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Contributions of photographs are needed
Seismic and seismic event refer to earthquakes, motions of the ground that can be hazardous to the occupants of buildings and the security and utility of structures such as bridges and tunnels.
Seismic retrofitting is the modification of structures to make them more resistant to ground motion and/or soil failure due to earthquakes. Other retrofit techniques are applicable to areas subject to tropical cyclones, tornadoes, and severe winds from thunderstorms. Methods to reduce hazards within households and also for general disaster preparedness are found in the related article Household seismic safety.
This article is intended to show the kinds of modifications made to existing structures to make them more resistant to catastrophic damage during major earthquakes. It is not intended as a manual of how to reenforce a structure, as that is best done engineering specialists and experienced and specialized craftspersons. There are some tasks, such as the adding of additional connections to foundations, that can be performed by an mechanically experienced homeowner. In some cases the structure may be sufficiently simple to allow the homeowner to follow simple instructions in a pamphlet. In other cases, especially with multiple level houses, and particularly those built upon slopes, a specialized and licensed expert should be consulted.
Seismic retrofit is primarilly applied to achieve life safety, with various levels of structure and materiel survivability determined by economic considerations:
The most common structures requiring extensive retrofit are bridges, road viaducts, towers, and large buildings.
Modifications fall into several catagories:
Generally required for large masonry buildings, excavations are made around the foundations of the building and the building (in piecemeal fashion) is separated from the foundations. Steel or reinforced concrete beams replace the connections to the foundations, while under these, layered rubber and metal isolating pads replace the material removed, these in turn are attached below to new or existing foundations. These allow the ground to move while the building, restrained by its inertial mass, remains relatively static. The pads absorb energy, transforming the relative motion between the ground and the structure into heat. While the pads tend to transmit some of the ground motion to the building they also keep the building positioned properly over the foundation. Careful attention to detail is required where the building interfaces with the ground, as at stairways and ramps, to ensure sufficient free motion without damage from compression or dismantling or falling from extension.
Dampers absorb the energy of motion and convert it to heat, thus "dampening" resonant effects in structures that are rigidly attached to the ground. In these 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.
A large tank of water may be placed on an upper floor. During a seismic event, the water 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.
Shock absorbers, similar to those used in automotive suspensions, may be used to connect portions of a structure that are free to move relative to each other and that may collide during an earthquake. Where a ridged connection could break or impose excessive strain on the buildings, and a loose connection could be dismantled, the shock absorbers allow the relative motion to be restrained by transfering and dissipating energy. This can be especially effective if the two structures have differing fundamental frequencies of resonance, as each structure may then assist in inhibiting the motion of the other.
Very tall buildings ("skyscrapers"), when built using modern lightwight materials, might sway uncomfortably (but not dangerously) in certain wind conditions. 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.
Frequently, building additions will not be strongly connected to the existing structure, but simply placed adjacent to it, with only minor continuity in flooring, siding, and roofing. 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. Proper construction will tie the two building components rigidly together so that they behave as a single mass.
The following is an image gallery of exterior reenforcement techniques. The methods are discussed in detail in the subsections below
Some historic buildings, made of unreinforced masonry, may have culturally important interior detailing or murals that should not be disturbed. 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.
Shown at right is an exterior shear reinforcement of a conventional reinforced concrete dormitory building. 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.
In other circumstances, far greater reinforcement is required. In the structure shown below — a parking garage over shops — the placement, detailing, and painting of the reinforcement becomes itself an architectural embellishment.
In many low rise structures, habitation or offices are built over a series of ground level garages, each walled into a compartment but with a large door opening on one side. The entire facade of one wall may thus be primarily composed of door openings. If a shock is applied along the axis of this wall, then that entire side of the building can collapse to one side. In these cases, the upper floors usually come down intact, but can crush occupants in spaces on the same floor as the garage.
Several failures of this type in one large apartment complex caused most of the fatalities in the 1994 Northridge earthquake.
A typical modification is to replace wood post and beam construction with welded or bolted steel beams, called "bents", or to replace one or more of the larger openings with a well-connected shear wall.
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.
Single- or two-storey 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 may have corroded to a point of weakness. A sideways shock can also slide the building entirely off of the foundations or slab.
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 likelyhood 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 build 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.
Some older low-cost structures are elevated on tapered concrete pylons set into shallow pits, a method frequently used to attach outdoor decks to existing buildings. This is seen in conditions of damp soil as it leaves a dry ventilated space under the house, and in far northern conditions of permafrost (frozen mud) as it keeps the building's warmth from destabilizing the ground beneath.
During an earthquake, the pylons may tip, spilling the building to the ground. This can be overcome by using deep-bored holes to contain cast-in-place reinforced pylons, which are then secured to the floor panel at the corners of the building. Another technique is to add sufficient diagonal bracing or sections of concrete shear wall between pylons
Reinforced concrete columns typically contain large diameter vertical rebar arranged in a ring, surrounded by lighter-gauge hoops of rebar. Upon analysis of failures due to earthquakes, it has been realized that the weakness was not in the vertical bars, but rather in inadequate strength and quantity of hoops. Once the integrity of the hoops are breached, the vertical rebar can flex outward, stressing the central column of concrete. The concrete then simply crumbles into small pieces, now unconstrained by the surrounding rebar. In new construction a greater amount of hoop-like structures are used.
One simple retrofit is to surround the column with a jacket of steel plates formed and welded into a single cylinder. The space between the jacket and the column is then filled with concrete, a process called grouting. Where soil or structure conditions require such additional modification, additional pilings may be driven near the column base and concrete pads linking the pilings to the pylon are fabricated at or below ground level. In the example shown not all columns needed to be modified to gain sufficient seismic resistance for the conditions expected. (This location is about a mile from the Hayward Fault Zone.)
Concrete walls are often used at the transistion between elevated road fill and overpass structures. The wall is used both to retain the soil and so enable the use of a shorter span and also to transfer the weight of the span directly downward to footings in undisturbed soil. If these walls are inadequate they may crumble under the stress of an earthquake's induced ground motion.
One form of retrofit is to drill numerous holes into the surface of the wall, and secure short L-shaped sections of rebar to the surface of each hole with epoxy adhesive. Additional vertical and horizontal rebar is then secured to the new elements, a form is erected, and an additional layer of concrete is poured. This modification may be combined with additional footings in excavated trenches and additional support ledgers and tie-backs to retain the span on the bounding walls.
Examination of failed structures often reveals failure at the corners, where vertical posts join horizontal beams. These corners can be reenforced with external steel plates, which must be secured by through bolts and which may also offer an anchor point for strong rods, as shown in the image at left. The horizontal rods pass across the beam to a similar structure on the opposite side, while the vertical rods are anchored after passing through a grouted anti-burst jacket. Another method is to simply add a great amount of small attachment points, as in the wall reenforcement method described above, with additional rebar and concrete. In one retrofit every corner joint has been surrounded by a block-like jacket. These blocks serve to transfer bending forces to new added jackets on the vertical and horizontal elements. The goal is to achieve the type of strength afforded by the new construction shown at right (this is not a retrofit).
Where moist or poorly consolidated alluvial soil interfaces in a "beach like" structure against underlying firm material, seismic waves travelling through the alluvium can be amplified, just as are water waves against a sloping beach. In these special conditions, vertical accelerations up to twice the force of gravity have been measured. If a building is not secured to a well-embedded foundation it is possible for the building to be thrust from (or with) its foundations into the air, usually with severe damage upon landing. Even if it is well-founded, higher portions such as upper stories or roof structures or attached structures such as canopies and porches may become detached from the primary structure.
Good practices in modern, earthquake-resistant structures dictate that there be good vertical connections throughout every component the building, from undisturbed or engineered earth to foundation to sill plate to vertical studs to plate cap through each floor and continuing to the roof structure. Above the foundation and sill plate the connections are typically made using steel strap or sheet stampings, nailed to wood members using special hardened high-shear strength nails, and heavy angle stampings secured with through bolts, using large washers to prevent pull-through. Where inadequate bolts are provided between the sill plates and a foundation in existing construction (or are not trusted due to possible corrosion), special clamp plates may be added, each of which is secured to the foundation using expansion bolts inserted into holes drilled in an exposed face of concrete. Other members must then be secured to the sill plates with additional fittings.
One of the most difficult retrofits is that required to prevent damage due to soil failure. Soil failure can occur on a slope, due to landslide or in a flat area due to liquification of water-saturated sand and/or mud. Generaly, deep pilings must be driven into stable soil (typically hard mud or sand) or to underlying bedrock. For buildings built atop previous landslides the practicality of retrofit may be limited by economic factors, as it is not practical to stabilize a large, deep landslide. The likelyhood of landslide or soil failure may also depend upon seasonal factors, as the soil may be more stable at the beginning of a wet season than at the beginning of the dry season. Such a "two season" Mediterranian climate is seen throughout California.
In some cases, the best that can be done is to reduce the entrance of water runnoff from higher, stable elevations by capturing and bypassing through channels or pipes, and to drain water infiltrated directly and from subsurface springs by inserting horizontal perforated tubes. There are numerous locations in California where extensive developments have been built atop archaic landslides, which have not moved in historic times but which (if both water-saturated and shaken by an earthquake) have a high probablility of moving en masse, carrying entire sections of suburban development to new locations. While the most modern of house structures built upon monolithic concrete slabs with post tensioning cables) may survive such movement largely intact, the building may be neither level nor properly located.
Natural gas and propane supply pipes to structures often prove especially dangerous during and after earthquakes. Should a building move from its foundation or fall due to cripple wall collapse, the ductile iron pipes transporting the gas within the structure may be broken, typically at the location of threaded joins. The gas may then still be provided to the pressure regulator from higher pressure lines and so continue to flow in substantial quantities; it may then be ignited by a nearby source such as a lit pilot light or arcing electrical connection.
There are two primary methods of automatically restraining the flow of gas after an earthquake:
Seismic retrofit techniques will vary with the nature of the structure, soil conditions, local topography, and distance from various faults. A nearby minor fault, capable of generating only a small earthquake, may be more dangerous to a structure than a distant major fault. In some cases, structures have been built spanning faults, and an appropriate retrofit may be to attempt to keep the portions together or to remove or make a spanning portion flexible.
Bridges have several failure modes.
Many short bridge spans are statically anchored at one end and attached to rockers at the other. This rocker gives vertical and transverse support while allowing the bridge span to expand and contract with temperature changes. The change in the length of the span is accommodated over a gap in the roadway by comb-like expansion joints. During severe ground motion the rockers may jump from their tracks or be moved beyond their design limits, causing the bridge to unship from its resting point and then either become misaligned or fall completely.
Motion can be constrained by adding ductile or high-strength steel restraints that are friction-clamped to beams and designed to slide under stress while limiting the motion relative to the anchorage.
Lattice beams consist of two "I"-beams connnected with a criss-cross lattice of flat strap or angle stock. These can be greatly strengthend by replacing the open lattice with plate members. This is usuallly done in concert with the replacement of hot rivets with bolts.
Many older structures were fabricated by inserting red hot rivets into pre-drilled holes; the rivets are then peened using an air hammer on one side and a bucking bar (an inertial mass) on the head end. As these cool slowly, they are left in an annealed (soft) condition, while the plate, having been hot rolled and quenched during manufacture, remains relatively hard. Under extreme stress the hard plates can shear the soft rivets, resulting in failure of the join.
The solution is to burn out each rivet with an oxygen torch. The hole is then prepared to a precise diameter with a reamer. A special bolt, consisting of a head, a shaft matching the reamed hole, and a threaded end is inserted and retained with a nut, then tightened with a wrench. As the bolt has been formed from an appropriate high-strength alloy and has also been heat-treated, it is not subject to either the plastic shear failure typical of hot rivets nor the brittle fracture of ordinary bolts. Any partial failure will be in the plastic flow of the metal secured by the bolt; with proper engineering any such failure should be non-catastrophic.
Unless the tunnel penetrates a fault likely to slip, the greatest danger to tunnels is a landslde blocking an entrance. Additional protection around the entrance may be applied to divert any falling material (similar as is done to divert snow avalanches) or the slope above the tunnel may be stabilised in some way. Where only small- to medium-sized rocks and boulders are expected to fall, the entire slope may be covered with wire mesh, pinned down to the slope with metal rods. This is also a common modification to highway cuts where appropriate conditions exist.
The safety of underwater tubes is highly dependant upon the soil conditions through which the tunnel was constructed, the materials and reenforcements used, and the maximum predicted earthquake expected, and other factors, some of which may remain unknown under current knowlege.
A tube of particular structural, seismic, economic, and political interest is the BART (Bay Area Rapid Transit) trans-bay tube. This tube was constructed at the bottom of San Francisco Bay through an innovative process. Rather than pushing a shield through the soft bay mud, the tube was constructed on land in sections. Each section consisted of two inner tubular tunnels, a central access tunnel of rectangluar cross section, and an outer oval shell encompasing the three inner tubes. The intervening space was filled with concrete. At the bottom of the bay a trench was excavated and a flat bed of crushed stone prepared to receive the tube sections. The sections were then floated into place and sunk, then joined with bolted connections to previously-placed sections. An overfill was then placed atop the tube. Once completed from San Francisco to Oakland, the tracks and electrical components were installed. The predicted response of the tube during a major earthquake was likened to be as that of a string of (cooked) spaghetti in a bowl of gelatin dessert). To avoid overstressing the tube due to differential movements at each end, a sliding slip joint was included at the San Francisco terminus under the landmark Ferry Building. The engineers of the construction consortium PBTB (Parsons-Brinkerhoff-Tudor-Bechtel) used the best estimates of ground motion available at the time, now known to be insufficient given modern computational analysis methods and geotechnical knowlege. This resulted in the slip joint being designed and constructed too short to ensure survival of the tube under possible (perhaps even likely) large earthquakes in the region. To correct this deficiency the slip joint must be extended to allow for additional movement, a modification expected to be both expensive and technically and logistically difficult. Other retrofits to the BART tube include vibratory consolidation of the tube's overfill to avoid potential liquifying of the overfill. Should the overfill fail there is a danger portions of the tube rising from the bottom, an event which could potentially cause failure of the section connections.
Elevated roadways are typically built on sections of elevated earth fill connected with bridge-like segments, often supported with vertical columns.
If the soil fails where a bridge terminates, the bridge may become disconnected from the rest of the roadway and break away. The retrofit for this is to add additional reinforcement to any supporting wall, or to add deep caissons adjacent to the edge at each end and connect them with a supporting beam under the bridge.
Another failure occurs when the fill at each end moves (through resonanant effects) in bulk, in opposite directions. If there is an insufficient founding shelf for the overpass, it may slip off. To fix this, the shelf is generally enlarged (often in concert with wall strengthening described above) and ductile stays may be added to attach the overpass to the footings at each end. These help keep the overpass centered in the gap so that it's less likely to slide off its founding shelf at one end.
In the extreme, large sections of roadway may consist entirely of viaduct, sections with no connection to the earth other than through vertical columns. When concrete columns are used, the detailing is critical. Typical failure may be in the toppling of a row of columns due either to soil connection failure or to insufficient cylindrical wrapping with rebar. Both failures were seen in the 1995 Great Hanshin earthquake in Kobe, Japan, where an entire viaduct, centrally supported by a single row of large columns, was laid down to one side.
Such columns are reinforced by excavating to the foundation pad, driving additional pilings, and adding a new, larger pad, well connected with rebar along side of or into the column. A column with insufficient wrapping bar, which is prone to burst and then hinge at the bursting point, may be completely encased in a circular or elliptical jacket of welded steel sheet and grouted as described above.
Sometimes viaducts may fail in the connections between components. This was seen in the failure of the Cyprus Viaduct in Oakland, California during the Loma Prieta earthquake. This viaduct was a two-level structure, and the upper portions of the columns were not well connected to the lower portions that supported the lower level; this caused the upper deck to collapse upon the lower deck. Weak connections such as these require additional external jacketing - either through external steel components or by a complete jacket of reinforced concrete, often using stub connections that are glued (using epoxy adhesive) into numerous drilled holes. These stubs are then connected to additional wrappings, external forms (which may be temporary or permanent) are erected, and additional concrete is poured into the space. Large connected structures similar to the Cyprus Viaduct must also be properly analyzed in their entirety using dynamic computer simulations.
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