Abstract

At present,Accelerated Bridge Construction(ABC)has been widely recognized and gradually promoted.This paper studies the mechanical behavior of a new type of pier-footing connection,namely socket connection,in the application of ABC.In the past decades,many scholars did a lot of research on socket connection.For example,Osanai et al.[1]studied the application of socket connection in the building structure and proposed that at least 1.5 times the column diameter(1.5D)could meet the requirements.M Mashal and A Palermo[2]studied the application of socket connection in bridge structure,not only verified the reliability of socket connection,but also suggested that the performance of 1.0D could be similar to that of cast-in-place structure.Haraldsson et al.[3]stated that the pier with 0.5D will be damaged by punching and shearing,and the footing cracks seriously,which means the vertical bearing capacity of the footing is insuf ficient.And then,scholars and engineers would like to believe that at least 1.0D was needed to ensure the performance of socket connection.In addition,Saiidi et al.[4-7]carried out not only quasi-static testing but also shaking-table testing in order to study socket connection and stated that socket connection may be a better choice for pier-footing connection or pier-cap beam connection.

As we all know,pile foundation is used extensively in China,so that the bottom plate is needed to be set under the pier when using socket connection,in order to meet the specification requirements of pile cap,that is,partial penetration.However,the configuration of socket connection above researches is usually not equipped with a bottom plate,that is,full penetration.Therefore,in order to apply the socket connection well in China,quasi-static testing of several specimens was conducted to study socket connection based on the existing research results,i.e.performance of socketed pier,reasonable embedment length and minimum reasonable embedment length were studied[8-10].The results showed that the socket connection could achieve the similar effect of the cast-in-place,and the bottom plate could further reduce the embedment length.Obviously,it is necessary to study the performance of the structure with merely increasing vertical load to further demonstrate that the embedment length can be less than 1.0D.Therefore,this paper designs two specimens to study the vertical bearing capacity of the structure.Specially,in order to determine the vertical bearing capacity of the socket wall,the socket wall of specimen A is constructed with smooth surface and a bottom plate is set to study the vertical bearing capacity of the bottom plate,while the socket wall of specimen B is constructed with rough surface but no bottom plate is set to.The specimen is shown in Figure 1.

The maximum axial force is loaded to 10 000 k N,which is equal to section capacity of pier approximately and around 5 times of the designed axial load.In addition,the load increment of each grade is 500 k N.During the test of specimen A,when the axial force was loaded to 1 000 k N,cracks were found in the bottom plate and the maximum width was 0.062 mm,the width of 0.084mm when loaded to 1 935 k N,and the width of 0.101 mm when loaded to 2 500 k N.When loaded to 10 000 k N,the crack width reached 0.45 mm.However,no obvious damage was found in the pier and the grouting material.For specimen B,when the axial force was loaded to 4 500 k N,a microcrack was found on the grouting material surface and its width was merely 0.033 mm,and when it was loaded to 10 000 k N,the maximum crack width of the grouting material was 0.08 mm.Simultaneously,the pier was not obviously damaged.Cracking is shown in Figure 2.

Figure 1　Configuration of specimens(unit:cm)

Figure 2　Cracks of footing

In addition,the Figure 3 can be obtained according to the force and displacement monitored by the actuator.The curves show that the specimens did not break up until 10 000 k N.Interestingly,the curves are actually very close,which may mean that the vertical stiffness and strength of the bottom plate and the socket wall are relatively close.It is necessary to point out that the displacement may contain around 7 mm of actuator slip displacement so that there is an approximate flat line in the front section of the curve.

Figure 3　Vertical loading curve

According to the test results,the following conclusions can be obtained.(1)The vertical bearing capacity of both the bottom plate and the socket wall is more than 10 000 k N.(2)Based on the result of force-displacement relationship,the vertical stiffness and vertical strength provided by the bottom plate and the socket wall are little different.(3)Under the condition of only vertical loading,0.7D can also meet the performance requirements.(4)It is reasonable to believe that if both the rough surface and the bottom plate are used in the socket connection,the vertical bearing capacity of the footing will be much larger than the section capacity of pier,so that only the pier can be damaged.