Earthquake Document: Detailed Evaluation

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Following the initial assessment, as time permitted, Division staff conducted a more detailed evaluation of 12 of the parks where damage was heaviest. Within these 12 parks were 479, or 81%, of the 592 homes down. These 12 parks were:

SAN BENITO

1. The Mission Oaks Mobilehome Park with 225 spaces and 120 homes down had homes installed with the following types of support systems:

	Total	Not Down	Down	% Failure
Steel Piers	75	14	61	81%
Concrete Piers	39	21	18	46%
Concrete Bock	111	70	41	37%
	225	105	120	53%

Five (5) homes within this park were equipped with Earthquake Resistant Bracing Systems (ERBS) and each performed without failure.

2. The Hollister Park with 10 spaces and 7 homes down had homes installed with the following types of support systems:

	Total	Not Down	Down	% Failure
Steel Piers	6	0	6	100%
Concrete Piers	0	0	0	0%
Concrete Bock	4	3	1	25%
	10	3	7	70%

3. The Country Trailer Estates with 4 spaces and 1 home down had homes installed with the following types of support systems:

Steel Piers	4	3	1	25%
Concrete Piers	0	0	0	0%
Concrete Bock	0	0	0	0%
	4	3	1	25%

SANTA CLARA

4. Dalys Mobilehome Park with 24 spaces and 1 home down had homes installed with the following types of support systems.

	Total	Not Down	Down	% Failure
Steel Piers	24	23	1	4%
Concrete Piers	0	0	0	0%
Concrete Bock	0	0	0	0%
	24	23	1	4%

SANTA CRUZ

	5. Rancho Cerritos with 144 spaces and 92 homes down had homes installed on the following types of support systems:

		Total	Not Down	Down	% Failure
	Steel Piers	33	11	22	67%
	Concrete Piers	106	38	68	64%
	Concrete Bock	5	3	2	40%
		144	52	92	64%

	Thirteen (13) homes in the park had ERBS and each one performed without failure.

	6. The Pinto Lake Estates with 174 spaces and 82 homes down had homes installed on the following types of support systems:

	Steel Piers	85	35	50	59%
	Concrete Piers	73	42	31	42%
	Concrete Bock	16	15	1	6%
		174	92	82	47%

	Four (4) homes in the park had ERBS and each one performed without failure.

	7. The Monterey Vista Estates with 122 spaces and 70 homes down had homes installed on the following types of support systems:

	Steel Piers	87	27	60	69%
	Concrete Piers	27	18	9	33%
	Concrete Bock	8	7	1	13%
		122	52	70	57%

	Fifteen (15) homes in the park were equipped with ERBS and each one performed without failure.

	8. The Green Valley Estates with 105 spaces had 57 homes down and had homes installed on the following types of support systems:

	Steel Piers	36	15	21	58%
	Concrete Piers	66	31	35	53%
	Concrete Bock	3	2	1	33%
		105	48	57	54%

	Two (2) homes in this park were installed on ERBS and each one performed without failure.

	9. The Colonial Manor with 71 spaces had 8 homes down and had homes installed on the following types of support systems.

	Steel Piers	5	3	2	40%
	Concrete Piers	49	45	4	8%
	Concrete Bock	17	15	2	12%
		71	63	8	11%

	Three homes in this park were installed on ERBS and each one performed without failure.

	10. The Meadow Manor with 276 spaces had 14 homes down and had homes installed on the following types of support systems:
	Steel Piers	10	0	10	100%
	Concrete Piers	0	0	0	N/A
	Concrete Bock	266	262	4	2%
		276	262	14	5%

	11. The Old Mill Mobilehome Park with 39 spaces had 14 homes down and had homes installed on the following types of support systems:

	Steel Piers	0	0	0	N/A
	Concrete Piers	32	20	12	38%
	Concrete Bock	7	5	2	29%
		39	25	14	36%

	Four (4) homes in the park were equipped with ERBS and each one performed without failure.

	12. The Freedom Mobilehome Park with 45 spaces had 13 homes down and had homes installed on the following types of support systems:

	Steel Piers	9	2	7	78%
Concrete Piers	31	25	6	19%
Concrete Bock	5	5	0	0%
	45	32	13	29%

The above represents a 0% failure rate of Earthquake Resistant Bracing Systems; a 64% failure rate for steel piers; a 43% failure rate for concrete piers; and, a 12% failure rate for concrete block. There was an unusual occurrence of the failure of concrete block in the Mission Oaks Mobilehome Park in San Benito County. Division inspectors returned to the park to look for a possible cause and found that the majority of concrete block support systems in the park were of lightweight (ornamental) concrete block rather than the conventional 8" X 8" X 16" building (structural) concrete block.

The figure below is representative of a failed block support system.

Although fewer than 4% of the homes in these 12 parks were equipped with Earthquake Resistant Bracing Systems (ERBS), their performance without failure is worthy of additional note. An ERBS is not designed to prevent damage or to prevent the home from being displaced from its pier or block support system, the ERBS is designed to reduce damage to the home. In each instance where homes were found to be equipped with ERBS the system performed as designed.

The HCD certified ERBS is designed to limit the fall of a home from its support system to a maximum of two inches (2'). Following an earthquake, homes equipped with ERBS may still have to be reinstalled upon their support systems. However, the reinstallation is greatly simplified since the home has not fallen completely to the ground and has not suffered significant structural damage. Homes with ERBS in the Loma Prieta earthquake, although their primary support systems had failed, were stable and were continued to be occupied.

While order, non-certified, and home-made ERBS reduced damage to homes on which they were installed, these devices did not perform as well as the newer certified devices. HCD is pleased with the industry support for the rigid certification standards and with the performance of the devices now that performance has been confirmed by more than the engineering theory initially applied in the certification program. HCD is also pleased that no performance conditions were observed of certified ERBS in the Loma Prieta earthquake that would necessitate a change in our regulations for certification.

In early 1990 HCD will be implementing legislation signed by the Governor in 1989 requiring a permit and inspection of each installation of an ERBS. This permit and inspection process will provide additional assurance to homeowners that the ERBS has been properly installed on their home.

The figure below is of a home with a failed primary support system and a functioning ERBS.

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