Full-scale test set up for a three-story skipped-floor system.

Simpson Strong-Tie conducted a series of tests to compare the differences between skipped-floor and tied-off systems. Four types of three-story shearwall systems were tested on a shake table in the state-of-the-art Simpson Strong-Tie Tyrell Gilb research laboratory in Stockton, California. The four systems include:

• All Floors Tied Off
• Second Floor Skipped
• All Floors Skipped
• Third Floor Skipped

The testing simulated a 6.7 magnitude earthquake ground motion to compare the drift for each story (inter-story drift) of each system. Building codes require the inter-story drift to be less than 2.5% of the story height. In this case, the tested shearwall systems measured 96 inches in height for each story, resulting in a 2.4-inch allowable inter-story drift.

The test results concluded that skipping floors has the potential to significantly increase inter-story drift. The three skipped-floor systems did not perform as well as the all-floors-tied-off system and in this case, did not meet code drift limitations on the lowest floor of a set of skipped floors.

The significant increase in the drift can be mainly attributed to:

• Effect of rod elongation accumulated at the lowest floor of a set of skipped floors.
• Accumulated elastic compression of the floor systems on the uplifting side of the wall.
• Accumulated compression (perpendicular to grain deformation) of top plates and sill plates on the uplifting side of the wall.

1st & 2nd Story Inter-Story Drift Comparison

1st Story Inter-Story Drift Comparison

For the All Floors Skipped and the 2nd Floor Skipped configurations, the 1st story is the bottom story of the group of stories tied together (see system illustrations below). This story experiences the greatest increase in inter-story drift. The graph below compares this drift with the Tied Off System.

click graph for larger view

2nd Story Inter-Story Drift Comparison

For the Third Floor Skipped configuration, the second story is the bottom story of the group of stories tied together (see system illustrations below). This story experiences the greatest increase in inter-story drift. The graph below compares this drift with the 2nd story drift of the Tied Off System.

click graph for larger view

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

• Introduction
• Animation: Skipped-Floor System vs. All-Floors-Tied-Off
• Issues To Consider With Skipped-Floor Systems
• Uplift Load Path: All-Floors-Tied-Off vs. Skipped-Floor Systems
• FAQs: Skipped-Floor Systems