# An Effective Beam Width Model of Slab in Thick Wall-Thick Slab Joints Subjected to Lateral Forces
## 1 Introduction  
In Japan, conventional reinforced concrete wall (WRC) structures, such as described in [PERSON] and [PERSON]'s work [1] and as shown in Figure 1, are composed of bearing walls, floor slabs, and protruding beams connected to the walls (called \"wall beam\" in the following contents). This type of structural system was invented in Japan after World War II as a result of the Japanese Government needing to provide a large number of residential buildings with good structural performance and a comfortable living environment [2, 3]. This type of system had many benefits such as ease of mass production, good fire resistance, and high seismic resistance, resulting in the WRC structural system still being commonly used in current practice [1, 3]. In particular, the high seismic performance is one of the most important features given that Japan is located in one of the most active seismic zones globally. In several major seismic events since the widespread adoption of WRC buildings, such as the Tohoku earthquake in 2011 and Kumamoto earthquake in 2016, it was observed that WRC structures incurred far less damage than other types of structures [4, 5]. However, due to the existence of wall beams in WRC structures, of which the depth must be greater than 450 mm according to the \"AIJ Standard for Structural Design of Reinforced Concrete Boxed-Shaped Wall Structures\" (abbreviated as the \"WRC Standard\" in this work) [3], small frames surrounded by walls and wall beams were formed, which led to the designfreedom of the available interior space and reduced structure plane. This resulted in the WRC structural system being less favorable from an architectural viewpoint. To provide more interior space and increase the flexibility of room layout for architectural design, thick wall-thick slab (TWTS) structures composed solely of thick walls and slabs, as described in [PERSON] et al. 's work [6,7] and as shown in Figures 2 and 3, have been proposed in Japan. In TWTS structures, the thickness of the slab was increased relative to conventional WRC buildings. However, the depth of wall beams was then decreased to match that of the slab. As such, the wall beams become visually part of the floor slab itself which results in larger interior space being provided as well as much easier construction. Furthermore, by retaining the bearing wall elements, it is expected that this system can still provide high seismic resistance similar to that of WRC structures. Based on the combined benefits (i.e., high resistance to lateral forces, simplifying the construction framework, and more open interior space for architectural flexibility) of the TWTS buildings, there is strong interest in adopting this structural system into practice in the near future, particularly for use in large-size residential housings and office buildings. However, since this is a new system, there are no design guidelines currently available specifically for TWTS components. As TWTS buildings are expected to behave similarly to WRC buildings, the performance criteria required for WRC buildings from the relevant standards [3] could be adopted. One of the key requirements is to predict the deformation angle of wall components to evaluate seismic safety. In all the factors affecting the deformation angle, the initial stiffness, which is defined as the stiffness before external excitations are applied, is generally recognized as a critical index regarding the initial seismic performance of structural components under earthquake loadings and thus paid lots of attention to. However, because the wall in the TWTS structures is always much stiffer than the flat slab component, the flat slab is the critical member to transfer the lateral loading and which the deformation refers to. Thus, the stiffness of the wall-slab joints under earthquake loadings is mainly determined by the flat slab component rather than that of the wall component.