ATS: Issues To Consider With Skipped-Floor Systems

Increased Component Sizes. In a skipped-floor system, the overturning forces transfer further up the building until a restraint is reached. As a result, elements at and below the restrained point have to resist higher uplift forces necessary to constrain the unrestrained stories below. This results in increased lumber, threaded rod, and bearing plate sizes. In a tied-off system, the incremental forces at each level are transferred directly into the tie-down system at that level.

Drift. Shear walls must be designed for both strength and lateral stiffness. A skipped-floor system that is designed only for strength probably will not be sufficient to satisfy drift requirements. Test results indicate that skipping floors has the potential to substantially increase inter-story drift. Drift analysis is important for all shear walls, but it is particularly critical in skipped-floor systems.

Redundancy. Even when they are appropriately designed, structural elements can perform unexpectedly during major seismic and high-wind events. In a skipped-floor system, multiple floors rely on a single component for their performance. If that element fails, the entire uplift resistance for all non-restrained floors below may be compromised. With an all-floors-tied-off system, the lower floors do not rely on the stability of the upper floors for their performance.

Construction Stability. When a system is designed to be tied off at each story, it allows the contractor to laterally restrain the building as construction progresses. In a skipped-floor system, the Designer or builder should consider a temporary bracing method during construction until the tie-down system is fully engaged, to prevent collapse or damage during a seismic or wind event.

To further understand the benefits of an all-floors-tied-off system, see these other pages: