A Study on Uncertainty in Seismicdesign and Method of analysis Devbrat bose (author)prof. Suhail qureshi (author) ABSTRACTAnearthquake is a natural inevitable unpredictable phenomena which takes placewhen high stresses built in the earth crust, are suddenly released as the crustbreaks with a few kilometers from theearth’s surface. The destruction caused on the structure is not only theshaking of structure , but also due to the action of lateral forces on thestructure which tend to bend the structure against the ground motion in orderto maintain the inertia of rest.
The input shaking causes the foundation ofa building to oscillate back and forth in a more or less horizontal plane. Thebuilding mass has inertia and wants to remain where it is and therefore,lateral forces are exerted on the mass in order to bring it along with thefoundation. For analysis purposes, this dynamic action is simplified as a groupof horizontal forces that are applied to the structure in proportion to itsmass and to the height of the mass above the ground. INTRODUCTIONStructures ofcivil engineering are mainly consideredto resist static loads. In General, theeffects of dynamic loads acting on the structure are not measured. This featureof neglecting the dynamic loads sometimes becomes the cause of disaster,particularly in case of seismic forces due to earthquake. Recently an example of this category isearthquake occurred on Jan.
26, 2001 in Bhuj (Gujarat). This has produced agrowing interest for earthquake resistant design of structures. Otherearthquakes in asia are Indian ocean earthquake (2004) , Kashmir and regions ofPakistan (2005) , bihar and Nepal earthquake (1934) , assam (1950) etc Earthquakes are catastrophic eventsthat occur mostly at the boundaries of portions of the earth’scrust called tectonic plates. Whenmovement occurs in these regions, along faults, waves are generated at theearth’s surface that can produce very destructive effects. Aftershocks are smaller quakes that occurafter all large earthquakes. They are usually most intense in size and numberwithin the first week of the original quake.
They can cause very significantre-shaking of damaged structures, which makes earthquake-induced disasters morehazardous. A number of moderate quakes (6+ magnitude on the Richter scale) havehad aftershocks that were very similar in size to the original quake.Aftershocks diminish in intensity and number with time. They generally follow apattern of being at least 1 large (within magnitude 1 on the Richter scale)aftershock, at least 10 lesser (within magnitude 2 on the Richter scale)aftershocks, 100 within magnitude 3 on the Richter scale, and so on. The LomaPrieta earthquake had many aftershocks, but the largest was only magnitude 5.0,with the original quake being magnitude 7.
1. In multistory buildings with floorsof equal weight, the loading is further simplified as a group of loads, eachbeing applied at a floor line, and each being greater than the one below in atriangular distribution .Seismically resistant structures are designed toresist these lateral forces through inelastic action and must, therefore, bedetailed accordingly. These loads are often expressed in terms of a percent ofgravity weight of the building and can vary from a few percent to near 50% ofgravity weight. Increased compression may exceed the axial compressive capacityof columns while decreased compression may reduce the bending strength ofcolumns. Literaturereview: Earthquakeengineering Research institute (EERI) has conducted the full survey of Bhujearthquake or also known as Kutch earthquake. This earthquake is considered inone of the most destructive and disastrous earthquake in the last fifty yearshistory of india . The important citiesaffected by the earthquake are Bhuj , Anjar, rajnagar , Gandhidham , Morbi etc.
where the maximum amount of damage and casualties has take place. It revealsthe structural weakness or inefficient in the form of design and planning ,improper analysis of calculation of dynamic loads which results in design deficiencyand inadequate constructions.In earthquake engineering, we deal with random variables and thereforethe design must betreated differently from the orthodox design. The orthodox viewpointmaintains that the objective of design is to prevent failure; it idealizesvariables as deterministic. This simple approach is still valid and applied todesign under only mild uncertainty. But when confronted with the effects ofearthquakes, this orthodox viewpoint seems so over trustful as to be worthless.In dealing with earthquakes, we must contend with appreciableprobabilities that failure will occur in the near future.
Otherwise, all thewealth of this world would prove insufficient to fill our needs: the mostmodest structures would be fortresses. We must also face uncertainty on a largescale while designing engineering systems—whose pertinent properties are stilldebated to resist future earthquakes about whose characteristics we know even Factors affecting analysis and design ofstructure due to various behavior of structures under seismic forces: Buildingbehavior :- The buildingis subjected to various types of forces , mostly due to vibration caused onaccount of earthquake forces. The foundation of structure or structural membersare not damaged due to impact forces or external pressure due to winds, but dueto action of inertial forces caused by various vibrations and shakening ofstructure of the building. The increased weight of structure has adverseeffects on design of building against seismic forces. It increases the inertialforces on the structure as mass of structure get increased and secondly itresults in buckling and crushing ofstructural members such as walls and columns when the earthquake forces tendsto bend the structural members or moved out the members from its initialposition . Fig :- showing behavior of building under seismic forces Influenceof soil :- Due to vibrationof structure, which is associated withthe ground motions , the amplification of acceleration and its frequency ofvibration coincides with the vibrations transferred to soil . This phenomena ofcoinciding of frequency is called resonance. Thus it is possible for thebuilding and the ground which it rest on it have same fundamental time periodand frequency of vibrations which wake the situation more adverse duringoccurrence of such conditions .
Thus , it is concluded from the fact that, toavoid such conditions the structure must ensures that it has differentfrequency and time period of vibrations to that of ground on which it restupon. Fig :- showing action of seismic waves on foundation soil Damping :- when the vibrational characteristics of structure coincides withthe vibrational characteristics of seismic waves , results in resonance whichis responsible for heavy destruction of structure. but, considering the factthat resonance in building is not same as in ideal cases which we have studied,rather they are damped resononating in nature.
Their damping behavior dependson the constructional material , connection type and other influence due topresence non structural members , which are only used to serve forarchitectural purposes and thus have no stiffness characteristics on thebuilding. Damping of structure is measured with reference to critical dampingin some percentage. Fig:- showing concept of damping Building motions and separations :- Earthquake-induced motions,even when they are more violent than those induced by wind, evoke a totallydifferent human response—first, because earthquakes occur much less frequentlythan windstorms, and second, because the duration of motion caused by anearthquake is generally short. People who experience earthquakes are gratefulthat they have survived the trauma and are less inclined to be critical of thebuilding motion. Earthquake-induced motions are, therefore, a safety ratherthan a human discomfort issue.
Lateral deflections that occur duringearthquakes should be limited to prevent distress in structural members andarchitectural components. Non load-bearing in-fills, external wall panels, andwindow glazing should be designed with sufficient clearance or with flexiblesupports to accommodate the anticipated movements. METHODS OF SEISMIC DESIGN:-Based onthe three criteria strength, stiffness and ductility the methods for seismicdesign are described below LATERAL STRENGTH BASED DESIGN: It is oneof the commonly used design method used nowadays.
The concept of this method isbased on providing the structure with sufficient lateral strength toresist various impacts on structure dueto seismic forces, assuming the condition of structure that it will behaveadequately in non-linear range . To achieve this , only simple constructiondetails are needed to satisfied for safe structural characteristics againstuneven failure DISPLACEMENT BASED DESIGN : In thismethod the structure is designed in such away that , to possess sufficientstrength an ductility so that it can transfer the vibrational energy byyielding and should bear the shock and shaking of structure effectively. Thismethod operates directly with deformation quantities hence gives better insighton the expected performance of the structures. This method of design has beenadopted by many countries in the form of their own design codes.
CAPACITY BASED DESIGN: Capacity baseddesign is one of such techniques which is new to field of seismic designof various structures . It is based onthe positioning of plastic hinges which is in predetermined positions andpredetermined sequences . The main objective of this method is to avoid thebrittle failure of structural member , which is achieved bytransforming brittle failure members into ductile modes ENERGY BASED DESIGN: This is the most capable andinnovative approach of earthquake resistant design.
this method is based on theconcept of total energy is dissipated by the kinetic energy , the elasticstrain energy and energy resisted through deformations and damping of thestructural members associated with it.SEISMIC ANALYSIS METHODS Main features of seismic method ofanalysis based on Indian Standard 1893(part 1): 2002 are described as follows Equivalent lateral force method: The approach of equivalent lateral force procedure , uses a simpletechnique of the structures fundamental period and anticipated maximum groundacceleration , or velocity , together with relevant factors , to determine themaximum base shear. The loads acting on the structure specifically horizontalloads are then distributed to the full height of the structure .Then thestructure is analyzed for static loads .Then the design forces are calculated in this static load analysis aregenerally less than the forces which are actually acting on the structure correspondingto the seismic forces. . Thedesign lateral forces or the design force for base shear are imposed on thestructure as per the clauses stated inIS 1893Response Spectrum analysis: Earthquakeacceleration show the irregularity of the ground motions, such as accelerationand velocity as a function of time .
these ground motion characteristicsprovide information related to the nature of ground motions. Thus it isnecessary, to have such a meaningful data which describes the criteria ofdesign purposes. This meaningful data is provided by the method called response spectrum method. this method can bemore efficiently defined as a graphic representation of maximum responses ofdamped single degree of freedom (SDOF),in which the mass spring system is continuously varying with naturalfrequencies as well as natural periods to a given ground motion characteristics. Objectives:Themain objectives are as follows · Tounderstand the fundamentals as well as the basic terminology of the earthquakeforces.
· Tounderstand the behavior of ground motions with respect to the structuralbehavior on the basis of characteristics such as frequencies and time period ofvibrations.· Tofamiliarize with the design procedure and codes adapted, according to thenature of earthquake forces .· Discussthe various aspects and their proper evaluation of various risks occur due toearthquake.· Toadopt the method which can able to analyze , and helps in managing destructioncaused due to earthquake. PROBLEM FORMULATION Uncertainty orrandomness in seismic design:-Thelargest uncertainty involved in the seismic design of structure are associatedwith the prediction of the potential of the earthquake , that may affect aspecific location on earth, any time with fixed period and their characteristics.· Duration of earthquake· Frequency of p-waves ands-waves· Magnitude and intensity ofearthquake· Maximum acceleration andground motion characteristics · Types and nature of waves · Location of epicenter and itsfocus RANDOMNESS IN CAPACITY AND DEMAND UNCERTAINITY IN DEMAND :- There are various unavoidable sources of uncertainty occurs in the expected demands on a structure. These sources of uncertainty include: Seismology . Ground motion characteristics Structural characteristics Modeling Structural Analysis Method UNCERTAINITY IN CAPACITY :- In recent years , strength of structure is calibrated in terms of capacity .
However , the previous studies and research has proved that , capacity of a structure is not only a function of strength of bearing loads and forces but also to proper dissipation of energy to nearby members without failure. Thus it is however difficult to finalize or calculate the overall capacity of a building or structure against these types of forces , which overall affects the design strength and load calculation , which is going to imposed on the structure. Thus uncertainty in capacity of structure include following difficulties : · composite action of joints · connection between various structural members · proportioning of ductile elements in structure · Members to serve for structural behavior as well as architectural importance. CONCLUSIONThestructure should be well analyzed for properly resisting those forces due toearthquake without failure. There are various techniques available anddocumented in various design standards. But the most effective technique shouldbe one which effectively detects the various characteristics of seismic forcesassociated with the ground motion characteristics and the nature of thoseforces when imposed on the structure. Also unconditionally they have easyconstructability and feasible in cost of construction REFRENCES1. Kumar, S.
L., Research Paper published at the PEC,1933. 2. “Study on Seismic Capabilities of Dhajji-Dewari Frames” M A Dar, J Raju, A.R Darand A H Shah, (AARCV) 2012 at India(Proceedings of International Conference on Advances in Architecture and CivilEngineering (AARCV 2012, 21st -23rd June 2012, Paper ID SAM206, vol1)) (ISBN 978-93-82338-01-7) 3. Coull , Alex, Smith , Bryan trafford., Analysis and design , Design of tall structures.
4.Taranath,S.B, CRC press , Taylor and francis group, design of tall structures 5.
Jain ,S. K. , paper published on CURRENT SCIENCE, VOL. 89, NO.
9, IIT Kanpur , Department of CivilEngineering