## 9 Building Occupancy and Use
### Structural Systems for Modular Buildings
#### Building Level
Structural efficiency is then optimized by leveraging the strength of steel against compression and tension forces, resulting in more robust and durable structures. A steel frame is also sometimes combined with a timber secondary structure to transfer heavy loads in multi-story timber buildings [86]. Figure 8: Structural solutions to support volumetric modules: (**a**) module stacking; (**b**) module anchoring to an exoskeleton; (**c**) module anchoring to the core. In general, most timber buildings exhibit a hybrid and complex structure, as mentioned by [PERSON] [107]. With only 78 examples, fully timber constructions are uncommon in all of the structural categories considered, representing 22.6% of all projects, while the remaining structures are hybrids. Most hybrid structures in timber buildings (54%) combine timber and concrete. Timber-concrete-steel structures account for 13.1%, and timber-steel structures for 5.4%. The classification also considers the presence of these materials in subsequent structural components [85]:  
* A podium or plinth (only the ground floor or a set of floors in the lower part of the building), which can be made of concrete or sometimes steel;
* The core, which can be made of timber (CLT or LVL), concrete, or an articulated set of one-dimensional metallic elements;
* Floor slabs, which can be made from a range of EWP or in a composite solution (timber with a layer of concrete or embedded precast concrete or steel beams);
* Lateral bracing and vertical/horizontal structural elements, which can be made of timber or steel;
* Others, mostly comprising external structural elements such as stairs and circulation areas. According to [PERSON] et al. [87], the use of concrete cores has increased over time (from 38% in 2009-2013 to 57% in 2018-2020) and is more prevalent in structures with more than 16 floors. Conversely, the use of CLT cores has decreased (from 63% to 43% over the same period). Additionally, their study revealed a discrepancy among nine-story structures: an increased use of CLT cores and a decline in concrete cores. However, there was a noticeable growth in the use of steel cores between 2017 and 2018, indicating a transition to hybrid models as systems evolve. The study's results make it challenging to determine whether the increased use of concrete cores results from taller building heights, increased beam-column system construction (which may require a core with more mass than CLT can provide), the trend of hybridization, or a combination of these factors. [PERSON] et al. [87] also noted a growing trend in the use of podiums (25% in 2009-2013 to 60% in 2018-2020). Interestingly, this measure seems not to correlate with a building's height or its program. According to [PERSON] et al. [48], using concrete podium structures offers advantages, such as accommodating ground-level commercial zones, providing open areas and large openings, and creating fire-resistant enclosures for substantial mechanical/electrical services and equipment. The timber structure is also outside the range of water splashing and earth moisture, and level transitions between inside and out can be created while providing structural protection for the timber without complex detailing. This type of structure also enables planners to create emergency staircases leading outside and access routes for emergency services without having to add special separate fire safety measures [86]. On the other hand, the motivation for using concrete in the core may be based on promoting lateral stiffness and strength in the structure, benefiting from concrete's inherent fire resistance, and reducing wind-induced sway, a commonly reported issue in tall buildings. In the case of Brock Commons Tallwood House, it was found that using concrete in the core also simplified the project approval process [48]. Nevertheless, integrating various materials in vertical structural components may also lead to challenges or issues.