ATS: Skipped-Floor Systems for Multi-Story Structures Frequently Asked Questions

What is ATS?

ATS stands for Anchor Tiedown System, and is a continuous rod system that extends from the foundation to the top of the structure and is restrained (tied off) at each level of a multi-story structure. It's designed to provide the load capacity and overturning resistance in shearwalls for multi-story buildings - which is especially critical during earthquakes and high wind events. To see more information about ATS, download the ATS catalog.

Why can't I use holdowns instead of ATS?

Holdowns can certainly be used to connect floor levels together, and anchor shearwalls to the foundation. However, tall, narrow walls (with high aspect ratios) are commonly used in today's multi-story buildings. Holdowns are not always able to resist the high uplift forces associated with these types of wall systems.

How does ATS transfer the uplift forces to the foundation?

The ATS system transfers forces by providing a restraint point (bearing plate and nut) at each floor level. As the sheathing lifts up the lumber, the bearing plate engages, transferring the loads into the threaded rod, which are then transferred into the foundation.

What is the Coupling Take-Up Device?

The Coupling Take-Up Device (CTUD) acts as a coupler, while simultaneously compensating for shrinkage and settlement due to dead load and construction loading. Utilizing the CTUD ensures that the shrinkage is localized and compensated for at each level instead of transferring gaps up the building, which occurs when each floor is not tied off.

What is the difference between a skipped-floor system and a system with all floors restrained?

Basically, a skipped-floor system is one where there is not a restraint (bearing plate and nut) at each floor level where the overturning forces can be transferred into the tiedown system. View this animation to see more information about the difference between a skipped-floor system and a system with all floors restrained.

Is it okay to skip floors?

Simpson does not recommend skipping stories. Our research and testing have shown there are many issues and safety concerns with a skipped-floor system. Please read below for additional information about this subject.

Are the load paths different for a skipped-floor system verses a tied-off system?

Absolutely. In a skipped-floor system, the overturning forces transfer up the building until a restraint point is reached. As a result, all elements at the restrained point have to resist the uplift forces for any non-restrained stories below. In a tied-off system, the incremental uplift at each level is transferred directly into the tiedown system at that level. View this animation.

Isn't the skipped-floor system less expensive than tying off at every floor because it has fewer components?

No. This is a common misconception, and one that is not well-communicated by supporters of skipped stories. In a skipped-floor system, the components at a restraint point must resist all the forces from the non-restrained stories below. As a result, larger lumber sizes, threaded rod sizes and bearing plates must be used, which contribute to the cost. During construction, temporary bracing is also required to avoid possible collapse during a high wind or seismic event. An all-floors-tied-off system does not have these additional costs.

Does skipping floors affect the performance of the system?

Yes. Third-party witnessed testing performed at Simpson's Tyrell Gilb Research Laboratory in Stockton, California, has proven that the performance of a building with skipped stories is significantly reduced when compared to a building that has a restraint at each level. In a skipped-floor system, the lower shearwalls rely on the upper shearwalls for stability, which can substantially increase inner-story drift in the lower levels. Testing has also shown that because a skipped floor structure lacks an uplift restraint connection at every floor, it may not be able to provide adequate uplift restraint, which is critical in resisting earthquakes or high velocity winds. Read more about testing.

Does skipping stories affect my shearwall drift calculation?

Yes. The rod elongation (PL/AE) is calculated and used in the d_a, or anchorage slip component of the shearwall drift equation. For example, if there are two stories dependent on one restraint point, the lower story will have to move twice the distance (which is the distance that the rod will elongate) to transfer its overturning forces, as compared to having its own restraint point.

How is building shrinkage affected by skipping floors?

When a continuous rod is utilized, the gap created between the nut and bearing plate is equal to the cumulative shrinkage of all levels below. Therefore, when a restraint does occur, the increased distance between the nut and bearing plate results in increased horizontal drift. In addition, it is possible that the rod may protrude from the top of the building if the shrinkage is excessive. Providing a bearing plate at each level, in combination with using a take-up device, ensures that the shrinkage is compensated for locally and that it doesn't accumulate up the building.

In a skipped-floor system, there are multiple levels relying on one component. What are the effects of this?

Skipping floors removes the redundancies that can help prevent sequential failures. Multiple levels must rely on a single component for their performance. If that element fails, the entire uplift resistance for all non-restrained walls below may be compromised.

How does skipping stories affect the system during construction?

Skipping stories prevents the lateral force resisting system from being engaged during construction. If a high wind or seismic event occurs during construction and alternate bracing is not used, the entire system may be at risk. Restraining every story as the building is built eliminates the need for temporary bracing, and may eliminate potential liability issues.

Where can I find additional information about skipped floors?

Please see our technical bulletin T-SKIPFLOORS08.

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