Dynamic load factor of concrete defects in hammer loading

Abstract

The most reliable way to check the safety of elements against the spalling of concrete defects is to apply a load of sufficient magnitude. Nevertheless, the determination of a sufficiently large load is qualitative and based on the technician’s intuition. Additionally, the response to impact loading is not the same as that to static loading. It would be useful to evaluate the response to a dynamic loading from an equivalent static loading. This paper investigates the dynamic load factor (DLF), defined as the ratio of the dynamic and static responses, adopting the response of a beam under impact loading and numerical analyses for more complex structures. Numerical results show that the most important variable in estimating the DLF is the ratio of the load time to the natural period of the first vibration mode. The DLF for displacements neglecting vibration damping can be estimated with sufficient accuracy in an undamped one-degree system. However, the DLF for stresses with damping, which is important for safety checks, is difficult to estimate. Nevertheless, its lower limit can be evaluated using the DLF of the undamped one-degree system. Although the risk of spalling is not considered to be high for defects with a first-order natural frequency exceeding 1000 Hz, a large load is likely to be applied by hammering. It is recommended not to hit hard to avoid excessive load during inspection.