Detailed Seismic Assessment Report. Islington Substation August 2011

Transcription

1 Detailed Seismic Assessment Report August 2011

2 Prepared for: Transpower New Zealand Prepared by: Opus International Consultants Reference: 5-C Date: 8 th August 2011 Status: Draft 1 Report prepared by: Reviewed by: Approved for Release: Jack Shepherd Robert Davey Mark Ryburn August 2011 i

3 Contents Executive Summary Introduction Purpose of the report Scope of work Sources of information Minimum Seismic Ratings for Existing Structures Site Conditions Performance in the Darfield and Christchurch Earthquakes Oil Filter/Compressor House General Description Initial Seismic Assessment Results Material Strengths Response Mechanisms and Critical Structural Weaknesses Method of Analysis Analysis Results Proposed Improvements Cost Estimates Condenser Building General Description Initial Seismic Assessment Results Material Strengths Response Mechanisms and Critical Structural Weaknesses Method of Analysis Analysis Results Proposed Improvements Cost Estimates Control Block General Description Initial Seismic Assessment Results Material Strengths Response Mechanisms and Critical Structural Weaknesses Method of Analysis Analysis Results Proposed Improvements Rough order of costs August 2011 ii

4 7 Other Structural Issues Identified on Site Conclusions References Appendix 1: ISA Report Appendix 2: Sketches August 2011 iii

5 Executive Summary Building Importance Level Seismic Policy Minimum Standard Detailed Assessment Standard Improvements Required Oil Filter/ Compressor House Condenser Building 4 75% NBS 100%NBS Nil 4 75% NBS 38%NBS Yes Control Block 4 75% NBS 28%NBS Yes A detailed assessment has been made of the earthquake performance of the Oil Filter/Compression House, Condenser Building and Control Block at Islington substation to establish whether or not they meet Transpower s minimum requirements for its essential buildings. This followed an initial screening assessment that indicated they potentially did not comply and therefore required further investigation. Transpower s minimum standard is 75% of the current Building Code requirement for a new building on the site (i.e. 75%NBS). The assessed ratings of the Condenser Building and Control Block are <75%NBS. Improvements are therefore required to these buildings to meet the minimum requirements. The Oil Filter/Compressor House assessed rating is 100%NBS therefore no improvements are necessary to meet the minimum requirements. The costs of proposed improvements are estimated to be $1,407,500. August

6 1 Introduction This report covers the detailed seismic assessment of the Oil Filter/Compressor House, Condenser Building and Control Block at Islington substation. 1.1 Purpose of the report The purpose of this report is to: Provide a structural description of the structures under consideration; Explain the methodology used for assessing the seismic performance of the structures; Assess the seismic performance of the structures and equipment contained within them; Identify areas where potential damage is most likely to occur based on the strength hierarchy established within the structural systems; Present the results obtained from the structural review; Make recommendations on required strengthening measures; and Provide a rough order of costs for any proposed strengthening schemes 1.2 Scope of work We carried out the following activities in order to complete this report: Site inspection of the Oil Filter/Compressor House, Condenser Building and Control Block; Review of the available documentation; Initial seismic evaluation of the structures to check preliminary risk classifications; Detailed seismic assessment of the high risk structures; and Concept design and rough order of cost estimate of improvement work if necessary. 1.3 Sources of information The seismic assessment of the Oil Filter/Compressor House, Condenser Building and Control Block structures are based on information obtained from the existing structural drawings provided by Transpower. A site inspection was carried out in May 2011 to identify the structures, check the currency of the drawings and identify potential site hazards. Other building data such as the classification of the buildings by importance level in accordance with NZS :2002 Standards [1] was obtained from Opus report dated September 2008 [2]. This classification was decided upon by Transpower following the Seismic Policy Guidelines [2]. 1.4 Minimum Seismic Ratings for Existing Structures The recommended minimum seismic requirements for Transpower substation structures are stated in the draft seismic policy prepared in September 2008 [2]. August

7 This draft policy takes into account the requirements for earthquake prone buildings prescribed in the Building Act 2004 and the requirements for lifeline facilities prescribed in the Civil Defence and Emergency Management Act The policy also incorporates the recommendations of the New Zealand Society for Earthquake Engineering (NZSEE) [3] to mitigate the risk to buildings that are essential to the functioning of the National Grid. Table 2 summarises the policy requirements for existing buildings. The minimum seismic standards of Territorial Authorities are deemed acceptable for less critical buildings (importance level 1, 2, and 3). These minimum requirements correspond to the Building Act requirements. For essential buildings (importance level 4), a higher seismic standard has been selected to ensure resilience of critical assets and meet Emergency Management Act obligations and NZSEE recommendations. Table 2. Proposed seismic policy requirements for existing Transpower substation buildings Importance level 4 Limit state Annual probability of exceedance %NBS Ultimate 1/1000 (74%) Serviceability 1/250 (75%) Driving documentation Civil Defence and Emergency Management Act 2002 NZSEE Recommendations 1, 2 and 3 Ultimate - 34% Territorial Authority minimum requirements Building Act 2004 Note that %NBS = the percentage ratio of the subject building seismic strength to the New Zealand Building Code strength requirement for a new building at the site. A building with 100%NBS for example meets current requirements for new buildings. August

8 2 Site Conditions No geotechnical data were available for the site. According to geological maps the site soils are likely to be postglacial alluvium. The soils are not expected to be prone to liquefaction or lateral spreading, however this should be confirmed by a geological investigation. 3 Performance in the Darfield and Christchurch Earthquakes The performance of the buildings in recent large earthquakes could provide useful imperical indications of their earthquake resistances depending upon intensity of load relative to the assessment criteria. The epicentres of the September 2010 magnitude 7.1 Darfield and February 2011 magnitude 6.3 Christchurch Earthquakes were located 27km West and 16 km South-East respectively from the Islington substation (see Figure 1). Strong motion recordings of the ground motions from these earthquake was made by the Geonet network at a station (TPLC) that is located approximately 3.4 km from the substation. While some distance apart, the ground motions recorded at TPLC station are likely to be similar to those experience at the substation site (actually higher for Darfield and lower for Christchurch). Figure 1: Locations of the September 2010 Darfield and February 2011 Christchurch Earthquake Epicentres Relative to and Geonet Station TPLC August

9 Acceleration Cd (g) Acceleration Cd (g) Seismic Review of Essential Buildings The earthquake accelerations produced by these ground motions are shown as response spectra in Figures 2 and 3. The Transpower minimum accelerations (75%NBS) used for the seismic assessment at the Islington site are also shown for comparison Darfield N-S Darfield E-W Transpower 75%NBS Period T (secs) Figure 2: Comparison of Darfield Earthquake Response Spectra (5% Damped) with Transpower Minimum Requirements at Islington Christchurch N-S Christchurch E-W Transpower 75%NBS Period T (secs) Figure 3: Comparison of Christchurch Earthquake Response Spectra (5% Damped) with Transpower Minimum Requirements at Islington August

10 In both instances the earthquake forces imposed on the structures at Islington substation were significantly less than forces used for this detailed assessment. Their good performances in these earthquakes cannot therefore be used as an indication that the buildings meet Transpower minimum seismic rating requirements. August

11 4 Oil Filter/Compressor House 4.1 General Description The Islington Oil Filter/Compressor House is a single storey, concrete walled building with a concrete roof. The approximate dimensions of the building on plan are 5.5m x 8.1m and 3.6m high. It was designed in 1953 by Ministry of Works New Zealand. The building was assessed to be in good condition in the May 2011 inspection. Drawings and photographs of the structure are included in the ISAR in Appendix 1. It was classified as an essential, importance level 4 building by Transpower. 4.2 Initial Seismic Assessment Results An initial screening seismic assessment was made based on a simple comparison of seismic design standards at time of construction with current Building Code new building standard [Appendix 1] which indicated that the building was potentially high risk. 4.3 Material Strengths The following material strengths have been adopted based on NZSEE [3] guidelines: Concrete compressive strength: 30MPa Reinforcement tensile strength: 270MPa 4.4 Response Mechanisms and Critical Structural Weaknesses Earthquake forces are resisted in both planes by reinforced concrete walls. The critical elements were identified as a short column between windows at high level. 4.5 Method of Analysis The detailed assessment followed the requirements of the NZSEE guidelines [3], using the force-based procedures. ULS forces were based on µ = 1.25 for shear. An equivalent static method of analysis was used in accordance with requirements of NZS [4]. 4.6 Analysis Results The analysis results are summarised in Table 3. Table 3: Analysis Results Building Element Action Limit State Capacity Side walls Shear SLS2 >100%NBS August

12 Side walls Shear ULS >100%NBS The capacity of the longitudinal walls is controlled by the shear strength at window opening level. The walls at this level act as short columns and therefore will be governed by shear mode of failure. These results indicate that the walls of the Oil Filter/Compression House meet Transpowers minimum requirements for earthquake collapse avoidance (ULS) and damage avoidance (SLS2). 4.7 Proposed Improvements Not required. 4.8 Cost Estimates Not applicable. August

13 5 Condenser Building 5.1 General Description The Islington Condenser Building is a large single storey, concrete framed building with reinforced concrete walls and a mezzanine floor which is located across 2No. bays. The plan dimensions of the building are approximately 18.5 x 61.1m, with a maximum height of 16.95m. In the longitudinal direction, reinforced concrete walls with significant structural openings provide stability whilst in the transverse direction this is catered for by reinforced concrete cantilever columns. The roof is supported by pinned base steel portal frames which sit atop the concrete columns and is constructed of lightweight metal decking overlaying steel and timber purlins. The concrete columns also support deep steel crane girders which spans the full length of the building supporting a large gantry crane. The foundations are reinforced pad foundations with ground beams tying them together in the transverse direction and supporting the walls in the longitudinal direction. The condensers are located at the level of the mezzanine floors and have their own foundations. It was designed in 1952 by Ministry of Works New Zealand. The building was assessed as in good condition in the May 2011 inspection. There was minimal earthquake damage at the link between the Condenser Building and the Control Room. Drawings and photographs of the structure are included in the ISAR in Appendix 1. It was classified as an essential, importance level 4 building by Transpower. 5.2 Initial Seismic Assessment Results An initial screening seismic assessment was made based on a simple comparison of seismic design standards at time of construction with current Building Code new building standard [Appendix 1] which indicated that the building was potentially high risk. 5.3 Material Strengths The following material strengths have been adopted based on NZSEE [3] guidelines: Concrete compressive strength: 30MPa Reinforcement tensile strength: 270MPa 5.4 Response Mechanisms and Critical Structural Weaknesses Earthquake forces in the longitudinal direction are resisted by the external reinforced concrete walls. These walls have many openings which are spaced at regular horizontal and vertical centres therefore forming frames with deep and short beams and columns. These components are vulnerable to shear failure. Forces in the transverse direction are resisted by the reinforced concrete cantilever columns and foundation beams. These components appear to be relatively well detailed for ductility for a 1950 s structure. It is expected then that plastic hinges could develop in the base of the column. In this case the anchorages of the beam and column reinforcing into August

14 the joints are relatively good, so that the capacity of the structure will be based on the beam and column rather than joint strengths. The major potential weaknesses are poor shear strength and poor confinement of the concrete and longitudinal reinforcement as a result of widely spaced ties (305mm centres). The foundations of the condenser units are stiff and strong enough to cater for the seismic forces which will be transferred from the mezzanine floor. For this reason the analysis of the transverse frames and longitudinal walls exclude mass from the mezzanine floor. 5.5 Method of Analysis The detailed assessment followed the requirements of the NZSEE guidelines [3], using the force-based procedures for the longitudinal direction and displacement-based procedures for the transverse frames. The criteria shown in Table 2 were used to determine the earthquake actions on the structure. Table 2: Criteria Used to Determine the Magnitude of the EQ Actions on the Building Components Building Element Action Limit State Criteria Transverse frames Flexure SLS2 ULS Ductility factor µ = 2.0 Concrete compressive strain at peak displacement response Transverse frames Shear SLS2 & ULS Shear force at member flexural capacities Side walls Shear SLS2 & ULS Ductility factor µ = 1.25 The analysis is based on the assumption that the crane will be parked at the end wall position when not in use. 5.6 Analysis Results The analysis results are summarised in Table 3. Table 3: Analysis Results Building Element Action Limit State Capacity Transverse frames Flexure SLS2 72%NBS Transverse frames Flexure ULS 89%NBS Transverse frames Shear ULS & SLS2 >100%NBS August

15 Side walls Shear SLS2 79%NBS Side walls Shear ULS 38%NBS The capacity of the longitudinal walls is controlled by the shear strength at window opening level. The transverse frames will respond in a column flexural mode as the shear strength of the beams and columns is sufficient to prevent shear failure. The displacement response of the frames is limited by concrete spalling strain. These results indicate that the longitudinal walls of the Condenser Building do not meet Transpowers minimum requirements for earthquake collapse avoidance (ULS). 5.7 Proposed Improvements The proposed improvement method is to infill the lower three levels of window openings with reinforced in-situ concrete. The infills must be dowelled into the existing structure. See Appendix 2 for a drawing indicating which windows are to be filled in. An alternative method would be to apply fibre reinforced epoxy coatings to the walls. Initial indications are that this would be more costly than the window infill option. However, this could be considered in more detail during design development stage. 5.8 Cost Estimates A rough order of costs has been prepared for the improvement scheme outlined above. Costs are exclusive of GST and construction management costs, and include the following: Structural works relating to the seismic upgrade scheme Site accessibility provision: o Main centre 0% o Provincial centre 15% o Remote 50% Enabling works provision: o Minor (moving furniture, internal walls etc etc) 5% o Medium (temporary screening to equipment, some coordination with Transpower operations, medium safety risk) 25% o Major (temporary removal or shutdown, relocation of equipment, building open to weather, works within minimum approach distance, likely to require on site Transpower representative, major safety risk involved) 100% or as assessed. A design contingency sum of 25% to allow for uncertainties associated with the conceptual status of the design An allowance of 10% for architectural finishing An allowance of 25% for preliminary and general costs, design costs, construction observation and consent fees. The current estimates are approximate, allowing for the most adverse conditions and design uncertainties. The Table 3 below summarises the results. August

16 Table 3: Summary of Costs for Improvement Work Item Cost Infill of panels; $7300 per window x 82 windows $599,000 Enabling works (minor) $30,000 Net costs $629,000 Design contingency (25%) $157,500 Architectural contingency (10%) $63,000 Preliminary and general, design, consent fees (30%) $189,000 Rounded total $1,038,500 August

17 6 Control Block 6.1 General Description The Islington Control Block is a two storey, reinforced concrete frame building with reinforced concrete walls. The first floor consists of a precast concrete slab supported by steel beams. Plan dimensions are approximately 23 x 34m and maximum height is 7.2m. The foundations consist of RC ground beams and pad footings. A link building is constructed between the Control Block and the Condenser Building. This structure is constructed 25mm away from the Condenser Building. It was designed in 1952 by The Ministry of Works. Its condition in the May 2010 inspection was assessed to be good however the link building between the Control Block and the Condenser Building showed signs of damage. Drawings and photographs of the structure are included in the ISAR in Appendix 1. It was classified as an essential, importance level 4, building by Transpower. 6.2 Initial Seismic Assessment Results An initial screening seismic assessment was made based on a simple comparison of seismic design standards at time of construction with current Building Code new building standard [Appendix 1] which indicated that the building was potentially high risk. 6.3 Material Strengths The following material strengths have been adopted: Concrete compressive strength: 30MPa Reinforcement tensile strength: 270MPa 6.4 Response Mechanisms and Critical Structural Weaknesses Earthquake forces in both the transverse and longitudinal direction are resisted by the external and internal reinforced concrete walls. These walls generally have many openings which reduces their strength. The link building is potentially vulnerable to pounding forces due to the displacement of the transverse frames of the Condenser Building. 6.5 Method of Analysis The detailed assessment followed the requirements of the NZSEE guidelines [3], using the force-based procedures. ULS forces were based on µ = 1.25 for shear. An equivalent static method of analysis was used in accordance with requirements of NZS [4]. 6.6 Analysis Results The analysis results are summarised in Table 2. August

18 Table 2: Analysis Results Building Element Action Limit State Capacity Longitudinal side walls Shear ULS 56%NBS Longitudinal side walls Shear SLS2 >100%NBS Transverse side walls Shear ULS 28%NBS Transverse side walls Shear SLS2 59%NBS The longitudinal walls do not meet the 75%NBS requirement largely due to the fact that there are many openings between ground floor and first floor along the north elevation. The shear resistance in the longitudinal plane is therefore reliant on the south elevation wall between ground floor and first. There is adequate strength provided in the longitudinal direction at first floor level and above due to the reinforced concrete internal wall. In the transverse direction there are many openings at both floor levels to the east and west elevations of the building. The shear resisting components are not adequate to cater for the seismic forces of 75%NBS. 6.7 Proposed Improvements The recommended improvement method is to infill the ground floor window openings of the North Wall elevation (9No. in total), the West Wall elevation (2No. in total) and the East wall elevation (5No. in total). Additionally at 1 st floor level, the West Wall elevation windows (4No. in total) and the East wall elevation windows (5No. in total) are proposed to be infilled with reinforced in-situ concrete. The infills must be dowelled into the existing structure. See Appendix 2 for a drawing indicating which windows are to be filled in. 6.8 Rough order of costs A rough order of costs has been prepared for the improvement scheme outlined above. Costs are exclusive of GST and construction management costs, and include the following: Structural works relating to the seismic upgrade scheme Site accessibility provision: o Main centre 0% o Provincial centre 15% o Remote 50% Enabling works provision: o Minor (moving furniture, internal walls etc etc) 5% o Medium (temporary screening to equipment, some coordination with Transpower operations, medium safety risk) 25% o Major (temporary removal or shutdown, relocation of equipment, building open to weather, works within minimum approach distance, likely to require on site Transpower representative, major safety risk involved) 100% or as assessed. August

19 A design contingency sum of 25% to allow for uncertainties associated with the conceptual status of the design An allowance of 10% for architectural finishing An allowance of 25% for preliminary and general costs, design costs, construction observation and consent fees. The current estimates are approximate, allowing for the most adverse conditions and design uncertainties. The Table 3 below summarises the results. Table 3: Summary of costs for strengthening schemes Item Cost Infill of panels; $6300 per window x 25 windows $201,500 Enabling works (medium) $22,000 Site accessibility (main centre) $0 Net costs $223,500 Design contingency (25%) $56,000 Architectural contingency (10%) $22,500 Preliminary and general, design, consent fees (30%) $67,000 Rounded total $369,000 The figures above are those for the strengthening scheme we believe to be most feasible. 7 Other Structural Issues Identified on Site Nil 8 Conclusions The results of the detailed seismic assessment of the Control Building are summarised in Table 2. Building Importance Level Seismic Policy Minimum Standard Assessed Standard Improvements Required Oil Filter/ Compressor 4 75% NBS 100%NBS August

20 House Condenser Building 4 75% NBS 38%NBS Yes Nil Control Block 4 75% NBS 28%NBS Yes The Condenser Building and the Control Block were assessed to be below the current NZ Building Code requirements for a new importance level 4 building on the site. Remedial works as detailed in Section 4 of this report to improve the building s earthquake resistance are required. As noted in Section 5 the Oil Filter/Compression House is at 100%NBS and therefore requires no improvements. August

21 9 References [1] AS/NZS , Structural Design Actions, General Principles, SNZ, [2] Structural Review Programme for Transpower Substation Buildings, Opus, [3] Assessment and Improvement of the Structural Performance of Buildings in Earthquakes, NZSEE, [4] NZS , Structural Design Actions, Earthquake Actions, SNZ, August

22 Appendix 1: ISA Report August

23 Appendix 2: Sketches August