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International Journal of Disaster Risk Reduction

International Journal of Disaster Risk Reduction 7 (2014) 28–38

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journal homepage: www.elsevier.com/locate/ijdrr

Assessing tsunami vulnerability of structures designedfor seismic loading

Sanket Nayak 1, Mudireddy Hari Obula Reddy, Rangoli Madhavi,Sekhar Chandra Dutta n

School of Infrastructure, Indian Institute of Technology Bhubaneswar, Samantapuri, Bhubaneswar 751 013, Odisha, India

a r t i c l e i n f o

Article history:Received 9 July 2013Received in revised form30 November 2013Accepted 1 December 2013Available online 11 December 2013

Keywords:EarthquakeTsunamiCoastal structuresSeismic resistantTsunami resilient

09/$ - see front matter & 2013 Elsevier Ltd.x.doi.org/10.1016/j.ijdrr.2013.12.001

esponding author. Mobile: þ9178944 07830; fail addresses: sanketiitbbsr@gmail.com,tbbs.ac.in (S. Nayak), hari.reddyiitbbs@gmaili.iit@gmail.com (R. Madhavi), scdind2000@giitbbs.ac.in (S.C. Dutta).obile: þ91 94373 10275; fax: þ91 674 2306

a b s t r a c t

Wave movement with large velocity triggered by strong earthquake occurring at the seabed is generally the primary cause of the tsunami. Occurrence of tsunami (like the oneduring the Sumatra earthquake in 2004 or the one during the Tohoku earthquake in Japanin 2011) causes devastating damages to the coastal structures and tremendous casualties.Seismic resistant design procedure is more popularly followed in various countries as perthe relevant seismic codes. It is the need of the hour to see whether the lateral load-resisting capability attributed through seismic design is sufficient to resist tsunamiloading. The present study using available design guidelines in various seismic codesand well accepted design literature for tsunami loading attempts to achieve this end in alimited form. The study may be helpful in providing a broad overview of tsunamivulnerability of coastal structures which are designed following the mandatory require-ments of seismic codes. Such tsunami vulnerability is attempted to be recognized in termsof critical height that corresponds to maximum inundation depth of tsunami wave whichthe structure may withstand because of being aseismically designed. Thus, the resultspresented in this study may prove useful in assessing and reducing tsunami vulnerabilityof coastal structures.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Earthquake and tsunami are the natural disasters,which cause huge loss of property in terms of lives andeconomy. The devastating damages of the present decadedue to the Indian Ocean tsunami in 2004 and the tsunamiin Japan in 2011, compelled the politicians, policy makers,economist and engineers to think deeply regarding tsunamiresilient design. From the past experiences, it has been

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observed that most of the tsunamis are triggered by earth-quakes. For example, Chile earthquake (1960), Alaska earth-quake (1964), Indonesia earthquake (2004) and Tohokuearthquake (2011), triggered tsunamis. Strong earthquakesresulting in deformation of larger area leads to severetsunamis than smaller earthquakes. Generally, earthquakeshaving focus deeper than 30 km, rarely cause tsunamis. Butsometimes certain earthquakes like the Chile (1960) andthe Indonesia (2004), which had a focal depth larger than30 km also triggered tsunamis. Generally, tsunami pos-sesses a lot of energy, move at high speed and can travelgreater distances. For example in a typical ocean havingdepth of 4 km, the travel speed of tsunami is nearly equal to700 kmph. However, after entering shallow water (less thana depth of 30 m) the tsunami waves travel at a speed of only60 kmph. The speed of tsunami waves further diminishes as

Fig. 1. Different terms related to tsunami [1,2].

S. Nayak et al. / International Journal of Disaster Risk Reduction 7 (2014) 28–38 29

it moves to shallower coastal water with increase in height.Such rise in height of tsunami waves to several meters nearthe coast is due to shoaling effect. As a result of this, whenthe tsunami reaches the coast it may develop into a rapidlyrising or falling tide i.e., a series of breaking waves. So, whiledesigning the structures along the coast, attention shouldbe given towards tsunami loading in addition to earthquakeloading in earthquake prone zones. But the analysis is verymuch complicated as both the forces are time dependentand the motion in each case is transient in nature. Someimportant parameters related to tsunami are presented inFig. 1. Though tsunami is a low probability rarely occurringevent but the devastating damage experienced during thelast two tsunamis (the 2004 Indian Ocean tsunami and the2011 Japan tsunami) compelled the human civilization tothink about the tsunami resilient design of structures.

The important parameters considered during seismicdesign are seismic weight, stiffness, ductility and redun-dancy. On the other hand, the parameters like strength andrigidity of the structure (at lower level), velocity of thewave, inundation depth and the exposed area on the waveside of the structure are taken into account during tsunamiresilient design. Well accepted detailed codal provisions[3–5] are available for seismic design of buildings. Thoughwell-established design procedures for tsunami resilientbuildings are not available but Federal Emergency Man-agement Agency0s Coastal and Construction Manual (FEMA55) [6], City and County of Honolulu Building Code (CCH)[7] and Structural Design Method of Buildings for TsunamiResistance (SMBTR) [8] (which is relevant Japanese designcode), provides some guideline towards the design oftsunamis resilient buildings. During analysis of tsunamiloading, most of the codes consider the following loads:(i) hydrostatic force (ii) buoyant force (iii) hydrodynamicforce (iv) surge force (v) debris impact force and (vi) wave-breaking force. The limited study attempted in this papermakes an effort in establishing an equivalence betweenseismic loading and tsunami loading. In fact, it has beenexplored in this study what inundation depth of tsunamiwave can be survived by a coastal structure which isdesigned for seismic load prescribed in that particularseismic zone. For evaluating seismic load, a few wellknown codes like IS 1893 (Part 1) [3], Eurocode 8 [4] andASCE 7-05 [5] are used. On the other hand, for evaluating

tsunami loads, the codes like FEMA 55 [6], CCH [7] andSMBTR [8] are used. This may be helpful to emergea broader picture regarding such tsunami vulnerability.The outcome of the study may prove beneficial for thecity planners and policy makers for disaster managementandmay also be helpful to the human civilization in reducingthe risk of damage during natural disasters in a broad sense.

2. Literature review

For the design of tsunami resistant structure, the calcula-tion of tsunami forces is necessary. So, many attempts havebeen made to calculate the Tsunami forces accurately. Thus,codes like FEMA 55 [6], CCH [7] and SMBTR [8] came up.FEMA 55 [6] provides the total flood load on a vertical wall(height Z2.2H) of a coastal residential building to be about11 times the hydrostatic force with inundation depth (H).According to SMBTR [8], the force per unit length of the wallis taken as an equivalent hydrostatic load with three timesthe inundation depth (H), for a tsunami wave with no break-up. Total force is approximately 9 times as of static loading ifthe building height is more than 3 times inundation depth.The pressure diagram will be truncated if it is less than 3H.A significant amount of research work on wave impactloading is being carried out experimentally as well as usingphysical modeling.

Mizutani and Imamura [9] conducted hydraulic experi-ments to measure the wave force of tsunami acting on theprevention structures along the coast such as seawalls andbreakwaters. They used four types of wave pressures,namely dynamic, sustained, impact standing and over-flowing. They proposed the formulation to estimate eachtype of wave forces for design of coastal structure. Yeh [10]proposed rational methodologies to determine designtsunami loads on onshore structures with finite breadth.Though the analytical solution proposed can be a usefultool for analysis but still it requires some simplificationsand assumptions. Thusyanthan and Madabhushi [11] con-ducted model testing of new tsunami-resistant housedesign and a typical Sri Lankan coastal house in a wavetank. They observed the well performance of the newdesign under tsunami loading while the typical coastalhouse was destroyed. Fujima et al. [12] investigatedtsunami forces on rectangular structures using a 7 m wide,

S. Nayak et al. / International Journal of Disaster Risk Reduction 7 (2014) 28–3830

11 m long, and 1.5 m deep wave flume. They used themeasured total force to formulate tsunami force equation.However, they reported that hydrodynamic formula wassuccessful for the structures near and far from shoreline,whereas, the hydrostatic formula was only successful forthe structures near shoreline.

Rao et al. [13] analyzed a two storey shelter buildingcomprising of six different types of structural configura-tions and made a comparative study of the response of thestructure being subjected to both earthquake and tsunamiforces. Lukkunaprasit et al. [14] carried out research on thecalibration of tsunami loading on a damaged building insouthern Thailand due to the 2004 Indian ocean tsunami.They have taken the weather monitoring building of theMeteorological station at Takua Pa, Phang Nga as the casestudy. They found that the building experienced minorstructural damage to the columns and girders; however,most of the nonstructural members like masonry infillwere damaged. From the study, they recommended avelocity suitable for computation of tsunami load forsouthern Thailand. Pimanmas et al. [15] described thedesign approaches for tsunami shelters for large debrisimpact and small-to-medium-sized debris impact. Santoand Robertson [16] experimentally investigated the forceson structural vertical elements (columns and walls) causedby a tsunami bore. They reported that for the multiplecolumn effect, forces generally increase due to the block-age of flow by adjacent columns. They also reportedthough symmetric shielding generally decreases the forceon shielded column but it was assumed that tsunami borewas perfectly perpendicular to the building. It was alsofound that tsunami inundation models are generally moreaccurate at predicting the bore height than velocity.

Triatmadja and Nurhasanah [17] conducted studiesusing physical modeling to investigate the influence ofthe presence of openings and protection of buildings toreduce the effect of tsunami attack. They found that theforce due to tsunami on a building though depends on butis not linearly correlated to the openings. The outcome ofthe study resulted with a simple equation to calculate thetsunami loading on a building having openings. Asai et al.[18] surveyed those structures that were damaged duringthe 2011 Great East Japan Earthquake. They rigorouslystudied the relationship between their damage, strengthand inundation depth to examine the design tsunamiload. Chock et al. [19] discussed the methodology usedfor evaluation of tsunami loading provisions obtainedfrom full-scale tsunami damage field observations.Again the findings of the study were used to developimproved building codes for USA [5] regions with tsunamihazards.

From the above study of the literature it may beconcluded that researches on tsunami loading againstthe structures based on damage observations are yetinsufficient in comparison to the seismic loading. Similar-ity of the tsunami with its seismic or wind counterpart liesin the fact that all of them are of lateral nature and they actover a relatively smaller interval of time as compared tothe life period of a structure. As most of the design codeshave guidelines for seismic and wind loading, an equiva-lence can be developed between them and tsunami

loading. In this context, the present study is an attempttowards facilitating the assessment of tsunami resistanceof structures through establishing equivalency betweentsunami and seismic loading as the structures are gener-ally designed to have seismic resistance as per existingcodal guidelines. It is well known that base shear deter-mination is the most important objective for carrying outseismic analysis and to study tsunami effect upon astructure, it is required to calculate the hydrodynamicforce. In this context, base shear is calculated using IS1893 (Part 1) [3], Eurocode 8 [4] and ASCE 7-05 [5],whereas, the hydrodynamic forces are calculated usingFEMA 55 [6], CCH [7] and SMBTR [8] in the present studyfor some typical low rise buildings. Seismic force andtsunami force are plotted suitably for the sake of compar-ison to yield an idea about the extent of tsunami resistanceof buildings designed for seismic forces.

3. Method of analysis

3.1. Details of the structure studied

Three buildings with a variation of number of storeyshave been chosen for the study. The building is consideredto have 12�12 m2 size in plan with three equal bays 4 meach and is structured with sixteen columns. Width ofinterior columns is 500 mm and exterior column is450 mm. The height of 1st storey is 3.5 m and that of2nd and 3rd storey is 3.2 m. In most of the cases twostorey building is considered in comparing betweentsunami and seismic loading. However, a sample studyhas also been presented encompassing single storied,double storied and triple storied buildings to see whetherthe nature of major broad findings changes with storeyheights. Further, it is well known that the maximumtsunami height that strikes the structures normally donot exceed 10–12 m [20]. In case of higher storey buildingto be designed for seismic loading, the expected seismicforce will be more than that of a building having lessnumber of storeys, as the former has more mass. So, thelateral strength of the column of a higher storey buildingwill be more than that of a lower storey building. Further,the tsunami force depends upon the exposed area. Theexposed area in both of the above cases remains almostsame as maximum inundation depth of tsunami remainssame. So, the higher storey building with larger strength ofcolumns will relatively be safer as compared to a lowerstorey building.Thus, it is thought that consideration ofbuildings having upto three storey may provide an overallcritical scenario.

3.2. Earthquake forces

The present analysis is carried out taking into consid-eration the shelters that are situated in the seismic zone Vfirst and then for other zones. For various values ofresponse reduction factor ‘R’ seismic forces are calculatedaccording to Indian seismic code IS 1893 (Part 1) [3] first.The response reduction factor or force modification factor‘R’ reflects the extent of inelastic excursion a structure mayundergo during severe earthquake. Varying the value of

S. Nayak et al. / International Journal of Disaster Risk Reduction 7 (2014) 28–38 31

response modification factor, ‘R’ seismic forces are calcu-lated using ASCE 7-05 [5]. Then calculation of seismicforces according to Eurocode 8 [4] also had done byvarying the behavior factor, ‘q’ value. The behavior factor‘q’ is broadly same as ‘R’ in the broad sense being definedas the ratio of the seismic forces that the structure wouldexperience if it responds completely elastically with 5%viscous damping, to the lateral strength which is beingactually provided.

3.3. Tsunami forces

While modeling for tsunami loading the first importantinformation to be known is the size and distribution ofseafloor deformation following an earthquake and theamount of energy released. The different codal provisionsfollowed for the calculation of tsunami loading are CCH[7], FEMA 55 [6] and SMBTR [8], respectively. It is a pointworth mentioning that tsunami loading prescribed inFEMA P646 [21] is primarily for structures made forvertical evacuation from tsunamis, and is not consideredin the scope of the present study.

3.3.1. Loading combinations for calculating tsunami-inducedforce

Depending upon the location and type of structuralelements, appropriate combinations of tsunami-inducedforce components (hydrostatic, hydrodynamic, surge,buoyant and debris impact force) should be used incalculating the total tsunami force. The surge and dragforces take into account the effective area for load transferto the lateral force resisting system. In the limitation of thepresent paper, two situations are considered. This is due tothe fact that a certain element may not be subjected to allof these force components simultaneously. Loadingcombinations significantly influences the total tsunamiforce and the subsequent structural behavior and design.Tsunami-induced loads are different from flood-inducedloads. Therefore, load combinations based on floodsurges are not directly applicable to tsunamis. The varioustsunami-induced force components namely; hydrody-namic, surge, and debris impact force have beenexplained below.

3.3.1.1. Hydrodynamic (drag) forces. In the case of 100%breakaway walls, the columns are exposed to thehydraulic bore and the drag forces are calculated basedon a drag coefficient of 2.0 for square columns, asrecommended by CCH [7] and FEMA 55 [6]. The non-breakaway walls are assumed to remain intact and thehydraulic bore impacts the entire surface of the building.In this situation, the drag coefficients are taken as 1.5 and1.25 using CCH [7] and FEMA 55 [6], respectively. Theexpression for drag force is given below.

FD ¼ ðρ=2ÞCdAu2 ð1Þ

in which; u¼ CðghÞ0:5 ð2Þwhere, FD¼hydrodynamic force; ρ¼density of water;Cd¼drag coefficient; A¼exposed area; u¼design velocity;C¼a constant whose value is suggested to be 2 to obtain

drag force in extreme tsunami event; h¼ inundation depth(height of tsunami from ground level).

3.3.1.2. Debris impact forces. A high-speed tsunami boretraveling inland carries huge amount of debris such asfloating automobiles, wood, boats, and ships. The impactof floating debris can induce significant forces on abuilding, leading to damage and collapse of thestructures. Both FEMA 55 [6] and CCH [7] codes accountconsistently for debris impact forces, using the sameapproach and recommend using the following equationsfor the estimation of debris impact force.

Fi ¼mbðdub=dtÞ ¼mbðub=ΔtÞ ð3Þwhere, Fi¼debris impact force; mb¼mass of the bodyimpacting the structure; ub¼velocity of impacting body;and dt ¼Δt¼ impact duration.

The recommended method for calculating normalimpact loads has been modified beginning with ASCE7-02 [22]. Previous editions of ASCE 7-98 [23] used theprocedure, which had been unchanged since at least 1972,relied on an impulse-momentum approach. This consid-ered a 1000 lb (4.5 kN) object striking the structure at avelocity of the flood water. A 1000 lb (4.5 kN) object can beconsidered a reasonable average for flood-borne debris.This represents a reasonable weight for trees, logs andother large woody debris that is the most common form ofdamaging debris nationwide. This weight corresponds to alog approximately 30 ft (9.1 m) long and just less than 1 ft(0.3 m) in diameter. The 1000 lb (4.5 kN) object alsorepresents a reasonable weight for other types of debrisranging from small ice floes, boulders, to man-made objects.

The mass of 1000 lb (4.5 kN) used is consistent with therecommendations of CCH [7] and FEMA 55 [6]. The onlydifference between CCH [7] and FEMA 55 [6] lies in therecommended values for the impact duration whichhas a noticeable effect on the magnitude of the force.For example, CCH [7] recommends the use of impactduration of 0.1 s for concrete structures, while FEMA 55[6] provides different values for walls and piles for variousconstruction types.

3.3.1.3. Surge force. Surge force is generated by theimpingement of the advancing water front of a tsunamibore on the structure. The surge force is applied over thefull length of the building in the direction of the tsunamifor non-breakaway walls. However, there is no formulationin FEMA 55 [6] for calculation of surge force. SMBTR [8]recommends the following equation for calculating thetsunami wave pressure for walls of height equal to orgreater than three times the inundation depth. If the wallheight is less than three times the inundation depth thenthe forces are truncated accordingly.

Qx ¼ ρg ð3h�zÞ ð4Þwhere, Qx¼tsunami wave pressure; ρ¼density of water;g¼acceleration due to gravity; h¼design inundationdepth; and z¼height of relevant portion from groundlevel where tsunami wave pressure is being calculatedsuch that, 0rzr3h.

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'R' = Response modification factor for seismic loading

Provisons for calculating tsunami loading

Fig. 3. Graphical representation of tsunami force vs. inundation depth

S. Nayak et al. / International Journal of Disaster Risk Reduction 7 (2014) 28–3832

Integration of the wave pressure formula for walls withheights equal to or greater than 3h results in the sameequation as the surge force formula recommended by CCH[7]. However, for walls of height less than 3h, CCH [7]recommends that the surge force has to be calculatedusing appropriate combination of hydrostatic and dragforce for a given situation. Interestingly, no clear guidelinesor formulation for such calculation has been provided. Inthis context, the present study restricts the use of thisformula up to a height of 3h.

The above mentioned tsunami force components donot act simultaneously on the structures. So, two separatecases of initial and post impact [24] are considered forcalculating the total tsunami force acting on the structureas shown in Fig. 2. The initial impact is the combination ofsurge and debris impact forces, further the hydrodynamic(drag) and hydrostatic force simultaneously with debrisimpact force are considered in post impact condition. Themaximum of initial and post impact force is considered asthe design tsunami force. Such a combination is madefrom the following considerations. When the tsunamiwave first hits the structure then it exerts the surge onthe structure. If such a wave carries any debris then alongwith surge force the extra force due to debris impact mayalso be exerted. However, after this moment the waterkeeps on flowing along the two sides of the structure forsome time and such a flow exerts hydrodynamic (drag)force. But during this period the flow may carry someboulder and thus, boulder impact may also take place. So,there may be criticality either in initial impact conditionwith surge and debris impact force acting simultaneouslyor in the post impact condition, hydrodynamic/drag andhydrostatic force acting simultaneously with debris impactforce. Thus, these two critical conditions are considered toevaluate maximum tsunami force which may be exertedon the structure.

Fig. 2. Proposed loading conditions; (a) initial impact and (b) post impact(Nouri et al. [24]), where, Fi, Fs, Fd and FHS are the debris impact, surge,drag, hydrostatic components respectively.

4. Result and discussions

The earthquake force mainly depends upon the char-acteristics of ground motion, stiffness, seismic weight andconfiguration of the structure; while in case of tsunami,the imparted forces on the structure depend upon thevelocity of wave, inundation depth and the area of thestructure exposed to tsunami wave. There are four seismiczones in India, starting from zone II to zone V in order ofseverity of expected ground motion. They are called as lowdamage risk zone, moderate damage risk zone, highdamage risk zone and very high damage risk zone in theorder of intensity of seismic shaking. Similarly, there arethree seismic zones ranging from zone 1 to zone 3 in

using CCH [7], FEMA 55 [6] and SMBTR [8] and seismic base shear forvarious ‘R’ values using ASCE 7-05 [5].

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q=1.5q=2q=3q=3.3q=3.6q=3.9q=4.4q=4.8q=4.95q=5.2CCHSMBTRFEMA

'q' = Behavior factor

Provisions for calculating tsunami loading

Fig. 4. Graphical representation of tsunami force vs. inundation depthusing CCH [7], FEMA 55 [6] and SMBTR [8] and seismic base shear forvarious ‘q’ values using Eurocode 8 [4].

S. Nayak et al. / International Journal of Disaster Risk Reduction 7 (2014) 28–38 33

Cyprus, Europe, as considered in Eurocode 8 [4]. However,such kinds of division of zones are not available for UnitedStates. The relevant code ASCE 7-05 [5] has provided

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'R' = Response reduction factor for seismic loading

Provisions for calculating tsunami loading

Fig. 5. Graphical representation of tsunami force vs. inundation depthusing CCH [7], FEMA 55 [6] and SMBTR [8] and seismic base shear forvarious ‘R’ values using IS 1893 (Part 1) [3].

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Fig. 6. Graphical representation of tsunami force vs. inundation depth using CCvalues using ASCE 7-05 [5].

contour lines for peak ground acceleration instead ofzoning. The detailed comparison of seismic base shearand tsunami force is presented in the form of graphs fordifferent seismic zones. For instance, for IS 1893 (Part 1)[3] zone V is considered, for Eurocode 8 [4] zone 3 isconsidered and for ASCE 7-05 [5], the region with thehighest value of Ss and S1 which are indicative of spectralacceleration of a typical short period system of 0.2 s and ofa typical medium period system of 1 s, respectively, areconsidered. The comparison of seismic base shear andtsunami induced lateral force is presented in Figs. 3–5 fortwo storey building. Further, Fig. 6 compares the seismicbase shear calculated using ASCE 7-05 [5] for site class Dwith different value of Ss and S1 and tsunami forcecalculated from above mentioned codal provisions fortwo storey building. Comparison of seismic base shearcalculated for different zones for a particular storey withtsunami force is presented in Fig. 7. Whereas, Fig. 8compares seismic base shear calculated for different stor-eys for a particular zone with tsunami force. On the otherhand, a summary of the observations on comparison ofseismic base shear and tsunami force for all zones in Indiaare presented in a tabular form in Table 1.

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Fig. 7. Graphical representation of tsunami force vs. inundation depth using CCH [7], FEMA 55 [6] and SMBTR [8] and seismic base shear for various ‘R’values using IS 1893 (Part 1) [3] for two storey.

S. Nayak et al. / International Journal of Disaster Risk Reduction 7 (2014) 28–3834

Further, Figs. 3–5 present seismic base shear calculateddue to different response modification factors as per ASCE7-05 [5], behavior factor as per Eurocode 8 [4] andresponse reduction factor as per IS 1893 (Part 1) [3],respectively. In each figure, tsunami load is also plottedas a function of inundation depth as per available guide-lines. It is noticed that at some inundation depth, termedas critical height hc in the present study, the tsunami forceand seismic base shear are equal in magnitude. From thegraphs in Figs. 3–5, this critical height hc can be deter-mined at the point of intersection of the tsunami forcecurve and the seismic base shear curve. Seismic base shearforce is greater than the tsunami force up to critical heightand less than the tsunami force after critical height. Astructure which is designed according to a seismic codecan, therefore, resist tsunami load generated by a tsunamiof height less than or equal to critical height. Otherwise,the structure has to be designed taking tsunami load intoconsideration. So, in a region if the required data for thepossible maximum inundation depth are available frompast records, then it may be concluded whether a structurein that region has to be designed according to seismic codeor the guidelines which take tsunami force into considera-tion. Further, such curves may also help to understand

whether already built up structures in coastal region willsurvive or not if a possible inundation depth of tsunami isannounced through a tsunami warning in case of such anevent. For example, let us consider that a masonry struc-ture is to be constructed in zone V of India. Then, seismicbase shear force is needed to be calculated using IS 1893(Part 1) [3] considering response reduction factor R equalto 1.5. On other hand, tsunami force is needed to becalculated according all three guidelines as India doesnot have any existing guidelines for calculating tsunamiloading. The graph presented in Fig. 5 exhibits a criticalheight hc of 1.86 m for R¼1.5, if tsunami loading iscalculated as per FEMA 55 [6] guidelines, while the samecritical heights come to be about 1.41 m if CCH [7] andSMBTR [8] are considered for tsunami loading. This clearlyindicates that the structure may survive a tsunami inun-dation depth of 1.5 m. Further, from Fig. 6 it is observedthat with decrease of Ss value, seismic base sheardecreases as well as the critical height for tsunami loading.It is also noticed from Fig. 7 that, with the decrease ofseverity of expected ground motion seismic base shear aswell as critical height for tsunami loading decrease. Fig. 8shows that critical height decreases with number ofstoreys as the design seismic force decreases with the

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Fig. 8. Graphical representation of tsunami force vs. inundation depth using CCH [7], FEMA 55 [6] and SMBTR [8] and seismic base shear for various ‘R’values using IS 1893 (Part 1) [3] for zone IV.

Table 1Variation of range of critical height with seismic zone, number of storey and response reduction factor R as specified in IS 1893 (Part 1) [3].

Critical height (hc)

Zone II (m) Zone III (m) Zone IV (m) Zone V (m)

R¼1.51 Storey 0.3–0.45 0.48–0.75 0.65–1.1 1–1.252 Storey 0.5–0.9 0.8–1.2 1.1–1.48 1.4–1.83 Storey 0.5–1 1.1–1.45 1.25–1.8 1.6–2.25

R¼31 Storey 0.25–0.35 0.25–0.48 0.4–0.6 0.5–0.82 Storey 0.25–0.45 0.5–0.75 0.6–1.1 1–1.253 Storey 0.25–0.5 0.7–1.1 1–1.25 1.2–1.6

R¼51 Storey 0.1–0.23 0.2–0.3 0.25–0.4 0.3–0.62 Storey 0.2–0.3 0.3–0.5 0.4–0.75 0.5–13 Storey 0.2–0.4 0.4–0.7 0.6–1 0.8–1.2

� Response reduction factor R¼1.5 is applicable for unreinforced load bearing masonry wall structures.� Response reduction factor R¼3 is applicable for ordinary RC moment resisting frame (OMRF), load bearing masonry wall structures reinforced with

horizontal RC bands, ordinary reinforced concrete shear wall and ordinary shear wall with OMRF.� Response reduction factor R¼5 is applicable for special RC moment resisting frame (SMRF), steel frame with eccentric braces, steel moment resisting

frame and ductile shear wall with SMRF.

S. Nayak et al. / International Journal of Disaster Risk Reduction 7 (2014) 28–38 35

Table 2Variation of critical height with seismic base shear for masonry structures.

Code combination Seismic base shear force (kN) indicatingthe limit of tsunami force to be resistedby traditional seismic design

Critical height (m)

Seismic code Tsunami guidelines

IS 1893 (Part 1) [3] FEMA 55 [6] 1127.43 1.86ASCE 7-05 [5] FEMA 55 [6] 2087.43 2.63Eurocode 8 [4] FEMA 55 [6] 1043.917 1.64IS 1893 (Part 1) [3] SMBTR [8] 1127.43 1.41ASCE 7-05 [5] SMBTR [8] 2087.33 1.868Eurocode 8 [4] SMBTR [8] 1043.917 1.2IS 1893 (Part 1) [3] CCH [7] 1127.43 1.41ASCE 7-05 [5] CCH [7] 2087.43 1.8Eurocode 8 [4] CCH [7] 1043.917 1.28

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number of storeys because of decrease in total seismicweight.

Further, to have an idea about how much differencemay be made in the observations and interpretations dueto use of codes and guidelines of different country, asummary of observations are presented for few bench-mark cases in Table 2. For instance, considering R¼1.5,from plots in Fig. 3, in which seismic base shear iscalculated according to ASCE 7-05 [5], critical heights fortsunami forces calculated according to FEMA 55 [6], CCH[7] and SMBTR [8] are presented in Table 2. Similarly,Fig. 4, in which seismic base shear is calculated accordingto Eurocode 8 [4], considering plot corresponding to q¼2also yields critical heights for tsunami forces calculatedaccording to FEMA 55 [6], CCH [7] and SMBTR [8]. Theseare included in Table 2, too.

From Table 2, it is found that critical heights obtainedby considering seismic base shear calculated according IS1893 (Part 1) [3], ASCE 7-05 [5] and Eurocode 8 [4] andtsunami load obtained by FEMA 55 [6], CCH [7] and SMBTR[8] lies in the range of 1.2 m–2.63 m for a masonrystructures. So, as an exemplary case, if the region in whichthe masonry structure needs to be constructed recordsmaximum inundation depth of 3 m which is greater thancritical height, then the designing of a structure in theabove mentioned region according to any seismic codewon0t be sufficient enough to prevent the failure ofstructure under tsunami loading. However, it may benoted that use of different codes and guidelines makedifference at least to some extent. Fig. 5 similarly yields acritical height of 1.86 m for seismic base shear to becalculated considering zone V of IS 1893 (Part 1) [3].

Similar calculations were carried out for all the seismiczones for one, two and three storied structures using IS1893 (Part 1) [3] to provide a wider picture about the issue.To have an insight on the effect of response reductionfactor R, three typical R have been chosen. They arenamely; R¼1.5, R¼3 and R¼5. Response reduction factorR¼1.5 is generally considered for unreinforced load bear-ing masonry wall structures, while response reductionfactor R¼3 is applicable for ordinary RC moment resistingframe (OMRF), load bearing masonry wall structuresreinforced with horizontal RC bands, structures withordinary reinforced concrete shear wall and those withordinary shear wall with OMRF. Further, a relatively higher

response reduction factor R¼5 may be considered forspecial RC moment resisting frame (SMRF), steel framewith eccentric braces, steel moment resisting frame andductile shear wall with SMRF. For each of these three casesthe variation of critical height for zones II, III, IV and V havebeen presented in Table 1. It is observed that as theseverity of the seismic zone increases, seismic base shearincreases resulting in increase in critical height of tsunamiwhich may be resisted, for a particular structure.

Further, critical height for a particular zone decreaseswith increase of response reduction factor R. Seismic baseshear for a particular structure at a particular location is afunction of response reduction factor R. It decreases withthe increase of R, as increased R indicates larger ductilitycapacity available for allowing more inelastic excursion ofthe structural elements. Thus, larger R implying lesserdesign base shear at elastic condition indicates a structurewill withstand tsunami wave of lesser height or inunda-tion depth. In fact, while resisting tsunami, it still notestablished whether inelastic excursion can be relied uponor not, because of lack of enough acceptable informationregarding exact nature of time variation of tsunami forcesavailable.

5. Summary and conclusions

The present study aims at assessing the tsunami vulner-ability of low rise structures by comparing tsunami forcewith seismic base shear for which normally they aredesigned for. For calculating seismic base shear variousseismic codes, such as, ASCE 7-05 [5], Eurocode 8 [4] andIS 1893 (Part 1) [3] for different response reduction factorsare computed and plotted. Similarly, the tsunami load actingon the same structures are obtained by using differentacceptable design provisions available for calculation oftsunami loading such as FEMA 55 [6], CCH [7] and SMBTR[8]. Such plots are superposed on plots of seismic base shearobtained. Tsunami load is plotted as a function of inundationdepth. The point of intersection of seismic base shear andtsunami load curves represent a particular inundation depthat which seismic base shear and tsunami load are equal. Thisparticular inundation depth is termed as the critical height oftsunami wave. If the the inundation depth of tsunami waveis lesser than this height, then seismic base shear will governthe design as this will be more than the lateral force exerted

S. Nayak et al. / International Journal of Disaster Risk Reduction 7 (2014) 28–38 37

by tsunami. On the other hand, once tsunami wave crossesthis critical height, the governing lateral force should beobtained due to the tsunami as the seismic base shear will belesser. The following broad conclusions can be obtained fromthe study.

(i)

If seismic base shear is calculated using ASCE 7-05 [5], itis observed that for response reduction factor rangingfrom 1.5 to 6, the critical height reduces from 2.5 m to0.8 m. On the other hand, when seismic base shear iscalculated using Eurocode 8 [4], it is observed that forbehavior factor (which is equivalent to response reduc-tion factor in most of the other codes, e.g., IS 1893 (Part1) [3]) varying from 1.5 to 5.2; the critical height reducesfrom 1.7 m to 1.2 m. Lastly, if seismic base shear iscalculated using IS 1893 Part 1 [3], it is observed that forresponse reduction factor ranging from 1.5 to 5, thecritical height reduces from 1.8 m to 0.7 m.

(ii)

So, it may be broadly concluded that typically for aresponse reduction factor of about 1.5 the criticalheight may be in range of 1.7–2.5 m, while for aresponse reduction factor of about 5, such range willbe about 0.7–1.2 m. In fact, response reduction factorof about 1.5 indicates masonry structure while thatabout 5 is applicable for either steel-framed structuresor reinforced concrete structures with adequate duc-tile reinforcement detailing. So, the study gives atleast a broad idea about the maximum height oftsunami wave which can be resisted by these twoextreme categories of structure if they are designed asper seismic codes.

(iii)

Similar calculations were carried out for other seismiczones of India, and for one, two and three storiedstructures using IS 1893 (Part 1) [3]. The critical heightsdetermined from graphs are tabulated in Table 1 forvarious typical response reduction factors. Table 1indicates that designed seismic lateral strengthincreases with severity of seismic zone as consideredpeak ground acceleration increases. Similarly, suchstrength increases with number of storeys, too, as withthe increase of number of storey, the seismic weight ofthe structure increases resulting in increased seismicbase shear. Hence, such structures will be able towithstand tsunami wave of greater height as reflectedfrom increase in critical height, hc, of tsunami wave,with number of storeys and severity seismic zones asobserved in Table 1.

(iv)

The critical height determined in this study plays a vitalrole in designing of coastal structures. For a particularstructure in a particular region, where data of maximuminundation depth is available from past records ornumerical analysis/simulations, the result presented inthis study may help to determine whether the structurerequires special design for tsunami loading over andabove the design made for adequate combination.

The structure which is designed according to seismiccode is safe under tsunami loading if the height of tsunamiwave does not exceed critical height of tsunami wave.If the data of maximum inundation depth of a particular

region indicate that inundation depth is greater than criticalheight, then structures need to be designed according to thecodal guidelines which take tsunami load into account. Thus,the present paper is a limited attempt to show how to assessthe tsunami vulnerability of structures which are designedfollowing seismic code. A number of graphs and tablespresented in the study may be of help to assess suchvulnerability at least in a broad sense pending a furtherdetailed study in this direction. The effect of buoyancy forcein increasing the tendency of overturning by momentarilyreducing the weight of the structure (as observed in 2011tsunami in Japan) is an issue deserving further research formore accurately estimating the effect of tsunami. The criticalheight calculated from the present study may further bevalidated through fragility curve developed after two greatevents of the 2004 Indian Ocean tsunami and the 2011 GreatEast Japan tsunami and also other small events such as in2009 Samoa and 2010 Chile.

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