Preload measurement of steel-to-timber bolted joint using piezoceramic-based electromechanical impedance method

Steel-to-timber bolted connections play an essential role in enabling glulam column and beam to work together. However, these connections are prone to bolt looseness due to shrinkage and creep effects of timber material during a long service life, which may cause consequent connection failures and structural safety hazards. Recently, detection and evaluation of bolt looseness have gained considerable attention, whereas the existing vision-based method is challenging to inspect early bolt looseness, and the percussion-enabled approach is susceptible to being affected by harsh environmental noises. Thus, in this study, the authors proposed a pre tightening torque measurement method for steel-to-timber bolted connection based on variations of electromechanical impedance (EMI) of designed Lead Zirconate Titanate (PZT) sensors. A theoretical formula is derived first based on the piezoelectric equation and boundary conditions to illustrate the feasibility of using EMI variations to reflect looseness levels. Moreover, a sensor that can be easily integrated into the steel-to-timber connection and work as a washer is designed and fabricated. Consequently, the environmental performance against temperature and humidity of designed sensors is investigated by preliminary tests. Based on preliminary tests results, a frequency range that showed lower environmental sensitivity is selected as the excitation signal in the following bolt looseness measuring experimental studies. Experimental results well validate the effectiveness of EMI method in the monitoring of pre-tightening torque of steel-to-timber bolted connections, showing a great potential to inspect preload looseness of glulam structures in field applications.