Causes Of Failure Of Structures During

EarthquakesEarthquake Resistant Design

& Construction

Effect Of EarthquakesThe effect of an earthquake can be classified as primary and secondary. Primary effects are direct effect on ground, geological features, water bodies and those on the building or other structures and the secondary effects are those which occur due to the primary effects such as fires, epidemics etc..

Landslides Earthquakes cause landslides where the hill slopes ale unstable due to badly fractured rocks which may consist of loose material . The effect is more pronounced in rainy season when the soil is wet than in dry season.

Effect Of EarthquakesGround Effects Earthquake-induced ground effects has been observed in the forms of around rupture alone the fault zone, landslides. settlement and soil liquefaction. Surface Faulting Surface faulting along the fault zone may be none, of very small extent. or may extend over hundreds of kilometers. Ground displacement along the fault may be horizontal, vertical or both, and may be a few centimeters or meters. Obviously a building directly traversed by such a rupture will be severely damaged or collapsed.

Effect Of EarthquakesLiquefaction Settlements Certain type of soils. such as alluvial or sandy soils are more likely to fail during an earthquake due to liquefaction. Liquefaction is a type of ground failure which occurs when saturated soil loses its strength and collapses or becomes liquefied It is more prominent if the foundation soil consists of uniform loose sands within a depth of about 8m below the ground surface and is either fully saturated by or submerged under water. The buildings resting on such land may tilt or sink . Rock Falls In fractured rock areas, the earthquake can also trigger rock fall when precariously supported rock pieces or boulders are shaken loose and roll down the hill slopes and damage buildings or infrastructures .

Causes Of Failure Of StructuresThe causes of failure of structures can be classified as:

Damage as result of problem of soils. Damage as a result of structural problems. Damage resulting from design and layout. Quality Of Materials and Construction Practices Resonance factor

Problem Of SoilsLiquefaction One of the most common causes of damage to structures is the result of liquefaction of the surrounding soil. When loose, saturated sands, silts, or gravel are shaken, the material consolidates, reducing the porosity and increasing pore water pressure. The ground settles, often unevenly, tilting and toppling structures that were formerly supported by the soil. During the 1955 Niigata, Japan earthquake, several four-story apartment buildings toppled over due to liquefaction . These buildings fell when the liquefied soil lost its ability to support them. there was little damage to these buildings and it was reported that their collapse took place over several hours.

Problem Of SoilsLandslidesWhen a steeply inclined mass of soil is suddenly shaken, a slip-plane can form, and the material slides downhill. During a landslide, structures sitting on the slide move downward and structures below the slide are hit by falling debris . Landslides frequently occur in canyons. along cliffs and mountains, and anywhere else that unstable soil exists.

The number of landslides is greatly increased wherever large earthquakes occur. Landslides can move a few inches or hundreds of feet. They can be the result of liquefaction, weak clays, erosion, subsidence, ground shaking, etc.

Problem Of SoilsWeak Clay The problems encountered at soft clay sites include amplification of the ground motion as well as vigorous soil movement, both of which can damage foundations. Several bridges suffered collapse during the 1989 Loma Prieta earthquake due to the poor performance of weak clay. Two parallel bridges were built in 1965 near Watsonville, CA were 800ft long with spans ranging from 80 to 120 ft. The surrounding soil was a very soft clay. During the earthquake, the soft saturated soil in was violently shaken. The soil pushed against the piles, breaking their connection to the superstructure and pushing them away from the cap beam so that they punctured the bridge deck). Investigators found long, oblong holes in the soil, indicating that the piles were dragged from their initial position during the earthquake.

Structural FailuresFoundation Failure Usually, it is the connection to the foundation or an adjacent member rather than the foundation itself that is damaged during a large earthquake. Engineers occasionally design foundations to rock during earthquakes as a way of dissipating energy and of reducing the demand on the structure; however, when the foundation is too small, it can become unstable and rock over. One of several factors that determine a foundations ability to

withstand the forces of an earthquake is the building’s mass. All buildings can carry their own weight; even poorly constructed ones can resist some additional lateral loads, such as those from a normal wind. But buildings are not necessarily designed or constructed to resist the irregular, multidirectional, and intense side-to-side loads that occur during an earthquake, particularly when earthquakes hit in a series of waves.

Structural Failures

Foundation ConnectionsThe major cause of’ damage to electrical transformers, variety of other structures and lifeline facilities during earthquakes is the lack of a secure connection to the foundation. Houses need to he anchored to the foundation with hold—downs connected to the stud walls and bolts connected to the sill plates. Otherwise, the house will fall off its foundation. If during the earthquake, the longitudinal reinforcement did not have sufficient development length to transfer the force to the footings; insufficient confinement reinforcement in the footings and columns and the lack of a top mat of reinforcement will result in pulling out of the column connection to the footing.

Structural FailuresFlexural FailureFlexural members are often designed to form plastic hinges during large earthquakes. A plastic hinge allows a member to yield and deform while continuing to support its load; however, when there is insufficient confinement for reinforced concrete members and insufficient unsupported width to thickness [b/t] ratios for steel plates, a flexural failure will occur instead. Often, flexural damage is accompanied by compression or shear damage, as the capacity of the damaged area has been lowered.

Structural FailuresColumn Beam Anchorage Failure

The lack of adequate anchorage between the support beam and the column allows the beam to be disconnected from the column: it also reduces the confinement of broken concrete. It is possible that the design drawings did not specify the anchorage of the reinforcement bars, or that the contractor did not put them according to correct drawings. These kind of details indicate the need for precise control of the design as well as on-the-job verification of the reinforcement before the concrete is cast.

Design And LayoutSoft StoreyGenerally the upper levels of a building often remain intact while the lower floors crumble. This is because the concentration of forces is at the ground floor, where most soft floors are located. Wherever they are, however, soft floors represent a break in a buildings structural continuity. With fewer walls and little infill, soft floors are typically less rigid than the building constructed on top of them, making soft floors and the columns that support them susceptible to failure.

A soft story, however, does not always occur on the bottom floor. During the 1995 Kobe, Japan earthquake, many tall buildings had mid-story damage, often due to designing the upper floors for a reduced seismic load.

Design And LayoutTorsional MomentTwist in buildings, called torsion, makes different portions at the same floor level to move horizontally by different amounts. This induces more damage in the columns and walls on the side that moves more Many buildings have been severely affected by this excessive torsional behavior during past earthquakes. It is best to minimize this twist by ensuring that buildings have symmetry in

plan (i.e., uniformly distributed mass ).It this twist cannot be avoided, special calculations need to e done to account for this additional shear forces in :he design of buildings; the Indian seismic code has provisions for such calculations. But, for sure, buildings with twist will perform poorly during strong earthquake shaking.

Design And Layout In general, buildings with simple geometry in plan have performed well during strong earthquakes. Buildings with re-entrant corners, like those U, V. H and i- shaped in plait , have sustained significant damage. Many times, the bad effects of these interior corners in the plan of buildings are avoided by making the buildings in two parts. For example, an L-shaped plan can be broken up into two rectangular plan shapes using a separation joint at the junction .

Design And Layout

Quality Of ConstructionConstruction materials is a primary factor affecting the vulnerability of a structure. Stone or brick laid in a weak mortar such as mud are always weaker compared to masonry built using strong binders such as a cement mortar. Although sufficient for carrying the gravity loads, mechanical characteristics of materials used for construction of masonry buildings are not sufficient to resist the additional bending and shearing effects induced in the structural system by the lateral forces generated in an earthquake.

Excessive thickness of horizontal mortar: vertical joints not filled with mortar: continuity in vertical joints: bricks not being soaked into water before construction: are some of the examples of inferior construction techniques in masonry construction. Similarly the use of too much water in concrete: improper compaction of concrete: honeycombing in concrete: low cover to reinforcing steel bars: improper placement of steel bars. are some of the examples in poor quality of RC construction.

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Resonance FactorSmall Buildings: Small building are more affected, or shaken, by high frequency waves (short and frequent). For example, a small boat sailing in the ocean will not be greatly affected by a low-frequency swell where the waves are far apart. On the other hand several small waves in quick succession can overturn, or capsize, the boat. In much the same way, a small building experiences more shaking by high frequency earthquake waves. Tall High Rises: Large structures or high rise buildings are more affected by low-frequency, or slow shaking. For instance, an ocean liner will experience little disturbance by short waves in quick succession. However, a low-frequency swell will significantly affect the ship. Similarly, a skyscraper will sustain greater shaking by long-period earthquake waves than by the shorter waves.

Resonance Factor

Resonance Factor

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Submitted By : Vinayak Bansal (08632)