Effect of Tsunami on Building

7/31/2019 Effect of Tsunami on Building

1/33

EFFECT OF TSUNAMI ON BUILDINGS

A Seminar Report

Submitted in partial fulfillment of the

requirements for the award of the degree

of

MASTER OF TECHNOLOGY

in

CIVIL ENGINEERING

(With specialization in Structural Engineering)

By

Abhinav Gupta

(Enrollment No. :-10523001)

DEPARTMENT OF CIVIL ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY ROORKEE

ROORKEE 247667, INDIA


2/33

CONTENTS

Page No.

CANDIADATES DECLARATION

ACKNOWLEGEMENT

CONTENTS

ABSTRACT

Chapter: 1 Introduction 2-5

1.1 General 2

1.2 Generation mechanisms 2

1.3 Characteristics 3

1.4 Effect on buildings 5

Chapter: 2 Effect of Tsunami on Buildings 6-14

2.1 Tsunami-induced hydraulic bores 6

2.2. Tsunami-induced forces on structures 7

2.3. Debris impact force 11

2.4. Existing Design Codes 14

Chapter: 3 Tsunami-induced forces 15-23

3.1. Hydrostatic force 17

3.2. Buoyant force 17

3.3. Hydrodynamic (drag) force 18

3.4. Surge force 19

3.5. Debris impact force 20

3.6 Uplift Forces on Elevated Floors 22

3.7 Additional Gravity Loads on Elevated Floors 23

3.8 Wave breaking force 24

Chapter: 4 Structural Analysis 25-26

Conclusion 27

References 28


3/33

CANDIDATES DECLARATION

I hereby certify that the work presented in this seminar report entitled Effect of

Tsunami on Buildings, in partial fulfilment of the requirements for the award of

the Degree of Master of Technology in Civil Engineering with specialization in

Structural Engineering, submitted to the Department of Civil Engineering,

Indian Institute of Technology Roorkee, is an authentic record of my own work

carried out under the supervision of Dr. A.K. Jain, Structural Engineering

Group, Department of Civil Engineering, Indian Institute of Technology

Roorkee, India.

The matter embodied in this project report has not been submitted by me for the

award of other degree or diploma.

Date:

Place: Roorkee (Abhinav Gupta)


4/33

ACKNOWLEDGEMENT

I wish to affirm my earnest acknowledgement and indebtedness to my

supervisor Dr. A.K. Jain, H.O.D., Department of Civil Engineering, for his

intuitive and meticulous guidance and perpetual inspiration in completion of this

seminar work. I want to express my profound gratitude for his co-operation in

scrupulously scrutinizing the manuscript and his valuable suggestions

throughout the work.

Date:

Place: Roorkee (Abhinav Gupta)


5/33

1

ABSTRACT

Tsunami waves represent extreme, often catastrophic events, which significantly and

adversely impact coastal areas. In spite of the lower frequency of occurrence comparing to

storms and storm-induced surges, tsunami-induced coastal flooding often leads to massive

casualties and tremendous economic losses and pose a significant threat to coastal and near

coastal structures. While extensive research has been conducted on the impact of

hydrodynamic forces on classical coastal protection works (breakwaters, seawalls, reefs,

etc.), there is limited research on their impact on structures such as buildings and bridges

located inland. This report deals with the present study in the field of tsunami induced

loading on coastal and near coastal structures and the loadings considered by different

codes to help design structures to withstand forces generated by tsunami-induced hydraulic

bores, including debris impact.


6/33

1. INTRODUCTION

1.1 General

A tsunami is the Japanese fo

waves caused by the displace

Earthquakes, volcanic eruptio

underwater nuclear devices)

meteorite ocean impacts or si

water all have the potential to

1.2 Generation mechani

The principal generation me

substantial volume of water

usually attributed to earthqu

rarely by meteorites and nucl

gravity. Tides do not play any

Fig 1.1

1.2.1 Tsunami generate

Tsunami can be generated wh

overlying water. Tectonic

2

r 'harbour-wave' and it means a big wave

ent of a large volume of a body of water, us

ns and other underwater explosions (includi

, landslides glacier calving and other m

milar impact events, and other disturbances

generate a tsunami.

sms

chanism (or cause) of a tsunami is the di

or perturbation of the sea. This displace

kes, landslides, volcanic eruptions, glacier

ar tests. The waves formed in this way are t

part in the generation of tsunamis.

Tsunami Generation Mechanism.

by seismicity

en the sea floor abruptly deforms and vertic

arthquakes are a particular kind of eart

r a series of big

ually an ocean

g detonations of

ass movements,

above or below

splacement of a

ent of water is

calving or more

hen sustained by

lly displaces the

quake that are


7/33

3

associated with the Earth's crustal deformation; when these earthquakes occur beneath the

sea, the water above the deformed area is displaced from its equilibrium position. More

specifically, a tsunami can be generated when thrust faults associated with convergent or

destructive plate boundaries move abruptly, resulting in water displacement, owing to the

vertical component of movement involved.

Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength

(often hundreds of kilometres long, whereas normal ocean waves have a wavelength of

only 30 or 40 metres),which is why they generally pass unnoticed at sea, forming only a

slight swell usually about 300 millimetres (12 in) above the normal sea surface. They grow

in height when they reach shallower water, in a wave shoaling process described below. A

tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.

1.2.2 Tsunami generated by landslides

In the 1950s, it was discovered that larger tsunamis than had previously been believed

possible could be caused by giant landslides. These phenomena rapidly displace large

water volumes, as energy from falling debris or expansion transfers to the water at a rate

faster than the water can absorb.

1.3 Characteristics

Tsunamis cause damage by two mechanisms:

The smashing force of a wall of water travelling at high speed, and The destructive power of a large volume of water draining off the land and

carrying all with it.

While everyday wind waves have a wavelength (from crest to crest) of about 100 metres

(330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a

wavelength of about 200 kilometres . Such a wave travels at well over 800 kilometres per

hour, but owing to the enormous wavelength the wave oscillation at any given point takes

20 or 30 minutes to complete a cycle and has an amplitude of only about 1 metre .This

makes tsunamis difficult to detect over deep water. Ships rarely notice their passage.


8/33

4

Fig. 1.2 Characteristic of Tsunami Waves

As the tsunami approaches the coast and the waters become shallow, wave shoaling

compresses the wave and its velocity slows below 80 kilometres per hour. Its wavelength

diminishes to less than 20 kilometres, its amplitude grows enormously. Since the wave still

has the same very long period, the tsunami may take minutes to reach full height. Except

for the very largest tsunamis, the approaching wave does not break, but rather appears like

a fast-moving tidal bore. Open bays and coastlines adjacent to very deep water may shape

the tsunami further into a step-like wave with a steep-breaking front.

When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level

is termed run up. Run up is measured in metres above a reference sea level. A large

tsunami may feature multiple waves arriving over a period of hours, with significant time

between the wave crests. The first wave to reach the shore may not have the highest run

up.

Drawback

The first part of a tsunami to reach land is a troughcalled a drawbackrather than a

wave crest. The water along the shoreline recedes dramatically, exposing normally

submerged areas.

A drawback occurs because the water propagates outwards with the trough of the wave at

its front. Drawback begins before the wave arrives at an interval equal to half of the wave's

period. Drawback can exceed hundreds of metres.


9/33

5

1.4 Effect on buildings

Tsunamis are rare events, high-impact natural disasters.

One of the important elements that need significant improvement is the estimation offorces generated by tsunami-induced bores, as well as water-borne debris.

Currently, there are no clearly established procedures to address the aforementioned forces.

Moreover, significant disagreement on existing empirical formulae fostered new research

interest in an effort to properly address the inclusion of both tsunami-induced forces and

the impact of debris into design codes.

(a) (b)

(c) (d)

Fig.1.3. Tsunami damage in Thailand and Indonesia (December 2004 Indian Ocean

Tsunami): (a) severe structural damage, Khao Lak, Thailand; (b) column failure of

a reinforced concrete frame, Phuket, Thailand; (c) column failure due to debris impact,

Banda Aceh, Indonesia; (d) punching failure of infill walls, Banda Aceh, Indonesia.


10/33

2. Effect of Tsunami on

This chapter deals with th

infrastructure located in the v

conducted on the impact of

(breakwaters, seawalls, reef

such as buildings and bridge

art knowledge with respect

including debris impact.

2.1 Tsunami-induced hy

As tsunami waves advance t

height increases while celer

shoreline, inundating low-lyi

hand, tsunami inundation can

the case of no breaking tsuna

(a)

(c)

Fig 2.1 Tsunami wave in Kh

(a) water recedes; (b) wavesthe shoreline; (d) tsunami wa

6

Buildings

estimation of tsunami-induced hydrodyn

icinity of the shoreline. While extensive re

hydrodynamic forces on classical coastal

, etc.), there is limited research on their im

s located inland. This chapter summarizes

to forces generated by tsunami-induced

draulic bores

oward the shoreline and water depth decre

ity decreases. Tsunami waves may break

g coastal areas in the form of a hydraulic b

also occur as a gradual rise and recession of

i waves.

(b)

(d)

ao Lak, Thailand (December 2004 Indian Oc

approach the shoreline; (c) tsunami wavesves inundate the shoreline.

amic forces on

search has been

rotection works

act on structures

the state-of the-

hydraulic bores,

ases, their wave

offshore or at

re. On the other

the sea level for

ean Tsunami )

break close to


11/33

7

The width of the continental shelf, the initial tsunami wave shape, the beach slope, and the

tsunami wave length are the parameters which govern the breaking pattern of tsunami

waves. Tsunami waves have a larger horizontal length scale compared to the vertical.

Consequently, implementing shallow water wave theory (i.e., depth-integrated equations of

momentum and mass conservation with the assumption of a hydrostatic pressure field)

seems to be a reasonable method for representing tsunami wave propagation.

2.2. Tsunami-induced forces on structures

Broken waves induced larger hydrodynamic horizontal forces on a test pile compared to

waves breaking at the pile location. As previously mentioned, broken tsunami waves

inundate shoreline in the form of a hydraulic bore, which is a fast moving body of water

with an abrupt front. However, mechanisms of impingement of broken tsunami waves on

structures located inland are not yet well understood.

Comprehensive experimental investigation of the interaction of bores and dry-bed

surges with a vertical wall was performed by Raichlen and Ramsden (1990). In these

experiments, three flow conditions were analyzed:

(1) Turbulent bores (initial still water downstream of the gate);

(2) dry-bed surges (no initial water depth downstream of the gate); and

(3) Solitary waves.

Forces and overturning moments due to bores and dry-bed surges were recorded and

calculated, respectively. The results of Ramsdens (1990) studies are not applicable to the

estimation of impulsive forces. It was observed that the pressure distribution during

impact is essentially non-hydrostatic. The experiment also demonstrated that the transition

from undular to turbulent bores led to a discontinuous increase in water-surface slope,

followed by an increase in measured run-up, pressure head, and exerted forces and

moments.

It was shown that recorded forces gradually increased to an approximately constant value

for both the case of a surge and a bore. No impulsive (shock) force exceeding the

hydrodynamic force was observed. However, an initial impulsive pressure equal to

three times the pressure head, corresponding to the measured run-up, was recorded.


12/33

8

Ramsden (1990) further derived empirical formulae for the maximum force and

moment exerted on a vertical wall due to the bore impact [Eqs. (2.1) and (2.2)].

where F is the force on the wall; Fl is the force on the wall due to a run-up equal to twice

the wave height, assuming hydrostatic pressure; H is the wave height at the wall; h is still

water depth; M is the moment on the wall; and Ml is the moment corresponding to Fl .

Fig.2.2 Measured and non-dimensional force history for a square column

(2.1)

2.2

Cr=

.5

2

= 1.325 + .347

+ 1

58.5

+ 1

7160

= 1.923 + .454

+

18.21

+ 1808


13/33

9

Arnason (2005) measured forces exerted on rectangular, rhomboidal, and circular

structures due to a hydraulic bore on a dry bed. It was observed that the surge force

overshot the hydrodynamic force in the case of a square column for small bore heights

(Fig.2.2). However, no overshoot was recorded for the case of circular and rhomboidal

columns.

Nouri(2007) Conducted experiments with the objective of estimating bore-induced forces

on free-standing structural components. The effect of other parameters such as upstream

obstacles, flow constrictions, and debris impact was also investigated. The structural

components were subjected to a dam-break flow generated by impoundment depths (h0) of

0.5, 0.75, and 0.85 m.

Fig.2.3 Time history of exerted forces on a circular structure.

Figure 2.3 provides the base shears for impoundment depths of 0.5 m, 0.75 m, 0.85 m, and

1.0 m. The first abrupt rise in force is caused by the initial impact (surge force) of the

hydraulic bore on the structure. With increasing upstream water depth, the surge force

increases. This increase is partly due to the larger impoundment depth and the increase in

bore front slope with increasing impoundment depth. Immediately following the initial

impact, there is a drop in the base shear. For the 0.75 m. 0.85 m, and 1.0 m impoundment

depths, the reduction in force ranges between 55% and 60% of the initial impacting force.

For the 0.50 m impoundment depth, the drop in the base shear force is approximately 30%

of the initial magnitude. This drop is followed by a gradual increase caused by the run-up

of the hydraulic bore. In all cases, the run-up force was equal to or greater than the initial

impacting load. The run-up is followed by a semi-steady state of flow characterizing the

drag force. Excluding the 1.0 m impoundment depth, the drag force represented the largest

force component in the loading history. Figure 2.4 shows the individual force components

for a 1.0 m impoundment depth, along with the corresponding bore height.


14/33

10

Figure 2.4 Time-History of Force Components on Circular Section

Figure 2.5 (a) provides the pressure-time history for the circular section along the height

while Figure 2.5 (b) provides the pressure distribution corresponding to the individual

force components.

Figure 2.5 Pressures: a) Time-History along Height of Column; b) Distribution

Corresponding to Forces.

Pressures are shown at 50 mm, 100 mm, 200 mm, and 250 mm from the base of the

circular section. At the instant of the initial impact of the hydraulic bore, the pressure

distribution is approximately triangular, as indicated by the surge force component at 12.4

s. The pressure distributions become increasingly constant at the point of the run-up and

drag force components, shown at 14.4 s and 16 s, respectively. Variations in the velocity

along the height of the bore are partly responsible for variations in pressure for the drag

force component.


15/33

11

2.3. Debris impact force

Matsutomi (1999) performed small and full-scale experiments on impact forces generated

by driftwood on rigid structures. Dam-break waves generated in a small flume carried

pieces of lumber to the point of impact on a downstream wall. Also, full-scale experiments

in which wooden logs impacted a frame were conducted in open air, and impact forces

were measured. An empirical formula for estimating the impact force, F, was derived

using regression analysis of collected data [Eq. (2.3)].

(2.3)

Fig.2.6 Impact forces of wood logs for bores and surges.

where w is the specific weight of wood, D is the diameter of the log, L is the length of the

log, CM is a coefficient which depends on the flow passing around the receiving wall

( 1.7 for bore or surge, and 1.9 for steady flow), u is the velocity of the log at impact, and

if is the yield stress of the log. Figure 2.6 shows the design chart based on Eq. (2.3).

= 1.6

.

.


16/33

12

Currently, three basic models are proposed for estimating the forces due to the impact of

debris on structures, which are used by a few design codes. In these models, the impact

force is calculated based on the mass and velocity of debris, while ignoring the mass and

rigidity of the structure. However, other than the mass and velocity of debris, each model

needs an additional parameter. The three models and their corresponding additional

parameters are

Contact stiffness stiffness between debris and structure,

Impulsemomentum stopping time of debris after impact and time history of impact,

Work energy distance travelled from where initial contact occurs, to where debris

stops.

Haehnel (2002) used a single-degree-of-freedom model with effective collision stiffness as

an additional parameter. They reviewed the current models discussed above and

demonstrated that none of the additional parameters are independent. Hence, the three

models are equivalent, provided that additional parameters are appropriately selected.

Further, small and large-scale experiments were performed in order to develop the single-

degree-of-freedom model. Small-scale tests were performed in a flume where wooden logs

were released into the flow and impacted a load frame located further downstream.

Fig.2.7 Effect of impact orientation on force.

Large-scale experiments were performed in a large basin where water was stationary and

logs were placed on a movable carriage. The effect of parameters such as added mass of


17/33

13

the water and the eccentricity and obliqueness of the collision were also considered. It was

found that the maximum impact force, Fi, max, can be calculated using Eq. (2.4).

(2.4)

Where u is the impact velocity of the log, is the constant effective stiffness between the

log and the structure, and m1 is the mass of the log. Based on experiments, Haehnel13

found

the value of= 2.4 MN/m to be the representative for the upper envelope of the collected

data.

As per the experiments done by the University of Ottawa in cooperation with the Canadian

Hydraulics Centre in Ottawa, Canada to simulate debris impact loading, a wooden log, 445

mm long and with a 90 mm x 90 mm cross-section, was used. The debris caused a

significant increase in the base shear recorded by the dynamometer mounted at the base of

the circular section. A spike is evident a short time after the initial impact of the hydraulic

bore. For the 1.0 m impoundment depth, an increase in the base shear force of 695 N

occurred over a rise time of 0.0075 s, whereas the base shear force increased by 430 N

over a period of 0.008 s for the 0.75 m impoundment. The results shown for the 0.75 m

impoundment demonstrate a second peak a short time after the initial debris impact. This

phenomenon was caused by a bounce back effect of the wooden log causing asubsequent impact. The second peak was always smaller in magnitude; however, the rise

time was similar to the first debris impact. This bounce back effect was observed for

other impoundment depths as well.

Figure 2.8 Debris Impact Loading on Circular Section

Fi,max=Max(x)=u


18/33

14

2.4. Existing Design Codes

The design of structures in flood-prone areas has previously been investigated. However,

few existing codes specifically address the design of onshore structures built in tsunami-

prone areas. Design codes that specifically address tsunami-induced forces were

introduced in order to suggest provisions for designing infrastructure in tsunami-prone

areas. Post-tsunami field investigations of the December 2004

Indian Ocean Tsunami are indicative of the extreme loads generated by tsunami induced

floods, and have outlined the need for developing new design guidelines. Recent research

work indicated that tsunami-induced loads are comparable or can exceed earthquake loads.

Tsunami-induced forces and the impact of debris are not properly accounted for in the

current codes, and significant improvement is needed.

At present, only four design codes and guidelines specifically account for tsunami induced

loads as listed below:

FEMA 55: The code is adopted by the Federal Emergency Management Agency (2003),

the United States, and recommends formulae for tsunami-induced flood and wave loads.

The City and County of Honolulu Building Code (CCH, 2000) : The code,developed by the Department of Planning and Permitting of Honolulu, Hawaii, United

States, makes provisions for regulations that apply to districts located in flood and

tsunami-risk areas.

Structural Design Method of Buildings for Tsunami Resistance (SMBTR, 2005): The

code is proposed by the Building Centre of Japan and outlines the structural design for

tsunami refuge buildings.

The City and County of Honolulu Building Code (CCH, 2000) and the Federal Emergency

Management Agency Coastal Construction Manual (FEMA 55, 2003) are two documents

that provide some guidance to engineers. The forces explicitly cited for a tsunami event

include buoyant forces, hydrostatic forces, hydrodynamic forces, debris impact forces, and

surge or wave breaking forces. There are significant differences between the two

documents. CCH (2000) determines surge forces generated by a tsunami bore-type wave,

specifically for wall-type structural components. FEMA (2003), on the other hand,

considers wave breaking, which is typical of coastal floods and storm events. The FEMA


19/33

15

(2003) document does not specifically address tsunami bores, which possess characteristics

similar to those experienced during the December 24, 2004 Indian Ocean Tsunami. The

other significant difference lies in the estimation of the flow velocity used in estimating the

drag force. In CCH (2000), the bore velocity is estimated to equal the depth of water at the

building. FEMA (2003), on the other hand, provides a significantly higher velocity in the

area near the shoreline during a tsunami event. The flow velocity is estimated as 2 ,where ds is the design flood depth. The consequence is larger drag forces in comparison to

the estimates given by CCH (2000). Only FEMA (2003) provides load combinations for

the given force components; however, these combinations are explicitly formulated for

flood scenarios and include wave breaking forces. Nistor et al (2008) proposed loading

combinations (Figure 2.8) that specifically consider a tsunami event including the effects

of a bore-type wave.

Figure 2.8 Proposed Tsunami Loading Combinations: a) Initial Impact; b) Post Impact

(Nistor et al. 2008)

The first loading combination (Initial Impact) considers surge and debris impact forces as

the main lateral load components. This represents the first impact of the tsunami bore. The

second in the sliding and overturning resistance of a structure.


20/33

16

3. Tsunami-induced forces

A broken tsunami wave running inland generates forces which affect structures located in

its path. Three parameters are essential for defining the magnitude and application of these

forces:

(1) Inundation depth,

(2) Flow velocity,

(3) Flow direction.

The parameters mainly depend on:

(a) Tsunami wave height and wave period,

(b) Coast topography,

(c) Roughness of the coastal inland.

The extent of tsunami-induced coastal flooding, and therefore the inundation depth at a

specific location, can be estimated using various tsunami scenarios (magnitude and

direction) and modelling coastal inundation accordingly. However, the estimation of flowvelocity and direction is generally more difficult. Flow velocities can vary in magnitude

from zero to significantly high values, while flow direction can also vary due to onshore

local topographic features, as well as soil cover and obstacles.

Forces associated with tsunami bores consist of:

(1) Hydrostatic force.

(2) Hydrodynamic (drag) force.

(3) Buoyant force.

(4) Surge force.

(5) Impact of debris.

(6) Uplift Forces on Elevated Floors.

(7) Additional Gravity Loads on Elevated Floors.

(8) Wave breaking force


21/33

A brief description of these fo

3.1. Hydrostatic force

The hydrostatic force is gene

planar surfaces. The hydros

(3.1), where is the seaw

inundation depth, and up is

proposed by CCH (2000) and

The point of application of t

from the base of the triangul

tsunami wave, the hydrostatic

3.2. Buoyant force

The buoyant force is the vert

body. Its magnitude is equa

submerged body. The effect

evident during post-tsunamidamage to structural elements

where is the fluid density

water displaced by the buildin

Figure 3.1 Buoyant fo

17

rces is further presented.

ated by still or slow-moving water acting pe

atic force per unit width, FHS, can be calc

ter density, g is the gravitational accele

the normal component of flow velocity.

accounts for the velocity head.

e resultant hydrostatic force is located at o

r hydrostatic pressure distribution. In the

force is significantly smaller than the drag a

ical force acting through the centre of mass

l to the weight of the volume of water

f buoyant forces generated by tsunami floo

field observations. Buoyant forces can gen, such as floor` slabs, and are calculated as f

FB =gV

including sediment (1200 kg/m3), and V i

g, i.e., the volume below the level of hmax .

ces on an overall building with watertight lo

(3.1)FHS= +

rpendicular onto

ulated using Eq.

ation, ds is the

quation (3.1) is

ne-third distance

ase of a broken

d surge forces.

of a submerged

isplaced by the

ding was clearly

erate significantllows:

(3.2)

s the volume of

wer levels.


22/33

18

3.3. Hydrodynamic (drag) force

As the tsunami bore moves inland with moderate to high velocity, structures are subjected

to hydrodynamic forces caused by drag. Currently, there are differences in estimating the

magnitude of the hydrodynamic force. The general expression for this force is shown in

Eq. (3.3). Existing codes use the same expression, but different drag coefficient values

(CD). For example, values of 1.0 and 1.2 are recommended for circular piles by CCH

(2000) and FEMA 55 (2003), respectively. For the case of rectangular piles, the drag

coefficient recommended by FEMA 55 (2003) and CCH (2000) is 2.0:

=

(3.3)

where FD is the drag force acting in the direction of flow, A is the projected area of thebody normal to the direction of flow, and u is the tsunami-bore velocity.

Figure 3.2 Hydrodynamic force distribution and location of resultant.

The flow is assumed to be uniform, and therefore, the resultant force will act at the

centroid of the projected area. As indicated, the hydrodynamic force is directly

proportional to the square of the tsunami-bore velocity. The estimation of the bore

velocity remains one of the critical elements on which there is significant disagreement inliterature.

The general form of the bore velocity is shown below [Eq. (3.4)]:

u = Cs (3.4)

where u is the bore velocity, ds is the inundation depth, and C is a constant coefficient.


23/33

19

Fig 3.3 Comparison of various tsunami-bore velocities as a function of inundation depth.

Various formulations were proposed by FEMA 55 (2003) (based on Dames and Moore

(1980)), Iizuka and Matsutomi (1999), CCH (2000), Kirkoz, Murty, Bryant, and Camfield

for estimating the velocity of a tsunami bore in terms of inundation depth (Fig. 3.3).

Velocities calculated using CCH (2000) and FEMA 55 (2003) represent a lower and upper

boundary, respectively.

3.4. Surge force

The surge force is generated by the impingement of the advancing water front of a tsunami

bore on a structure. Due to lack of detailed experiments specifically applicable to tsunami

bores running up the shoreline, the calculation of the surge force exerted on a structure is

subject to substantial uncertainty. Accurate estimation of the impact force in laboratory

experiments is a challenging and difficult task. CCH (2000) recommends using Eq. (3.5),

based on Dames and Moore (1980):

FS = 4.5gh2, (3.5)

where FS is the surge force per unit width of wall and h is the surge height.

The point of application of the resultant surge force is located at a distance h above the

base of the wall. This equation is applicable to walls with heights equal to, or greater than

3h. Structural walls with height less than 3h require surge forces to be calculated using anappropriate combination of hydrostatic and drag force for each specific situation.


24/33

20

Fig. 3.4 Tsunami wave pressure for structural design recommended by SMBTR (2005).

SMBTR (2005) recommends using the equation for tsunami wave pressure without soliton

breakup derived by Asakura (2000) [Eq. (3.6)]. The equivalent static pressure resulting

from the tsunami impact is associated with a triangular distribution where water depth

equals three times the tsunami inundation depth (Fig. 3.4):

qx = g (.hmax z), (3.6)

where qx is the tsunami wave pressure for structural design, z is the height of the relevant

portion from ground level (0 z 3h), is the mass per unit volume of water, and g is the

gravitational acceleration. A value of equal to 3.0 and 2.0 is used for walls and columns,

respectively.

The tsunami wave force may be composed of drag, inertia, impulse, and hydraulic gradient

components. However, SMBTR (2005) does not specify different components for the

tsunami-induced force, and the proposed formula presumably accounts for other

components.

3.5. Debris impact force.

A high-speed tsunami bore travelling inland carries debris such as floating automobiles,

floating pieces of buildings, drift wood, boats, and ships. The impact of floating debris can

induce significant forces on a building, leading to structural damage or collapse.

Both FEMA 55 (2003) and CCH (2000) codes account consistently for debris impact

forces, using the same approach and recommend using Eq. (3.7) for the estimation of

debris impact force:


25/33

21

= = 3.7

where Fi is the impact force, mb is the mass of the body impacting the structure, ub is the

velocity of the impacting body (assumed equal to the flow velocity), u i is approach

velocity of the impacting body (assumed equal to the flow velocity), and t is the impact

duration taken equal to the time between the initial contact of the floating body with the

building and the instant of maximum impact force.

The only difference between CCH (2000) and FEMA 55 (2003) resides in the

recommended values for the impact duration which has a noticeable effect on the

magnitude of the force. For example, CCH (2000) recommends the use of impact duration

of 0.1 s for concrete structures, while FEMA 55 (2003) provides different values for walls

and piles for various construction types as shown in Table 3.1.

Table 3.1 Impact duration of floating debris (FEMA 55 (2003)).

According to FEMA 55 (2003), the impact force (a single concentrated load) acts

horizontally at the flow surface or at any point below it. Its magnitude is equal to the force

generated by 455 kg (1000-pound) of debris travelling with the bore and acting on a 0.092

m2

(1 ft2) surface of the structural element.

Fig 3.5 Waterborne debris impact force.


26/33

22

The impact force is to be applied to the structural element at its most critical location, as

determined by the structural designer. It is assumed that the velocity of the floating body

goes from ub to zero over some small finite time interval (t). Finding the most critical

location of impact is a trial and error procedure that depends, to a large extent, on the

experience and intuition of the engineer.

3.6 Uplift Forces on Elevated Floors

Uplift forces will be applied to floor levels of a building that are submerged by tsunami

inundation. When computing the buoyant forces on a floor slab, consideration must be

given to the potential for increased buoyancy due to the additional volume of water

displaced by air trapped below the floor framing system. In addition, exterior walls at the

upper floor level will exclude water until their lateral resistance is exceeded by the applied

hydrostatic pressure. This can significantly increase the displaced volume of water

contributing to the buoyancy, as shown in Figure 3.6.

Hydrodynamic forces can also act vertically on floor slabs. During rapid inundation, rising

water will apply uplift to the soffit of horizontal structural components, adding to the

buoyancy uplift. The presence of structural walls and columns in a building will obstruct

the tsunami flow passing through the building.

Figure 3.6 Definition sketch for upward buoyant force exerted on an elevated floor.


27/33

23

The total uplift force on the floor system can be estimated using Equation 3.7:

Fu = 0.5 Cus Afuv2

(3.7)

where Cu is a coefficient (taken as 3.0), s is the fluid density including sediment (1200

kg/m

3

), Afis the area of the floor panel or floor framing component, and uv is the estimatedvertical velocity or water rise rate.

3.7 Additional Gravity Loads on Elevated Floors

During drawdown, water retained on the top of elevated floors, as shown in Figure 3.7,

will apply additional gravity loads that can exceed the loads for which the floor system was

originally designed. The depth of water retained, hr, will depend on the maximum

inundation depth at the site, hmax, and the lateral strength of the wall system at the elevated

floor. It should be assumed that the exterior wall system will be compromised at some

point so that water will inundate submerged floor levels. Because of the rapid rate of

drawdown, it is likely that much of this water will be retained in the upper levels (at least

temporarily) resulting in significant additional gravity load on the floor system. The

maximum potential downward load per unit area, fr, can be estimated using Equation 3.8:

fr = s g hr (3.8)

where s is the fluid density including sediment (1200 kg/m3), g is the acceleration due to

gravity, and hr is the maximum potential depth of water retained on the elevated floor

determined using Equation 3.9:

Figure 3.7 Gravity loads exerted on an elevated floor with water retained by exterior walls

during rapid drawdown.


28/33

24

hr = hmax h1 hbw (3.9)

where hmax is the maximum inundation level predicted at the site, h1 is the floor elevation

above grade, and hbw is the maximum water depth that can be retained before failure of the

wall due to internal hydrostatic pressure. For elevated floors without walls (such as a

parking structure with open guardrails) water may remain on elevated floors until it drains

of the structure.

3.8 Wave breaking force

Tsunami waves tend to break offshore and approach shoreline as a broken hydraulic bore

or a soliton, depending on the wave characteristics and coastal bathymetry. Consequently,

classic breaking wave force formulae are not directly applicable to the case of tsunami

bores. For approximate calculations following expression for wave breaking force may be

used:

Fbrkw =0.5 gCdbD (3.10)Where,

Cdb = shape coefficient (value = 2.25 for square or rectangular piles and 1.75 for circular

piles.

D = pile diameter

Hb = wave breaking height (Hb =0.78ds is the design still water depth).

[Note: In addition to the above mentioned forces that directly affect the structure a tsunami also

scours off supporting material at foundation of the structure which reduces the strength of

foundation of the structure. In a tsunami surge the leading wave may scour away much of the

supporting materials around the base of a structure and weaken the foundation so much that the

foundation may fail under subsequent drag load. The behaviour of tsunami surge scour is very

complex and dependent on the geometric properties of the substructure and material properties of

surrounding soil at the base.]


29/33

25

4. Structural Analysis

A simple, 10-storey ductile reinforced concrete moment resisting frame structure is

analyzed for tsunami and seismic loads. The tsunami inundation level is assumed to be 5

m. The seismic weight is approximately 4400 kN per floor and the storey heights are 3.65

m. Figure 4.1 shows a plan view of the structure, while Table 4.1 provides the force

components considered in calculating the tsunami loading.

Figure 4.1 Building Plan Layout (Palermo et al., 2007)

Table 4.1 Force Components for Tsunami Loading

The calculated elastic base shear for the building under seismic effects is approximately

13720 kN. A 5 m tsunami level would induce an approximate base shear of 23838 kN due

to the surge force during the initial impact and 12700 kN during the post impact caused by

the drag of the tsunami flow around the structure. If the velocity component is assumed to

be equal to the tsunami inundation level as assumed by CCH (2000), the post impact phase

6.

0

6.0

6.0


30/33

26

would generate a base shear of 1620 kN. The hydrostatic force is calculated as 2644 kN.

While this example is intended to provide an understanding of the tsunami forces imposed

on structures, it also highlights the importance of properly quantifying the tsunami force

components. The surge force is estimated as nine times the hydrostatic force; however, this

has not been widely accepted in the literature. Furthermore, the velocity generated by the

tsunami bore varies significantly, which affects the magnitude of the drag forces. This

example also assumes that all non-structural exterior elements remain intact. It is highly

probable that the first impact of the tsunami wave damages the exterior non-structural

components, reducing the lateral load that is transferred to the structure. As such, the non-

structural components act as a fuse for the lateral load resisting system.

[Note: The debris impact loading according to FEMA (2003) and CCH (2000) is negligible

in the calculation of the global base shear and has therefore been omitted.]


31/33

27

CONCLUSION

While tsunamis cannot be prevented, or their destructive effects entirely avoided, actions

can be taken to mitigate the risks of this hazard, thereby reducing the impacts on life,

physical structures and livelihoods. While it is recognized that most buildings cannot

withstand extreme tsunami loads, multi-story buildings of reinforced concrete and

structural steel that are built to withstand local seismic forces and/or extreme wind

conditions with limited structural damage, may offer protection from smaller tsunami

waves.

1. Tsunami-induced loading should be considered for near-shoreline structureslocated in tsunami-prone areas.

2. More guidance is required for structural engineers in order to estimate tsunamiloads on structures.

3. Improved estimates of bore velocities are required to provide more accurate dragand debris impact forces.

4. Based on the impoundment depths investigated, the experimental results indicatethat the surge force does not significantly overshoot the drag force as indicated by

current codes.

5. Pressure readings of the circular section indicate that the initial bore impact causesan approximate triangular pressure distribution along the height of the section.

6. Debris impacting structures can produce a bounce back effect, causing a secondlower amplitude impact.

7. Tsunami risk could be completely avoided by proper site selection.8. Further to minimize the hydrodynamic drag forces, structural systems like steel

frames with dampers, post-tensioned reinforced concrete frames and shear walls

may be provided.

9. However, to reduce the loads that act on the building, hard costal engineeringsystems like seawalls, bulkheads and revetments can be used.


32/33

28

REFERENCES

Arnason, H. (2005). Interactions between Incident Bore and a Free-StandingCoastal Structure, Ph.D. Thesis, University of Washington, Seattle.

Asakura, R., Iwase, K., Ikeya, T., Takao, M., Fujii, N. and Omori, M., (2000) Anexperimental study on wave force acting on on-shore structures due to over flowing

tsunamis, Proceedings of Coastal Engineering, JSCE, Vol.47, pp. 911-915.

CCH Building Code (2000), City and County of Honolulu, Honolulu, U.S. Dames and Moore, (1980), Design and Construction Standards for Residential

Construction in Tsunami-Prone Areas in Hawaii. Federal Emergency Management

Agency, Washington, D.C.

FEMA 55, (2003),Coastal Construction Manual, 3rd edition. Federal EmergencyManagement Agency, Washington, D.C.

FEMA P646 (2008). Guidelines for Design of Structures for Vertical Evacuationfrom Tsunamis, Federal Emergency Management Agency, Washington, D.C.

Haehnel, R.B., and Daly, S.F. (2002), Maximum impact force of woody debris onfloodplain structures. U.S. Army Corp of Engineers, Engineer Research and

Development Centre.

I.S. Code (2010),Tsunami resistant design of buildings and structures(Draft).Bureau of Indian Standards. New Delhi, India.

Matsutomi, H.(1999) A practical formula for estimating impulsive force due todriftwoods and variation features of the impulsive force, Journal of Hydraulic,

Coastal and Environmental Eng, JSCE, No.621/II-47, pp. 111-127.

Nistor, I., Palermo, D., and Nouri, Y. (2007), Tsunami-Induced Forces onStructures, Handbook on Costal and Ocean Engineering, California State

University, Los Angeles, USA. Ch-11, pp. 261-284.

Nouri, Y., Nistor, I., Palermo, D., and Cornett, A. (2007), Structural Analysis forTsunami-Induced Force and Debris Impact, Coastal Structures 2007, Venice,

Italy.

Palermo, D., and Nistor, I. (2008), Understanding Tsunami Risk to Structures,Science of Tsunami Hazards, Vol. 27, No. 4, pp. 1-11.

PDC (2005). Tsunami Awareness Kit, Pacific Disaster Centre,Kihei, HI, U.S.


33/33

Ramsden, J.D., and Raichlen, F. (1990), Forces on Vertical Wall Caused byIncident Bores, Journal of Waterway, Port, Coastal, and Ocean Engineering,

Vol.116, No.5, pp. 592-613.

Effect of Tsunami on Building

Documents