Tsunami Resisting Structure Design

Tsunami waves

Abstract:

In the past Lebanon has experienced some major earthquakes and tsunamis. Kids are taught at school that Lebanon’s capital Beirut was buried 9 times because of these catastrophic events. However, other than manmade disasters, modern situation has been calm and this small Mediterranean country has been tsunami free for quite some time. Unfortunately studies show that this situation will not remain as calm in the future and that the Lebanese coast is in danger of re-experiencing these waves. In this paper the historic Tsunami activity around the world will be reviewed and evidence shall be given on why this event will strike Lebanon after 1500 years of absence. In addition, the behaviors and characteristics of tsunamis will be presented, how they start and how they destroy and by knowing the effects they have on buildings and the damages they cause to their elements, we are able to design a structure with the purpose of resisting these loads. The aim of this study is to design a Tsunami Resisting Structure in accordance with the current guidelines and complying with the existing Standards and Codes, that is expected to provide safe refugee from Tsunami and earthquakes. An example shall be given and the applied loads shall be calculated in the following pages.

Introduction:

It is said in the bible that the Lord told Noah that a flood of waters shall be brought upon the Earth to destroy mankind, both man and beast shall drown (Genesis 6:17). Tsunamis have been happening since the beginning of time, in the past as well as in our days when that event occurred, as part of evacuation technique or simply by instincts for survival people try to escape by going inland or trying to reach higher altitudes. But the wave hits the land with great energy and speed that basically trying to race it horizontally is practically impossible and most casualties that happen during a tsunami is when people try to flee the coast and move inland to higher altitudes. In some areas an evacuation is not possible in time and the best chances people have is through a vertical evacuation by moving upwards to higher levels in a tsunami resisting structure. (FEMA P646A, 2009)

The idea of a vertical evacuation structure came to be after the tragic Sumatra earthquake and Indian Ocean tsunami in December 26, 2004. 300,000 lives were lost in the boxing day tsunami (Christian Meinig et al.,2005), and that number would have been so much worse if Reinforced Concrete Structure did not act as shelters for survivals. (FEMA P646A, 2009)

We learnt from past events, and if a concrete multistory house was able to resist, even it was temporarily, even if it was for a short term, then a structure can be designed to withstand Tsunami as well as earthquakes, because in most cases, these two hazards are connected.

Definition:

The Federal Emergency Management Agency (FEMA) and the National Oceanic & Atmospheric Administration (NOAA) explain tsunami as: “A Tsunami is a naturally occurring series of ocean waves resulting from a rapid, large-scale disturbance in a body of water”.). They can be caused by either volcano eruption under water, or earthquake or landslides etc. (FEMA P646A, 2009). When they travel in the ocean the waves and fast but short in height, but when they reach the coast, the waves gain in height.

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It was believed that a number of events led to these oceanic waves however after the study of past events it was concluded that the major cause for tsunami generations was due to tectonic movements. Katsuyuki Abe in his publication “Tsunami and Mechanism of great earthquakes” explains the relationship between these waves and earthquakes. Katsuyuki Abe said: “tsunamis are generated by tectonic deformations rather than by large submarine landslides and slumps.” (Katsuyuki Abe, 1972). When tectonic plates are active, plate deformation induces seismic activity that cause earthquakes (Stein and Klosko, 2002) and The source of the tsunami/earthquake can be tracked back to its origin by analyzing the arrival date of the first wave to the shore.

The boxing day 2004 Tsunami went down in history as the deadliest tsunami, with more than 230,000 casualties across 14 countries The earthquake triggering the tsunami was of magnitude 9, the most powerful earthquake in 40 years (ABC news, 2014). With its epicenter located at the Sumatra coast, between 2 tectonic plates, the Indian plate and the Burma plate. The two plates were so stressed that the earthquake happen when the Indian plate slipped under the Burma plate. (Sudhir et al., 2005)

Tsunami in Lebanon

No matter where we read or heard it, whether it was in the findings and studies published by the National Council for Scientific Research, Beirut, Lebanon or National Center for Geophysics, Lebanon or even in the August issue of the journal Geology 2007 or National Geographic News or Discovery News channel or the local newspapers, the context were the same: “It is just a matter of time before a destructive tsunami hits this region.”

An underwater survey placed Lebanon near an active fault that could generate an earthquake in the seafloor causing a devastating tsunami in the region. Elias Ata and colleagues researched that the same fault that was responsible for the strongest earthquakes in the Mediterranean is active and is in danger of producing a tsunami wave just like it did thousands of years ago in the 551 A.D. tsunami in Phenecia now known as Lebanon (Elias et al, 2007). They confirmed the presence of fresh seismic breaks in the sea floor and argued that the Mount Lebanon thrust is identified as repeated earthquakes with 1500 years of return. Since the last earthquake was in 551 A.D. with an active fault line and a 1500 recurrence period, an earthquake in water is bound to happen any day now, and a tsunami will hit the entire Lebanese coast from north to south.

Fortunately complying with the Lebanese building code for construction and design is deem to satisfy conditions for earthquake and seismic analysis, but the law does not impose tsunami design to developers and consultants and the idea of a tsunami resisting structure or a vertical evacuation model was somehow absurd in the region. However now that we have solid proof of a predicted tsunami, that idea doesn’t look absurd anymore.

Structural system

A basic concept design of a Tsunami/Earthquake proof structure follows a number of consideration and guidelines. After observing past events, we can conclude that the major element that contributes to the survival of houses was the material used. While most small residences in Australia are timber and lightweight systems or even brick veneer and load bearing wall system, the most robust model that can resist important horizontal or vertical loads is reinforced concrete and or Steel structures. Multistory building are heavier and thus can prevent sliding when horizontal action is applied and because above levels are not submerged they aid to resist lateral loads.

Another key component is Orientation, when buildings are constructed in a direction parallel to the ocean, the wave will have a greater surface of contact, and therefore the lateral force will hit the plan façade with the lowest inertia. When they are built perpendicular to the ocean, the contact surface is smaller and the pressure will act upon a direction of highest inertia.  An ideal design is to not resist entirely the wave and have the structure with 800mm thick reinforced concrete wall. It must be a combination between a robust design combined with weak members. In other word, the wave should pass through the building, without causing the collapse of above stories. In order for the wave to flow though the ground level, non-load bearing walls have to breakaway to make passage for the wave. These breakaway walls are masonry block walls, they collapse once struck by the wave.  (FEMA TB9, 99)

How vulnerable Structures are?

Observation to partially damaged houses indicated that all elements can fail under tsunami load. Foundation can be uplifted and soil can be scoured. Columns can be sheared causing one story collapse or leading to progressive collapses of all upper levels and impact force can completely dislocate columns. Beams can be bent lateraly due to horizontal action and were majorly cracked. Joints connecting two elements, such as vertical support with slab and foundation can fail entirely. Shear and bending failure in walls have been detected, and punching shear where an axial element punches through a plan system was observed as well. However studying only the elements that failed is not enough; engineers must also consider the components that survived. For instance, it was that facades that had openings such as windows and doors did not crumple as opposing to sides there were completely closed and thus have been ruined by the wave.

Calculation example

This example consists of determining the actions caused by a tsunami on a structure of 2 basements, parking of area 1800 m2 each, Ground Level , Mezzanine and ten stories. The site is located 200 m from the shoreline, at elevation 5 m from the sea level near Beirut, Lebanon

Elevation profile indicating the different slopes and location of the structure, taken from Google   Earth 3D

The slope of the terrain is essential to determine the wave parameters, specially height.

The Federal Emergency Management Agency FEMA, has set guidelines to design structure from tsunami waves, FEMA P646 and FEMA P646A, 2005 shall be demonstrated in the below paragraphs to calculate all the loads generated on our structure. But first we need to determine the wave’s properties.

A tsunami wave is defined by its inundation elevation R*, with is the height of the wave at its maximum penetration. Given the topography of our project, the wave will reach the steep slope and retrieve back, that’s 510m from the shore. To determine the height, we will assume that the height is 10m

Tsunami Loads

As per FEMA P646A, 2005 8 forces are acted upon the structure:

Hydrostatic, Buoyant, Hydrodynamic, Impulsive , Debris impact, Debris damming and Uplift forces, not to mention Additional gravity loads from retained water on elevated floors.

Hydrostatic force in the basement is when the water from tsunami is retained in the basement, and cannot escape because the basement wall outline prevent seepage, the water is trapped and the basements are compared to water reservoirs since the two basements are completely filled with water. A static analysis on reservoir will therefore be conducted with water height 12m (=6m+8m)

Fluid density ρs = 1.2 ρwater = 1200 kg/m3

pc is the hydrostatic pressure, pc = ρs.g.h

pc (at GL ) = ρs. h = 1200 x 8 = 9.6 T/m2

pc (at 1st Bas. ) = ρs. h = 1200 x (8+ 3) = 13.2 T/m2

p_c (at 2^nd Bas.) = ρ_s. h = 1200 x (8+6) = 16.8 T/m²

Buoyant Force

The uplift force as per Archimedes’ principle is as follow:

The reinforced concrete has a specific gravity of 2.5 T/m3, in our case the slab isn’t a solid slab but made of hollow blocks with self weight equal to 0.63 T/m2

Specific gravity of a 32 cm slab of hollow blocks is = 1.97 T/m3

Water has a specific gravity of 1.2 T/m3

When immersed in the water the slab won’t weight the same actuality it would weigh less due to the fact it has a buoyant force acting upward making it lighter.

And so in water, the specific gravity of the slab will become 1.97 T/m3-1.2 T/m3 = 0.77 T/m3

If we take into consideration the thickness of the slab 32 cm it becomes  0.77 T/m3 x 0.32m = 0.246 T/m2

The buoyant pressure decreased the specific gravity of the slab from 0.63 T/m2  to 0.246 T/m2 so it must be equal to 0.63 T/m2  – 0.246 T/m2 = 0.384 T/m2

The buoyant force will be applied upwards as uniform surface load 0.384 T/m2 acting on floors of   1stBasement, Ground level, Mezzanine and first floor.