Abstract
The segmented ductile-irons pipeline with rubber-gasket push-on joints is one of the most commonly used type of pipeline in the water supply system.For typical nonstructural components,such as the large-diameter pipelines inside the utility tunnels,the seismic dynamic response is af fected by both acceleration and displacement[1].The influence of the stif fness,strength reduction and energy dissipation characteristics of the push-on joints of pipelines on the seismic response of the pipeline during earthquakes cannot be ignored.
A series of pseudo-static tensile tests were performed on the water-filled pushon joints of the DN400 ductile iron water pipelines in the utility tunnel to investigate the axial mechanical behavior and failure mechanism of the joints.Figure 1 illustrates a typical configuration of the push-on joints of ductile iron pipeline,which consists of the bell,the spigot and the rubber-gasket.The waterfilled push-on joints of the ductile iron pipelines were subjected to both monotonic and cyclic tensile loadings in their axial direction in the experiments.A displacement control scheme was adopted in the tests with a loading rate of 0.1 mm/s.Figure 2 shows the loading protocol for the cyclic tensile tests.The loading amplitude gradually increases from 2.5 mm up to the failure of the joint,and two loading cycles were performed at each amplitude.During the axil tensile tests,water sealing condition was recorded at the pipe joint.When initial leakage with small water drops form at the joint,it is regarded that the joint suf fers a minor damage,which does not seriously affect the water transportation capability of the pipeline.With the further increase of the joint opening,serious water leakage occurs with water inside the pipeline continuously flowing out from the joint.It is regarded that the pipe joint suffers significant damage,which impairs the water delivery capacity of the pipeline.The axial tensile tests were terminated when significant damage to the push-on joint were observed during the tests.A new rubber gasket was replaced in the push-on joint after each set of tests.
Figure 1 Configurations of push-on joints of ductile iron pipeline(unit:mm)
Figure 2 Cyclic loading protocol of axial tensile tests of push-on joint
Figure 3 compares the experimental results from monotonic and cyclic axial tensile tests.It was found that under monotonic and cyclic loading the push-on joint exhibits similar axial stiffness,which primarily attributes to the shear stiffness of the compressed rubber gasket between the spigot and the bell.However,the axial resistance of the push-on joint from the monotonic test is much larger than that from the cyclic test.It is because the lubricant of rubber sealing ring was air drying and crusting before the monotonic tests.In the subsequent cyclic tests,the lubricant remained effective,which had significant influence on the initial axial tensile strength of the push-on joint.
For the axial cyclic tests,it can be seen that the equivalent axial joint stiffness reduces exponentially with the increase of the axial joint opening,as shown in Figure 4.
Figure 3 Axial force-joint opening response of push-on joints under different loading protocols
Figure 4 Axial effective tensile stiffness of pipe joint
Besides,the initial and severe water leakage occurred at axial joint openings of 40 mm and 47 mm,respectively,for both monotonic and cyclic tests.The results show that the loading method has insignificant effect on the critical joint openings corresponding to the initial and severe water leakage at the joint as shown in Table 1 and Figure 3.In this paper,the experimental results from this research are also compared with the tests results from domestic and foreign scholars[2-7],as shown in Figure 5.It should be pointed out that Zhong et al.[7]and Meis et al.[2]presented peak joint opening at the moment of“serious water leakage”,similar to this study.However,the peak joint openings from other studies are corresponding to the joint axial deformation when the pipe spigot is completely pulled out.
Table 1 Axial test results of push-on joint
Figure 5 Comparison of peak joint openings from this test with published results
A new finite element model for the push-on joint was developed in OpenSees in this study to simulate its nonlinear behavior.Figure 6 compares the hysteretic axial force-joint opening response of numerical simulation and tests under cyclic loadings.The joint models can generally capture the stif fness degradation and energy dissipation characteristics of the push-on joint under cyclic loadings.The experimental and numerical results can provide a sound basis for the future study on the seismic performance of water pipelines in utility tunnels.
Figure 6 Comparison of axial force-joint opening response betw een experimental and numerical result