ABSTRACT
In suspension bridges, cable clamps play a leading part in connection between the main cable and the suspender cable. The dead and live load acting on the stiffened girder of suspension bridge deliver to the main cable through the suspender cable. The clamp is the key node to connect suspender cable and main cable. The cable clamp are made of cast steel. After installation of the bolts, high tension which is applied on the bolt, makes pressure between the clamps and the main cable, preventing the clamps to slide along the main cable. When the gap in the cable body gets smaller, reduction of the pre-tension on bolts is very considerable. When suspension bridge serves, the large descent of the pre-tension in bolts, effecting as holding two pieces of the clamps together, could make the clamps displace along the main cable. Therefore, it’s vital to obtain the pre-tension in bolts. In this paper, a new testing method has been come up with, aiming at measuring the pre-tension in bolts by ultrasonic testing instruments. Ultrasonic testing has the characteristics of harmless to the human body, no damage to the measured object, fast measurement speed and high sensitivity, so it is widely used in recent years. The ultrasonic senor placed at one end of the bolt can transmit and receive signals at the same time. At present, the main application of ultrasonic detection is to transmit narrow pulse ultrasonic wave along the bolt. The ultrasonic wave transmit time can be obtained by detecting and calculating the ultrasonic echo signal. Test equipment is divided into three parts, that are mechanical module, data acquisition module and data analysis module. In the mechanical module the jack and pump are responsible to pull off the bolt. Data acquisition module is composed of ultrasonic sensor, pressure ring and temperature sensor. Data analysis module is responsible for real-time data analysis and record. Set jack on the bolt, and pull the bolt using the oil pump corresponding with the jack. Using the instruments relationship curve between tension and sound transmit time is recorded when the jack is tensioned hierarchically. The testing procedure is divided into two stages. At first stage, the tension on the jack increases gradually, and pressure on the jack is smaller than the pre-tension force on the bolt. At this point, the measured ultrasonic transmit time increases, but the change is small. At second stage, the pressure provided by the jack is larger than the pre-tension force on the bolt. It is defined as over tension stage. At this stage the force on the nut is zero, and the force on the jack equals to the pre-tension of the bolt. Compared with tension stage increment of the length of bolts is significant. In tension stage and over tension stage, the increments of bolt length may be significantly different, where the point of difference is bolt preload. Before and after the pressure on the jack reaches to the pre-tension of the bolt, increment of the length of bolts changes obviously, and then the pre-tension of the bolt can be obtained. Finally, in order to test the validity of the method, experiment is done. The pressure ring is installed under the bolt nut to get the actual tension of the bolt. At the same time, the new method is used to obtain the tension of the bolt. The method can be tested by comparing actual tension and measured tension. Ten compared data are recorded. There is 1 bolt on which the error of actual tension and measured tension is less than zero ton. 1 bolts error is less than 1 tons. 4 bolts error are less than 2 tons, etc. Considering the 5 tons error of pressure on the jack, differences between the target force and the measured tension are in the range of 5 tons, so the test procedure is valid.
