Structural behavior

The challenges of designing and sizing the elements that make up a shade structure.

Structural engineer Craig G. Huntington, president of Huntington Design Associates Inc., describes the challenge of designing and sizing the elements that make up a shade structure: “Shade structures are generally intended to be economical structures, and, as such, it’s often difficult to include a proper membrane analysis in their budget.”

“There is a common expedient taken by designers of small shade structures I have seen, which results in extremely unconservative assumptions about the loads that act on the supports,” Huntington says. “Consider, for example, an uncurved, three-sided canopy supported by a post at each of its three corners. Let’s say it has a plan area of 300 square feet, and wind acting up or down against its surface of 20 pounds per square foot. This suggests a total vertical load of 300 x 20 = 6,000 pounds, which is then assumed to distribute equally to the three corners, or 2,000 to each post. So far, these assumptions are reasonable.

But then these designers go astray by assuming that the horizontal load acting at the top of each of the three posts is equal to this vertical load; that is, 2,000 pounds acting horizontally at the top of each post. Remember, though, that the fabric (and cables at the edge of the fabric) are pulling in a nearly horizontal direction against the top of the post.

The loading on the tops of the posts is directly in line with these cables, and the 2,000-pound vertical load at the post tops is therefore associated with a much larger (typically 3-5 times) horizontal load at the top of the post. Taking this simple approach, the designer is grossly underestimating the horizontal load at the top of the post, and seriously undersizing his posts or any tieback cables.”

Huntington suggests that shade structures don’t necessarily require a structural engineering consultant, but they do need to be conceived and detailed with a good understanding of their structural behavior.

Bruce N. Wright, AIA, is a consultant to designers and architects and a frequent contributor to Fabric Architecture.