Typical retrofit situations and solutions
Collapse of low buildings with openings at ground floor level
This type of collapse is known as soft story collapse. In many buildings the ground level has different functions with respect to the upper levels. Non-elevated residential structures may be built over a parking lot that may have large gates on one or more sides. Hotels may have raised floors in the grounds that allow for a large main entrance or ballrooms. Office buildings may have businesses or warehouses at street level, which often have large windows to display merchandise.
Traditional seismic design considers the lower floors of a building to be stronger than the upper floors and where this is not verified, the structure will not respond to earthquake oscillations in the expected way, and the first floor will likely collapse under the weight of the upper floors. With modern design methods, it is possible to calculate the behavior of a "weak" floor and structurally remedy its potential failures. In 1994 in Los Angeles, collapses of this type at a large apartment complex caused many deaths in the 1994 Northridge earthquake.
Beam-column joints
Connection joints between beams and columns are a common structural weakness, which must be addressed by retrofitting. Before the introduction of modern anti-seismic codes in the 1970s, beam-column joints had normally not been dynamically studied or designed but to resist non-oscillating static loads. Some laboratory tests have confirmed the seismic vulnerability of these connections, which today can be defined as "poorly" designed.[12][13][14][15] A failure of the connections between beams and columns can typically lead to the catastrophic collapse of a frame-based building, as has been frequently observed in recent earthquakes[16][17].
Various strengthening solutions have been proposed for the joints between reinforced concrete columns and beams, which have been tested in the last 20 years. Philosophically, several seismic retrofit strategies discussed above can be applied for reinforced concrete joints. Casing with steel or concrete has been a widely used retrofit technique until the arrival of composite materials such as carbon fiber reinforced polymer. Composite materials such as carbon FRP and aramid FRP (mixture of Kevlar and Twaron) have been extensively tested for use in seismic retrofit with a certain degree of success.[18][19][20] A novel technique includes the use of selective weakening of the beam and the addition of external post-tensioning to the joint[21] in order to obtain a hinge flexural ("flexural hinging") in the beam, something more desirable in terms of anti-seismic project.
For example, a large number of weld failures in the beam-column joints of low- to medium-rise steel buildings during the 1994 Northridge earthquake have shown the structural deficiencies of these post-1970 'modern design' welded connections, which are very rigid and oppose the moment of force.[22] A SAC research project [4] has documented, tested and finally recommended some retrofit solutions for these welded steel joints that resist the moment of forces. For these welded joints, solutions have been developed such as: a) increasing the strength of the weld, and b) adding a steel hook ("steel haunch") or conforming flanges in the shape of a dog bone ("dog-bone shape flange").[23].
Collapse of load-bearing structures within the floor diaphragm
In the United States, wooden floors constructed as boxes supported by relatively narrow and deep beams (beams) of good quality wood are frequently used, covered with inexpensive and light wooden boards (plywood) diagonally, to form an economical platform for the floor that will be covered with the final surface of the floor (carpets, tiles, linoleum, parquet, thin wood beams, vinyl). In many structures these are all aligned in the same direction. To prevent these narrow beams from bending on one side, blockers are used at each end, and to give additional strength, diagonal wooden braces can be placed between the beams at one or more points along their length. At the external edge, a single blocking depth is typically used and a perimeter frame of beams is placed, so that it forms a box in close contact with the wall (wood or brick).
If the placement of the blocks, nails or screws is inadequate, the beams (which have a tall and narrow shape) can be turned on their side by the movement forces of the supporting structure, with the weak part facing up. In this position, the top sheet of the floor falls on the part of the roof, losing its original strength and the structure can completely collapse. As part of a retrofit, the placement of blocks between the beams can be duplicated, especially in the most external parts of the building. It may be appropriate to add additional nails between the radier of the perimeter wall erected over the peak diaphragm, although this will require exposing the radier plate by removing the internal fill or portion of the sides.
Foundation slippage and cripple wall damage
One- and two-story residential structures built with wood frames on a perimeter or rock foundation are relatively safe in an earthquake, but in many structures built before 1950 the radier plate ("sill plate") between the concrete foundation and the floor diaphragm (perimeter foundation) or a thick rock foundation ("studwall") may not have been sufficiently bolted. Additionally, screws or connecting structures between wood and rock (that have not been substantially protected from corrosion) may have corroded to a point of weakness. A lateral shake can cause the building to slide entirely off the foundation or solid rocks.
Frequently these buildings, especially built on a moderate slope, are erected on a platform connected to perimeter foundations thanks to low resistant walls called ("cripple wall" or pin-up). This same structure can fail in its connections to itself at the angles, leading the building to move diagonally and collapsing the low walls.
Presence of many support sticks in shallow holes
Some older, inexpensive structures are raised on concrete piles that are driven into shallow pits, a method often used to add balconies, terraces or platforms to pre-existing buildings. This is frequently done in places where the land is flooded or was swampy, especially in tropical conditions, because it leaves a dry and ventilated space under the house (apart from preventing animals from climbing into the house), and at the opposite extreme, in sub-polar or mountain conditions where permafrost (frozen mud) forms because it prevents the heat from the building from destabilizing the underlying terrain. During an earthquake, pylons can bend or fall, flipping the building toward the ground. This problem can be avoided by using holes that are dug to a depth where reinforced pylons are built, which are then secured to the floor plate at the corners of the building. Another technique is to add sufficient beams and diagonal supports to hug the columns, or to place reinforced concrete wall sections between the pylons (starting from a certain depth).
Explosion of reinforced concrete columns
Reinforced concrete columns typically contain large vertical reinforcing bars of very thick corrugated steel (the contraction of "reinforcing bars") placed in a ring, surrounded by others of smaller diameter. After having analyzed many of the damages caused by the earthquakes, it has been understood that the weakness was not in the vertical bars, but in the inadequate strength of the horizontal connectors ("fences") and their small number. Once the integrity of the horizontal connectors is broken, the vertical reinforcement can bend outward, placing the central concrete column in tension. Then the concrete simply cracks until it breaks into small pieces, which are not held together by the reinforcement that surrounds them. In new constructions, a greater number of link bars are placed, which are also thicker.
A simple form of retrofit is to surround the column with a thick band or "jacket" of steel sheets that are cast and welded until they form a single cylinder. Subsequently, the space between the sleeve and the column will be filled with concrete, in a process called "grouting" in English. When the soil or structural conditions necessitate this additional modification, further piers may be applied near the base of the column and concrete pads attaching the piers to the pylon will be fabricated at or below ground level. In the example shown, not all columns needed to be modified to gain sufficient seismic resistance to withstand the expected conditions. (This town is about a mile from the Hayward fault.).
Explosion of reinforced concrete walls
Concrete walls are frequently used at the transition between elevated highway fills and bridges or structures that pass over other roads (avoiding their interruption). The massive concrete wall is used to retain the ground and eliminate the need for support pylons for the highway bridge, thus allowing a shorter crossing and the super-elevated structure to rest directly on the foundation in a part of the ground that has not been disturbed by being elevated. If these walls are inadequate, they can collapse due to the stress caused by the wave movement of the earth induced by an earthquake.
One form of anti-seismic reinforcement is to drill numerous holes in the surface of the wall, and secure short sections of L-shaped steel rods to the surface of each hole with epoxy adhesive. The cement wall is covered with other vertical and horizontal rebar beams, forming a framework, a wooden formwork is placed, and an additional layer of concrete is poured. To this modification, other earth steps and other cement walls with foundations in excavated trenches can be added, as well as buttresses and other support structures to retain the "air" section of the road between adjacent walls.
Resins for brick walls and fiberglass reinforcements
Structures built with bricks can be reinforced with layers of fiberglass and resins (such as epoxy or polyester). On lower floors they can be applied over the entire exposed surface, while on higher floors they can be confined to narrow areas around windows and door openings. Lining the brick walls with these layers adds resistance to tension and movement, which gives us a wall that is more resistant to bending on one side, or moving forming waves that make the wall explode in the typical X shape. Efficiently protecting an entire brick building requires extensive computer analysis and engineering. For example, some windows and doors on the ground floor might need to be closed with brick walls, it might even be necessary to place buttresses, and on the contrary, the upper floors would have to be lightened, expanding the windows vertically (turning them into window-doors). In a brick building it is not easy to determine the points to be treated, and it often becomes necessary to demolish parts that form an "L" and that have a different resonant behavior than the rest of the building.
Many times, placing a thick steel frame on doors and windows increases their resistance, preventing the wall from bursting through the weak point constituted by the opening. The semicircular arch and round windows can be a solution that reinforces the ground floor of the building.
On the contrary, if the brick walls are not load-bearing, and the structure is supported by reinforced concrete columns, metal or rubber insulators can be placed between the columns and the base of the first floor. In this case, the walls of the first floor can be eliminated, placing windows or very thin walls of not thick brick. In this case it may be necessary to place the elevators and stairs in a structure external to the building, with a flexible connection to the building.
Lifting
Where humidity or at the point of contact with alluvial soils, for example in "beach-type" structures that are located immediately on top of firm material (such as rocks), seismic waves traveling in the sand or loose mud part can be amplified, exactly as happens with water waves that reach a steep beach. In these steep conditions, peaks of vertical acceleration have been measured that reach more than double the force of gravity. If a building is not well secured to foundations that are in turn solidly connected to the rock (puncturing it) it is possible that the building is thrown, with its foundations or separated from its foundations, thrown into the air, normally with serious damage that occurs when it lands or when it hits its foundations (which being strong points tend to break the ground). In buildings well anchored to the ground, Attached structures such as small towers, added upper floors, roofs and porches may detach from the primary structure.
Good construction practice for modern structures that can be declared earthquake-resistant dictates that there must be good vertical connections between each component of the building (unless it is a "sacrifice" point, to protect the structure from further damage), from undisturbed or engineered ground (for example loose earth or sand reinforced with many pylons, as in the Burj Al Arab tower on an artificial island in the United Arab Emirates) to the foundation, through the radier") (in English sill plate), then through the supporting columns, the platforms until reaching the roof structure.
On the foundation and radier the connections are typically made with a steel cage or with corrugated steel sheets, nailed to wooden elements, using special high-strength nails.
Land consolidation
One of the most difficult retrofits is required to prevent damage due to landslide or liquefaction. Landslides can occur near a slope, a ravine (as in a landslide), and even in a flat area due to the liquefaction of soil composed of water-saturated sand or mud. Generally, deep piles must be driven into unstable terrain (typically mud or sand) until rock is found or the ravine is stabilized.
Another, very expensive system, in flat areas (river alluviums) is to build a vast and deep underground "floating" foundation platform with thick reinforced concrete walls, which can have internal spaces and floors that can serve as warehouses, underground parking, drainage tunnel and/or for viability or air raid shelter, which can join several buildings and add to the seismic isolation at the points of contact with external buildings. An example of this is the housing, commercial and cinema center "Leonardo" which is located near the Rome-Fiumicino Airport.
It may not be worth applying retrofit to buildings that are located above previous collapse areas, as it is excessively economically burdensome, because very often it is not practical (or it is simply impossible) to stabilize a collapse that can be very vast and deep. The possibility of a landslide or liquefaction of the land occurring may depend on climatic factors, given that the land may be more stable at the beginning of the wet season compared to the beginning of the dry season. This type of "two-season" situation is seen in the Mediterranean climate, which also exists in California.
In some cases, the best thing to do is to reduce the amount of water entering the terrain, for example from higher altitudes on a hill or mountainside, by capturing surface and foothill water and diverting it through channels or tubes, and extracting water that infiltrates past the barrier directly and through subsurface wells by inserting perforated horizontal tubes. There are numerous locations in California where large housing developments have been built on archaic landslide zones, which have not moved in historical times, but which (if they become saturated with water and are simultaneously struck by an earthquake) have a high probability of moving en masse, taking entire sections of a suburban sector to new positions. Although the most modern of the housing structures (well tied to monolithic concrete foundations reinforced with post-tensioning cables) can survive these displacements largely intact, the building will no longer be in its proper location (it could have approached a ravine "Ravine (geography)" or be on land that now covers a highway).
Utility Cables and Pipes: Risks
Natural gas and propane pipes leading to structures often create hazards during and after an earthquake. If a building moves from its foundation or falls due to the collapse of its walls, the ductile iron tubes that carry gas into the structure can break, typically at the location of the welded joints. Gas may continue to arrive from the lines at high pressure and continue to flow in substantial quantities; This enormous volume of fuel gas can then be ignited by an external flame igniter source such as a water heater pilot flame or an electrical arc that is sending sparks, such as a broken power line.
There are two main methods of automatically shutting off gas flow after an earthquake, which are installed on the low pressure side of the regulator, and usually downstream of the gas meter.
• - A metal ball in a box can be placed on the edge of a hole. During the seismic shock, the ball will roll into the hole, closing it by gravity, thus preventing further passage of gas. The ball can later be put back into place, using an external magnet. This device responds only to ground movement.
• - An excess flow sensitive device can be used to close a valve if the gas flow exceeds a certain limit (much like the circuit breaker in an electrical circuit). This device will work regardless of seismic movement or not (for example due to the rupture of a pipe during excavations), but it will not respond to minor losses that may be caused by an earthquake.
It seems that the safest configuration is the one that envisages using each of these devices in series.
Tunnels
Unless the tunnel penetrates a fault that can slip (cutting the tunnel into two sections), the greatest danger to tunnels is a landslide or avalanche that can block an entrance. Additional protection around entrances (in major tunnels) can be applied to deflect any falling material. The slope above the tunnel can also be stabilized in some way. Where it is estimated that only small and medium-sized rocks may fall, the entire slope can be covered with a network of thick steel wire, nailed to the slope with metal bars. This procedure is also a common modification in places where highways cut through mountains or hills and where the appropriate conditions are verified that make it necessary and possible.
Underwater subway galleries
The safety of any type of underwater metropolitan depends highly on the conditions of the ground through which the tunnel was built, on the materials and reinforcements used, on the maximum intensity of the earthquake that can be anticipated and on other factors, some of which may be unknown to current science.
A particularly interesting metropolitan area from a structural, seismic, economic and political point of view is BART (Bay Area Rapid Transit). This subway was built at the bottom of the San Francisco Bay, through an innovative process. Instead of pushing a rotating steel shield through the mud of the bay, the tunnel was prefabricated in sections built on land. Each section (rectangular) consists of two circular section tunnels, a central rectangular section access tunnel, and an external oval shield that covers the three internal tunnels. The space between the tunnel tubes was filled with concrete. The bottom of the bay was excavated and covered with a flat bed of crushed rocks prepared to receive the sections of the metropolitan. The sections were floated on the surface to their installation location and sunk, then joined with welded connections to the previously placed sections. Filling was deposited on the precast to cover it and keep it firm. Once completed from San Francisco to Oakland, the rails and electrical components were placed. It was expected that the response to the earthquake by the metropolitan during a major earthquake could be compared to that of a bundle of spaghetti cooked in a deep plate of gelatin. To avoid excessive stress to the structure of the subway due to different movements at each end, a system of sliding joints was installed in the terminal part in San Francisco "San Francisco (California)") under the historic Ferry Building.
The engineers of the PBTB construction consortium (Parsons Brinckerhoff-Tudor-Bechtel) used the best ground motion calculations available at the time, which are currently considered insufficient due to modern computational analysis methods and geotechnical knowledge. Unexpected vertical settlement of the tunnel has reduced the amount of landslide that can be absorbed by the tunnel without opening holes and flooding the structure. These factors have given rise to the fact that the slip joint in San Francisco is now too short to ensure the survival of the metropolitan, especially under the pressure of the filling ground, which is necessary to prevent the sections of the metropolitan from "floating", leaving the bottom and remaining uncovered and supported irregularly on the bottom, which can lead to the rupture of the structure.