Seismic retrofit
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Contributions of photographs are needed - particularly some of the methods and procedures to provide additional structural security to house structures and security of household furnishings and appliances.
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.
Introduction
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 craftpersons. 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.
There remains much that can be done to enhance home safety by the occupants, even if they are inexperienced mechanically, and some of these methods (such a securing water heaters) may be shown in some detail. Pamphlets showing application of these methods in greater detail are also available from public agencies, but often these show only a minimal and simplistic installation. Effective securing of household equipment and furnishings may require some thought - mostly along the lines of "what could happen", "how heavy is this", and "where are the strong points to secure to". It does little good to screw the back of a bookcase to a wall if the back is (as is common) weakly attached to the bookcase structure. Similary, an expansion bolt through drywall may simply pull through if the load is heavy - one must locate and used stout screws into the underlying wood stud structure.
Preparedness goes beyond mere structure. In a large event it is expected that police, firefighting, and medical assistance will be overwhelmed. The recommendations are that a household should have sufficient water and preserved food to live independently for three days. There are shops specializing in providing emergency equipment supplies, and containers to the homeowner.
There is also considerable neglect at the community level :
- Are you and your neighbors prepared to assist one another? Do you know who has first aid skills?
- Can you do CPR?
- Do you know emergency treatment for broken bones, burns, and bleeding?
*Do you know how to shut off your gas supply (and have an appropriate wrench nearby the valve)?
- Do you know what to do about a downed and live high voltage line touching an occupied vehicle?
- Do you have several fire extinquishers available?
At one time, the Boy Scouts were a popular organization among families with youth. This organization would encourage their members to learn such important matters, especially as it was related to the possibility of nuclear attack or natural disaster during the Cold War. This organization's motto is Be Prepared. Unfortunately, following a move of headquarters from New York City to the U.S. state of Texas, they became involved in certain social issues that lead to their loss of respect and decline in membership in many areas - particularly in the region of Northern California, one likely location of a major earthquake disaster.
The U.S. Civilian Defense Corps - a product of |, was mostly dismantled in the later stages of the cold war, once it appeared that the probability of a major nuclear war was low. At the present time there is a vacuum of preparedness, organization, and abilities to deal with a large event, the appropriate civil organizations are starved for resources, and the political and legislative forces are concerned with other matters ranging from self-important to nonsensical, such as political redistricting and the suppression of the teaching of evolution in the schools.
There are many levels of detail in approach to securing a residence, ranging from providing large steel beams to reenforce a large garage door opening down to putting small dots of "earthquake" wax under small valuable items. As the article is currently over the recommended 32k character limit, some of these household tips will later be moved to a separate article.
Levels of modification
Seismic retrofit is primarilly applied to achieve life safety, with various levels of structure and materiel survivability determined by economic considerations:
- Life safety only. The goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passers by, that shelved contents will not fall upon occupants, and that the structure can be safely exited. Under severe sesmic conditions the structure may be a total economic loss, 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 useability. The structure is to be undiminished in its utility, altough it may be necessary to perform extensive repair or replacement of components in preparation for the next major seismic event. This is typically the lowest level of retrofit applied to hospitals, fire fighting stations, public safety (police) command centers, and the like and is the preferred level of retrofit and design for transportation infrastructure such as rail and highway roadways, bridges, and tunnels.
- Primary structure undamaged and the structure is undiminished in utitity for its primary application. A high level of retrofit, this ensures that any required repairs are only "cosmetic" - for example, minor cracks in plaster, drywall and stucco. This is the preferred level of retrofit for hospitals.
- Structure unaffected. This level of retofit is preferred for historic structures of high cultural significance.
The most common structures requiring extensive retrofit are bridges, road viaducts, towers, and large buildings.
Types of modifications
Modifications fall into several catagories:
Isolation
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
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.
Slosh tanks
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
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.
Active damping with fallback
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.
Reinforcement
Connections between buildings and their expansion additions
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.
Exterior reinforcement of building
The following is an image gallery of exterior reenforcement techniques. The methods are discussed in detail in the subsections below
Exterior column addition
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.
Infill shear 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.
Massive exterior structure
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.
Shear failure in lowest storey
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.
Shear failure within floor diaphragm
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.
Sliding off foundation and cripple wall failure
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), with metal joins between panels, and with well attached stucco covering to enhance its performance.
Multiple piers in shallow pits
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 column burst
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.
Reinforced concrete wall burst
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.
Reinforced concrete post to beam connections
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).
Lift
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.
Soil
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.
Utilities provided to structures
Natural gas and propane pipelines 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 remain available 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 restraining the flow of gas after an earthquake:
- A caged metal ball may be arranged at the edge of an orifice. Upon seismic shock, the ball will roll into the orifice, sealing it. The ball may later be reset by the use of an external magnet.
- A flow-sensitive device may be used to close a valve if the flow of gas exceeds a set threshhold (very much like an electrical circuit breaker). This device will operate independently of seismic motion.
Objects within buildings
Any massive or tall object within a structure should be securely fastened to that structure, through bolts, screws, or strap hinges, so that the object does not move around during an earthquake.
Securing appliances and wood stoves
Usually, appliances using natural gas are connected to the ductile iron supply pipes with a flexible corrugated tubing made from a relatively soft material, such as brass. It is important that the appliance be properly secured, or at least restrained in its maximum motion, so that it cannot extend and tear the flexible connection.
Free standing natural gas stoves — which in older homes may not be surrounded by cabinetry, but are simply set down upon the kitchen floor — require some form of restraint. If necessary to move the stove small distances for cleaning, such restraint may be provided by sturdy cables which must be attached to strong locations on both the stove and the wall of the kitchen. Stoves attached within cabinets may be a hazard if the cabinets themselves are weakly attached to the building structure.
Gas fireplace inserts, both decorative and functional, must also be restrained. Some form of restraining clip should be installed that may be easily removed when necessary.
Wood Franklin stoves and pellet stoves, often placed upon a hearth in front of a fireplace or with a separate pipe, are often quite heavy, and must be secured to a substantial member of the flooring system.
Water heaters of any type, gas or electric, must be rigidly attached to the building structure, as when filled with water they are all quite heavy. When the building moves, the inertia of the water mass is sufficient to break the connecting water pipes, or to slide the bottom of the tank about, with the gas or electrical connections now at risk. All water heaters should be secured in every horizontal direction and at the top and bottom with wood or metal blocking and corrosion-resistant steel straps.
Securing furnishings
Bookcases not only need to be secured, it is also necessary to secure the shelves' contents. Each shelf should be secured from sliding out as a unit, and the shelf, if used for book storage, should have a lip at the lower edge. The space between shelves should be such that the books will not tip forward off of the shelf (pivoting about the lip), but rather should first strike the shelf above (which itself must be securely fastened). A book is easily removed by lifting it up slightly so that it will slide over the lip.
Small decorative items should be secured using Velcro or a putty known as "earthquake wax". It may be necessary to remove felt padding from the bottom of the object in order to apply either substance.
Kitchen cabinents
Most modern cabinets are factory built, or custom built to specification, and then installed within the kitchen. Older buildings or some modern custom work may have "built in place" cabinetry. Even if the cabinets have a modern appearance, they may be a simple "reface" of this older type. Security is important and it may be necessary to add additional blocking to the cabinet structure so that the cabinet may be affixed to a structural member of the wall.
Child proof latches are also useful to retain the contents within the cabinets. While very old cabinets were sometimes made with a mechanical latch that had to be manually operated, more modern cabinets typically use gravity, friction, or magnetism to keep the cabinet door lightly closed. The child proof latch allows the door to be partially opened but prevents complete opening until operated by hand. Of course, caution must be used when opening such cabinets after a seismic event.
Structures to be modified
Seismic retrofit techniques will vary with the nature of the structure, soil conditions, local topography, and distance from various faults. A close proximity to a 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 keep the portions together or remove or to make flexable a spanning portion. Diferent considerations apply to bridges, tunnels, underwater tubes, elevated roadways, small residential houses, low rise structures, medium rise structures (8-12) stores, and high rise structures. Different techniques will be applied depending upon the materials used in the construction of the structure.
Structures to be modified
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
Bridges have several failure modes.
Expansion rockers
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
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.
Hot rivets
Many older structures are 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 shear failure typical of hot rivets nor the 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.
Tunnels
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.
Underwater tubes
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. 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.
Cuts
Fills
Elevated roadways
Fill and overpass
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.
Viaducts
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.
Houses
Wood frame
Masonry
Low rise
Medium rise
High rise
Massive masonry
Tunnels
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 bolders 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.
Underwater Tubes
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 interest (structurally, seismically, economically, and politically) 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 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 is 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 emplaced sections. Once completed from San Francisco to Oakland, the tracks and electrical components were emplaced. 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 Jello® (a 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 San Francisco's landmark Ferry Building. At the time, 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 defficiency the slip joint must be extended to allow for additional movement, a modification expected to be quite expensive (several hudreds of milions of U.S. dollars) and also technically and logistically difficult. Owing to trickle down budget constraints and deficits, the current Governor of California, Arnold Schwarzenegger has proposed using funds allocated for this BART tube enhancement, and other transportation and seismic retrofit funds to be redirection to the completion of the eastern replacement span of the San Francisco-Oakland Bay Bridge, currently estimated to cost far more than its original estimates - this as an alternative to using state highway funds for the latter - funds that would come from the entire state. While state funds have been used to pay for repairs of damage due to past earthquakes troughout the state there is a reluctance by some political factions to prepare for future events in the central coastal region using statewide funds, particulary given the regional politics involved in this and other matters, such as north to south water exports and general and sometimes bitter conservative/liberal political divisons.
Cuts
Fills
Elevated roadways
Fill and overpass
Elevated roadways are typically built on sections of elevated earth fill connected with bridge-like segments, often supported with vertical columns. This type of structure is subject to several kinds of failure. If the soil fails where a bridge-structure terminates, that may become disconnected from the roadway and break away or cause a larger failure of the type seen in viaducts. The retrofit for this is to add additional reenforcing to any supporting wall, if such a wall is present, or to add deep cassons adjacent to the edge at each end and then connect them with a supporting beam under the overpapass. Another failure is when the fill at each end moves (through resonanant effects) in bulk, in opposite directions. If there is 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 aid in keeping the overpass centered in the gap and so less likely to slide off its founding shelf at one end.
Viaducts
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 entire columns either due to soil connection failure or due to cylindrical bursting of the concrete and consequent hinging, due to insufficient cylindrical wrapping with rebar. This was seen in particular in the large earthquake at Kobe, Japan called the Great Hanshin earthquake, where an entire viaduct, centrally supported by a single row of large columns, was laid down to one side due to pylon and soil failures.
Such columns are reenforced by excavating to the foundation pad, driving additional pilings, and adding a new, larger pad, well connected with rebar up 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 eliptical jacket of welded steel sheet and grouted as described above.
Sometimes viducts may be inadequate 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, causing 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 reenforced concrete, often using stub connections that are glued (using epoxy adhesive) into numerous drilled holes. These stubs are then connected to addtional wrappings, external formswork (which may be temporary or permanent) is 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, a task now possible using dynamic computer simulations on supercomputers or large connected clusters of smaller computers.
Houses
Wood frame
Masonry
Low rise
Medium rise
High rise
Massive masonry
External link
- ABAG Home Quake Safety Toolkit From ABAG, the Association of Bay Area Governments, their web site includes much valuable information and interactive analysis tools. If you know or can reasonably estimate in the worst case the expected shaking index for your area you can still use the included home safety evaluation quiz, even if you are not located within the San Francisco Bay Area. There are other sections generally applicable for any potential level of seismic activity, such as securing furnishings. This is an especially valuable reference for any resident of an area subject to seismic activity