MHS Challenges Natural Disasters- Earthquakes

By : Eric Hunting
Part 2

Under the forces of an earthquake a building's structure is deformed mostly by strong horizontal movements which create great shearing forces on its joints. The framing 'wracks', pulling back and forth, square frames pulled into rhombus shapes with alternating joints pulled out and in. For nailed joints the effect is akin to having someone rapidly flexing the joined pieces of wood with great force or wedging a crow-bar between them. This loosens the nails by pulling them back and forth, widening the split between the fibers that was first made when the nail was driven into the wood and weakening the connection. Such structures usually fail in an earthquake at the nail points, the wood finally splitting and cracking apart as nails are twisted or the nails being pulled out. After an earthquake home owners are often frustrated when authorities will not allow them back into their homes to recover their goods even when the buildings, from the outside, don't look too damaged. But the emergency workers know that all the nailed joints in the structure may be greatly weakened, with perhaps many cracks being hidden behind the cladding of the home, ready to fail at the least vibration or live load.
Mobile homes are a little better in the face of this sort of damage in part because they are normally no more than a single story high. This reduces the top mass of the structure greatly and hence the amount of shearing force applied as the structure shakes. They are also engineered to tolerate the conditions of road transport so they tend to be made to resist wracking better. But it's largely because of one of their great weaknesses; their primitive foundations. Most mobile homes have no more foundation than a set of little metal pyramid piers which stand on plastic or cement pads on the ground. When an earthquake hits these are quickly broken away, the structure then being free to slide around on the ground as the earth shakes under it. Unfortunately, in the process the underside and lower edges of the structure-not to mention any trailer wheels- can be quite torn up and then the single greatest flaw of mobile homes -their very poor repair ability comes into play to doom a structure nature itself didn't completely destroy.The standard techniques for reinforcing stick frame structures against earthquake damage involves control of wracking and hence the deformation caused by shearing forces. This is usually done by bracing the corners of frames with various forms of 'gussets' or adding diagonal tension members across frames to tie opposing corners.
These components are typically made of steel and are often bolted in place rather than being nailed in place. Diagonal bracing was common with the balloon framing of the late 19th century but disappeared as lumber shortages compelled builders to rely on shorter length timber.
Old Japan homes commonly employed post & beam construction with nail-less joinery. This had many advantages in that earthquake prone region. Nail-less joinery typically relies on a 'positive' or 'compression' interface between wooden components.
Thus one of the stronger properties of the wood -it's compression strength perpendicular to its grain- hold structures together and is doing the work of resisting shearing forces in response to the shaking of an earthquake. Joinery in the traditional Japanese home was often deliberately left a little 'loose', allowing the structure to be more flexible in response to earthquake shaking. This allows the full resilience of wood to come into play, structures flexing greatly but in the end withstanding the abuse. No surprise that this form of construction was employed through most of Japan's history. Let's now consider the way MHS behaves in an earthquake. Like nail-less post & beam structures made of wood, MHS relies on a positive interface between its components. All its components are bolted or clamped together. Thus it should exhibit the same resilient characteristics of the wooden structure and nail-less joinery with the benefit of the increased tensile strength of metals. Like the old style Japanese house, the MHS house will flex greatly but in the end it will return to its original shape.

However, MHS's aluminum profiles and clamped joint connectors are more rigid than wood and would not bend to as great a degree as wood will without suffering permanent deformation. But at the same time it is going to resist -due to its tensile strength- that high degree of bending better than wood can and so may subject its joints to less shearing force. In essence, MHS should behave more like the steel frames of skyscrapers which, as is well known, is one of the safest forms of structure in an earthquake. Where MHS may actually do more poorly than wood is in conditions of constant vibration or extremely frequent tremors. Aluminum is subject to a greater potential for metal fatigue because its molecular structure tends to retain deformation.
This metal fatigue will eventually lead to cracking of the components which cracks occurring at connection points. But these are conditions more typical of a vehicle suffering the vibration from decades of constant use. That is unlikely for any stationary structure.
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