Gisborne District Council · 2016. 1. 13. · The single level building achieves 67% NBS under longitudinal earthquake loadings. Resisting transverse seismic loading the eastern end

Gisborne District Council GDC Two Storey Office Block(1954)

Seismic Assessment

September 2011

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Seismic Assessment

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

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Seismic Assessment

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 ........................................................................................................................... 10

Appendix B: Calculations....................................................................................................................... 24

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.

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Seismic Assessment

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.

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Seismic Assessment

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 2004.

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.

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Seismic Assessment

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.

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Seismic Assessment

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.

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Seismic Assessment

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.

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Seismic Assessment

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.

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Seismic Assessment

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.

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Seismic Assessment

Appendix A: Drawings

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Gisborne District Council GDC Office and Chambers

Seismic Assessment

Appendix B: Calculations

Documents

Gisborne District Council · 2016. 1. 13. · The single level building achieves 67% NBS under longitudinal earthquake loadings. Resisting transverse seismic loading the eastern end