Initial Assessment

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In the days immediately following the Loma Prieta earthquake HCD supervisory and inspection staff from the Division of Codes and Standards (Division) were dispatched to the area from several assigned areas within Northern California. The Division's Southern California staff were placed on standby to be moved into the disaster area in the event they were needed. Unlike previous disasters where Division staff were employed in the "damage assessment" activity necessary to obtain declarations of a disaster from the Governor and the President, extensive media coverage led to early declarations at both the State and Federal level.

Since Division staff were not required to participate in extensive damage assessment activities, they were able to concentrate on the damage to manufactured housing within mobilehome parks where HCD has a combination of primary and secondary enforcement responsibility. HCD has secondary enforcement responsibility for the Mobilehome Parks Act where local governments have opted to assume primary responsibility. In the event that local governments choose not to provide enforcement within their jurisdictions, the Mobilehome Parks Act provides that HCD shall have the primary enforcement responsibility within that jurisdiction. Of the State's nearly 6,000 mobilehome parks, HCD has primary enforcement responsibility in more than half of the parks.

The figure below is representative of a manufactured home considered to be 'down", i.e., the home's support system had failed to a sufficient degree that the home would be required to be reinstalled:

Within the disaster area Division staff found manufactured homes "down" in twenty-seven (27) mobilehome parks within the hardest hit counties of San Benito, Santa Clara and Santa Cruz. These parks reflected a mixture of both HCD and local enforcement jurisdictions.

Within San Benito, Santa Clara and Santa Cruz Counties there are 193 mobile home parks containing 24,438 manufactured homes. Seventy-five (75) of the parks (39%) and 9,243 of the spaces (38%) are subject to HCD enforcement jurisdiction with the balance subject to local government jurisdiction. There were reportable damages to manufactured homes in approximately twenty-seven (14%) of the parks within these counties. The Division's initial assessment of the performance of manufactured home support systems was conducted within these twenty-seven (27) parks. The initial count of the homes down within these parks was:

In addition to the homes that went down during the Loma Prieta earthquake, four (4) homes were destroyed. Two (2) of these homes were structurally damaged beyond repair and two (2) others were burned.

The figure below is of one (1) of the manufactured homes destroyed by fire during the Loma Prieta earthquake:

While the cause of the fires that destroyed two (2) mobilehomes during the Loma Prieta earthquake is not known, it is suspected that the gas piping to either the home itself or to one of the home's gas appliances were ruptured and the escaping gas ignited. Gas appliances within the home are required to be secured in place; however, previous experience indicates that homeowner replacements of appliances often overlook this requirement of HCD and Federal regulations. Another requirement of HCD regulations is that the gas piping connector to the home itself be flexible and of a six-foot (6') length. The purpose of this unusual connector, listed and tested specifically for this application, is to accommodate movement of the home without fracture of the connector.

Of the 592 homes that went down in the 27 parks surveyed, 301 were homes installed on steel piers (51%), 223 were installed on concrete piers (38%) and 68 were installed on concrete blocks (11%). All of these support systems for manufactured homes are approved under HCD regulations.

The steel pier is a favorite of many manufactured housing installers. They are relatively light in weight and their triangular construction allows them to be stacked one upon another thereby taking up little space in the installer's truck.

The figure below is of a manufactured home that had been installed with steel piers:

While the steel pier is favored by many for its light weight and easy adjustability, it provides little bearing surface for the manufactured home to rest upon. On the other hand, the steel pier has considerable ability to hold vertical weight. If the horizontal forces of wnc am earthquake could be overcome by tie-downs or some other means, the steel pier would be an excellent method of supporting a manufactured home.

Unfortunately, there are few areas of California that are not subject to the horizontal 10 us of wind and earthquake. Horizontal movement of the home during high winds or in an earthquake sometimes causes the steel pier to topple over, or for the home's frame to slip away from and off of the steel pier's relatively small bearing surface.

The figure on the previous page shows a steel pier still sitting vertical and nearly square upon its pressurized wood footing. In this particular circumstance it appears that the home has moved several inches horizontally without toppling the pier. Had the homeowner not left the wheelhubs and axles on the home after installation, preventing the home from falling completely to the ground, the steel pier would have punctured the floor of the home. This is a relatively common problem in an earthquake and many believe that the combination of vertical and horizontal forces in earthquakes first lift the home from the pier, shift the home horizontally a few inches, then gravity slams the home to the ground. The figure on the previous page would seem to support that theory in this case.

Until 1980, manufactured homes, then known by their still common name "mobilehomes' were vehicles under California law. Since these homes were treated as vehicles the wheelhubs and axles were prohibited by law from being removed from the home. Since 1980, manufactured homes have become transportable structures under California law and their towbars, axies and wheelhubs may be removed. Many businesses have developed over the years since 1980 that deal with the purchase, removal and recycling of the axies and wheelhubs from manufactured homes. It has even become popular in recent years to exclude these amenities from the price of new manufactured homes and they are removed from the homes by the dealer or installer at the time of delivery and installation of the home.

There are still a number of homeowners who advocate leaving the axles and wheethubs on the manufactured home and a few others who believe that the wheels and tires, properly inflated of course, should also remain on the home for stability during earthquakes. Although the maintenance of these seemingly obsolete appendages may seem ridiculous to most homeowners, particularly with recent concerns for energy conservation and recycling, the figure on the previous page makes the issue worthy of consideration.

The axle and wheelhub of the home shown on the previous page actually prevented the home from falling completely to the ground. Had the home gone to the ground, there is no question that the pier shown in the figure, as well as several others, would have punctured the floor of the home and caused considerable damage. The home, at the point of axle connections, was held approximately twelve inches (12") from the ground by this assembly of parts actually designed to move the home down the road.

The concrete pier is similar in design to the steel pier, only much heavier. The pier has a concrete base of triangular design with an adjustable steel bolt in the center of the pier to facilitate adjustment. While this type of pier has most of the characteristics of the steel pier and performs about the same as a steel pier, it is quite heavy and cannot be stacked. In most areas of the State the concrete pier does not have the same degree of popularity as it appears to have within the disaster area where its use was second only to concrete block.

The figure below is of a manufactured home that had been installed with concrete piers:

223, or 38%, of the homes down were installed on concrete piers and, like steel piers, concrete piers are an approved method of supporting manufactured homes under current HCD regulations.

Sixty-eight (68), or 12%, of the manufactured homes found by Division inspectors to be downed by the Loma Prieta earthquake were installed on concrete block. There were two (2) different methods of stacking the block, 1) block against block from the footing material to wood shims at the point of contact with the home's chassis, and 2) using intermediate wood spacers between the blocks. There was no identifiable difference in the performance of these two (2) methods. Unlike steel and concrete piers, concrete blocks provide a greater bearing surface at both the footing and at the manufactured home's chassis. This greater bearing surface is believed to be responsible for the improved performance of this type of support system in an earthquake.

The figure below is representative of concrete block support systems:

One of the problems noted with concrete block, where the support system failed, was the breaking of the blocks where earthquake forces raised the home and the home came crashing down on the block. In similar instances with steel piers, in the unlikely event that the chassis came down squarely on the pier, the steel piers buckled. There were no noted instances of the failure of concrete piers under vertical pressure.