## 8 Technical Challenges and Potential Solutions
### Discontinuous Horizontal and Vertical Diaphragm Issues  
As an essential structural component, the floor cassettes and walls must carry vertical loads and transmit horizontal loads effectively. In recent years, researchers have shown that the horizontal diaphragm rigidity relative to the vertical diaphragm is insufficient, or the distribution of the vertical diaphragm is uneven. The rigid assumption of the diaphragm would result in significant errors in the displacement, internal force, and natural vibration characteristics of the structure [159; 160]. Besides the flexural and shear rigidity, the IMCs' features significantly impact the in-plane rigidity and force transmission capacity [161; 162]. On the other hand, the IMU is typically connected at their corners in PFISBs, with apparent discontinuities in the horizontal and vertical directions. Therefore, as shown in Figure 12, there is a clear difference between the PFISB and traditional floor force transmission mechanisms. In addition to the vertical diaphragms, such as wall panels/corrugated steel walls/steel braces, the horizontal diaphragm, such as the floor and ceiling cassette, exerts importance on the IMC under lateral forces. The primary disadvantage of the vertical and horizontal diaphragm's discontinuity is the lateral rigidity reduction, regardless of the connection efficiency. Since PFISBs are increasingly used in multi- and high-rise structures, engineers use lateral stabilizing systems [111]. Yet, the non-rigid diaphragm complicates the IMC between the IMU and lateral stabilizing components. Therefore, if the calculation is still based on the rigid assumption, the real force transmission characteristics cannot be correctly reflected [3; 163; 164]. Thus, the discontinuous diaphragm influence has become much more complicated, as recently found by the authors in their experimental and analytical works [165].  
Figure 12: Cluster columns, beams, and discontinuous diaphragms.  
The B2 tower's hat-truss bracings [33], the base isolation system [90], the double skin plate shear wall [87], the hybrid coupled wall [88], and the light steel wall with concrete [89] are approaches to achieve the continuous vertical diaphragm and improve the lateral capacity. However, most of these need extensive onsite construction, reducing the PFISBs' sustainability. As a result, additional innovations are required to resolve the difficulties above without impairing the PFISBs' performance. Moreover, research on the horizontal diaphragms in traditional structures is currently focused on: (1) the deviation analysis of the rigid diaphragm hypothesis [166; 167]; (2) the classification of the diaphragm stiffness [168]; (3) the micro factors [169; 170] as well as the macro factors [171; 172] for the in-plane behavior of the diaphragm; and (4) the numerical modeling of floor systems, in particular the behavior of the IMC [173]. The above studies have shown that the in-plane diaphragm's performance would depend not only on the material but also on the IMC and structural configuration details, providing some reference for future research on the PFISBs' vertical and horizontal diaphragm. In order to explore the effects of connections on the in-plane diaphragm stiffness or global structural responses [3], some studies [174] were conducted considering the simplification method [175]. However, there was still a lack of comprehensive research on the diaphragms, unique connections, and the structural arrangements.  
As a result of the discontinuous diaphragm, an exact diaphragm analysis model is required rather than a rigid assumption, particularly in buildings with a decreased lateral stiffness. More research on the vertical and horizontal discontinuous PFISB diaphragm is needed, including a comparative experimental, numerical, and theoretical evaluation of the in-plane mechanism.