## 1 Introduction  
Earthquakes can cause building collapse, and people may be buried by ruins [1, 2]. Relevant statistics show that being buried is the most common cause of earthquake-related injuries [3]. Therefore, it is necessary to conduct training to improve rescue efficiency and mitigate casualties after earthquakes. To achieve this, the ability to accurately simulate ruin scenarios is required. The reliability of simulated ruins is also an essential factor for efficient training [4]. To date, several studies on the collapse simulation of buildings have been conducted using the finite element (FE) method. Such studies typically focus on the collapse resistance of structures, rather than the formation of ruins, such as the analyses of the progressive collapse resistance caused by accidental load [5, 6, 7, 8], the simulations of seismic collapse processes, etc. [9, 10, 11, 12, 13]. Few studies that simulate the distribution of ruins induced by structural collapses have been conducted, due to the high computational workload, and the difficulty of simulating large deformations and nonlinear behaviors using the FE method [14, 15]. Currently, there are two methods for performing studies on ruins induced by structural collapse:
* Shaking table test Shaking table tests have been used to study the ruin distribution in many studies [16; 17; 18; 19; 20]. For example, [PERSON] [20] analyzed the collapse process and distribution of ruins using a \(\frac{1}{4}\)-scale shaking table test of a three-story reinforced concrete frame. In general, the structures used in such tests are highly simplified approximations of real-world structures. Owing to the high cost and limited experimental capacity of shaking table tests, small-scale models are often required, thereby limiting the reliability of the results. * Numerical simulation methods The discrete element method (DEM) [21; 22], applied element method (AEM) [15; 23] and physics engines are the most common numerical simulation methods for ruin simulations. A physics engine is a piece of computer software, based on [PERSON]'s laws of dynamics, that provides an approximate simulation of complex physical behaviors, such as rigid body dynamics (including collision detection), soft body dynamics and fluid dynamics. [24]. Some physics engines have a relatively low computational demand [25]. In recent years, physics engines have been widely used to simulate structural collapse and ruin distributions [26; 27; 28; 29; 30; 31]. For example, [PERSON] et al. [28] simulated seismic damage in urban areas, based on a multi-story concentrated-mass shear model and PhysX. Bullet Constraints Builder (BCB) [29] is a building collapse simulation software, which is developed based on the open-source physics engine Bullet [32] and Blender Python script. Bullet [32] is an open-source, real-time physics engine developed by [PERSON] in 2003, which can simulate collision detection, and soft and rigid body dynamics. The BCB can simulate the dynamic behavior of structures at the large-deformation stage, and the distribution of ruins. Furthermore, the simulation results are generally consistent with ruin scenarios in real-world earthquake events [30]. Although the physics engine-based building collapse simulation methods have advantages in simulating large structural deformations, the simulation results at the small-deformation stage are less reliable. For example, due to the difficulty of determining constraint damping in Bullet, and the oversimplification of the rebar model in BCB, the simulation results of the BCB and FE methods are quite different at the small-deformation stage. However, the process of the structural collapse is often sensitive to the initial state of the collapse. Therefore, ensuring the accuracy of collapse mode simulations using physics engine-based methods is difficult. To address the challenges mentioned above, a hybrid framework for simulating building collapse and ruin scenarios, based on an FE model and a physics engine, is proposed.