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In North America, most building codes recognize two types of building design and construction: 1) engineered design and construction and 2) design and construction by conventional rules.

Engineered construction uses fully designed systems, by engineers, to resist the calculated imposed loads. Conventional construction is based on a set of prescriptive rules derived from traditional construction practice that have evolved to reflect research findings, and past performance. Conventional construction is limited by building codes to smaller buildings with limited occupancies. Large houses and most non-residential buildings are engineered.

The inertial forces generated by the ground movement of the earthquake, concentrate lateral forces in the roof and floors where the mass of the building is greatest. The forces in the roof and floors must be resisted by walls and the entire structure must be adequately connected to the foundation. The following components of wood-frame construction are critical to the resistance of seismic forces:
-Anchorage of the foundation,
-Strength and ductility of walls,
-Strength and continuity of the horizontal floors, roof and ceilings,
-Interconnection of all the framing elements.

Conventional construction guidelines typically prescribe minimum roof, wall and floor constructions and the connections between them. Braced walls that meet minimum requirements with respect to sheathing length and type of sheathed wall must be spaced at regular intervals in the building. Anchor bolt requirements are specified to ensure that the structure is adequately tied to the foundation.
Typical construction of often exceeds the minimum conventional requirements in the building codes. For example, wall sheathing is often thicker than specified minimum thickness and typically there are more walls in a building than the minimum braced wall requirements.

In engineered design, a lateral load path is established and each element of the load path is designed and detailed to resist the calculated earthquake force. Roofs and floors are designed as diaphragms and some of the walls in the building will be designed as shearwalls. The design of shearwalls and diaphragms includes ensuring that:
-Structural wood sheathing (OSB or plywood) is thick enough to resist the calculated forces,
-Nailing is adequate to transfer the shear forces in the sheathing to the roof, floor or wall framing,
-Blocking is specified at the edges of the structural sheathing in the diaphragms and shearwalls, if necessary, and
-Framing members around the perimeter of the diaphragms and shearwalls are strong enough and properly spliced to resist the calculated tension and compression forces.

Engineered design also requires adequate connections between all of the elements in the load path. Therefore, additional nails or special framing anchors are typically required to connect the diaphragms to the shearwalls. Special “hold-down” connections are used to hold fown the corners of the shearwalls and additional anchor bolts are usually required to connect the shearwall to the foundation.

Wood-frame buildings have properties that naturally enhance their performance in earthquakes, although to ensure that wood-frame buildings are safe in earthquakes, good construction practices must be followed. Some of these properties are described in the following:

Strength and Stiffness:
The lateral forces of an earthquake tend to distort the building so the walls rack (become unsquare). Braced walls or shearwalls are critical for providing racking resistance during an earthquake. Walls constructed with plywood or OSB structural sheathing are very effective in resisting the racking forces of earthquakes. In locations where strong earthquakes are possible, the stiffness and resistance of the walls can be augmented by increasing the thickness of the structural panels and increasing the number or size of the nails. In addition, research and experience have shown that ‘non – structural’ elements contribute to the lateral resistance of the structure. For example, interior finishes, partitions, and many types of exterior cladding contribute to the lateral resistance of the structure.

Ductility:
Compared to other materials such as masonry and concrete that have to be carefully designed and detailed to ensure good seismic performance, wood systems are inherently more ductile. Ductility is the ability of the structure to yield and to deform without collapse. It is desirable for a building to have some flexing capability when subjected to the sudden loads of an earthquake because the flexing allows the building to dissipate energy. The numerous nailed joints are very effective in providing ductility to wood-frame buildings.

Weight:
Wood-frame construction is lightweight. Concrete walls used in ICF (insulated concrete form) wall construction are about 7 times heavier than typical wood-frame walls. Since forces in an earthquake are proportional to the weight of the structure, lightweight wood-frame buildings that are properly designed and built can be expected to perform very well in earthquakes.

Redundancy:
Buildings that have numerous load paths are considered structurally redundant and provide an extra level of safety in earthquakes. Structures supported by heavy frames rely on relatively few structural members and connections. A design or workmanship flaw in any one component may mean overloading of adjacent load paths. Typical wood-frame construction is comprised of hundreds of structural elements and thousands of nail connections. This means that the failure of one load path can often be compensated for by adjacent members and joints.

Connectivity:
Wood-sheathed walls can resist the racking forces of an earthquake, but a building must also be designed and built to resist sliding or overturning. In either case, the building must be adequately secured to the foundation. The connection of the walls, floors and roof framing make the house a single solid structural unit and is an important feature for holding a house together during an earthquake.

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