Seismic retrofit - Wikipedia, the free encyclopedia. Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1.
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US, Japan etc.) and late 1. Turkey, China etc.). In view of the imminent problem, various research work has been carried out. State- of- the- art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world - such as the ASCE- SEI 4. Whilst current practice of seismic retrofitting is predominantly concerned with structural improvements to reduce the seismic hazard of using the structures, it is similarly essential to reduce the hazards and losses from non- structural elements. It is also 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. This is typically done by the addition of cross braces or new structural walls.
Reduction of the seismic demand by means of supplementary damping and/or use of base isolation systems. This strategy recognises the inherent capacity within the existing structures, and therefore adopt a more cost- effective approach to selectively upgrade local capacity (deformation/ductility, strength or stiffness) of individual structural components. Selective weakening retrofit. This is a counter intuitive strategy to change the inelastic mechanism of the structure, while recognising the inherent capacity of the structure.
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However, with the development of Performance based earthquake engineering (PBEE), several levels of performance objectives are gradually recognised: Public safety only. The goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passersby, and that the structure can be safely exited. Under severe seismic conditions the structure may be a total economic write- off, requiring tear- down and replacement. Structure survivability. The goal is that the structure, while remaining safe for exit, may require extensive repair (but not replacement) before it is generally useful or considered safe for occupation. This is typically the lowest level of retrofit applied to bridges.
Structure functionality. Primary structure undamaged and the structure is undiminished in utility for its primary application. A high level of retrofit, this ensures that any required repairs are only . This is the minimum acceptable level of retrofit for hospitals. Structure unaffected. This level of retrofit is preferred for historic structures of high cultural significance.
Techniques. They could be tightened and loosened to support the house without having to otherwise demolish the house due to instability. The bolts were directly loosely connected to the supporting frame of the house.
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External post- tensioning. Under the PRESS (Precast Seismic Structural Systems). An extension of the same idea for seismic retrofitting has been experimentally tested for seismic retrofit of California bridges under a Caltrans research project . It should be noted that external pre- stressing has been used for structural upgrade for gravity/live loading since the 1.
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This earthquake engineering technology, which is a kind of seismic vibration control, can be applied both to a newly designed building and to seismic upgrading of existing structures. Steel or reinforced concrete beams replace the connections to the foundations, while under these, the isolating pads, or base isolators, replace the material removed. While the base isolation tends to restrict transmission of the ground motion to the building, it also keeps the building positioned properly over the foundation. Careful attention to detail is required where the building interfaces with the ground, especially at entrances, stairways and ramps, to ensure sufficient relative motion of those structural elements.
Supplementary dampers. In addition to adding energy dissipation capacity to the structure, supplementary damping can reduce the displacement and acceleration demand within the structures. In some cases, the threat of damage does not come from the initial shock itself, but rather from the periodic resonant motion of the structure that repeated ground motion induces. In the practical sense, supplementary dampers act similarly to Shock absorbers used in automotive suspensions. Tuned mass dampers. These are typically employed to reduce wind sway in very tall, light buildings.
Similar designs may be employed to impart earthquake resistance in eight to ten story buildings that are prone to destructive earthquake induced resonances. During a seismic event, the fluid in this tank will slosh back and forth, but is directed by baffles - partitions that prevent the tank itself becoming resonant; through its mass the water may change or counter the resonant period of the building.
Additional kinetic energy can be converted to heat by the baffles and is dissipated through the water - any temperature rise will be insignificant. Active control system.
A solution to this problem is to include at some upper story a large mass, constrained, but free to move within a limited range, and moving on some sort of bearing system such as an air cushion or hydraulic film. Hydraulic pistons, powered by electric pumps and accumulators, are actively driven to counter the wind forces and natural resonances.
These may also, if properly designed, be effective in controlling excessive motion - with or without applied power - in an earthquake. In general, though, modern steel frame high rise buildings are not as subject to dangerous motion as are medium rise (eight to ten story) buildings, as the resonant period of a tall and massive building is longer than the approximately one second shocks applied by an earthquake. Adhoc addition of structural support/reinforcement. 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. Connections between buildings and their expansion additions.
As a result, the addition may have a different resonant period than the original structure, and they may easily detach from one another. The relative motion will then cause the two parts to collide, causing severe structural damage.
Seismic modification will either tie the two building components rigidly together so that they behave as a single mass or it will employ dampers to expend the energy from relative motion, with appropriate allowance for this motion, such as increased spacing and sliding bridges between sections. Exterior reinforcement of building. In this case, the solution may be to add a number of steel, reinforced concrete, or poststressed concrete columns to the exterior. Careful attention must be paid to the connections with other members such as footings, top plates, and roof trusses. Infill shear trusses.
In this case, there was sufficient vertical strength in the building columns and sufficient shear strength in the lower stories that only limited shear reinforcement was required to make it earthquake resistant for this location near the Hayward fault. Massive exterior structure. In the structure shown at right . Damage in San Francisco due to the Loma Prieta event. This collapse mode is known as soft story collapse. In many buildings the ground level is designed for different uses than the upper levels. Low rise residential structures may be built over a parking garage which have large doors on one side.
Hotels may have a tall ground floors to allow for a grand entrance or ballrooms. Office buildings may have stores in the ground floor which desire continuous windows for display. Traditional seismic design assumes that the lower stories of a building are stronger than the upper stories and where this is not the case. Using modern design methods, it is possible to take a weak story into account. Several failures of this type in one large apartment complex caused most of the fatalities in the 1. Northridge earthquake. Typically, where this type of problem is found, the weak story is reinforced to make it stronger than the floors above by adding shear walls or moment frames.
Moment frames consisting of inverted U bents are useful in preserving lower story garage access, while a lower cost solution may be to use shear walls or trusses in several locations, which partially reduce the usefulness for automobile parking but still allow the space to be used for other storage. Beam- column joint connections. Prior to the introduction of modern seismic codes in early 1. Laboratory testings have confirmed the seismic vulnerability of these poorly detailed and under- designed connections. Philosophically, the various seismic retrofit strategies discussed above can be implemented for reinforced concrete joints. Concrete or steel jacketing have been a popular retrofit technique until the advent of composite materials such as Carbon fiber- reinforced polymer (FRP).
Composite materials such as carbon FRP and aramic FRP have been extensively tested for use in seismic retrofit with some success. Various retrofit solutions have been developed for these welded joints - such as a) weld strengthening and b) addition of steel haunch or 'dog- bone' shape flange. Discovery of these unanticipated brittle fractures of framing connections was alarming to engineers and the building industry. Starting in the 1.
Many engineers believed that steel moment- frame buildings were essentially invulnerable to earthquake induced damage and thought that should damage occur, it would be limited to ductile yielding of members and connections.