## 1 Introduction  
To diminish the existing vulnerabilities of infrastructure to seismic hazards and fulfill the ever-increasing structural demands of modern codes, the earthquake engineering community has steadily worked over the years to enhance knowledge on the behavior of structures under severe earthquakes. Experimental evaluation of existing and new seismic design methodologies can be one of the most effective ways to help engineering practice understand, accept, and utilize new information in their designs. Several experimental procedures can be used to simulate and measure the behavior of structural systems and components under simulated earthquake loads, including quasi-static testing, hybrid simulation and shake table testing. Quasi-static tests do not simulate ground motion and cannot reproduce rate-dependent effects and hybrid simulation relies on computer models for inertia effects, leaving shake table testing as one of the most direct and effective structural testing methods [1]. Structural response generated during shake table tests are the most consistent with the actual structural responses that occur during earthquakes. In fact, these types of experiment have the advantage of reproducing recorded accelerations of real earthquakes with high reliability while also maintaining the real dynamic rate characteristics, which include all inertial effects of the test specimen. These realistic conditions are highly useful in validating and calibrating models for seismic design, in performing evaluation of structures and in conducting seismic risk assessment [2, 3]. Despite the aforementioned advantages of shake table testing, some distinctive challenges must be taken into consideration and overcome to successfully carry out such testing. Many of the difficulties are linked to practical implementations and economic constraints. Avoiding scaling effects by testing entire systems at full-scale [4; 5; 6] is desired, but such endeavors require large facilities and expensive specimens. Space limitations in experimental facilities, equipment capacity and limited funding generally impose limits on the scope of the experiments. As a result, shake tables of reduced size and limited payload capacity are typically utilized, which in turn results in testing on reduced-scale specimens and on structural subassemblies instead of entire systems. To be as realistic as possible, even at reduced scale the specimens are often large-scale. Whenever the shake table experiments include large-scale specimens, a substantial amount of mass needs to be provided in order to achieve sufficient inertia to obtain the desired dynamic characteristic as well as to be able to generate sufficient forces to reach strength capacity of the specimen. This is especially important for the realistic cases intended to evaluate seismic performance levels approaching or exceeding collapse [7]. However, the downside of providing the needed mass can introduce safety issues caused by instability of the test model or by collapse under the simulated seismic excitations [1]. Accordingly, alternative methods and devices have been developed for externally supporting and securing the additional mass to the specimen and transmitting the inertial forces to the test model [8]. The various options come with benefits and drawbacks, whereby the implementation of a particular system will depend on the test objectives and the available resources [1]. Despite the wide variety of solutions, the inertial loading systems have not necessarily satisfactorily addressed all the concerns associated with safety, setup time, and cost. This article aims to propose and validate a new configuration of the inertial mass system to mitigate some of the shortcomings of past shake table testing configurations. The relevant system characteristics and the dynamic equations of motion are presented and discussed in detail. The new system has been used with success in dynamic experiments of cantilever-type reinforced concrete (RC) bridge columns pushed to near-collapse performance levels.