Mayor and Councillors. Community Services Manager

Transcription

1 Report To: From: Mayor and Councillors Community Services Manager Date: 08 December 2014 Document: File reference: Meeting date: 17 December 2014 Subject: Earthquake Strengthening of Hauraki House Recommendation: THAT the report be received, and THAT Council proceed with the design, tender documentation, consenting and tendering stages of the seismic upgrade, reroofing, repainting of Hauraki House in 2014/15,and THAT this stage be funded from Council s Property Budget, and THAT a report be presented to Council at the end of the tendering stage for Council to consider costings and project timing before works commence. Purpose This report is to update Council on the earthquake strengthening work and capital projects identified for Hauraki House, Paeroa. Hauraki House Page 1 of 8

2 Rear, older section of Hauraki House Background Council staff undertook a pro-active approach towards determining seismic assessments of its buildings in line with the Hauraki District Council (HDC) Earthquake Prone Building Policy Hauraki House was rated to perform at 23% NBS in an initial assessment and a detailed assessment of strengthening was therefore the option to be considered for this building as outlined in the HDC policy. Adopted Methodology GDC Consultants Ltd carried out the following inspections and testing during the month of December 2012 in completing this assessment: Review of available drawings and IEP report prepared by GDC Consultants Ltd. Non-intrusive inspections of the building; observation of important structural connections and seismic characteristics noting any obvious deficiencies. No as built drawings or records were available. Determine the nature of the design, primary structural characteristics and identify the lateral force-resisting systems. A detailed site measurement of visual structural elements above ground. Scanning of the concrete structure to identify, verify structural and steel reinforcements. A number of structural drawings were obtained for use in this investigation. These drawings were used as a reference for this exercise. Page 2 of 8

3 Three dimensional (3D) structural modelling and quantitative analysis. Building Description Hauraki House comprises three parts built at different times. The main part is a two storey building rectangular in shape which was built in It is a reinforced concrete frame structure with a concrete slab floor. First floor is a double T concrete slab floor. There are two concrete staircases from the ground floor to the first floor within the building on the side of the back entrances. Staircase walls are reinforced masonry walls while the remaining internal walls are timber framed. The roof structure is steel trusses with a light weight iron roof. Then there was a single storey timber framed lean-to built after the main part along the front and northeast of the main two storey building. In around 1996, the west half of the front lean-to was demolished and replaced by a two storey steel frame structure on concrete floor. The building is currently occupied by Hauraki District Council, Hauraki District Civil Defence EOC and Hauraki District Library. The detail assessment was carried out only for the original concrete frame structure which is built in The site inspection did not reveal any distress in walls, columns, etc. indicating that the building is founded on satisfactory bearing soils. A significant amount of lateral resistance in the longitudinal and transverse directions is provided by the reinforced concrete frame. Modelling and Analysis of Building The structural quantitative seismic assessment has been based on the NZSEE 2006 guidelines for the assessment and improvement of the structural performance of buildings in earthquakes and the draft document Detailed Engineering Evaluation Procedure issued in July The technical assessment assumptions are included in Appendix A. The structural quantitative analysis is based on the three dimensional computer modelling of the building. The model was developed using the geometry obtained from the available As-Build drawings and site measurement undertaken on 3 rd of December The structural analysis program used to model the structure is StaadPro. The 3D model captures the effects on the overall response caused by the building s stiffness on lateral resistances in both longitudinal and transverse directions. The assessment is based on the equivalent static method as outlined in NZS 1170:2002 Part 5, and the appropriate seismic loading has been applied accordingly. The following parameters from AS/NZS 1170 were used to calculate the seismic actions of the building: Building importance level 2 Design Life 50 years Annual probability of exceedance for ultimate limit state 1 in 500 years Annual probability of exceedance for serviceability limit state 1 in 25 years Local seismic hazard factor 0.16 Based on our engineering judgement, the building ductility factor of 1.25 at ultimate limit state (ULS) and 1 at serviceability limit state (SLS) was assumed for the analysis. The External Factors effecting the Risk of the building for Earthquake Action Seismic restraint on ceilings, services, plant, partitions and furniture have not been investigated. It would be prudent to conduct a detailed study on these items. The Christchurch earthquakes have shown a number of in-ceiling services, and the ceiling grid itself, can fall during shaking. We have concentrated our detailed assessment on the primary resisting elements contained within the structure and identify other elements that may need securing and excluded in our assessment are: (a) Concrete staircases (b) Any attachments such as water tanks, boilers etc. Page 3 of 8

4 The Christchurch earthquake events have shown that many staircases have failed while the building itself stands due to improper detailing of landing to stair connections. The staircases are the main element of escape from a building for an event such as an earthquake, fire, etc. Considering the age of the building the stair case landing construction could be inappropriate for earthquake action. Therefore it is recommended to modify stairs to increase their performance by introducing sliding joints. Building Seismic Rating. The seismic analysis was carried out for both ultimate limit state (ULS) and serviceability limit state (SLS), however the capacity check based on force-based method was carried out for the ultimate limit state only. The overall seismic rating of the building is based on the building s weakest elements. The detailed assessment s computation for ultimate limit state is summarised below on table 1: Longitudinal- NBS % Transversal- NBS % Building rating 14% 10% Table 1: Summary of seismic performance building, in each of the principal loading directions (at importance level 2) The critical structural elements mode of failure is summarized as follow: Structural Component Deficiency Classification Reinforced Concrete Column Flexure/Shear Failure Critical Reinforced Concrete Slab Flexure/Shear Failure Non-Critical Beam Column Joint Shear Failure of Joint Bond Failure of Longitudinal Critical Non-Critical Bars Stair Case Shear failure of landings Critical Concrete Pad Foundation Foundation Rocking Non-Critical Table 2 Summary of Classification Component Deficiencies Issues and options Dangerous, Earthquake-Prone and Insanitary Buildings Policy 2010 This earthquake-prone building policy under the Building Act 2004 embodies a passive approach that reflects Council s determination to reduce earthquake risk over time in a way that reflects the low risk level determined from a report by the Institute of Geological & Nuclear Sciences and is acceptable in social and economic terms to its ratepayers. While theoretically all buildings under this policy are required to be assessed, buildings that can be shown to have been design or strengthened to NZS 4203:1992 and subsequent codes will not be required to be assessed unless they have an identifiable critical structural weakness. Defining the risk is therefore very important in defending a passive approach. The Hauraki District Council commissioned the Institute of Geological & Nuclear Sciences (GNS) in late 2005 to develop an overview of the earthquake risk for the Hauraki District, including damage to buildings and human casualties. The December 2005 report; summarised below, concludes that Page 4 of 8

5 the Hauraki District is an area of low seismicity. There has been no updated research showing anything since 2005 to alter that conclusion. Earthquake Risk - Hauraki District Earthquake risks to the buildings and people of the Hauraki District were estimated by subjecting the District to a one million year, synthetic catalogue of earthquakes that represents the seismicity of New Zealand. For each of the approximately five million model earthquakes the ground shaking throughout the Hauraki District was estimated, taking into account local ground conditions. Damage to buildings, collapse, and casualty levels were then estimated using models based on historical data from New Zealand and abroad. Casualty estimates were made twice for each earthquake, once for daytime conditions and once for night-time. A scenario earthquake on the Kerepehi Fault was also considered. The buildings of Hauraki District were divided into two broad classes, residential and workplace, with respective estimated replacement values of $1,400 million and $680 million The population was estimated to be 15,500 during a typical working day and 16,000 at night. Hauraki buildings range in type and age reflecting the building codes relevant at the time of construction. Construction methods comprise wood, unreinforced masonry, brick buildings and modern double storey steel and concrete buildings. There are no multi level buildings (greater than two stories) within the District, reflecting its typically rural nature and low density population. For modelling purposes the buildings were further subdivided into four fragility classes; unreinforced masonry, timber framed, pre-1980 reinforced concrete, and 1980 onwards reinforced concrete. The probabilities of experiencing various levels of losses and casualties (dead plus seriously and moderately injured) due to all earthquakes that could affect the district are summarized in the table on page six. For a scenario earthquake on the Kerepehi Fault it was estimated that the mean dollar loss would be $60 million and the probable number of casualties zero to two. Policy on Alterations to High Risk buildings Where an application for a building consent is received for an upgrade or alteration of a building that is deemed to be high risk, the applicant will be required to have an evaluation carried out by a suitably qualified person to determine if the building is likely to be earthquake-prone. If it is found that the building is earthquake-prone an engineering consultant suitably experienced in this aspect of structural design will need to provide as part of the application, a detailed assessment of how the building will be upgraded in accordance with this policy. Once an application activates the policy, Council will require any necessary upgrade, even if a building owner chooses not to undertake the building work set out in the application. High Risk buildings are defined in the definitions as below. Policy Definitions High Risk buildings are defined as buildings classified as level 3 or 4 buildings in Table 3.2 of AS/NZS :2002. Council has defined its risk categories based on AS/NZS :2002. Those risk levels being:- 1) Buildings with special post-disaster functions of importance level 4 (e.g. hospitals). 2) Buildings that contain people in crowds of importance level 3 (e.g. theatres, auditoriums). 3) Heritage buildings. (HI or II) 4) All others - Importance level 1 and two. Page 5 of 8

6 Earthquake-prone building classified as level 3 or 4 buildings in terms of AS/NZS :2002 will be required to be strengthened to at least 67% of the relevant building code and thereafter as near as is reasonably practical to that of a new building. Policy on Alterations to existing Buildings Where an application for a building consent is received for an alteration to a building to a value greater than $100, inclusive of GST the applicant will be required to have an evaluation carried out by a suitably qualified person to determine if the building is likely to be earthquakeprone. If it is found that the building is earthquake-prone an engineering consultant suitably experienced in this aspect of structural design will need to provide as part of the application, a detailed assessment of how the building will be upgraded in accordance with this policy. Once an application activates the policy, Council will require any necessary upgrade, even if a building owner chooses not to undertake the building work set out in the application. A Certificate for Public Use (where applicable) and a Code Compliance Certificate will not be issued until the building achieves the level required. The value of all the applications for a building consent to make an upgrade or alteration to the entire building, in a two (2) year period, will be accrued when determining if the value is greater than $100, Budget Implications GDC Consultants Limited were engaged to provided Rough Order Costs (ROC) for the 2 stages of, 1. Design and Tender stage 2. Seismic Upgrade The estimates for these stages are as follows, Stage One-Design and Tender Documents, Consenting fees ROC Range $37,000-$49,000 Two-Establishment $4,500- $6,500 Foundations $14,500-$20,500 Load Bearing Walls using Shot Crete $81,000-$117,000 Wall Bracings $13,500-$19,500 Roof Bracing $18,000-$26,000 Fire Engineer Compliance $9,000- $13,000 Reinstatement and Decoration $15,500- $22,500 Engineer Fees $10,500- $15,500 Project Management $9,000- $13,000 Contingency sum $13,500 - $19,500 Stage Two Total $189,000-$273,000 Total Project ROC $226,000 - $322,000 Note: These additional projects identified for Hauraki House are currently unfunded and if approved would be funded from the Corporative Property Budget. Project ROC Range New Roof over the centre section of Hauraki House $160,000 Repainting of entire outside of Hauraki House $60,000 Replacement of sections of internal Carpet $40,000 per year for 2 years Note: These projects identified for Hauraki House were identified in the LTP and would be funded from the Corporative Property Budget Page 6 of 8

7 The reroofing of the centre of Hauraki House and the outside repainting has been put on hold until a decision is made whether or not to undertake the seismic upgrade of the building. The logical sequence for these named projects, if decided to proceed would be, 1. Design/tender documentation stage of the seismic upgrade, reroofing, repainting 2. Consenting stage 3. Tendering stage 4. Reroofing 5. Seismic Upgrade 6. External repaint 7. Recarpeting The reroofing, external repaint and recarpeting is already allowed for in existing budgets. The seismic upgrade is an unbudgeted item and would be funded from Council s Property Budget. Decision Making Process Due to Council s Dangerous, Earthquake-Prone and Insanitary Buildings Policy 2010, Council will be required to earthquake strengthen Hauraki House to a minimum of 67% of the relevant building code when it applies for a building consent to undertake the reroofing of the central section of Hauraki house. Staff are recommending to Council that outlined capital works be undertaken over 2014/15 and 2015/16 to spread the cost, minimise disruption to day to day activities and have the ability to still manage existing property project commitments. The following stages are recommended, 2014/15 1. Design/tender documentation stage of the seismic upgrade, reroofing, repainting 2. Consenting stage 3. Tendering stage 2015/16 1. Reroofing 2. Seismic Upgrade 3. External repaint 2015/ /17 1. Recarpeting Conclusion Staff are recommending to Council that outlined capital works be undertaken over 2014/15 and 2015/16 with projects combined to gain cost and disruption efficiencies, and are staged as, 2014/15 1. Design/tender documentation stage of the seismic upgrade, reroofing, repainting 2. Consenting stage 3. Tendering stage 2015/16 1. Reroofing 2. Seismic Upgrade 3. External repaint Page 7 of 8

8 2015/ /17 Recarpeting The reroofing, external repaint and recarpeting is already allowed for in existing budgets. The seismic upgrade is an unbudgeted item and would be funded from Council s Property Budget. Steve Fabish Community Services Manager Page 8 of 8