Abstract
The Federal Emergency Management Agency (FEMA) stages manufactured housing units (MHUs) at key locations to be used during disasters when other housing accommodations cannot be arranged for individuals or families. Due to the unpredictability of natural disasters and the limitations of the manufactured housing industry to produce large numbers of MHUs in very short periods of time, it is necessary for FEMA to store a large number of units for many months or years. With these long storage periods come degradation issues with frames, tires, bearing, and braking systems. These types of issues are also of interest to the U.S. Department of Transportation (DOT) Federal Motor Carrier Safety Administration (FMCSA) because regulated carriers normally transport these units and 49 CFR 393.75 Tires specifies tire loading restrictions for manufactured homes.
To compare the running gear (tires, brakes, drums, bearings, axles, springs, and spring mounting hardware) currently specified in the 2014 Rugged Based Performance Requirements (RBPR) and the newly recommended running gear specified in the upcoming revision to the RBPR, the Oak Ridge National Laboratory (ORNL) outlined a Longevity Test Plan, which focused on the assessment of component longevity (failure rates by component type).
The Test Plan also studied the forces imparted to the MHU from the running gear from interaction with the roadway for both the running gear in the 2014 RBPR and the running gear specified in the upcoming RBPR revision. Since the assessment of component longevity and failures are also issues of concern for FMCSA, both agencies collaborated to support the research presented in this report. The tests and data collection were conducted at the Navistar Proving Grounds (NPG) in New Carlisle (near South Bend), Indiana, during the May-November 2018 period.
Four alternatives for the suspension system of a typical three-bedroom MHU were considered in this test. Alternative 0 was the MHU that was selected from the FEMA staging area in Selma, Alabama. Empirical data had shown that, on average, this alternative experiences one tire failure every 150 miles (or 0.0067 tire failures per mile traveled). Before testing, Alternative 0 was equipped with similar running gear hardware, although in new condition and installed following the manufacturer’s specifications. This became the Baseline Unit, or Alternative 1. Another unit, also selected from the FEMA staging area in Selma, Alabama, was equipped with better axles (i.e., axles that were rated for 7,000lbs as opposed to 6,000lbs for the Baseline Unit) and better tires. This became the Upgrade Unit, or Alternative 2. At the end of the Longevity Test, the Upgrade Unit was mounted with radial tires. This became Alternative 3.
Alternative 1 (Baseline Unit) had, on average, one tire failure every 251 miles (or 0.0040 tire failures per mile traveled). So, by simply following the correct manufacturer’s specifications for the assembly of the transportation system, a reduction of 41% in the per mile tire failure was achieved. In the same test, Alternative 2 (Upgrade Unit) showed, on average, one tire failure every 728 miles traveled (or 0.0014 tire failure s per mile traveled). When the Upgrade Unit was mounted with radial tires in the last part of the test, no tire failures were observed for the 2,400 miles tested. That is, in the case of the radial tires (Alternative 3) the tire failure was less than one per 2,400 miles traveled (or less than 0.0004 tire failures per mile traveled).
The data collected for the Longevity Test showed that the maximum accelerations were registered in all cases at the center of the MHU in the vertical direction. The overall maximum was registered by the Baseline Unit, Alternative 1. If that value is normalized to 100%, then the Upgrade Unit (Alternative 2) showed, on average, 81% at the same location, and Alternative 3 (radial-tire test) 59%. That is, the upgraded transportation system showed a reduction of 19% and 41% in the vertical accelerations transmitted to MHU at its center when regular and radial tires, respectively, were used.
As part of the Test Protocol, NPG conducted a visual inspection of the MHUs and transportation system at the end of each test day. ORNL reviewed all the inspection reports in an attempt to determine if there was any difference in the damage that the Baseline and Upgrade MHUs showed during the test. The Upgrade Unit always showed lower accelerations transmitted to the MHU than the Baseline Unit. In most cases, these differences were statistically significant. The FEMA inspection report, conducted by a FEMA certified inspector at the end of the tests, included a shorter list of damaged items for the Baseline Unit than for the Upgrade Unit, although similar type of damages was described for both units. This may be an indication that, although lower than those of the Baseline Unit, the accelerations transmitted by the Upgrade Unit were above the threshold at which MHU elements start to fail.