Part 1
Perhaps the most destructive of nature's hazards, earthquakes havewreaked havoc on human habitation since the dawn of civilization and, while no form of construction can be considered truly immune to damage from this powerful force, in the most earthquake prone regions many different methods of coping with it have evolved. In general there are three ways structures can be made to withstand the forces of earthquakes; resistance, resilience, and dynamic response. Resistance basically means making a structure so physically strong and massive that it resists the damage the shock forces of an earthquake produce. This approach has tended to be difficult to employ because it often calls for large volumes of material. When the limit of resistance is reached, it may mean total and sudden structural failure, in which case the massive amount of collapsing material itself becomes the chief hazard. Underground homes have been the most successful with the strategy of resistance because their subsurface location allows the structure to communicate the shock waves of an earthquake like the earth around it rather than simply withstanding them by brute strength. This makes them some of the most earthquake resistant structures of all.Historically, this has been the most successful approach for building in earthquake prone areas. Japan is infamous for its frequent earthquake activity and its traditional approach to dealing with this hazard for housing was to rely on relatively light post & beam wooden construction which could be shaken violently but return to its previous state with little damage. If structures did fail, light composition meant there was less potential injury from falling material and often the structures would not fail completely, leaving opportunity for refuge and escape. Post & beam construction also offered the advantage of relatively quick repair after an earthquake, the nail-less joinery leaving surviving materials and components intact for reuse. This approach is also the approach common to steel framed skyscrapers and is responsible for the adage that skyscrapers arefar safer than other structures during an earthquake.
Dynamic Response is the high tech strategy for dealing with earthquakes. It is based on designing systems of components which allow a structure to respond dynamically to the forces of an earthquake. This may take the form of springs of cushions in a system of foundation piers. Or it might take the form of special limited motion sliding plates that allow the earth to slip under the structure as shock waves pass through it rather than transmit them to the structure. Another approach is to mount systems of mass dampers or shock absorbers in different locations in the structure so that it dissipates the energy of shock waves passing through it. The cost, sophistication, and perpetual maintenance overhead of these systems tends to limit their use to the largest and most critical of buildings.
Conventional stick frame homes and prefab mobile homes rely on the strategy of resilience for tolerating earthquakes and their predominantly lumber composition works fairly well in this respect. Natural lumber is a very resilient material. Their limitation is the use of nailed joints. Wood is exceptionally strong except in one respect; under tension force in the direction perpendicular to its grain. Wood is essentially a fiber material. It's like a very stiff rope made up of individual strands of fiber. Pull the rope in a direction parallel to the direction of the fibers and it is very strong. But pull a rope sideways, perpendicular to the direction of the fibers, and the individual fibers readily come apart and the rope unravels.
This is why pieces of wood will sometimes split when you drive a nail into them. A nail pushes between the fibers of the wood, breaking their weak adjacent bonds, and if it's a particularly hard or thin piece of wood -or the nail is very thick-has a potential to split the wood, generating a crack that spreads in parallel to the grain. This is why homes tend to be made with softer woods like pine, the softer wood easily crushed under the point-compression of a nail and so offering less resistance to the penetration of nails and cracking less frequently. The friction between the nail and the wood along the surface area of the nail is what holds nailed connections together. The strength of this connection is generally determined by the length of the nail and thickness of wood, which increases the surface area of the nail in contact by the wood. This is generally a fairly strong connection but it’s a connection that relies on the weakest aspect of the wood; the bond between its fibers. Once broken, the bond is never restored and the wood is permanently damaged. This is why one can't re-use a nail hole. And this is also why it takes many nails spaced out over a large area to effect a very secure joint.
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