How do Multi-Family Buildings withstand earthquakes and wind loading?Jun 15 2017 · 0 comments · Nishkian Chamberlain, Wood Frame ·0
Wood is often seen in multi-family mid-rise buildings as the most economical construction material. While the purpose of joists, beams, and wood stud walls are easily understood in their role carrying the gravity loads of a building, the lateral load resisting elements and how they work can be more confusing.
Lateral Load Resisting Shear Walls
When lateral loads due to wind forces or a seismic event hit a building, the loads travel through the floor, collecting into and being resisted by wood shear walls. The top and bottom plates of the shear walls act as continuous collectors, moving the lateral loads from the diaphragm into the shear wall. Plywood sheathing, the nailing of the plywood to wood studs, and anchor bolts at the sill plate resist the shear forces. As the lateral loads move from a horizontal plane (i.e. the floor) to a vertical plane (i.e. the wall), the lateral loads also create vertical tension and compression forces, which are resisted by end posts and a hold-down system located at each end of the shear wall.
Figure 1: Components of a Shear Wall
At the beginning of a project, as unit layouts and floor plans are being set, it is important to ensure that the building will have an appropriate shear wall layout. In multi-family buildings, corridors usually afford a sufficient length of interior shear walls, but exterior shear walls must be carefully coordinated between the client, architect, and structural engineer. Façade features, window sizes, and window locations can all critically affect exterior shear wall designs.
Stacking Shear Walls
Structurally, it is always more efficient to stack shear walls from the top of the building to the foundation (or podium). This allows the components that resist the compressive and uplift forces to be continuous. When shear walls do not stack, the building code requires that components be designed for an increased load, and extra framing members and connections are required to transfer the loads.
Figure 2: Stacked Shear Walls
The shear wall design is not just determined by the number of shear walls, but also the length of each shear wall. As shear walls get shorter, the hold-down system gets loaded more heavily. The minimum length of shear wall permitted on a project depends on the floor-to-floor height – the taller a floor, the longer a shear wall must be. This aspect ratio is determined by taking the height (h) and dividing it by the shear wall length (bs). Shear walls cannot have an aspect ratio greater than 3.5 but as a rule of thumb, aspect ratios should be less than 2. Where the aspect ratio exceeds 2, the wall’s shear capacity is penalized. In other words, shear wall lengths should always aim to be greater than half the floor height.
Nishkian Chamberlain has extensive experience with multi-family construction and would be happy to help you find cost-effective solutions to your construction and development needs. Please do not hesitate to contact us at NCInfo@Nishkian.com, or give us a call at (310) 853-7180. You can also go to our Contact page to connect with any one of our offices in your region.
Figure 3: Aspect Ratio