## 9 Building Occupancy and Use
### Structural Systems for Modular Buildings
#### Building Level
\begin{table}
\begin{tabular}{c c} \hline \hline
**Type of Structural System** & **Sub-System** \\ \hline \multirow{4}{*}{One-dimensional} & Exoskeleton \\  & Post-and-beam \\  & Frame structure & Post-and-beam with linear bracing \\  & Linear systems & Post-and-slab \\  & & Post-and-slab-band \\ \hline \multirow{4}{*}{Two-dimensional} & Crosswall and party wall \\  & Honeycomb \\  & Panel + beams \\  & Bearing wall & Panel + box beams \\  & Panel + truss \\  & Panel + beams + columns \\  & Panel + columns \\  & Panel + external frame (balconcies) \\ \hline \multirow{4}{*}{Three-dimensional} & Space modules \\  & Volumetric modules & Space modules \\  & Modular systems & Space modules + external frame (balconcies) \\ \hline \multirow{4}{*}{Combination} & Frame + panel \\  & Frame + space modules \\  & Exoskeleton + space modules \\  & Light frame + mass timber \\  & Panel + beams + external frame (balconcies) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sub-categories of structural systems. Adapted from [PERSON] et al. [85]. enhances stability and ensures a robust connection between the prefabricated modules and the building core, providing a secure and integrated structural system. Timber has a strength (parallel to grain) that is similar to that of reinforced concrete; hardwood is slightly stronger, and softwood is slightly weaker. Nevertheless, timber cannot rival the compression strength of modern high-strength concrete. Timber is less stiff than concrete, and both materials are far less stiff or strong than steel. However, timber has a low density compared to these other conventional structural materials. Due to this, when considering strength-to-weight and modulus-to-weight ratios, softwood demonstrates a comparable performance to steel. This implies that timber is notably structurally efficient in constructions, or components of constructions, where a significant portion of the load to be withstood is the self-weight of the structure. Examples are roofs, some bridges, and tall buildings' gravity load-resisting systems [105]. In designing tall timber buildings under specific circumstances, such as in seismically active regions, an additional challenge arises beyond the effects of gravity and wind loading. It is related to the dissipation of seismic energy within the structure, as timber is a non-ductile material. In a full-timber construction, the dissipation of seismic energy occurs at the connecting points between the timber elements, such as nails, screws, bolts, hold-downs, etc. Structural timber members can be combined with other materials, such as steel and concrete, to cope with the disadvantages found within tall buildings' structural and seismic design that have been mentioned [106]. In multi-story timber construction, access cores (such as emergency exits and those used to brace buildings), firewalls, or entire reinforced concrete base stories are now often combined with timber structures, and steel-reinforced concrete frames or compartmental-ized structures with a building envelope made of highly insulated timber panel elements are increasingly being used [86]. Combining concrete with timber in multi-story buildings offers several advantages over solid wood structures. This combination allows for longer spans, providing better performance in terms of vibration, deflection, and sagging due to increased structural stiffness. It enhances fire safety and soundproofing and minimizes vertical subsidence. Additionally, the concrete layer protects the underlying timber during construction and potential interior leaks. However, composite timber-concrete structures can be heavier if additional mass is not needed for soundproofing purposes [86]. Steel, known for its structural strength, is often used in situations that require support for long spans and heavy loads. By incorporating timber in this context, design flexibility is expanded, allowing for the creation of innovative, sustainable, and aesthetically pleasing structures.