Part (2)

Effects of Earthquakes on Structures and Planning Considerations

• The Nature of Earthquake Hazard

• Architectural and Structural Considerations

• The Effects of Earthquakes on Buildings

• General Goals in Seismic-Resistant Design

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The Nature of Earthquake Hazard

1- Ground Shaking: The shaking resulting from an earthquake is not life threatening in itself; it is the consequential collapse of structures that is the main cause of death, injury, and economic loss.

2- Ground Failure: Ground failure can primarily cause any of the following:

A- Tsunamis: Tsunami or sea waves, which may threaten coastal regions. They are caused by the sudden change in seabed level that may occur in an offshore earthquake.

B- Liquefaction: Loss of strength in saturated granular soil due to the build-up of pore water pressure under cyclical loading.

C- Landslides: Which are often triggered by liquefaction of a soil stratum.

D- Fault Movement: Which can prove troublesome to structures directly crossing a fault. However, the number of structures directly over a fault break is small compared with the total number of structures affected by the earthquake. Faults are mainly a problem for extended facilities such as pipelines, canals, and dams.

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E- Fires: They break out following earthquakes. They can be caused by flammable materials being thrown into a cooking or heating fire or broken gas lines. Fires can easily get out of control since the earthquake may have broken water mains or blocked roads firefighters need to use.

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1- Damage Due to Ground Shaking

Fig. (2.1) Damage due Ground Shaking

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2- Damage Due to Ground Failure

A- Due to Surface Faulting

Fault, 1980 El Asnam Earthquake

Overturned Train, 1980 El Asnam Earthquake

Collapsed Bridge, 1976 Guatemala Earthquake

Damage to A building, 1971 San Fernando Earthquake

Fig. (2.2) Damage due to Surface Faulting

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B- Due to Liquefaction

Tilting of Buildings, 1964 Niigata Earthquake

Collapsed Bridge, 1964 Niigata Earthquake

Linear Fissure, 1977 Caucete Earthquake

Sand Blows

Fig. (2.3) Damage due to Liquefaction

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Liquefaction

Liquefaction Process Liquefaction is a process by which sediments below the water table temporarily lose strength and behave as a viscous liquid rather than a solid. The types of sediments most susceptible are clay-free deposits of sand and silts; occasionally, gravel liquefies. The actions in the soil which produce liquefaction are as follows: seismic waves, primarily shear waves, passing through saturated granular layers, distort the granular structure, and cause loosely packed groups of particles to collapse (Fig. 2.4). These collapses increase the pore-water pressure between the grains if drainage cannot occur. If the pore-water pressure rises to a level approaching the weight of the overlying soil, the granular layer temporarily behaves as a viscous liquid rather than a solid. Liquefaction has occurred.

Fig. (2.4) Sketch of a packet of water-saturated sand grains illustrating the process of liquefaction. Shear deformations (indicated by large arrows) induced by earthquake shaking distort the granular structure causing loosely packed groups to collapse as indicated by the curved arrow.

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In the liquefied condition, soil may deform with little shear resistance; deformations large enough to cause damage to buildings and other structures are called ground failures. The ease with which a soil can be liquefied depends primarily on the looseness of the soil, the amount of cementing or clay between particles, and the amount of drainage restriction. The amount of soil deformation following liquefaction depends on the looseness of the material, the depth, thickness, and areal extent of the liquefied layer, the ground slope, and the distribution of loads applied by buildings and other structures.

Liquefaction does not occur at random, but is restricted to certain geologic and hydrologic environments, primarily recently deposited sands and silts in areas with high ground water levels. Generally, the younger and looser the sediment, and the higher the water table, the more susceptible the soil is to liquefaction. Sediments most susceptible to liquefaction include Holocene (less than 10,000-year-old) delta, river channel, flood plain, and aeolian deposits, and poorly compacted fills. Liquefaction has been most abundant in areas where ground water lies within 10 m of the ground surface; few instances of liquefaction have occurred in areas with ground water deeper than 20 m. Dense soils, including well-compacted fills, have low susceptibility to liquefaction.

Effect of Liquefaction on the Built Environment

The liquefaction phenomenon by itself may not be particularly damaging or hazardous. Only when liquefaction is accompanied by some form of ground

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displacement or ground failure is it destructive to the built environment. For engineering purposes, it is not the occurrence of liquefaction that is of prime importance, but its severity or its capability to cause damage. Adverse effects of liquefaction can take many forms. These include: flow failures; lateral spreads; ground oscillation; and increased lateral pressure on retaining walls.

Flow Failures

Flow failures are the most catastrophic ground failures caused by liquefaction. These failures commonly displace large masses of soil laterally tens of meters and in a few instances; large masses of soil have traveled tens of kilometers down long slopes at velocities ranging up to tens of kilometers per hour. Flows may be comprised of completely liquefied soil or blocks of intact material riding on a layer of liquefied soil. Flows develop in loose saturated sands or silts on relatively steep slopes, usually greater than 3 degrees. Lateral Spreads

Lateral spreads involve lateral displacement of large, surficial blocks of soil as a result of liquefaction of a subsurface layer. Displacement occurs in response to the combination of gravitational forces and inertial forces generated by an earthquake. Lateral spreads generally develop on gentle slopes (most commonly less than 3 degrees) and move toward a free face such as an incised river channel. Horizontal displacements commonly range

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up to several meters. The displaced ground usually breaks up internally, causing fissures and scarps to form on the failure surface. Lateral spreads commonly disrupt foundations of buildings built on or across the failure, sever pipelines and other utilities in the failure mass, and compress or buckle engineering structures, such as bridges, founded on the toe of the failure. Damage caused by lateral spreads is severely disruptive and often pervasive. For example, during the 1964 Alaska earthquake, more than 200 bridges were damaged or destroyed by spreading of floodplain deposits toward river channels. The spreading compressed the superstructures, buckled decks, thrust stringers over abutments, and shifted and tilted abutments and piers. Lateral spreads are particularly destructive to pipelines. For example, every major pipeline break in the city of San Francisco during the 1906 earthquake occurred in areas of ground failure. These pipeline breaks severely hampered efforts to fight the fire that ignited during the earthquake; that fire caused about 85% of the total damage to San Francisco. Thus, rather inconspicuous ground-failure displacements of less than 2 m were in large part responsible for the devastation that occurred in San Francisco.

Ground Oscillation Where the ground is flat or the slope is too gentle to allow lateral displacement, liquefaction at depth may decouple overlying soil layers from the underlying ground, allowing

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the upper soil to oscillate back and forth and up and down in the form of ground waves. These oscillations are usually accompanied by opening and closing of fissures and fracture of rigid structures such as pavements and pipelines. The manifestations of ground oscillation were apparent in San Francisco’s Marina District due to the 1989 Loma Prieta earthquake; sidewalks and driveways buckled and extensive pipeline breakage also occurred.

Loss of Bearing Strength

When the soil supporting a building or other structure liquefies and loses strength, large deformations can occur within the soil which may allow the structure to settle and tip. Conversely, buried tanks and piles may rise buoyantly through the liquefied soil. For example, many buildings settled and tipped during the 1964 Niigata, Japan, earthquake. The most spectacular bearing failures during that event were in the Kawangishicho apartment complex where several four-story buildings tipped as much as 60 degrees. Apparently, liquefaction first developed in a sand layer several meters below ground surface and then propagated upward through overlying sand layers. The rising wave of liquefaction weakened the soil supporting the buildings and allowed the structures to slowly settle and tip. Settlement

In many cases, the weight of a structure will not be great enough to cause the large settlements associated with soil

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bearing capacity failures described above. However, smaller settlements may occur as soil pore-water pressures dissipate and the soil consolidates after the earthquake. These settlements may be damaging, although they would tend to be much less so than the large movements accompanying flow failures, lateral spreading, and bearing capacity failures. The eruption of sand boils (fountains of water and sediment emanating from the pressurized, liquefied sand) is a common manifestation of liquefaction that can also lead to localized differential settlements.

Increased Lateral Pressure on Retaining Walls

If the soil behind a retaining wall liquefies, the lateral pressures on the wall may greatly increase. As a result, retaining walls may be laterally displaced, tilt, or structurally fail, as has been observed for waterfront walls retaining loose saturated sand in a number of earthquakes.

Can Liquefaction Be Predicted?

Although it is possible to identify areas that have the potential for liquefaction, its occurrence cannot be predicted any more accurately than a particular earthquake can be (with a time, place, and degree of reliability assigned to it). Once these areas have been defined in general terms, it is possible to conduct site investigations that provide very detailed information regarding a site’s potential for liquefaction.

Mapping of the liquefaction potential on a regional scale has greatly furthered our knowledge regarding this hazard.

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These maps now exist for many regions of the United States and Japan, and several other areas of the world. Liquefaction potential maps are generally compiled by superimposing a liquefaction susceptibility map with a liquefaction opportunity map. Liquefaction susceptibility refers to the capacity of the soil to resist liquefaction, where the primary factors controlling susceptibility are soil type, density, and water table depth. Liquefaction opportunity is a function of the intensity of seismic shaking or demand placed on the soil. Frequency of earthquake occurrence and the intensity of seismic ground shaking produced by those events are the major factors affecting liquefaction opportunity. To develop an opportunity map, an earthquake source model is required that includes locations of seismic source zones and quantitative estimates of the number and magnitude of the expected earthquakes in those zones.

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Tsunamis

Tsunami is a Japanese term that means “harbor wave”. Tsunamis are the result of a sudden vertical offset in the ocean floor caused by earthquakes, submarine landslides, and volcanic deformation. Tsunami Initiation:

A sudden offset changes the elevation of the ocean and initiates a water wave that travels outward from the region of sea-floor disruption. Tsunami can travel all the way across the ocean and large earthquakes have generated waves that caused damage and deaths.

Fig. (2.5) Tsunami Initiation

The speed of this wave depends on the ocean depth and is typically about as fast as a commercial passenger jet (about 700 km/hr). This is relatively slow compared to seismic waves, so we are often alerted to the dangers of the Tsunami by the shaking before the wave arrives. The trouble is that the time to react is not very long in regions close to the earthquake that caused the Tsunami.

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Fig. (2.6) Tsunami in Deep Water Tsunamis pose no threat in the deep ocean because they are only a meter or so high in deep water. But as the wave approaches the shore and the water shallows, all the energy that was distributed throughout the ocean depth becomes concentrated in the shallow water and the wave height increases.

Fig. (2.7) Tsunami in Shallow Water

Typical heights for large Tsunamis are on the order of 10’s of meters and a few have approached 90 meters. These waves are typically more devastating to the coastal region than the shaking of the earthquake that caused the Tsunami. Even the more common Tsunamis of about 10-20 meters can “wipe clean” coastal communities.

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Architectural and Structural Considerations

Building Configuration:

In recent years, there has been increased emphasis on the importance of a building’s configuration in resisting seismic forces. Early decisions concerning size, shape, arrangement, and location of major elements can have a significant influence on the performance of a structure. Since the design professional plays a large role in these early decisions, it is imperative that the architect thoroughly understand the concepts involved.

Building configuration refers to the overall building size and shape and the size and arrangement of the primary structural frame, as well as the size and location of the nonstructural components of the building that may aspect its structural performance. Significant nonstructural components include such things as heavy nonbearing partitions, exterior cladding, and large weights like equipment or swimming pools.

In the current UBC, elements that constitute both horizontal and vertical irregularities are specifically defined, so it is clear which structures must be designed with the dynamic method and which structures may be designed using the static analysis method.

The code states that all buildings must be classified as either regular or irregular. Whether a building is regular or

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not helps determine if the static method may be used. Irregular structures generally require design by the dynamic method, and additional detailed design requirements are imposed depending on what type of irregularity exists. The following sections describe some of the important aspects of building configuration.

A. Torsion

Lateral forces on a portion of a building are assumed to be uniformly distributed and can be resolved into a single line of action acting on a building. In a similar way, the shear reaction forces produced by the vertical resisting elements can be resolved into a single line of action. For symmetric buildings with vertical resisting elements of equal rigidity, these lines of action pass through the same point. If the shear walls or other vertical elements are not symmetric or are of unequal rigidity, the resultant of their shear resisting forces, the center of rigidity, does not coincide with the applied lateral force. Since the forces are acting in opposite directions with an eccentricity, torsion force is developed, which is in addition to the lateral load alone.

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Fig. (2.8) Development of Torsion

When the force on a vertical element caused by the eccentricity acts in the same direction as that caused by the lateral load directly, they must be added. However, when the torsional force acts in the opposite direction, one cannot be subtracted from the other.

The UBC requires that even in symmetrical buildings a certain amount of accidental torsion be planned for. This accounts for the fact that the position of loads in an occupied building cannot be known for certain. The code requires that the mass at each level is assumed to be displaced from the calculated center of mass in each direction by a distance equal to 5 percent of the building dimension at that level perpendicular to the direction of the force under consideration.

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Fig. (2.9) Torsion’s Analogous Simplification

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The importance of understanding the concept of torsion will become apparent in the following sections.

B. Plan Shape & Dimensions:

Irregularities in plan shape can create torsion and concentrations of stress, both of which should be avoided whenever possible. One of the most common and troublesome plan shapes is the re-entrant corner.

During an earthquake, the ground motion causes the structure to move in such a way that stress concentrations are developed at the inside corners. In addition, since the center of mass and the center of rigidity do not coincide, there is an eccentricity established that results in a twisting of the entire structure as discussed in the previous section. Of course, building shape is often dictated by the site, the program, or other requirements beyond the control of the architect or engineer. In the cases where such shapes are unavoidable, there are ways to minimize the problem. The portions of the building can be separated with a seismic joint, they can be tied together across the connection, or the inside corner can be splayed.

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Fig. (2.10) Problem Plan Shapes

A second common problem that arises with building plans is a variation in the stiffness and strength of the perimeter.

Fig. (2.11) Solution to Re-entrant Corners

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Fig. (2.12) Variation in Perimeter Stiffness

Even though a building may be symmetric, the distribution of mass and lateral resisting elements may place the centers of mass and rigidity in such a way that torsion is developed.

During an earthquake, the open end of the building acts as a cantilevered beam causing lateral displacement and torsion. There are four possible ways to alleviate the problem.

In the first instance, a rigid frame can be constructed with symmetric rigidity and then the cladding can be made nonstructural. Secondly, a strong, moment-resisting or braced frame can be added that has stiffness similar to the other walls. Third, shear walls can be added to the front if this does not compromise the function of the building. Finally, for small buildings, the structure can simply be designed to resist the expected torsion forces.

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The ratio of plan dimensions should not be inordinately large to prevent different types of forces acting on different plan sections. If this cannot be achieved, then seismic joint should be provided in such a building.

C. Elevation Design

The ideal elevation from a seismic design standpoint is one that is regular, symmetrical, continuous, and that matches the other elevations in configuration and seismic resistance. Setbacks and offsets should be avoided for the same reason as re-entrant corners in plan should be avoided; that is, to avoid areas of stress concentration. Of course, perfect symmetry is not always possible due to the functional and aesthetic requirements of the building, but there are two basic configurations that should (and can) be avoided by the architect early in the design process.

The first problem configuration is a discontinuous shear wall. This is a major mistake and should never happen. Discontinuities can occur when large openings are placed in shear walls, when they are stopped short of the foundation, or when they are altered in some other way. Since the entire purpose of a shear wall is to carry lateral loads to the foundation and act as a beam cantilevered out of the foundation, any interruption of this is counterproductive. Of course, small openings like doors and small windows can be placed in shear walls if proper reinforcement is provided. Two common examples of discontinuous shear walls are shown. In the first, the shear wall is stopped at the second

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floor level and supported by columns. This is often done to open up the first floor, but it creates a situation where stress concentrations are so great that even extra reinforcing cannot always resist the build-up of stress.

Fig. (2.13) Discontinuous Shear Walls

The second example is also a common design feature where the second floor and floors above are cantilevered slightly from the first floor shear wall. Even though the shear wall continues, the offset also creates an undesirable situation because the direct load path for the lateral loads is interrupted, and the floor structure has to carry the transfer of forces from one shear wall to the next.

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In all cases of discontinuous shear walls, the solution is simple: shear walls should run continuously to the foundation. Another serious problem with building configuration is the soft story. This occurs when the ground floor is weaker than the floors above. Although a soft story can occur at any floor, it is most serious at grade level because this is where the lateral loads are the greatest. The discontinuous shear wall discussed in the previous section is a special

Fig. (2.14) Soft First Stories

case of the soft story. Others can occur when all columns do not extend to the ground or when the first story is high compared with the other floors of the structure. A soft story can also be created when there is heavy exterior cladding above the first story and the ground level is open. Of course, there are usually valid reasons for all of these situations to occur. For example, a hotel may need a

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high first story, but shorter floors above for the guest rooms.

When earthquake loads occur, the forces and deformations are concentrated at the weak floor instead of being uniformly distributed among all the floors and structural members.

There are several ways to solve the problem of a soft story. The first, of course, is to eliminate it and try to work the architectural solution around the extra columns or lower height. If height is critical, extra columns can be added at the first floor. Another solution is to add extra horizontal and diagonal bracing. Finally, the framing of the upper stories can be made the same as the first story. The entire structure then has a uniform stiffness. Lighter, intermediate floors can be added above the first between the larger bays so they do not aspect the behavior of the primary structural system.

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Lightweight Construction:

Other things being equal, the greater the structural mass, the greater the seismic forces. In contrast to wind design, seismic design calls for lighter construction with a high strength-to-weight ratio to minimize the internal forces. Ductility:

The ductility of the structure can be considered as a measure of its ability to sustain large deformations without endangering its load-carrying capacity. Therefore, in addition to seismic strength, the ductility of the structure should be given serious consideration.

• The required ductility can be achieved by proper choice of framing and connection details.

• Ductility is improved by limiting the ratio of reinforcement on the tension side of beams.

• Using compression reinforcement in beams enhances ductility.

• Using adequate shear reinforcement enhances ductility.

• Provision of spiral reinforcement or closely spaced ties improved ductility.

Adequate Foundations:

Differential settlement of buildings is to be minimized through proper design of footings. Earthquake oscillations can cause liquefaction of loose soils, resulting in an uneven settlement. Stabilization of the soil prior to building

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construction, or the use of deep footings are some remedial measures needed to overcome such a problem. Short Column Effects:

Frequently a column is shortened by elements, which have not been taken into account in design, such as the partial-height infill walls. This creates very large shear forces due to the short length of the column when subjected to very large bending moments. Even if very strong stirrups are used it is difficult to save such columns, therefore such collapses have been frequent. The only possible solution is to use different structural concept; eliminating such partial heights of infills. The shear force is given by the following relation V = 2 M (plastic) / L. Fig. (2.15) shows failure due to short column effect.

Fig. (2.15) Failure due to Short Column Effect Separation of Structures:

The mutual hammering received by buildings in close proximity of one another has caused significant damage.

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The simplest method of preventing damage is to provide sufficient clearance so that the free motion of the two structures can occur. Joints and Connections:

Joints are often the weakest link in a structural system. It is necessary to provide strong horizontal confining reinforcement within the joint zone. Joints in reinforced concrete frames are often responsible for collapses in earthquakes. Inadequate Shear Strength:

To enhance shear capacity one should first use suitable amount of stirrups and ties to prevent the brittle type of failure associated with shear. Diagonal reinforcement is recommended for deep members to resist diagonal tension. Materials and Workmanship:

It is obvious that no design can save the structure if bad materials are used or if workmanship is not good. The best available quality design codes are deemed useless unless quality control is kept starting from the design process and ending up with the site execution. Bond, Anchorage, and Splices:

Bond, when effectively developed, enables the concrete and reinforcement to form a composite structure. If the area of concrete surrounding the bar is small, splitting is the common mode of failure. One should avoid splices and

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anchorage at the location where the surrounding concrete is extensively cracked (i.e., plastic hinges). Detailing of Structural Elements:

Closely spaced stirrups and ties are used in columns and walls, to hold the reinforcement in place and to prevent buckling of longitudinal bars. Closely spaced stirrups and ties are used in potential hinge regions of beams, to ensure strength retention during cyclic loading. Detailing of special transverse steel through beam-column joints in ductile frames to maintain the integrity of the joints during adjacent beam hinge plastic deformation. Detailing of Non-Structural Elements:

Te tendency of non-structural elements to be damaged, as the building sways need to be addressed. To overcome such problems, either separation is kept between structural and non-structural members, or the forces resulting from the attachment of structural elements need to be taken into consideration.

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The Effect of Earthquakes on Buildings When an earthquake occurs, the first response of a building is not to move at all due to the inertia of the structure’s mass. Almost instantaneously, however, the acceleration of the ground causes the building to move sideways at the base causing a lateral load on the building and a shear force at the base, as though forces were being applied in the opposite direction. As the direction of the acceleration changes, the building begins to vibrate back and forth.

Fig. (2.16) Building Motion During an Earthquake

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Theoretically, the force on the building can be found by using Newton’s law, which states that force equals mass times acceleration. Since the acceleration is established by the given earthquake, the greater the mass of the building, the greater the force acting on it. However, the acceleration of the building depends on another property of the structure- its natural period. If a building is deflected by a lateral force such as the wind or an earthquake, it moves from side to side. The period is the time in seconds it takes for a building to complete one full side-to-side oscillation. The period is dependent on the mass and the stiffness of the building. In a theoretical, completely stiff building, there is no movement, and the natural period is zero. The acceleration of such an infinitely rigid building is the same as the ground. As the building becomes more flexible, its period increases and the corresponding acceleration decreases. As mentioned above, as the acceleration decreases, so does the force on the building. Therefore, flexible, long-period buildings have less lateral force induced, and stiff, short-period buildings have more lateral force induced.

Natural periods vary from about 0.05 sec. for a piece of furniture such as a filing cabinet to about 0.1 sec. for a one-story building.

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Fig. (2.18) Fundaments Periods

A rule of thumb is that the building period equals the number of stories divided by 10.

As the building moves, the forces applied to it are either transmitted through the structure to the foundation, absorbed by the building components, or released in other ways such as collapse of structural elements. The goal of seismic design is to build a structure that can safely transfer the loads to the foundation and back to the ground and absorb some of the energy present rather than suffering damage. The ability of a structure to absorb some of the energy is known as ductility, which occurs when the building deflects in the inelastic range without failing or collapsing. The elastic limit is the limit beyond which the structure sustains permanent deformation. The greater the ductility of a building, the greater is its capacity to absorb energy.

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Ductility varies with the material. Steel is a very ductile material because of its ability to deform under a load above the elastic limit without collapsing. Concrete and masonry, on the other hand, are brittle materials. When they are stressed beyond the elastic limit, they break suddenly and without warning. Concrete can be made more ductile with reinforcement, but at a higher cost.

Resonance

The ground vibrates at its natural period. The natural period of ground varies from about 0.4 sec. to 2 sec. depending generally on the hardness of the ground.

The terrible destruction in Mexico City in the earthquake of 1985 was primarily the result of response amplification caused by the coincidence of building and ground motion periods. Mexico City was some 400 km from the earthquake focus, and the earthquake caused the soft ground under downtown buildings to vibrate for over 90 seconds at its long natural period of around 2 seconds. This caused tall buildings around 20 stories tall to resonate at a similar period, greatly increasing the accelerations within them. This amplification in building vibration is undesirable. The possibility of it happening can be reduced by trying to ensure that the building period will not coincide with that of the ground. Other buildings, of different heights and with different vibrational characteristics, were often found undamaged even though they were located right next to the damaged 20 story buildings. Thus, on soft (long period) ground, it would be best to design a short stiff (short period) building.

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General Goals in Seismic-Resistant

Design and Construction

• If basic, well-understood principles are ignored and short cuts taken, disaster can occur.

• Many tall buildings that survived major earthquakes show that adherence to theses principles can produce structures out of which people can be sure of walking alive, even if some structural damage has occurred.

The philosophy of earthquake design for structures other than essential facilities has been well established and proposed as follows.

A- To prevent non-structural damage in frequent minor

ground shaking.

B- To prevent structural damage and minimize non-structural damage in occasional moderate ground shaking.

C- To avoid collapse or serious damage in rare major ground shaking.