Studying On Joint Connection Condition Monitoring Method Of Fastener Type Formwork Support Frame Based On Piezoelectric Active Sensing Technology

Fastener type formwork support frame is the most widely used form of formwork support in the whole formwork support system with the largest inventory.It has many advantages,such as fast construction speed,simple installation,reasonable force,etc.,and has become one of the most commonly used forms of formwork support in engineering construction.The support frame fastener node is the most complex and concentrated part in the whole support system.Once the damage occurs,it will lead to the failure of the joint connection,and then promote the collapse of the whole formwork support system,resulting in the loss of people’s lives and property.On the other hand,the aging of materials,the bolt screw is not in place,false screw,empty screw when installing the formwork support,and the fatigue damage caused by the complex site construction load in the use stage of the formwork,the cyclic use of the formwork and other factors will inevitably cause the loosening of the fastener node connection.However,in the actual construction process,because the formwork is a temporary structure,its safety is easy to be ignored,resulting in frequent accidents caused by the collapse of formwork support frame.In this paper,based on the piezoelectric active sensing technology,the joint connection state monitoring method of fastener formwork support frame is studied to ensure the safe operation of formwork support frame and reduce the occurrence of construction accidents,which has certain practical significance and practical value.For fastener type formwork supporting frame node connection state monitoring of real-time,noise resistance,sensitivity and robustness problem,by theoretical analysis,finite element numerical simulation and model test,wavelet packet energy method,time inversion technique and compressive resistance three different kinds of racks fastener node connection status monitoring method is proposed based on piezoelectric active sensing technology,Specific study contents are as follows:(1)The common piezoelectric materials and their main performance parameters are introduced,and the basic principle of piezoelectric effect is briefly described.Then,the basic theory of the wavelet packet analysis method,time inversion method and piezoelectric impedance technology are respectively introduced.(2)Based on Hertz contact theory,the contact mechanics model of the coupler-steel tube was established,and the relationship model between the contact area of the inner surface of the coupler and the steel tube and the joint connection state was deduced.The finite element software was used to model the fastener node of the formwork support frame,and the numerical simulation was carried out to verify the relationship model,which provided a theoretical basis for subsequent experimental study.(3)on the basis of the above relationship model,combined with the present applications of piezoelectric sensor technology,put forward three kinds of targeted different fastener node connection status monitoring method,which is based on wavelet packet energy method,the time reversal method and piezoelectric impedance technology,and designing a fastener node model,the experimental study is carried out based on three methods respectively,Verify the feasibility and effectiveness of the three monitoring methods.The specific content is as follows:(1)A method of monitoring node connection state based on wavelet packet energy method is proposed.Fasteners node of the closer connection,,the greater the contact area of fastener inside surface and steel pipe and across the interface of signal,the more of the energy value after wavelet packet decomposition of.So,based on wavelet packet energy method,according to the loose index RMSD can realize the loose status of fastener node of formwork supporting frame.The joint model of formwork support frame with straight Angle fastener is designed and made,and the monitoring test of different joint states is carried out.Through theoretical analysis and model test,the effectiveness and repeatability of the method based on the wavelet packet energy method are verified.The method can realize the online real-time monitoring of the joint tightness of formwork support frame in practical engineering,and has the advantages of simplicity,operability and strong.(2)Considering the complexity of monitoring conditions on the construction site,a method with good noise robustness is proposed,that is the monitoring method of connection state of support frame fastener node based on time inversion method.The closer the connection of the coupler node is,the larger the contact area between the coupler and the steel pipe is,the more stress wave signals passing through the contact surface are,and correspondingly,the larger the focusing peak value after time inversion is.The time inversion technique is used to monitor the joint state of right-angle fastener formwork support frame according to the size of the focusing peak value.The experimental results show that the method is effective and repeatable,and has certain anti-noise performance,which is more suitable for complex construction site monitoring.(3)Considering that it is difficult to detect the tiny loosening of joint connections and that the construction load,wind load and the accuracy of instruments and equipment will cause errors to the monitoring results,a joint state monitoring method of fastener node based on piezoelectric impedance technology and BP neural network is proposed.The tighter the connection of the fastener node is,the greater the stiffness of the nodes and the larger the admittance peak value extracted by impedance method is.Based on the high sensitivity of the piezoresistive method and the strong self-learning ability of the BP neural network,the right-angle fastener formwork support frame was monitored according to the damage index I.The experimental results verified the effectiveness,sensitivity and robustness of the proposed method.