Seismic retrofit
Seismic retrofit is the modification of structures to make them more resistant ground motion and/or soil failure due to earthquakes. Other retrofit techniques are applicable to areas subject to tropical cyclones, tornados, and severe winds from thunderstorms.
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 reenforced 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 are means of absorbing the energy of motion and converting it to heat. This will "dampen" resonant effects and have been mentioned above in the use of isolation dampers. The following examines the use of this principle in structures that are rigidly attached to the ground. In this case, the threat of damage does not come from the motion itself but rather from the periodic resonant motion of the structure that may be induced by repeated ground motion due to resonance of the surface of the earth in response to the initial and (for a short time continuing) earthquake shocks.
Slosh tanks
In many cases the damage from an earthquake comes not from the initial shock but from the following rythmic oscillations of the ground, not directly, but by their induction of resonant motion in the structure. This typically occurs in buildings of about eight to ten stories, with resonant frequencies of about one cycle per second, that correspond to the induced motion of the earth due to the earthquake. By placing a large tank of water with appropriate baffles on an upper floor, the resonate energy can be damped by the sloshing of the water, which by its mass may also change the resonant period and can form both a counter-resonance (a dissonance) and a dissapation of the energy by convertsion to heat. (Owing to the thermal mass of the water, the rise in temperature will be trivial and inconsequential.)
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. Without any connection the two building components may collide and so pound against each other. 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 dissapating 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 have been known to sway uncomfortably (but not dangerously) in certain wind conditions. This motion has been known to induce nausea in occupants. 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, 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 that of the approximately one second shocks applied by an earthquake.
Reenforcement
Connections between buildings and their expansion additions
Frequently when an addition is made to a building it will not be strongly connected to the existing structure, but simply stands adjacent to it, with only minor continuity in flooring, siding, and roofing. As it is likely that the addition may have a different resonant period than the original structure they may easily detach from one another. The relative motion will then cause the two parts to pound together, which may cause severe strutural damage. Proper retrofit (and original construction) will tie the two building components rigidly together so that they behave as a single mass.
Exterior reenforcement of building
Exterior column addition
Some buildings such as theaters, made of unreenforced 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 columns - steel, reenforced concrete, or poststressed concrete to the exterior. Careful attention must be paid to the connections with other members such as roof trusses.
Infill shear trusses
Shown in the gallery below is an exterior shear reenforcement of a conventional reenforced 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 reenforcement was required to make it earthquake resistant for this location, near the Hayward fault.
Massive exterior structure
In other circumstances, far greater reenforcement is required. In the structure containing a parking garage over shops the placement, detailing, and painting of the reenforcement becomes itself an architectural embellishment - in many ways more visually interesting than was the original structure.
Shear failure in lowest storey
In many low rise structures such as one or two stories of 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 be composed of mostly door openings. If a shock is applied along the axis of this wall the 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. It was this type of failure that caused most of the fatalities in the Northridge earthquake. A typical modification to replace wood post and beam construction with welded or bolted steel beams, called "bents", or to eliminate one or more of the garage or carport openings, replacing it with a well connected shear wall. In the Loma Prieta earthquake some buildings in San Francisco, built on leveled sand dunes or on filled marsh, failed with moderate to severe damage to the lower garage and utility floor. This was owing to the fact that the buildings were constructed of stucco over rough wood sheathing, rather than the more modern use of plywood, which has far better shear resistance.
Shear failure within floor diaphram
Floor structures in wooden buildings are usually constructed upon relatively deep spans of wood (called joists), covered with a diagonal wood plank 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, especially at the outer edges of the building, may be doubled, by inserting an additional block between each joist. It may be appropriate to add additional nails between the sill plate of the perimeter wall erected upon the floor diaphram, although this will require its exposure by removal of 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 diaphram (perimeter foundation) or studwall (slab foundation) may not be sufficiently bolted in, or have any bolts at all if it is an older building. While modern attachment hardware is made corrosion resistant, 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 reenforced in sheer and that the sheer panels are well connected to each other through the corner posts. This requires using 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. Buildings built from the beginning for earthquake resistance will typically use OSB (Oriented Strand Board), usually not appropriate for retrofit but used in new construction in greater amounts, with metal joins between panels, and with well attached stucco covering to enhance its performance.
Multiple piers in shallow pits
In some older low cost structures, rather than using continuous perimeter foundations the building is 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 is most often seen in southern climates. The entire building can fall to the ground by the tipping of these pylons. This construction technique is also used far northern conditions of permafrost, permanently frozen mud and is intended to avoid transferring the building's heat to the ground, which if thawed will not support the building.
These weaknesses can be overcome by using deep bored holes to contain cast in place reenforced pylons that are secured to the floor panel - generally at the corners of the building. Another technique is to add sufficient diagonal bracing or sections of concrete sheer wall between pylons to prevent tipping.
While earthquakes may be frequent in some regions subject to permafrost, such hazards are not usually considered in north American locations such as the area around the central Mississippi river. There is however, a deep seismic region called the New Madrid Fault Zone, capable of generating large earthquakes. As this area is in general unprepared for seismic events, with many masonry buildings of poor resistance, the results could be unexpectedly and tragically devastating
Reenforced concrete column burst
Reenforced concrete columns typically contain large diameter vertical rebar arranged in a ring, surrounded by lighter gauge hoops of rebar. The principle design stresses in older structures were vertical. Upon analysis of failures due to earthquakes it was realized that the weakness was not in the vertical bars but rather in inadequate strength and quantity of hoops, as these prevent the bursting of the column under bending stresses. Once the integrity of the hoops are breached at some point the vertical rebar can flex outward, stressing the central column of concrete in compression or hinging - it simply crumbles into small pieces, now unconstrained by the surrounding rebar. In new construction a greater amount of hoop-like structures are used. In retrofits, one simple fix is to surround the column with a steel jacket of steel plates formed and welded into a single circular or elliptical surround about the column. The space between the jacket and the column is then filled with concrete pumped through a hose, a process called grouting. This may be performed in concert with the prior driving of pilings and the fabrication of concrete pads below grade level, where soil or structure conditions require such additional modification.
Reenforced 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 in moving the fill. One form of retrofit is to drill numerous holes into the surface of the wall. Into these holes are placed short L shaped sections of rebar, secured to the hole using epoxy adhesive. Additional vertical and horizontal rebar is secured to the added elements, a formswork erected, and and additional layer of concrete is poured. When cured, the forms are removed, revealing a thicker, stronger wall structure. 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.
Reenforced 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 illustration at right. 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 the second picture to the right, 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.
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 the return contact under the influence of gravity. Even if it is well founded, higher portions such as upper stories or roof structures or attached structures such as attached 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, such as firm sandstone or harder rock. 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" mediteranian climate is seen in central and southern 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 to speed drainage of excess moisture. There are numerous locations in California where extensive developments have been built atop archaic landslides - landslides that have not moved in historic times but if both water saturated and shaken by an earthquake have a high probablility of moving en mass, carrying entire sections of suburban development to new locations. While the most modern of house structures may survive such movement largely intact, the building may be neither level nor properly located. Only recently have such matters been considered in the permitting of housing developments, and in the U. S. even that restraint is rather weak due to the effects of developer contributions to fund the electorial campaigns of local county and city supervisorial candidates. There is even one location in Daly City, California (just south of San Francisco_ where a developer was allowed to build an extensive suburban neighborhood directly atop the rubble field above and around the infamous San Andreas Fault!
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.
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 vertcal and transverse support while allowing the bridge span to expand and contract with temperature changes. The change in the length of the span is accomodated over a gap in the roadway by expansion joints - interdigitated comb-like structures. Under 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 fall entirely or at the least become misaligned. Motion can be constrained by adding ductile steel restraints, or high strength steel restraints that are friction clamped to beams and are designed to slide under stress while limiting the motion relative to the anchorage.
Latice beams
Latice beams consist of two I beams connnected with a cris-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 by loosing heat to the surrounding plate they are left in an annealed (soft) condtion, while the plate, having been hot rolled and quenched during manufacture, remains relatively hard. Under extreme stress the hard plates can sheer the soft rivets, resulting in failure of the join.
The solution is to burn out each rivet with an oxgen torch. The hole is then prepared to a precice 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, the assembly being tightend 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 soft 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 and when properly engineerd and 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 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 may consist entirely of viaduct, long 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
Please note that this is a work in progress. Contributions of illustrations are especially welcome