Citation: Leger, V., T. Azuhata (2022), Structural performance evaluation of cyclone resilient houses
damaged due to tsunami after the January 2022 Hunga Tonga – Hunga Ha’apai volcanic eruption, Synopsis
of IISEE-GRIPS Master’s Thesis.

STRUCTURAL PERFORMANCE EVALUATION OFCYCLONE RESILIENT HOUSES DAMAGED DUE TO
TSUNAMI AFTER THE JANUARY 2022 HUNGATONGA-HUNGA-HA’APAI VOLCANIC ERUPTION
Victorina Stephanie Nodis LEGER1
MEE21714

Supervisor: Tatsuya AZUHATA2
ABSTRACT

Natural Hazards have been occurring more frequently in Tonga compared to past years due to effect of
Climate Change. Before the January 2022 event, strong storm sea surges from a Tropical Cyclone
destroyed coastal areas at ‘Eua Island that acted like a tsunami disaster. After the event of January 2022,
there have been more frequent earthquakes occurred near Tonga and more coastal protection weredamaged as well as bulk of the housing stock at the coastline. The Cyclone Resilient Houses (CRH) that
had been popularized in the past 40 years for housing reconstruction programs are now questionable
after the January 2022 event. For this purpose, the study is to perform structural performance evaluationof the CRH timber shear walls using the AS 1684.2 – 2010 Capacity of Wall Bracings on timber framed
structures. This is assessed against the demand lateral forces from tsunami in addition to wind andearthquakes based on the Australia standard AS 1170.2 and AS 1170.5 - 2002 and tsunami force using
the Japanese guideline. The analysis result shows that the capacity of the shear walls is safe in the caseof tropical cyclones because of the adequate nailed connections. The CRH is also safe from earthquakes
because the CRH has added capacity from steel strap bracing and stud ties. As for tsunami load, it is too
big to compare against the capacity of the CRH and therefore CRH can never withstand tsunami withinundation depth over 1m. Therefore, the priority focuses on reinforcing strength of CRH in terms oftsunami by raising floor heights above ground to no more than 3m with concrete or wooden poles thatwould not increase tsunami wave pressures when it reaches the building. Moreover, this can be ignored
if relocation can be made to build CRH on higher grounds and/or build coastal protection and planting
mangroves over the coastal area of the inundated areas. Nevertheless, this draws attention into enforcing
structural performance evaluation of buildings to suggest future disaster mitigation planning of housing.
Keywords: Cyclone Resilient Houses, Performance evaluation, Tsunami, Earthquakes, Horizontal force
1. INTRODUCTION
In the 2017 World Risk Index Report, Tonga is ranked as the second most as-risk country in the worldin terms of exposure to natural hazards. Tonga is a small group of islands located in the Pacific, of whichapproximately 170 islands with a population of 100,700 inhabiting 36 of the islands according to the
Tonga Census Report of 2016. The main island with the capital city is home to 74% of the population.
Tonga was highly affected after a most recent disaster on January 15, 2022, by a violent eruption of
Hunga-Tonga-Hunga Ha’apai (HTHH) undersea Volcano at 5:10pm local time. The eruption thengenerated a powerful Tsunami that hits various coastal areas of the Kingdom of Tonga rendering heavy
damage to infrastructures across the Kingdom. A lot of the damage is associated with the tsunami1
2

Building Control Division of the Ministry of Infrastructure Tonga.
Building Research Institute.

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affected locations for the building infrastructure and the common damages identified were buildings
with only the foundations remaining. After the Initial Damage Assessment carried out by theGovernment Ministries in Tonga, the observed maximum inundation depth recorded was up to 10m high.
Moreover, the target of this study focuses on the Cyclone Resilient Houses (CRH) that have been usedin Tonga in the past 40 years for Housing Reconstruction Programs. It was also developed through atime where no building regulations were in place, and building construction and type were all traditional.
The CRH also known as the Hurricane house was becoming common in Tonga in the past however,some suffered severe damages from the tsunami after the 2022 HT-HH volcanic eruption. Therefore,
there is a need to re-evaluate the structural performance of the CRH against tsunamis in addition tocyclones and earthquakes to suggest future disaster mitigation planning of housing.
2. DATAThe focus of this study is based on the inundated areas after the January 2022 event, and where the
Cyclone Houses were observed with damages. They are (1) Kanokupolu village under the capital islandof Tongatapu, (2) ‘Atata Island and (3) ‘Eua Island. The damages to the CRH in relation to theinundation depths were identified in the ‘Eua Island and the ‘Atata Island. Figure 1 shows the locationof the CRH at ‘Ohonua, ‘Eua within 50m from the shoreline without any coastal protection in the area.

𝜂 = 8𝑚𝜂 = 7𝑚Figure 1: Damaged CRH at ‘Ohonua, ‘Eua with different inundation depth
In the ‘Atata Island, the damages observed to the CRH were different in comparison to the damages
seen at the ‘Eua Island. Figure 18 shows the location of the damaged CRH according to different
inundation depths. The damages observed were different at high points and low points of the island.
At high points, no structural damage was observed as shown in Figure 2. However, at low points of
the Island where inundation depth reached 6 m, the damaged CRH showed some structural damage
to the shear wall panels as shown in the Figure 2.

Low portion

High portion𝜂 = 6𝑚𝜂 = 1𝑚Figure 2: Damaged CRH with different inundation depths at low and high points of ‘Atata Island2

3. ANALYSIS MODEL
The Cyclone Resilient House is a 2-room timberframed structure on RC slab and foundation. It is
designed to 70m/s design wind speed and in Wind
Region C high cyclonic areas. The shear wall panels of
the structure are the focus of the analysis in thisresearch based on the damages occurred after theJanuary 2022 event whereby the sub-structure was still
firm while the super-structure was badly damaged dueto the lateral force induced by the tsunami. The floor
plan of the CRH is outlined in Figure 3.

Figure 3: Floor plan and elevation of the CRH
The structure of the timber shear wall comprises of three things: timber frame, plywood cladding andthe nail for connections of the two. The timber-framed structure is based on the Australia Standard
AS1720 – 1997. This 100mm x 50mm sized timber is framed according to Figure 4 for each wall sections
in both x and y directions.

Figure 4: Shear walls from timber framed to bracing and nailed connections to timber panels
3.3. Horizontal Design Forces
Step 1: WIND
The design wind load that Tonga follows as per the National Building Code (NBC) of Tonga - 2007 is
the Australia Standard AS/NZS1170.2 – 2002 for Structural Wind Actions. The NBC has a standard

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design wind speed, VR = 70m/s that is used for all wind design load. The following procedure outlinedin that standard is what was used in this study.
Step 2: EARTHQUAKEThe NBC of Tonga adopt the Australia and New Zealand Standard NZS 1170.5 – 2002 for Earthquake
Design Actions as there is currently no seismic design code for the country. There was an update to the
NBC for Tonga in 2019 which utilities specific hazard maps that was developed only on the capitalisland of Tongatapu under the Multi Hazard Data Risk Assessment (MHDRA) for Tongatapu Project
funded by the Asia Development Bank (ADB). In this update, ground motion is defined as a single valueof 0.7g across the country with a 10% probability of exceedance in 50years, equivalent to a 500-year
period.
Step 3: TSUNAMI
In Tonga, currently there is no guidelines regulated for Tsunami Loads or have it mentioned in the NBC.
We know that it has been just recently that Tonga started to have frequent tsunami and earthquakes, and
the Government of Tonga is now considering the effects of the tsunami on the private housing. Therefore,
for the purpose of this study the Japanese guideline for the tsunami forces on buildings was adopted andused.
4. RESULTS AND DISCUSSION
4.1. Horizontal Load Carrying Capacity of the CRH
The analysis procedure adopted for this research is based on the Australian standard, “Residential
timber-framed construction (AS 1684.2 – 2010).” The AS 1684.2 – 2010 sets out the requirements forthe construction of conventional stud-framed walls considering wall frame members. The total capacityof bracing walls shall be the sum of the bracing
capacities of individual walls. The two types ofstructural wall bracing in the CRH are of the (1) steel
bracing and (2) timber sheet bracing in Figure 5. It
also provides the unit bracing capacity (kN/m) for
the typical wall bracing systems. The CRH uses
Type B bracing unit using Pryda Strap Brace(SB103) or Pryda Speedbrace with design capacityof 3kN/m for wall heights up to 2.7m as shown in
Table 4. The location and distribution of bracing
shall be provided in both directions as shown in
Figure 6.

Table 4: Types of steel bracing usedin the CRH

Figure 6: Length of Wall bracing – timber panel4

Total Length
X-direction: 19.08 m (7.098 m)
Y-direction: 11.022 m (16.5 m)
Red: Timber panel
Blue: Steel strap
Figure 5: Type of timber sheet

bracing used in the CRH

The structural performance evaluation of the CRH in comparison of Demand and Capacity is shown in
Table 6.
Table 6: Capacity vs Demand of the CRH
CAPACITY
Direction
Timber Panel
Steel Strap
Total
X (Longtidunal)
114.4816.05
130.53
Y (Transverse)
66.13
24.75
90.88
DEMAND
DirectionWind
Earthquake
Tsunami (h=1.0m) [*]X (Longitudinal)
29.72
73.08
52.92
Y (Transverse)
61.52
73.08
105.84

Note: [*] represents amplification factor a = 1.5
The result in Table 6 shows that for wind hazard, the capacity of the CRH can overrule and will be safe
during a tropical cyclone with heavy wind gusts. As for earthquake hazard, the capacity of the CRH is
also higher than the demand therefore the CRH would be considered safe during an earthquake. The
tsunami destructive forces can only be compared to the capacity of the CRH when the tsunami
inundation depth is 1m. For inundation depths of more than 1m, the tsunami forces are way above thecapacity of the CRH. The tsunami load is very large that the destroyed houses were located near the
shore in less than 100m from shoreline. For CRH located in higher grounds, the tsunami force isobstructed and will slowly reduce the tsunami forces by the time it reaches that location. For this reason,
the CRH can never be recommended for housing reconstruction for tsunami resilient houses as evident
from the results in Table 6.
The following can be concluded from the analysis results:i.
Shear walls capacity depends mainly on the nail-joint properties although not specificallyenforced in the construction of the CRH. Increasing the number of nail joints leads to increasing
the shear wall racking capacity.
ii.
CRH can also be used as an Earthquake Resilient House.
iii.
CRH is not safe for tsunami when the tsunami inundation depth exceeds 1m. CRH should not
be recommended to build at coastlines because of its vulnerability to tsunami disaster.
iv.
Alternatives in place to substitute for CRH in the case of tsunami shelter reconstructions.
v.
Cost review analysis of different type of structures.
5. CONCLUSIONSThe objective of this study was to fully assess the current capacity of the CRH that had been used overthe past 40 years in Tonga for post-disaster housing reconstruction activities. Currently in Tonga, there
is no certain guidelines for structural performance evaluation of buildings. The two methods used in thisstudy are very useful to use even for the MOI when they evaluate damages of public infrastructures so5that they can provide what retrofitting techniques can be used. Although the retroffiting method was notcovered in this study but it is important for a start to develop guideline for performance evaluation ofdifferent type of structures. This study covered methods for timber framed structure. The primary
analysis of the CRH focused on the shear wall panels which was also noted in the results that the capacityof the nailed connections alone cannot overcome the earthquake demand until the steel strap bracings
were used. However, for the tsunami lateral forces it was too big to even compare to the capacity of the
CRH or any other engineered building in that location. Therefore, other alternatives can be taken into
account such as the use of other material type of structures and increasing floor height above ground of
CRH or any building in general to reduce tsunami wave pressure projected on the surface area. The
result of the cost analysis of the CRH shows that it is becoming more expensive to construct now
compared to when it was introduced and not affordable for the government to bare and be able to build
a lot of houses to accommodate all the affected households Therefore, there is a need to investigate otheralternatives such as utilizing local materials that will eliminate the cost of delivery of materials fromabroad. To do this, we need more detailed approach into research and to conduct tests to evaluate
performance of the local construction materials, and we also need more engineered suggestions on how
to use them to meet minimum requirements set out in the Tonga National Building Code.
6. ACTION PLAN
The limitations of DRM measures within government in terms of both structural and non-structuralcountermeasures needs to be addressed.
(1) Non-structural Measures:• Establish provisions for tsunami guidelines on• Develop tsunami hazard maps for Tonga through
Structural Design of Tsunami Evacuation (ortsunami simulation and numerical analysis andimportant) Buildings.
regular refinement of hazard maps but especially • Introduce and enforce establishment offor earthquakes to include updates on active
guidelines for structural Performance Evaluation
faults, surface deformation measurements and soof timber framed structures following theforth.
Australia standard AS 1684.2 – 2010 followed in• Develop country hazard zoning maps for Tonga
this study and other guidelines for Performance
to be used in the NBC and to inform future
Evaluation of RC structures using Stera 3D oramendments to the building code.
other Australia standards.
(2) Structural Measures:• Establish Vulnerability Assessment for theConstruction of barriers like seawalls, dikes andbuilding sector considering combination of therevetment with considerations to include
exposure
hazard maps
andstructural
natural based solutions like planting mangroves
characteristics of buildings.
at coastal areas.
ACKNOWLEDGEMENTS
This research was conducted during the individual study period of the training course “Seismology,
Earthquake Engineering and Tsunami Disaster Mitigation” by the Building Research Institute, JICA,
and GRIPS.I would like to express my sincere gratitude to my supervisor Dr. Tatsuya Azuhata for his
valuable support and experience which helped me to complete my research successfully, and to IISEE
staffs for their great assistance always.
REFERENCES
Standards Australia, Committee TM-002, Timber Framing., 2010, AS 1684.2 – 2010: Australia
Standard for Residential timber-framed construction Part 2: Non-Cyclonic Areas, Third Edition.
Government of Tonga, World Bank Group, GFDRR, 2022, Global Rapid Post Disaster Damage
Estimation (GRADE) Report: The January 15, 2022 Hunga Tonga-Hunga Ha’apai Eruption andTsunami, Tonga.
Kabeyasawa, T., 2022, Recent Study and Guidelines for Forces on Buildings by the tsunami, Lecture
Note of the International Training Program on Seismology and Earthquake Engineering

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