Gisborne District Council

Transcription

1 Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment September 2011

2 P a g e 1 PROJECT: Gisborne District Council GDC Two Storey Office Block(1954) Specific Seismic Assessment PROJECT ADDRESS: Gisborne District Council 15 Fitzherbert Street Gisborne 4040 New Zealand PREPARED FOR: Gisborne District Council PREPARED BY: Kelvin Landon BE (Hons) MIPENZ CPEng IntPE DOCUMENT HISTORY: Date: Author: Issue/Revision: Notes: 02 September 2011 KPL A Issued for comment 07 September 2011 KPL B Changed building name Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

3 P a g e 2 Contents Disclaimer... 2 Executive Summary... 3 Introduction... 4 Building Location... 5 Building Description... 5 Criticality... 6 Base Information... 6 Initial Evaluation Procedure... 7 Detailed Structural Analysis Method... 7 Detailed Structural Analysis Results... 8 Comment... 8 Conclusion... 9 Appendix A: Drawings Appendix B: Calculations Disclaimer This report has been prepared solely for the use of Gisborne District Council. The findings are not intended for use by other parties, and may not contain sufficient information for the purposes of other parties or for other uses. Our professional services are performed using a degree of care and skill normally exercised, under similar circumstances, by reputable consultants practicing in this field at this time. No other warranty, expressed or implied, is made as to the professional advice presented in this report. Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

4 P a g e 3 Executive Summary The Gisborne District Council Two Storey Office Block(1954) has previously been assessed as an earthquake-prone building which meets only 19% of the New Building Standard (NBS). This study independently evaluated the NBS compliance with an intensive structural analysis and agrees with the above value of 19% NBS. The current Gisborne District Council policy on earthquake-prone buildings requires that they be strengthened to at least 67% NBS. However, the building in question houses the Civil Defence Emergency Management Group for the Gisborne region and is therefore required to be more resilient to earthquakes than a typical building. It is recommended that the building in question is demolished and replaced with a new structure rather than attempt to strengthen it. Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

5 P a g e 4 Introduction Over the years design standards for buildings in New Zealand have changed. The impact of this is that many older buildings, like the Gisborne District Council (GDC) Two Storey Office Block(1954), do not meet current design standards; this structure has been previously been identified as an earthquake prone building under the terms of the Building Act A building is earthquake-prone under the Building Act 2004 if, having regard to its condition and to the ground on which it is built, and because of its construction, the building will in a moderate earthquake be likely to collapse, causing injury or death to persons within that, or any other property, or damaging other property. The latest requirement is for buildings that do not meet more than one third of current code standards to be labelled earthquake prone. This then means owners have to take some action in accordance with rules and timetables. Although the building code requires that they are bought up to at least one third of code the actual level is set by local authorities and Gisborne District Council has a minimum requirement of two-thirds of code. This is also in line with the recommendations of the New Zealand Society for Earthquake Engineering, supported by IPENZ, which has recommended the minimum requirement for any strengthening undertaken be set higher at 67 per cent. However, critical buildings in New Zealand, those that have important contents or that are expected to function after a big earthquake, such as hospitals and civil defence centres, have higher standards applied. The GDC Two Storey Office Block(1954) current code compliance has previously been assessed using the Initial Evaluation Procedure(IEP)method. This is an approximate method that assumes design techniques and materials typical of the building construction era to estimate a compliance value. The purpose of this study is to establish the level of current code compliance (%NBS) by structural analysis using actual design and construction data. This calculates a %NBS value that can relied on to a higher level of confidence than the IEP method. Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

6 P a g e 5 Building Location The Gisborne District Council (GDC) Two Storey Office Block(1954) forms part of the Gisborne GDC civic buildings located on the corner of Fitzherbert Street and Ormond Road in Gisborne. Only the original structure, in the north eastern corner of the site as highlighted in Figure 1 below, has been considered as part of this assessment. Figure 1: Building location Building Description The structure in question consists of two distinct buildings, designed in November 1954 and constructed soon after, taking the form of a 20m x 20m two storey reinforced concrete frame building with an adjacent single level building of approximately 7m x 12m. The single level portion of the building was originally used as the Council chambers. Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

7 P a g e 6 Criticality The seismic performance of this building is required to be higher than a typical office building as it is houses the local Civil Defence Emergency Management Group and is considered a post disaster critical building. This increases the design demand on the building by 40%. Base Information A limited set of drawing for this structure, see Appendix A, was obtained from the GDC archives. No specifications were available and the drawing set does not appear to be complete. The drawings are limited to the primary structure only; much of the secondary structure and architectural detail is not documented and therefore many assumptions and approximations have been made to complete the analysis. This lack of information is unlikely to have affected the outcome of this study as the estimation which has been made is considered typical and the results conclusive. Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

8 P a g e 7 Initial Evaluation Procedure This building was originally analysed by Strata Group Consulting Engineering using the New Zealand Society for Earthquake Engineering (NZSEE) Initial Evaluation Procedure (IEP). This is an approximate method requiring a degree of engineering judgement designed to inexpensively assess a building s seismic capacity as percentage of New Building Standard. The IEP assessment as carried out by Strata Group appears to be carried out correctly; the author of this report agrees with the results of the initial IEP assessment and has therefore elected to not repeat this analysis. Typically the expected seismic performance of a building is estimated and described, using the New Zealand Society Earthquake Engineers (NZSEE) method, as a percentage of New Building Standard (NBS). NBS is simply described as the requirements of the current New Zealand Building Code (NZBC). The expected structural performance of building is therefore described as %NBS where 100% NBS indicates full compliance with current NZBC. The IEP for the GDC Two Storey Office Block(1954) gave a result of 19% NBS. Detailed Structural Analysis Method Due to both approximate nature of the IEP process and the ramifications of a low value it was elected to carry out a more detailed assessment using actual design documentation, data, material and a mathematically based structural analysis. The structural analysis was carried out in two phases. The first analysis was a simple 2D analysis looking at a central frame. The majority of this phase was undertaken by hand calculations and is therefore simplistic in nature. The results were inconclusive, therefore it was considered necessary to complete a more detailed 3D computer analysis carried out using SAP 2000 structural analysis software. The 3D structural analysis utilizes a computer model of all primary structural members including shear walls, beams and columns. Each structural member has loads due to gravity applied. Seismic ground movement develops forces with the structural member and accordingly an equivalent loading is also applied within the model to reflect the effect on the structure. Building geometry has a significant effect on the loads induced in individual members. Any non-symmetry within the structure typically causes building rotation and an associated increase in the loadings on the perimeter structural frames. Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

9 P a g e 8 Detailed Structural Analysis Results Use of the 3D structural analysis enabled calculation of the loads applied to individual members. The existing central frames provide capacity of approximately 25% of the capacity required by New Building Standard. The perimeter frame members attract higher loads and being of a similar member design to the central frame achieve lower compliance. The higher stressed members provide only 15% of the design capacity required by NBS. Therefore the 19% NBS (as an approximate average of 15 and 25) obtained from the IEP is considered to be a fair estimate of the compliance of this building with current code requirements. The single level portion of the structure appeared to perform better, however its exact performance is difficult to estimate due to limited information available on the drawings. The single level building achieves 67% NBS under longitudinal earthquake loadings. Resisting transverse seismic loading the eastern end is supported by a reinforced concrete portal frame providing 67%NBS capacity. It is assumed the western end of the building is supported by the existing main two level structure. Strengthening of the western end is therefore required. The details of the roof structure need further investigation but with strengthening of the western end it appears viable to achieve 67% NBS. Comment Overall the structure appears lightly reinforced and well below current requirements; the initial 19% NBS is considered to be in the appropriate order of the actual %NBS capacity of the building. Strengthening of this building is expected to be a technically challenging task considering both design and construction. Without calculating a strengthening regime, it is expected that most members would need strengthening. Due to its age and existing design it is expected that due to the level of strengthening required it would be uneconomic to upgrade this building to either 67 or 100% NBS. Further to this, as the building appears to have a limited historic value and is not listed on any registers, rebuilding must be considered as the preferable economic option. Strengthening of this building to current code requirements is possible however the cost would be significant and the outcome would impact on the space, affecting the use of the building. It would also require the building to be vacant for an extend period of time whilst the work is undertaken. If strengthening of this building was to proceed the external masonry cladding would also need to be considered as this may fall from the building in a seismic event. Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

10 P a g e 9 Conclusion The GDC chambers and offices achieve 19%NBS. This is not considered an acceptable level of performance in line with Gisborne district council. The building will require strengthening work if it is continued to be used in the long term by Gisborne District Council. Replacement of the building must be considered as an alternative to strengthening. The single level structure (original chambers) is a robust structure and appears it could viably be strengthened to 67% NBS. Further investigation including destructive investigation would be required to confirm existing hidden details and the viability of strengthening. Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

11 P a g e 10 Appendix A: Drawings Gisborne District Council GDC Two Storey Office Block(1954) Seismic Assessment

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25 P a g e 24 Appendix B: Calculations Gisborne District Council GDC Office and Chambers Seismic Assessment

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