One of the places Ruilong Li mentioned when he blogged recently about wind engineering facilities that test buildings to destruction was the Insurance Institute for Business & Home Safety (IBHS) Research Center in South Carolina. Last summer, in conjunction with the American Modern Insurance Group, the center conducted a series of tests to evaluate the effect of high winds on structures attached to manufactured homes.
Carports, awnings, and porches are frequently attached to manufactured homes, despite recommendations from manufacturers to the contrary. Typically they are designed to lower building standards than those that apply to the homes they are attached to, and most building codes allow them to be built to withstand lower wind speeds. As a result, they fail frequently and at lower windspeeds than the buildings to which they are attached. The host building-and quite often other structures nearby-can be damaged when they fail, and the cumulative insurance losses can be considerable.
Since June 1976, every new manufactured home has had to comply with codes adopted by the U.S. Department of Housing and Urban Development (HUD). In response to lessons learned from Hurricane Andrew, the HUD Code wind load requirements were revised in 1994 and incremental construction standards for three wind load zones were introduced. Manufactured homes built to the most basic standard-for HUD Code Zone I-may only be located in that zone.Homes manufactured to the more stringent HUD Code Zone III are designed for the highest wind loads but may be located in any of the three zones.
The IBHS tests were designed to demonstrate effective mitigation measures for strengthening carports and their associated connections, and to demonstrate the superior performance of homes manufactured to HUD Code Zone III standards.
IBHS researchers attached carports to homes manufactured to HUD Code Zone I and Zone III standards and subjected them to steadily increasing uniform wind flows and turbulent windstorm simulations until local or global failure occurred. A sequence of similar carports with incremental enhancements was then tested to determine the relative benefits of various mitigation measures. During the tests a window and a door were opened on each home's windward side to evaluate their performance during a high-wind event.
Carports with poor connections between supporting posts and the ground are extremely vulnerable to wind uplift forces when they exceed the weight of the structure, and failure can occur at very low wind speeds. Because such weaknesses are relatively easy to identify and fix, they were not included in the tests. Beyond such post anchorage issues, the tests noted several types of carport failure:
- roofs buckling at mid-span
- carport separating from home
- beams supporting the roof eave buckling
- connection between roof pans and support beams and channels failing
- wind-induced vibrations cracking roof pans
Analysis of the test results suggested design improvements to address these issues.
The results are not just applicable to carports. They apply equally to other lightweight aluminum attached structures, such as porches, awnings, and canopies and to similar structures attached to traditional site-built homes.
For a fuller description of the tests and analysis of the results, read the IBHS Summary Report.