Testing in multi-story structures yields high performance

Visit any major city in the United States and you’ll come across new infill housing projects.

The attraction of urban living and the increase in housing demand have spurred the exponential growth of high-density, multi-story structures. As these mid-rise buildings gain popularity, new design methods are needed to accommodate for lateral and uplift forces. Since the design of new three- and five-story structures tend to favor many window and door openings, providing adequate overturning resistance can be challenging.

Typically with less wall space, site-built shear walls with traditional floor-tofloor hold-downs often do not provide the necessary restraint.

Continuous tie-down systems have emerged as a solution for light-frame multi-story construction. These systems typically consist of a continuous rod that extends from the foundation to the top of the structure and use either a bearing plate and nut or hold-down device to provide restraint.When using a continuous tie-down system, there are four primary design issues to consider: load path, shrinkage, drift, and the performance and effects of skipped stories.

Load path

In continuous tie-down systems, uplift forces are collected continuously and cumulatively in a central member, usually a steel rod that is anchored to the foundation.Overturning loads are transferred to posts through boundary nailing and travel upward through the system until finding a point of restraint at the top of each story (or in a skipped-floor application, at the top of a set of skipped stories).

Another distinct feature of continuous tie-down systems is that of cumulative and incremental loading. The rod at the lower shear wall is designed for the cumulative tension load, but the bearing plate or hold-down device is designed to resist only the incremental (story) uplift from the shear wall below.

Shrinkage

Wood structures will naturally shrink, which can negatively impact the performance of the overturning restraint system. Spaces can develop between the restraint and wood plates.When a shear wall tries to overturn, these spaces must be closed through vertical movement before the tie-down system can be engaged. This can result in additional drift, and can adversely affect the shear wall system performance. Shrinkage-compensating devices are often used to fill these spaces. (Calculations are required to determine whether shrinkage compensation is needed.) Drift—Inter-story drift is another factor that is affected by the performance of the continuous tie-down system.Drift is measured by evaluating anchorage slip and rotation, bending and shear deformation, fastener slip, elongation of rods, and deformation of the wood framing.

Depending on the type of tie-down system used (continuous versus skipped stories), drift limitations will vary.

The performance and effects of skipped stories

Recently, there has been much discussion as to which tiedown system performs better—a continuous tie-down system or a skipped-story system. A skipped-story system uses a continuous rod as a method to connect two or more stories together and relies on a common point of restraint to provide overturning resistance.

With a tied-off system, each story is restrained independently.

Laboratory testing

Simpson Strong-Tie performed a series of tests to measure the performance of these two systems. Four types of shear wall assemblies were tested. Each wall structure was three stories tall and 8 feet wide with 12-inch platform framing, utilizing Simpson’s Anchor Tiedown System (ATS). To limit the variables, the same wall system was tested for both skipped-story and tiedoff conditions with the exception of the rod size, which was based solely on requirements for stress (not drift).

Therefore, the skipped-story systems had the same rod size for all stories, while the tied-off system used different rod sizes based on the load demand at each floor.

The following four systems were tested: first-story skipped (no uplift restraint provided at top of first story), second-story skipped (no uplift restraint provided at top of second story), first- and second-stories skipped (no uplift restraint provided at top of the first and second stories), and all stories tied-off (uplift restraint provided at top of each story).

The testing simulated the Rinalid ground motions obtained from the 1994 Northridge, Calif., earthquake, which measured 6.7 on the Richter scale and resulted in 0.84g’s of peak ground acceleration.

Each configuration was tested twice and the average response of the largest cycle of inter-story drift was recorded (see photo right).

Test results

The test results showed that skipping stories has the potential to increase inter-story drift significantly, especially at the lowest story of any set of skipped stories that are tied together by a common restraint. The first- and second-stories skipped system and the first-story skipped system did not meet code drift limitations on the lowest story of the common restraint. The all stories tied-off assembly, however, did comply with drift limits.

With a skipped-story system, incremental uplift of each story must travel up the building until it finds the point of restraint. For example, the lower shear walls rely on the upper shear walls for stability as they transfer overturning forces up the building. As a result, elements at the restrained point have to resist cumulative loads for the entire system below that point.

In a skipped-story system, other factors must also be addressed, including lack of redundancy, shrinkage, construction stability, and drift.

Understanding the key issues

As mid-rise buildings continue to be one of the fastest growing construction segments, these structures will need to be designed with a system that can provide adequate lateral-load resistance.

Load path, shrinkage, drift, and construction sequencing can affect the integrity and performance of the system.

By understanding the key issues, you’ll be able to determine the best design solution for your multi-story structure.