Detailed Damage Assessment of Aitutaki Residential Housing by Cyclone Pat

Detailed Damage Assessment of Aitutaki Residential Housing by Cyclone Pat . Prepared by Ministry of Infrastructure & Planning (MOIP) 15 March 2010 Detailed Damage Assessment of Aitutaki Residential Structures in the wake of Cyclone Pat Background A day after Cyclone Pat struck the Island of Aitutaki; a Ministry of Infrastructure & Planning (MOIP) Team consisting of an engineer- Ata Herman and three Building Inspectors -Loius Teiti, Joe Akaruru & Chalie Tamangaro, was deployed (from the 11 to 13 February 2010), to the island to carry out an Initial Damage Assessment of the all building infrastructures – from government, commercial, tourism, community and residential. The initial assessment reports was broad in scope and focuses on a general overview of the type of building damage that have occurred, primarily to assist in determining the relief and immediate response requirements. Introduction From 22 February 2010 - twelve days after Cyclone Pat, at the request of the Aitutaki Recovery committee, a MOIP Team was deployed to Aitutaki for the second time to undertake a more detailed damage assessment of residential buildings only. The Team consists of three engineers- Ata Herman, Nemia Tagi & Paul Maoate, building inspector – Joe Akaruru and a GIS/Survey/database personnel – Alistair Newbigging. The task of the team was to undertake field assessment survey of all the residential buildings that had incurred damages by Cyclone Pat wind force. The assessment was very much guided by the initial damage assessment carried out previously, especially identifying the building locating and sites, and name of occupiers before and during the cyclone period. In support of the teams’ efforts, Pat Arioka from Cook islands Red Cross and Reboama Samuel from the Emergency Management Unit of the Police department, were of great assistance in cross-referencing of database to include household names and location identifications. Scope The assessment was based on a more detailed data acquisition of wind damage to only residential buildings on the Island. Significant wind damages to roof structures were observed for many of the buildings with each one of them recorded on templates with associated photographic identifications. The data was also mapped onto Geographic Information System (GIS) format. The report will have a study site map, satellite photos of sites, and brief damage description to the cause of structural failures. 2 Typical Building Structural Systems In residential buildings, the primary structural systems are made up almost entirely by exterior load bearing walls and non-load bearing wall, roof structure and diaphragm and foundation. The overall building integrity depends not only on the strength of these components but also the adequacy of the connections between them In the design of structures for wind loading, it is necessary to provide a continuous load path from the roof down into the (concrete) foundation. In a noticeable number of failed structures examined this continuous load path was not present. From general observation, adequately constructed homes stood very well against Cyclone Pat wind forces. Where there was evidence of a breakdown in the load transfer path, the damage extent ranged from considerable to total, depending on the type of construction involved. The roofing systems of all residential buildings investigated were predominantly constructed with light timber trusses or roof rafter system. Roof Framing Systems The roof trusses and rafter cables were found to be less than satisfactory under the wind forces, the connection between the purlin and the truss or rafter was inadequate. Substandard workmanship in the fixing of the purlin to trusses was evident. (improper use of a single vertically driven nail as a tie-down) especially to newer constructed buildings. The much older housings with timber framed roof structure were found to have timber rots, General Observed Performance of Buildings In general, older housing performed poorly with the majority of structural damages occurring on the structural was associated with older construction (>25 years). The evidence after examining of the older structures have shown deterioration in the tie down components due to corrosion, rot, insect attack and leading to reduction in strength and/or non existence of tie downs along the critical load path. The Cyclone Pat’s wind force creating severe structural damages, also caused the damaged structure components such as roofs being detached when wind driven forces threw these debris on to other adjacent buildings thus instigating damages to that building. The duration of intense winds also maximized the potential for fatigue failure of roofing, claddings, fixings and generally roof structures of all buildings damaged. In some cases of failure of older housing, it was apparent that the house had some upgrading undertaken. For example, a new roof was fitted with new roofing’s but the roof failure occurred at the purlin to truss or rafter connection which still had only the original inadequate nail connection. For housing in flat exposed areas a 90 mm long Type 17 screw is required along with pre-drilling. Strapping of purlin to rafters would have been alternative and secured connection. 3 The roof structural connections have primarily contributed to failure of most of these structures. The poor construction practice was also evidence in regards to inadequate tie down fixings with only the use of nails without the use of cyclones ties or strapping. The lack of maintenance by the occupants also leads to failure from corrosion of fixings and structural frame elements. Findings of Structural damages The finding based on detailed assessment is divided into: 1. Structural Assessment 2. Non- Structural Assessments 3. General Assessments Structural Assessments In the design of structures for wind loading, it is necessary to provide a continuous load path from the roof down into the (concrete) foundation. In a noticeable number of structures examined this continuous load path was not present. The perimeters, including corners, of roofs typically experience the highest uplift pressure during wind cyclones. Loss of roofing was observed at the perimeters and corners in numerous cases. It was also observed that when roofing was lost it was that the roofing was nailed too far apart. Using nail spacing that meets or exceeds the code minimum would have significantly reduced loss roofing iron in most of the buildings. The nail spacing for corrugated roofing should be nailed at every second nip or 5 nails per sheet/purlin or 3 type screw per sheet/purlin. A common failure that was observed was the loss of claddings on the gable end walls. One likely cause, is that air entered through openings or other means and pressurized the inside of the house space thus dislodging one or more sections of claddings. The damage to the roof cladding permitted wind-driven wind and airborne debris to enter the building and resulted in additional costly damage to the interior and contents. There were cases where the entire truss system lifted off the roof and the top plate came off with it. The truss uplift forces were transferred adequately to the top plate, but the top plate had no tie down mechanism to transfer the load to the wall, since the top plate was not anchored properly. The result was then a complete loss of the roof system. Many of these failures occurred to old limestone built wall structures. These limestone buildings were built in the sixties and seventies. Many of the leanto attachments or extensions to the main buildings were commonly heavily damaged. Therefore, support columns/posts must be anchored to both the roof and concrete foundation, particularly in high wind regions such as the hills. Non-Structural Assessments Structures with adequate roof ventilations were observed to have performed better due to the ability of the ventilation to relieve induced internal pressure. 4 Extensive damage was caused and induced by airborne debris. The debris consisted largely of failed roofing materials, but also included felled trees and branches. General Assessments From the observations it was evidence that hip roofs performed significantly better than gable roofs. Workmanship Clearly, not all tradepeople were well qualified in the construction of buildings structural systems and connections necessary to resist wind loads. Where quality workmanship was observed, the performance of buildings was improved Repair/Retrofit of Partially Damaged and Undamaged Buildings In many buildings, it was observed that damage occurred in one part of the building and that the reminder of the building was structurally undamaged. Based on this scenario, it was concluded that the repair of only the ‘damage portions’ of buildings may leave the remainders of the building vulnerable to damage from future cyclones. Electrical components Any homes still containing old wiring should be rewired as soon as possible, and have their fuses replaced with Circuit Breakers and Residual Current Devices, as required by Cook Islands Electrical standard and regulations. Re-wiring, especially to reconstruction of category 3 buildings with most of its roof structure destroyed, would involve the complete removal of all old cables, old steel conduit and replacing with new cables. All original sockets, lights switches, light fittings and other electrical fittings are replaced with modern fittings. New mains cables are installed and the dodgy old fuse board and fuses are thrown out and replaced with a modern recessed switch board, circuit breakers and residual current devices (RCDs). Power meters that were once part of the interior fuse board can be re-located to a new meter board outside, so that the meter reader no longer requires access to the home. Finally an electrical inspector will check certain aspects of the job and sign off the Certificate of Compliance. Categorised of Building Damages The detailed assessment followed suit from the initial assessment undertaken previously but with the categorizing of assessed damaged residential buildings in-line with the four categories as follows: 5 • Category 1: Houses with minor damage to roof and walls (0-25%).

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