A standard building comprises of two major sections: (1) substructure which includes the foundation walls, footings and piers or slabs; and (2) superstructure which includes the body and the roof.
Prior to the 1994 Northridge earthquake, with the exception of some buildings that were built after the 1987 Sylmar earthquake and were voluntarily bolted to the foundation, in most buildings across United States, the two major sections of the house were just stacked on top of each other with absolutely no attachment between them whatsoever and with very minimal attachment between the sub-parts such as between the roof to the body and between the footings and piers to the foundation walls.
This factor has resulted in movement and displcaement and collapse of many buildings and at times of large earthquakes or hurricanes and tornados.
The Northridge earthquake resulted in major uniform code changes all across the US in 1996. The new building codes enacted after 1996 require that all parts of a structure to be positively attached to each other through designed foundations and seismic anchoring and strapping. Buildings being built now have their foundation systems attached to the body which is then attached to the roof in one continuous line so as to transfer the earthquake's shear force to the ground without damaging the building.
As such all new buildings are designed to be made of shear walls which are then bolted to the foundations according to certain engineering criteria. The illustration below shows all parts of a building that need to be tied together: (1) the foundation is attached to (2) the first floor which is tied to (3) the second floor which is tied to (4) the roof which is attached to (5) the adjacent wall of the building which is then attached back to (6) the foundation in one continuos fashion with the help of plywood, heavy lumber and specifically designed steel hardware commonly known as Simpson ties in the industry.
To achieve this, every single part of the building system, most importantly the weakest and most susceptible part of the building, the substructure, has to be sewn together and then attached to the superstructure to create a continuous path so that a building rides out the shear movement of the earthquake until it passes through.
The body is also tied together by way of specifically designed material and hardware to prevent movement.
HUD also adopted the new guidelines adopted by other government agencies in 1996 requiring that not only all the new constructions comply with the new codes, but also that in certain cases, the old houses that were built before 1996 to be upgraded to the new standards meaning that any factory-built house which was the subject of the code had now to be strapped to its permanent concrete foundation.
In 1996 HUD revised its 1989 version of its code and provided new guidelines entitled Permanent Foundations Guide for Manufactured Housing (PFG). This Guideline which was for the benefit of design and construction and inspection professionals. Read.
In May 21, 2009, HUD provided additional guidelines in regards to PFG for lenders and HUD personnel entitled HUD Handbook 7584. Read.
The 1996 code update, brought two significant changes to the equation. First the use of portable and adjustable metal piers for portable houses was disallowed. Second, the manufactured houses that were previously just sitting on these stands had now to be positively attached to a PERMANENT FOUNDATION system by way of one of the following methods: (1) HUD's own national standards; (2) Local standards that meet or exceed HUD's national standards; or (3) Specifically designed system approved, inspected and documented by local and HUD officials.
In the old HUD Code, adjustable steel piers were used and the foundation system was constructed in separate pieces.
Note that in new the HUD Code, adjustable steel piers are no longer used and all the foundation system is tied together and attached into concrete poured into the ground according to material, size and specific designs outlined in PFG.
However the 1996 edition of the HUD's PFGFMH no longer allows for this type of installation as the structures that were previously installed in this manner were damaged in the 1994 Northridge earthquake. The San Jacinto fault, presents great hazard from ground rupture in the urbanized San Bernardino area.
Even if the 1996 code allowed for this type of installation in general, because of the many deficiencies and code violations that can be clearly viewed below in the very pictures that were provided by the same persons who approved the installation, the house should have never been certified as code compliant when so many dangerous situations exist.
The figures above indicate that not only as of 1996 the type of support that is under this house is no longer allowed in general, but it has been specifically disallowed in areas where designated as high risk.
Historically, the San Jacinto fault zone, which passes through Rialto, San Bernardino, and Colton and continues south into the San Jacinto Valley in Riverside County, is the most active fault zone in southern California.
The last earthquake in the area occurred in 1923. During the 33 years from 1890 to 1923, the local San Bernardino region experienced five significant earthquakes with estimated local magnitudes of 6 or greater. Each one of these earthquakes resulted in energy release roughly equivalent to the energy released in the 1971 San Fernando earthquake, which caused 64 deaths and cost $550 million in damage (Kachadoorian, 1971).
The Fema map below shows that San Bernardino County is situated in the most susceptible area prone to earthquakes.