User:TenOfAllTrades/SeismicRetrofit

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Cross & Associates	Wikipedia
Seismic Retrofitting: Seismic retrofitting is the modification (i.e., strengthening) of existing buildings to make them more resistant to seismic activity, ground motion, or soil liquefaction caused by earthquakes.	Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes.
We are James K. Cross & Associates, Inc. … the premier Southern California based earthquake contractor that specializes in seismic retrofitting (i.e., foundation structural strengthening) of existing residential and commercial structures. We have been providing quality retrofits and foundation replacement and repair services to the greater Los Angeles area since 1985. Why Retrofit? To improve the chances of your home surviving an earthquake, you should take action ahead of time to strengthen the lower areas off your home that are most likely to fail during an earthquake. This includes strengthening the connection (i.e, bolting) between the house to the foundation, and reinforcing the "cripple wall" between the foundation and main structure. Other strengthening measures include adding bracing between the floor joists and adding "framing anchors" between the floor joists and the cripple wall.
Before we proceed in our discussion, however, it is important to keep in mind that there is no such thing as an "earthquake proof" structure. That having been said, the fact remains, however, that performance can be greatly improved by proper retrofitting of the existing structure.	It is important to keep in mind that there is no such thing as an earthquake-proof structure, although seismic performance can be greatly enhanced through proper initial design or subsequent modifications.
Reinforcement: The most common form of seismic retrofit to buildings is adding reinforcement to the lower areas of the building to resist seismic forces. Such strengthening may be limited to connections between existing building elements or may involve adding primary resisting elements such as walls or frames in the lower stories.	Reinforcement The most common form of seismic retrofit to lower buildings is adding strength to the existing structure to resist seismic forces. The strengthening may be limited to connections between existing building elements or it may involve adding primary resisting elements such as walls or frames, particularly in the lower stories.
Earthquake Related Building Failure. Sliding Off The Foundation: Single or two story wood-frame domestic structures built on a perimeter or slab foundation are, for the most part, relatively safe in an earthquake. But, in many structures built before 1950, the sill plate between the concrete foundation and the floor diaphragm (in the case of perimeter foundations) or stud wall (in the case of slab foundations) may not be sufficiently connected (bolted in). Also, older attaching components (e.g., metal bolts) may have corroded to a point of weakness. In such cases, a sideways shock (shear), such as an earthquake, can cause the building to slide entirely off the foundation or slab.	Sliding off foundation and cripple wall failure Single or two story wood-frame domestic structures built on a perimeter or slab foundation are relatively safe in an earthquake, but in many structures built before 1950 the sill plate that sits between the concrete foundation and the floor diaphragm (perimeter foundation) or studwall (slab foundation) may not be sufficiently bolted in. Additionally, older attachments (without substantial corrosion-proofing) may have corroded to a point of weakness. A sideways shock can slide the building entirely off of the foundations or slab.
Cripple Wall Failure: Often such buildings, especially if constructed on a moderate slope, are erected on a platform connected to a perimeter foundation through a low stud wall called a "cripple wall". This low wall structure itself may fail during an earthquake, collapsing the cripple wall and causing the building to move diagonally. The likelihood of failure of the cripple wall can be reduced by the proper installation of "shear panels", which are plywood sheets nailed to the cripple walls. This protects and strengthens the cripple wall. Basically, it is a sheet of plywood nailed onto the wood framing of the cripple wall to prevent it from collapsing in an earthquake. But, not just any sheet of plywood will do. Current earthquake safety standards specify that only structural grade sheet plywood be used for shear paneling. This grade of plywood is made without interior unfilled knots and with more, thinner layers than common plywood. The earthquake safety standards also specify the type of nails to be used, the required nail spacing, the spacing of the framing and other factors that affect the strength of a shear wall.	Often such buildings, especially if constructed on a moderate slope, are erected on a platform connected to a perimeter foundation through low stud-walls called "cripple wall" or pin-up. This low wall structure itself may fail in shear or in its connections to itself at the corners, leading to the building moving diagonally and collapsing the low walls. The likelihood of failure of the pin-up can be reduced by ensuring that the corners are well reinforced in shear and that the shear panels are well connected to each other through the corner posts. This requires structural grade sheet plywood, often treated for rot resistance. This grade of plywood is made without interior unfilled knots and with more, thinner layers than common plywood. New buildings designed to resist earthquakes will typically use OSB (oriented strand board), sometimes with metal joins between panels, and with well attached stucco covering to enhance its performance. In many modern tract homes, especially those built upon expansive (clay) soil the building is constructed upon a single and relatively thick monolithic slab, kept in one piece by high tensile rods that are stressed after the slab has set. This poststressing places the concrete under compression - a condition under which it is extremely strong in bending and so will not crack under adverse soil conditions.
Soft Story: Whenever a habitable area is built over an open area, such as a garage, the open area is called a "soft story." Many homes, apartments, and commercial buildings are built in this configuration. The term "soft story" refers to the open area that is not able to transmit shear (side movement) forces to the story above. Like a cripple wall with no shear paneling, a "soft story" that has not been properly reinforced can fail causing partial or complete collapse of the building. Under current California Law, when selling a property, the existence of cripple walls without shear paneling and the existence of an unsupported "soft story" must be disclosed to the buyer. Where this type of problem is found, James K. Cross & Associates can reinforce the "soft story" by adding shear walls or other reinforcing elements such as steel framing to correct the problem.
Shear Failure Within The Floor Diaphragm. Floors in wooden buildings are usually constructed on tall wooden beams called "joists", and covered with diagonal wood planking or plywood to form a sub-floor on which the finish floor is laid. Typically, these joists are all aligned in the same direction and, if not properly supported, can tip over on their side. To prevent the joists from tipping over onto their side, blocking is used at each end, and for additional stiffness, blocking or diagonal wood or metal bracing is placed between the joists at one or more points along their spans. At the outer edge it is typical to use a single depth of blocking and a perimeter beam overall. If the blocking or nailing is inadequate, each beam can topple over by the shear forces on the building, much the same as a row of dominoes. In this toppled position, the joists lack most of their original strength and the structure may collapse. As part of a seismic retrofit James K. Cross & Associates will reinforce the blocking and take additional steps to prevent the floor joists from toppling, including cross bracing and/or securing vertical wall elements into the foundation using specialty connectors and bolts glued with epoxy cement into holes drilled in the foundation.	Floors in wooden buildings are usually constructed upon relatively deep spans of wood, called joists, covered with a diagonal wood planking or plywood to form a subfloor upon which the finish floor surface is laid. In many structures these are all aligned in the same direction. To prevent the beams from tipping over onto their side, blocking is used at each end, and for additional stiffness, blocking or diagonal wood or metal bracing may be placed between beams at one or more points in their spans. At the outer edge it is typical to use a single depth of blocking and a perimeter beam overall. If the blocking or nailing is inadequate, each beam can be laid flat by the shear forces applied to the building. In this position they lack most of their original strength and the structure may further collapse. As part of a retrofit the blocking may be doubled, especially at the outer edges of the building. It may be appropriate to add additional nails between the sill plate of the perimeter wall erected upon the floor diaphragm, although this will require exposing the sill plate by removing interior plaster or exterior siding. As the sill plate may be quite old and dry and substantial nails must be used, it may be necessary to pre-drill a hole for the nail in the old wood to avoid splitting. When the wall is opened for this purpose it may also be appropriate to tie vertical wall elements into the foundation using specialty connectors and bolts glued with epoxy cement into holes drilled in the foundation.