier to cause the overall collapse of the bridge structure.How to avoid premature failure of the plastic hinge area becomes one of the keys to the design of the pier.All over the world,the lateral restraint reinforcement of the plastic hinge are specified from the aspects of reinforcement ratio and structural measures,and the core concrete is restrained to improve the compressive strength and meet the ductility required for the design[1].On this basis,many scholars have carried out many researches to solve the inherent defects of the concrete material in the plastic hinge area.
As listed in Table 1,six full-scale bridge pier specimens are designed,and the test piers(SFDCP)are partially reinforced with steel fiber fine stone concrete,which avoids the damage of the lower plastic hinge region with the most unfavorable force,and the bearing capacity is improved by 18%.We verified the feasibility of using steel fiber fine stone concrete replacing 25%reinforcement of the plastic hinge area on the foundation of the common piers.Finally,a simplified calculation formula is proposed to estimate the reasonable plastic hinge length of the SFDCP which match well with the test results.The SF-1 specimen is shown in Figure 1.
Figure 1 Dimension and reinforcement bars of specimen(unit:mm)
Table 1 Test parameters
RRRS:clamping ratio of replacing section.
Figure 2 Hysteresis curve of SF-1
The test parameters and hysteresis curve of SF-1 are both shown in Table 1 and Figure 2.
The SF-1 exerts a good restraining torsion effect in the plastic hinge region,the torsional peak load reached 55.84 k N·m,comparing to 34.58k N·m of the ordinary pier C-1.
Figure 3 Schematic diagram of equivalent plastic hinge length
Figure 4 Schematic diagram of the equivalent plastic hinge moving up[3]
In this paper,the torsion-bending ratio of the test is 0.47,and the superposition of torsion under small torsion-bending ratio has little impact on the position and length of the plastic hinge of the ordinary concrete specimen[4],that is,the evenly distributed torque has less influence on the bending curvature of the section,and the plastic hinge moves up in Figure 3 and Figure 4.There is the following formula:
Then we find a formula that can estimate the length of the SFDCP different from some norms[5]which only considers the ordinary piers.
And a new formula to estimate the plastic hinge position of the SFDCP:
Both Eq.(2)and Eq.(3)agree well with the test results.Each error is less than 24%.
The plastic hinge moves up in the SFDCP,so that the overall bearing capacity of the pier body is fully exerted,and the torsional bearing capacity is improved by more than 18%.
The 50%reduction of the amount of the reinforcement in the steel fiber fine stone concrete reinforcement area has no obvious influence on the resistance of the SFDCP.
In this paper,for the plastic hinge moving up,the formula for determining the plastic hinge moving amplitude is proposed and the plastic hinge length formula recommended by the specification is further modified.For the test results,a good fitting effect is obtained,which is suitable for steel fiber fine stone concrete replaced pier design.