Dynamic behavior of reinforced concrete slabs under rapid loading and low velocity impact
Date of Issue2016-11-04
School of Civil and Environmental Engineering
In addition to static loading, reinforced concrete structures may experience various kinds of dynamic loadings such as earthquake, impact or blast. These types of dynamic loadings can lead to different loading rates on structure and result in dynamic structural response which is different from the static behavior of structure. Therefore, it is desirable to gain better understanding of the influence of loading rate on structural behavior. Nowadays, impact problems on structures have gained greater attention in structural engineering. The impact loads caused by vehicle impact, crash of aircraft, natural hazard, terrorist attack or accidental impact may cause significant damage to structures. Nevertheless, there is still no satisfactory guideline for structural design and evaluation concerning structures subjected to impact loads. Hence, research work is needed to fill the gap between the increasing engineering demand and the limited knowledge in this field. The objective of this thesis is to experimentally and analytically evaluate the behavior of reinforced concrete slabs subjected to rapid loading and low velocity impact. In this study, an experimental program was designed and carried out. In this experimental program, thirty-nine reinforced concrete slab specimens were tested in three groups. In the first group, eighteen slab specimens with six types of designs were tested under static (0.0004 m/s), medium (0.4 m/s), and high (2m/s) rate loadings to study the influences of loading rates on the behavior of reinforced concrete slab. In the second group, six slab specimens with the same types of designs as the slab specimens used in the first group were tested under low velocity impact (5.43 m/s). A comparison study between the results from the rapid loading tests in the first group and the impact tests in the second group concerning damage process, failure mode, strain rate, and energy absorption capacity was carried out to illustrate the correlation between these two types of experiments. In the third group, fifteen reinforced concrete slabs were tested under low velocity impact to study the influences of impact velocity, mass, diameter and nose shape of the impactor on the behavior of slab. From the experimental results of impact experiment, it is found that the assigned impact energy is strongly related with slab damage. Therefore, a finite element (FE) analysis is followed using LS-DYNA software to numerically study the energy capacity of reinforced concrete slabs under impact. The influences of various parameters on the energy capacity of reinforced concrete slab are evaluated. Both rapid loading and impact experiments are costly. The FE analysis also requires lots of modeling efforts and time of calculation. Therefore, simplified analytical models were proposed for obtaining the load-displacement relationship of reinforced concrete slabs under dynamic loads and predicting the response of reinforced concrete slabs under low velocity impact. The analytical load-displacement relationship is based on a quadrilinear moment-curvature relationship and the Critical Shear Crack Theory which could consider both the strength softening of slab after punching shear failure and the loading rate effect. The model for assessing the impact response of slab is a 2 degrees of freedom (2-DOF) mass-damper-spring system based on structural dynamics. This 2-DOF model is further utilized to perform a probability analysis of reinforced concrete slab subjected to low velocity impact to investigate how the slab response and damage are affected by the inherent uncertainties existed in the impact problem.