Message from Chairman PEC Engr. Dr. Muhammad Akram Sheikh

ENGINEER PEC September 2007Essentials of Earthquake DesignsRichter maximum wave amplitude defined as logarithm tobase ten and recorded on seismograph. The rise of oneJunaid Sultan Khanmean ten rise of the wave were period of half second. Theinstruments record the components of groundEarthquakeacceleration, two in horizontal direction and one in thevertical.Earthquake kills ten thousands people every year. Theknowledge has now increased and the structures are Special geological engineering studies for each sitedesigned using quantitative dynamic analysis, using consisting of occupancy, map showing faults (active) andspectra of defining parameters. From the wave readings existing structural type are to be carried out. The quantumobserved in different stations the position of the center of of studies depends upon the type of structure but revolveearthquake can be calculated. The seismic waves radiate around two pre-requisites.from the source below the ground surface as opposite 1. Measurements of density, water content, shearsides of the fault, rebound to decrease the strain energy instrength, behavior under flooding, attenuation in situthe rocks. The point is called focus and directly above thisand laboratory of boreholes samples.is called the epicenter. Earthquakes with foci 70 kms or 2. Determination of wave speeds.less are shallow and below 300s km are called deep. In short prediction of ground acceleration, duration,Smaller earthquakes, known as after shocks follow most frequency strong portion of earthquake is compulsory forearthquake of the shallow origin. Foreshocks before major designing a structure. The response of the structure isearthquake sometime predict the main shock.complicated and it depends on several factors such asamplification, soil condition and source distance.Major earthquakes were attributed to volcanoes Moderate earthquake with long duration may cause morepreviously, but global geology explained the plate damages. In addition when the frequency of structure andtectonics phenomena. The lithosphere, which is outermost ground motion is close, ground motion is amplified.part, consists of stable rocks (plates) 80 km in depth andthe movement of these plates produces seismic activities. FoundationsThree types of elastic waves cause shaking and produceearthquake. The forth wave is called P-wave and its motion The super-structure interfaces foundation and under staticis same as sound wave i.e. alternately pushes and pulls load generally the vertical loads are transferred tothe rock or other medias. The slower wave is called S- supporting soil. In seismic areas the loads imposed canwave and at ground level it produces both vertical and exceed the vertical loads of a structure and also producehorizontal motion. These waves can not travel through uplift coupled with horizontal forces and movements atliquids such as oceans. The speed of both waves depend foundation level. Experience has shown that building onupon the density of rock, the first to reach are P-waves rock suffer very little damage in respect to building on deepeffecting like a sonic boom and some seconds later the S- soils. The liquefaction, which kills the supporting soilwaves arrive with components of side to side motion, pressure, is also worked-out. The design base shear wascausing horizontal and vertical motions. The S- waves are formulated to be directly proportional to seismic zonedestructive. Both waves are dependent on density and factor, importance factor and variable further proportionedelastic modulus. The third type is called surface- wave, to site coefficient and fundamental building period. Allfurther sub-divided into love-wave and Raleigh-wave. The these factors, which can be examined from the earthquakelove-wave moves ground side to side without vertical codes, are used in the formula for design shear.movement, while the Raleigh-wave moves in a verticalplane. Both types are slower in speed than body waves of LiquefactionP&S wave types. Nevertheless, love-wave is faster thanRaleigh-wave with unpredictable pattern.Loose saturated sand deposits are exposed to soilliquefaction and causes settlement of structure andThe shaking at ground level is more pronounced than in landslides. Loose sand deposits have tendency todeep basement, tunnel and mines as amplitude of waves compact and decrease in volume. Water can not drainis doubled at the surface. Earthquake waves are affected rapidly and there is increase in pore water pressure.by both soil conditions and topography. Seismic waves in Shaking increase the pore water pressure more than overaddition to simple shake-up has rotational element having burden pressure, the sand shear strength shall reduce andstrong effect due to torsion forces. Earthquake forces is in liquefied stage. The sand boils at surface during andamage structures directly and indirectly resulting in fires, earthquake. Evaluating the potential of soil is difficult taskabrupt changes in soil pressures, displacement, and standard penetration, cone penetration and otherlandslides, sea waves and changes in levels.methods are used to determine design parameters. For allnew constructions, a choice is to be made betweenMagnitudedesigning for liquefaction or move the project. Sometime itis preferable to modify the soil conditions by:-Wadati in Japan originated in 1931 and later, Charles 1. Excavation and replacement of soils.Richter in California in 1935, invented “Richter scale”. The 2. Compaction piles and grouting.18