Lecture (3 )

Prepared by :

Assc. Prof Nasser El-Shafey

2016- 2017

When loose, saturated sands, silts, or gravel 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

Liquefaction can cause problems as soil loses it’s ability to

resist shear and flows much like quick sand. Anything

relying on the substrata for support can shift, tilt, rupture, or

collapse.

1-Liquefaction

Damage of Structure As A Result Of Soil Problems

Liquefaction-caused building

failure in Niigata,

Japan.

Liquefaction-

caused building

failure in Niigata,

Japan

Liquefaction Adapazari, Turkey

1999

1995 Kobe, Japan earthquake,

increase pore pressure pushed

quay wall bridge near the west

end toward the river, allowing

the soil and western-most pier

to move one meter Eastward.

2- Landslides

When 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.

In 1970 an earthquake off coast of Peru produced a landslide began 80 miles away from earthquake. The slide was large (estimated it's height at about 30 meters), killing more than 18,000 people.

Gravity retaining wall pushed outward by landslide

One of about 75 homes

damaged as a result of the

Turnagain Heights slide.

Collapse of Tsu Wei Bridge due to landslide during the Ji Ji, Taiwan earthquake.

Slope instability –Niigata, Japan 2004

IMPORTANCE OF GROUND CONDITIONS

Local soil conditions, particularly deep layers of soft soil as may

be found in river valleys or near estuaries, significantly amplify

shaking. They also modify the frequency content of seismic waves

by filtering out higher frequency excitations

DAMAGE AS A RESULT OF STRUCTURAL PROBLEMS

Engineers will 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

1. Foundation Failure

Large longitudinal displacements of bridge span causing damage to abutment

2. Foundation Connections

Structures need to be well anchored to foundation.

The longitudinal reinforcement did not have sufficient

development length to transfer the force to the footing

Soft Story

Soft Story

Soft First Story is a discontinuity of strength and stiffness for lateral load at the ground level.

3. Soft Story

Soft Storey at Ground Floor

Soft story (stiffening structural elements which are present in the upper stories are missing at the ground floor)

1) Building Plan

2) Heavy mass at top should be avoided

Smaller water tank at top is preferred

NO YES

3) Large projections not allowed 4) Floating column not allowed

Short Column

Short Column

4. Torsional Moments

Curved, skewed, and eccentricallysupported structures often experience a torsional moment during earthquakes.

5. Shear

Most building structures use shear walls or moment-resisting

frames to resist lateral forces during earthquakes. Damage to

these systems varies from minor cracks to complete collapse.

M. McKinley-ALASKA

6. Flexural Failure

Flexural 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 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.

Flexural damage tocolumns

Insufficient transverse reinforcement toprevent buckling of the vertical reinforcement

7. Connection Problems

When a bridge superstructure moves off its expansion joint or when the connections between building columns and beams fail, the result is too often the COLLAPSE of thestructure.

Special structural systems 1-Rigid frames 2-Shear walls 3-Frames and cores 4- Coupled framed-shear wall systems

5- Cores-coupled shear wall

6- Tubular systems7- Perforated tubes.8- Tube in tube. 9- Bundle tube.

Structural system Number of floors

Frames 20

Shear walls 35

Frames & shear walls 50

Cores-coupled shear walls 55

Perforated tube 60

Tube in tube 65

Bundle tube 90

The response spectrum is a convenient method for illustrating

and quantifying how natural period of vibration and damping

of a building affects its response to earthquake shaking.

SEISMIC FORCES

Natural period of vibration of building has huge effect on maximum horizontalacceleration, and magnitude. Buildings with T=0.2 to 0.7 seconds are amplifyingground acc. by almost a factor of 3.0. As natural periods become longer, from 0.7to 1.7 seconds, peak accelerations reduce towards the same intensity as the peakground acceleration. Beyond 1.7 seconds, maximum accelerations continue todiminish until at T=4.0 seconds building acceleration is only 0.3 of the maximumground acceleration.

Earthquake effects on buildings depend on:Mass of structure Stiffness of structureDuctility of structures Foundation typeSoil conditions Earthquake zone

1.Symmetry: The structure is almost symmetrical in plan along

two orthogonal directions regarding the lateral stiffness and

mass distribution.

2.Recesses: shall not exceed 5% of the floor area.

3.Aspect Ratio: Lx / Ly <4.0.

4.Torsional regularity: eccentricity between C.M. & C.R.

< 15% Li for each direction (Li=Lx &Ly for x & y directions).

Plan Regularity

Quasi static analysis: Regular buildings; H <100 m & H/B <5.

Response Spectra: Regular buildings; H=100-150 m or H/B >5.

Dynamic analysis: Irregular buildings or H>150m.

Earthquake methods of Analysis

Elevation RegularityAll lateral force resisting elements such as cores, shear walls or frames should be continuous from foundation up to the last floor or setback or recess level.Stiffness and mass regularity: Difference between two successive floors does not exceed 75% for Lateral stiffness and 50% for mass.Capacity Continuity: ratio between actual to required resistance of columns in framed structures should sustain for successive floor. The masonry walls effect should be considered as such.Vertical geometric regularity.

Simplified Modal Response

Spectrum Method