1.0 INTRODUCTION 1.0 INTRODUCTION
There are several types of sea waves, which the force, speed and high of the waves are different There are several types of sea waves, which the force, speed and high of the waves are different (Table 1). Some of these waves are destructive to structures, and some are only minor waves (Table 1). Some of these waves are destructive to structures, and some are only minor waves which have no effect on structures.
which have no effect on structures.
The force that induced by tsunami waves are much enormous and faster than other The force that induced by tsunami waves are much enormous and faster than other waves. It
waves. It doesn’t resemble the normal sea waves, because tdoesn’t resemble the normal sea waves, because tsunami waves have a longer sunami waves have a longer wavelength, which generally consists of a series of waves with periods ranging from minutes to wavelength, which generally consists of a series of waves with periods ranging from minutes to hours, with wave heights of tens of metres can be
hours, with wave heights of tens of metres can be generated by large events.generated by large events.
WAVES WAVES TYPES TYPES DESCRIPTION DESCRIPTION Deep water
Deep water A number of waves with different length.A number of waves with different length.
Straight, long, powerful, and travel for great distance.Straight, long, powerful, and travel for great distance.
Destructive Destructive
High waves with short wavelength and vertical eclipses.High waves with short wavelength and vertical eclipses.
Have powerful backwashHave powerful backwash
Dragging objects back to the sea.Dragging objects back to the sea.
Internal Internal
Formed when there is a disturbance in between two masses of water withFormed when there is a disturbance in between two masses of water with
different density. different density.
Become turbulent and high waves when hit the shore.Become turbulent and high waves when hit the shore.
Kelvin Kelvin
Formed due to lack of the wind.Formed due to lack of the wind.
On Pacific Ocean sea.On Pacific Ocean sea.
High, wide, and warmer waves than the surrounding water.High, wide, and warmer waves than the surrounding water.
Shallow Water Shallow Water
In shallow waters at a depth less than 1/20In shallow waters at a depth less than 1/20thth of their wavelength.of their wavelength.
Two types:Two types:
o
o Tidal.Tidal. o
o Seismic / Tsunami.Seismic / Tsunami.
Surging Surging (Wind-Driven) (Wind-Driven)
Generated by intense wind in the sea.Generated by intense wind in the sea.
It has low energy, but travel for a long distance, and break away from theIt has low energy, but travel for a long distance, and break away from the
shore. shore.
TABLE 1
TABLE 1 Different type of ocean wavesDifferent type of ocean waves
Source: Marine Insight, April 30
Tsunami waves are caused due to earthquakes occurred beneath the ocean. The speed of the tsunami is extremely fast in open water, and then it will become increase in height significantly in shallow water, which causing dangerous and devastating effect to the shore it hits. Although the impact of tsunamis is limited to coastal areas, their destructive power can be enormous and they can affect entire ocean basins.
1.1 HISTORY OF TSUNAMI CASES
Throughout the history, several tsunami cases have been reported occurred in every parts of the world. Each time it occurred, it causes major losses to human lives and structures. Several prevention and mitigation way have been produced to reduce the losses, but because of the
devastating effect it brings, still major losses were reported.
Some examples of the devastating effects of tsunami force when it occurred is shown below in Table 1.2
LOCATION DATE CAUSES EFFECTS
Sumatra, Indonesia 26 December 2004 Earthquake with magnitude of 9.1 off the
coast of Sumatera
i. 50 m tsunami.
ii. Several other countries affected
iii. USD 10 Billion damages iv. 23,000 people dead
North Pacific Coast, Japan 11 March 2011 Earthquake with magnitude of 9.0 i. 10 m high waves ii. 18,000 people killed.
iii. USD 235 Billion damages. iv. Nuclear emergency due to
leaking on the nuclear power plant
Northern Chile 13 August 1868
Series of two earthquake with magnitude of 8.5
i. 21 m high waves. ii. 25,000 deaths. iii. USD 300 Million
damages. Table 1.2 Major tsunami occurrences and the effects.
Historical studies are important for future prevention method. These studies will give more data and knowledge in preparing engineers in designing a structure that can resists the tsunami loads. These studies also need to be done thoroughly so that the number of losses can be reduced in the future.
1.2 AIMS AND OBJECTIVES
The aims and objectives of these dissertations are
To study and understand the waves force effects, the risk categories on structures, and
the different failure modes on structures.
To summarize the way to quantify the force of tsunami affecting a structure and
comparing the equations from other research that has been done.
To study in details the dynamic effects of the tsunami impact force on structure and the
differences from static force.
To create a finite element modelling based on dynamic tsunami forces acting on
structure.
2.0 LITERATURE REVIEW
As what have been described in the previous chapter, tsunami force causing very devastating effects on structure. According to history, billions of dollars were loss in one event of a tsunami attack. The total loss will also increase in re-building the structures again, and thus the importance of engineers needs to find a way for structure that can resists the tsunami force.
Several factors can trigger the formation of tsunami waves. According to Federal Emergency Management Agency (FEMA), the factors that triggered tsunami are:
Underwater Earthquakes. Volcanic Eruption.
Submerged or aerial landslides.
According to history, majority of the tsunami was caused by the vertical displacement that occurred at the bottom of the ocean. Because of the displacement, it created a high speed over thousands of kilometre of tsunami waves (Palermo and Nistor, March 2008).
But, to understand the forces that being induces by the tsunami, we need to understand the types of wave force and failure that will happen to structure under such loads. These types of forces also were determined based on the location and types of the structure.
Before the Chilean tsunami that occurred in 1960, dynamic pressure wasn’t taken into consideration when designing a structure to withstood tsunami attack. Dynamic pressure is more significant than static pressure in understanding the forces that being induced by the tsunami.
Figure 2.1 Japan Tsunami
2.1 FORCE CATEGORY
The forces that induced by tsunami are categorised into several parameters. In defining the magnitude and application of tsunami-induced forces, these parameters are important to take into consideration (FEMA, 2008).
Inundation depth. Flow velocity. Flow direction
And these parameters also depend on several other factors that are:
Wave height. Wave period.
Coastal topography.
Roughness of the coastal inland.
Categories of the forced that induced by tsunami are (Palermo and Nistor, March 2008):
Hydrostatic forces. Buoyant forces.
Hydrodynamic forces. Surge forces.
Impact forces.
Breaking wave forces.
2.1.1 Hydrostatic Forces
This force is generated by a slow moving wave acting perpendicular on a surface. This force is usually used on structure such as seawalls and dikes, but not being used for buildings. It is applied 1/3 from the base from pressure distribution, and this force is smaller than the drag and surge force in the case of broken tsunami waves. But, this force is important when coastal flooding from tsunami which is similar to rapid-rising tide occurred.
for hmax> hwelse hmax hw
Fh = Hydrostatic force
pc = Hydrostatic pressure at centroid
hmax = Maximum water height above the base of wall
hw = Height of wall panel
2.1.2 Buoyant force
Buoyant force is come from the Archimedes principle. In case of tsunami-induced force, it is the vertical force acting through the centre of mass of a submerged body. The magnitude is considered equal to the weight of water volume that is displaced by the submerged body. This force has significant effect on the floor slab of structure, wood frame buildings, empty above-ground and below-above-ground tanks.
FB = Buoyant force ρ = Density of water = 1000 kg g = Gravity V = Volume 2.1.3 Hydrodynamic forceIt is also known as drag force. This force occurs as tsunami bore moves inland with moderate to high velocity and flows around structures. It is assumed to be uniform, which act at the centroid of area. This force is varies, because it depends on the shape of the structure where the flow occurred.
FD = Drag force CD = Drag Coefficient ρ = Density of water = 1000 kg A = Area u = Flow velocityWidth to Depth Ratio Drag Coefficient From 1 – 12 1.25 13 – 20 1.3 21 – 32 1.4 33 – 40 1.5 41 – 80 1.75 81 – 120 1.8 >120 2
Table 2.1 Drag coefficient value based on the width to depth ratio
2.1.4 Surge Force
This force is generated by the surge of the water from the tsunami bore acting on a structure. It depends on the geometry of the structural element and velocity of the tsunami. The magnitude of this force has four times more than the hydrostatic value.
In japan, another approach has been used to determine the surge force (Building Centre of Japan). It is quite identical to the surge force equation. They adopted the equation made by Keulegan (1950).
√
Fs = Surge force ρ = Density of water = 1000 kg g = Gravity h = Height of surge b = Base u = Velocity 2.1.5 Impact forceThe impact force came from the debris that has been collected by the tsunami bore and strike against buildings and structures. The debris can be as small as sand to as big as a ship, thus can induced very significant force on buildings, which will lead to structural failure and collapse. This force is being assumed as single concentrated load that act horizontally at the flow of the surface.
Figure 2.2 The debris that was drag by the tsunami
There are several equations that have been developed to determine the impact force on a structure.
Which is based from the impulse-momentum approach.
∫
The duration of the impact on a structure is depends on the type of the construction (FEMA). Type of Construction Duriation (t) of impact (sec, s)
Wall Pile
Wood 0.7 – 1.1 0.5 – 1.0
Steel N/A 0.2 – 0.4
Reinforced Concrete 0.2 – 0.4 0.3 – 0.6 Concrete Masonry 0.3 – 0.6 0.3 – 0.6
Table 2.2 Duration of impact on type of materials of construction
Source: FEMA CCM
American Society of Civil Engineering (ASCE) has developed other equation to determine the impact force.
FI = Impact force
m = Debris mass
u = Object impact velocity CI = Importance coefficient
CO = Orientation coefficient
CD = Depth Coefficient
CB = Blockage Coefficient
R max = Maximum response ratio for impulsive load
2.2 RISKS CATEGORIES
To determine the categories of risks that can happened from tsunami’s, historical data need to be gathered and studied for more understanding and to have more accurate measure in reducing the risks. American Society of Civil Engineers (ASCE) has come up with a building code for building and structures (Table 2.3 (a) and (b)) so the structure will not fail or collapse the second
a tsunami wave forces hit the structures.
But currently, there are no national standards for engineering design for tsunami effects that can be used. Because of this, the design risks of tsunami on coastal zone are still not clear and broad for construction to follow from the design codes. So, as a result, the risks categories are being made in accordance to the structures importance and hazards it can bring if collapse or fail (Table 2.3 (a) and (b)). For example, the nuclear plant in Japan that failed during the Japan tsunami in 2011, it has a disastrous effects to lives if it ultimately failed. So, these risks studies need to be followed to prevent for future tragedies.
RISKS
CATEGORY DESCRIPTION
I Building and other structures that has low risks to humans
II All buildings and other structures except those listed on categories I, III, IV
III
Buildings and other structures with potential to cause a substantial economic impact and/or mass disruptions to of day-to-day civilians lives in the event of failure
IV Buildings and other structures designated as essential facilities Table 2.3(a) ASCE 7 risks categories
Use or Occupancy of Buildings and Structure
Risks category for tsunami Buildings and other structures that represent a low hazard to human life in the
event of failure I
All buildings and other structures except those listed in Risk Categories I, III, and
Buildings and other structures , the failure of which could pose a substantial risk to human life, including, but not limited to:
Buildings and other structures where more than 300 people congregate in
one area
Buildings and other structures with day-care facilities with a capacity
greater than 150
Buildings and other structures with elementary school or secondary
school facilities with a capacity greater than 250
Buildings and other structures with a capacity greater than 500 for
colleges or adult education facilities
Any other occupancy with an occupant load greater than 5,000 based on
net floor area
Buildings and other structures, not included in Risk Category IV, with potential to cause a substantial economic impact and/or mass disruption of day-to-day civilian life in the event of failure.
Buildings and other structures not included in Risk Category IV (including, but not limited to, facilities that manufacture, process, handle, store, use, or dispose of such substances as hazardous fuels, hazardous chemicals , hazardous waste, or explosives) containing sufficient quantities of toxic or explosive substances
where the quantity of the material exceeds a threshold quantity established by the authority having jurisdiction and is sufficient to pose a threat to the public if released.
III
Buildings and other structures designated as essential facilities, including, but not limited to:
Health care facilities with a capacity of 50 or more resident patients. Hospitals and other health care facilities having surgery or emergency
treatment facilities.
Fire, rescue, ambulance, and police stations. Designated tsunami vertical evacuation refuges.
Designated emergency preparedness, communication, and operation
centres and other facilities required for emergency response.
Power generating stations and other public utility facilities required in an
emergency.
Aviation control towers and air traffic control centres. Telecommunication centres.
Buildings and other structures, the failure of which could pose a substantial hazard to the community.
Buildings and other structures (including, but not limited to, facilities that manufacture, process, handle, store, use, or dispose of such substances as
hazardous fuels, hazardous chemicals, or hazardous waste) containing sufficient quantities of highly toxic substances where the quantity of the material exceeds a threshold quantity established by the authority having jurisdiction and is
sufficient to pose a threat to the public if released.
Buildings and other structures required to maintain the functionality of other Risk Category IV structures.
Table 2.3(b) ASCE 7 risks categories in details
Source: American Society of Civil Engineers (ASCE), February 2011
2.3 FAILURE MODES OF STRUCTURES UNDER TSUNAMI FORCE
Under tsunami loads, building will fails and collapse. So, it is importance to know the types of failure modes on a structure under this destructive force. These failure modes can be studied from previous tsunami’s tragedies and thus, engineers can improve and finding ways so that the structure won’t from failed under these conditions.
From the Indonesian tsunami occurred on December 2004, three different types of structural failure was observed which is sliding, overturning, and undermining through scouring (Dias and Mallikarachchi, June 2006). These failure modes are different on each structure, because it all depends on the wave height (Table 2.4) and structure height.
Wave
Height Structure Height Types of failure Image
2 – 4 m Single-storey building Sliding > 4m Multi-storey building Undermining through scouring
Table 2.4 Variety of structures behaved under tsunami loading
Source: Dias and Mallikarachchi, June 2006
From historical tsunami’s event, engineers can study the failure modes that can occur under the loads. From these studies, mitigation method can be developed and identified so that the structure can be more resistance to tsunami attack.
Further studies need to be done to know more structural failure modes under tsunami loading. From the Tohoku tsunami tragedy, different types of failure modes were observed (Table 2.5). These failure modes are far more devastating, since it destroyed bigger, taller, and more reinforced structures than the structures in Indonesia tsunami. So, this types of failure modes need to be considered more when designing important structures.
Types of Structure Failure Modes Image
Two-storey reinforced
Three-storey steel building Lateral pushover
Concrete warehouse Flow stagnation pressurization
Concrete wall Combination of hydrostatic and hydrodynamic loading
Treatment Plant Bore Impact
Table 2.5 Failure modes on reinforced structures
2.3 PREVIOUS RESEARCH
There are several previous researched that has been done to studied the force that being induced by the tsunami. These studies are important since it will create a more understanding and better
method in designing structure that can resists the tsunami loads in the future.
For my work, I will look at two studies that have been made by S. Mizutani and F. Imamura, and D. H. Cammilleri. Both studies are important since it shows the force that induced by tsunami using different equations and methods.
2.3.1 Dynamic Wave Force of Tsunamis Acting on a Structure
Studies that made by S. Mizutani and F. Imamura focused more on structures along the coastlines such as seawalls and breakwaters, and done by doing hydraulic experiment. They introduced the types of wave’s forces acting on seawalls and breakwaters and categorised it into four which is dynamic, sustained, impact standing, and overflowing.
They observed that the existence between impact standing and overflowing wave pressure have a very large value on short period of time at a local point. They also concluded that the impact standing value, due to collision of the reflected and incident waves is related to wave celerity and run-up height. They also proposed new equations to estimates the maximum value of kinetic, sustained, impact standing, and overflowing wave pressure. Below are the proposed equations from the observations.
2.3.1.1 Dynamic wave pressure
The dynamic wave’s pressure was developed by Fukui et al. in 1962, which is used to estimate
the maximum dynamic pressure applied on a structure. He empirically proposed this equation to determine the maximum dynamic wave pressure on structure.
Where pdm = maximum kinetic wave pressure
c = celerity
h = initial water depth H = incident wave height
ρ
ω
= density of seawater g = acceleration of gravityK = kinetic wave coefficient = 0.12 2.3.1.2 Sustained wave pressure
This formula considered that the maximum sustained wave pressure psm related to the maximum
kinetic wave pressure pdm. This formula also taking into consideration the angle of slope of the
structures.
2.3.1.3 Impact standing wave pressure
This formula was observed and it was concluded that impact of standing waves is related to the collision of reflected and incident waves. It takes into consideration the relationship between run-up height and sustained wave pressure, wave celerity and dynamic wave pressure.
(
√
)
2.3.1.4 Overflowing wave pressure
It was observed that the maximum overflowing wave pressure occurred when the overflowing wave collide on the back of the structure. They suggested several important parameters that is used in the equation.
Where: pom = maximum overflowing wave pressure
Vm = maximum velocity
Hw = water depth on the crest
θ
2 = angle of the back slope2.3.2 Tsunami and Wind-Driven Wave Forces in the Mediterranean Sea
This paper was made by Denis H. Cammileri for the maritime engineering magazine. He studied and made comparison the impact of the force generated between the wind-driven waves and tsunami wave, and compared this with several methods. He uses several theories in obtaining the force for tsunami wave for his research such as Keulegan(1950) (Figure2.3), Morrisons (Figure 2.4), and Ambraseys (1962) (Figure 2.5) method. Since this is more on comparison between two types of wave, I will focus more on the tsunami wave from this paper.
He concluded that, tsunami wave are a non-breaking waves, which has higher energy when it strikes the shoreline than it does when it reaches certain distance. The tsunami wave also slows down, increase in height, and wavelength decreases when it approaches the land. By the models and calculations that he made, he suggested that the wave loading may be between 9 -18 times the hydrostatic forces and this value doesn’t considered the effect of debris impacts on structure.
Figure 2.3 Kuelegan methods for shallow water relationship
Figure 2.4 Morrison method consists of two parts, drag and inertia force
