Analysis of Manufactured Home Vulnerability in Extreme Winds

Abstract

Fatality rates are higher in mobile and manufactured homes (MMH) than in permanent site-built homes during tornadoes. The physical processes driving the enhanced fatality rates have not been fully explored, but must be understood to drive effective mitigation measures. Over a two year period from 2019 to 2020, detailed forensic assessments were conducted of tornado damage to mobile and manufactured homes in Alabama. These post-tornado studies were analyzed to evaluate the relative performance of common MMH anchorage systems. Experimental tensile testing was conducted on tie-down straps collected from field samples. Finally, a numerical model was developed using the commercial software SAP2000 to perform a parametric assessment of the demand on common anchorage systems in MMH. Overall, the study finds that a primary driver of the high fatality rates is the typical failure sequence of these homes. MMH usually experience anchorage failures prior to any other structural failures, resulting in an increased risk of rolling or lofting. The brittle anchorage failures lead to complete destruction of the superstructure at low wind speeds, increasing the likelihood of fatalities occurring. This study finds that several factors contribute towards this non-ideal failure sequence, including defective tie-down straps, premature ground anchor pullout, inadequate code requirements, and the increasingly common use of alternative, pan-style anchorage systems. Identification of these factors guides future mitigation and education efforts, and gives targeted directions for future research.