Revisiting Earthquake Lessons - Manufactured Homes
Thursday, October 4, 2018
Author: Kelly Cobeen
Structure Type: Manufactured Homes (Mobile Homes)
Earthquakes: 1994 Northridge, 2014 South Napa
Lesson: In the 1994 Northridge Earthquake, substantial and wide spread damage occurred to manufactured homes that shifted on or fell off of their supports. This resulted in significant loss of habitability and loss of homes due to fire. The 2014 South Napa Earthquake served as a reminder that this type of behavior will continue until substantive steps are taken to mitigate this behavior.
The 1994 Northridge Earthquake
The Northridge Earthquake confirmed a past pattern of poor earthquake performance due to manufactured homes falling off of their support systems. 5421 out of 9095 homes surveyed in 69 affected parks required reinstallation (4466 fell off supports, 955 shifted on supports) (HCD, 1994). Homes that have fallen off of their support systems, as a minimum, are generally considered uninhabitable until they have been reinstalled and leveled at their intended location. In addition, extensive damage can occur to surrounding porches, carports and other similar structures, as well as utility connections. A worst-case scenario for manufactured homes is fire ignition, particularly where homes fall or slide into utility hookups, causing severing of gas and electrical lines. The close proximity of homes aids rapid spread of fire. In the Northridge Earthquake 175 homes burned in the HCD-surveyed parks, including 76 homes that slid and severed utility lines and 17 that burned due to damaged gas-fired water heaters (HCD, 1994). Figure 1 shows a mobile home park that in the 1994 Northridge Earthquake lost at least 36 homes to fire.
Following the Northridge Earthquake
Due to poor performance observed in the Northridge Earthquake, the State of California passed legislation requiring the installation of bracing for newly installed manufactured homes or homes relocated to a new lot. There are no requirements for anchorage of existing homes that are not relocated. The regulations that were implemented are found in California Code of Regulations, Title 25, Chapter 2, Article 7, Section 1336. While intended to improve earthquake performance, the provisions were defined in terms of design to a wind pressure of 15 psf., except where homes were rated for higher wind loads. Practice is to use proprietary Engineered Tie-Down (ETS, Figure 2) systems to meet these requirements. These provisions require that manufactured homes be anchored to their foundations. “Foundations,” in manufactured homes are typically short sections of 2x framing or plywood placed on top of the existing grade. Those familiar with earthquake design principles will identify anchorage to a small block of wood sitting on top of the grade a conceptually inappropriate earthquake resisting system. Note that strap systems with soil anchors that are used as tie-down systems in some other parts of the country are seldom found in California mobile home parks.
The 2014 South Napa Earthquake
The South Napa Earthquake served as a limited check on the performance of regulations put in place following the Northridge Earthquake, and demonstrated that bracing systems installed with the intent to improve earthquake performance do not necessarily do so. While this event was smaller and affected far fewer homes, 157 out of 829 (19%) homes in five affected parks overseen by HCD required reinstallation. This included 11 out of 52 (21%) of homes identified by HCD as having an ETS and 19 out of 70 (27%) of homes identified by HCD as having state regulated Earthquake Resistance Bracing Systems (ERBS, Figure 3) (FEMA, 2015). Homes were almost exclusively installed on 2x framing or plywood “foundations” placed on top of the existing grade. Six homes were destroyed by fire, with fire ignition being attributed to severed utility lines. Figure 4 shows a manufactured home following the 2014 South Napa Earthquake. The home has fallen off of its supports into the utility hookup. A fire was ignited but was put out. The exit door for the home is blocked by the adjacent exit stair.
The performance seen in the 1994 Northridge and 2014 South Napa Earthquakes is very likely to be repeated in future earthquakes for several reasons. First, there is a very large stock of existing homes that are not required to improve seismic bracing, and so will remain vulnerable. Second, measures put in place to improve performance where homes are installed new or relocated are conceptually inappropriate and were shown ineffective in the 2014 South Napa Earthquake. More stringent federal minimum standards that could potentially improve performance have been put in place, but have not been implemented in California. The more stringent standards require consideration of wind uplift concurrent with horizontal wind loads, thereby removing the ability to rely on friction to transmit horizontal forces to supporting grade. Because regulations governing installation of manufactured homes fall outside of the purview of the California Building Standards Commission, it is not possible for the engineering community to pursue change through the standard code change process. As a result, substantive change will require either legislation or reinterpretation of existing regulations by HCD.
CSSC 1994. A Compendium of Background Reports on the Northridge Earthquake for Executive Order W-78-94 (SSC Report No. 94-08), California Seismic Safety Commission, 1994.
CSSC 1995. Northridge Earthquake: Turning Loss to Gain (SSC Report No. 95-01), California Seismic Safety Commission, 1995.
EERI 2004. The San Simeon, California, Earthquake of December 22, 2003, Earthquake Engineering Research Institute, 2004
FEMA 2015 Performance of Buildings and Nonstructural Components in the 2014 South Napa Earthquake (FEMA P-1024), Federal Emergency Management Agency, 2015.
FEMA 2017. NETAP Webinar: “Improving Earthquake Performance of Manufactured Homes.” Presented by the Applied Technology Council, September, 2017.
HCD 1994. California Department of Housing and Community Development letter to the California Seismic Safety Commission, June 23, 1994, and as quoted in Northridge Earthquake: Turning Loss into Gain.
USGS 1994. Dwelling and Mobile Home Monetary Losses Due to the 1989 Loma Prieta, California, Earthquake with an Emphasis on Loss Estimation (USGS Bulletin 1939-B), U.S. Geological Survey, 1994.
Figure 1. Manufactured home fire damage in the 1994 Northridge Earthquake. Photo credit: NIST GCR-98-743
Figure 2. Engineered tie-down system (ETS).
Figure 3. Earthquake-resistant bracing system (ERBS).
Figure 4. Manufactured home fallen onto utility hookups in 2014 South Napa Earthquake.