Engineering Solutions for
Earthquake Strengthening
Win Clark, Structural Engineer
Heritage New Zealand Pouhere Taonga
October 2015
Earthquakes cannot be prevented but their impact can be mitigated
Topics to be covered:
• What is an Earthquake-prone Building ?
• Current Proposals in the Building Amendment Bill:
Building (Earthquake-prone Buildings) Amendment Bill
• Current status of NZSEE Guidelines Review:
Assessment and Improvement of the Structural Performance of Buildings in Earthquakes
• Structural Assessment and Strengthening:
Engineering Objectives.
• Developing techniques for Structural Strengthening and
Securing.
• 7 simple steps to consider when strengthening a building.
Earthquake-prone Buildings what are they ?
Definition of Earthquake-prone Buildings in the
Building Act
Section 122 of the Building Act 2004:
1.(a) ultimate capacity exceeded in a moderate earthquake.
1.(b) likely to collapse causing:
- Injury or death
- damage to any other property
2. Does not apply to a building used for residential purposes, unless:
- comprises 2 or more storeys, and
- contains 3 or more household units
Definition of a “Moderate Earthquake”
an earthquake that would generate shaking at the
building site that is of the same duration as, but is onethird as strong as the earthquake shaking that would be
used to design a new building at that site.
Therefore:
an earthquake-prone building will have strength that is
33% or less of the seismic loading that meets the
standard NZS 1170 Part 5: 2004 for the particular
building site.
Earthquake Prone Buildings What are the likely scenarios ?
•Is a probability of 51% “likely” ? or is a lesser percentage
still “likely” ?
•Prior to the earthquake, the tectonic mechanism that
generated the 22nd February event had a 16% contribution
to the seismicity of the Christchurch area.
•Peak shaking on the 22nd February lasted for just over
second.
4-
•Peak Ground Acceleration (PGA) horizontally and vertically
was in the order of 2 x gravity (2g).
•This acceleration was significantly greater than the Building
Code 500-year mean return period design event.
Earthquake Prone Buildings • What would be the effect of a longer duration event, but
lower PGA ?
Similar to what is expected in an Alpine Fault rupture
What do we mean by “collapse” ?
• The whole building, or parts of the building ?
• Can we calculate the collapse load for any existing building ?
Colombo Street, Sydenham
Canterbury Earthquake 22nd February 2011
Deaths due to Masonry Building Failure
On 22nd February the official toll due to the earthquake was 185
From A Risk Framework for Earthquake Prone Building Policy by Tony Taig for DBH July 2012
Territorial Authorities Earthquake-prone Building Policy
• Active or Passive Approach to identifying Earthquake-prone
Buildings.
• Initial Seismic Assessment (ISA)
this is a broad screening process to identify potentially Earthquake-prone
Buildings. Includes an IEP (Initial Evaluation Procedure).
• Territorial Authorities (TAs) notify building owners.
• Building owners to engage an experienced engineer to carryout
a Detailed Seismic Assessment (DSA)
• The DSA will determine the %NBS (New Building Standard)
• If %NBS is less than 33% the TA will issue the building owner
with a notice to fix in an agreed time frame (Section 124 of BA)
Seismic Assessment Continuum
Complex structural analysis
Simple structural analysis
Access to drawings
COST
Exterior inspection only
SEISMIC ASSESSMENT
RELIABILITY/
CONFIDENCE
Level of judgement required
ISA
DSA
Proposed changes to the Building Act
The proposed changes are set out in the:
“Building (Earthquake-prone Buildings) Amendment Bill”
Bill No. 182-2, Select Committee Report 2/09/15
The most significant changes include:
• Definition of “Earthquake-prone”:
- includes buildings that pose a risk to adjoining properties.
- includes hostels, boarding houses and residential houses
more than 2-storeys and contain more than 3-units.
- excludes residential houses (except as above),
farm buildings, retaining walls, wharves, tunnels and
monuments.
•Regulations to define the term “ultimate capacity” when used
to describe an earthquake-prone building.
The most significant changes to the Building Act include:
• Seismic Risk
- earthquake-prone building identification and remediation
process to be based on seismic risk.
- The seismic risk of an area would be defined by its hazard factor
(Z factor from NZS 1170.5 Loadings Standard) as follows:
+ High Seismic Risk area - Z factor greater than or equal to 0.3
+ Medium Seismic Risk area - Z factor between 0.15 and 0.3
+ Low Seismic Risk area - Z factor less than 0.15
- Timeframe for action on earthquake-prone buildings is to vary
depending on level of seismic risk.
The most significant changes to the Building Act include:
• Priority Buildings are defined as buildings that:
- are generally used for health or emergency services or as
education facilities
- contain unreinforced masonry that could fall on to busy
thoroughfares in an earthquake (parapets)
- the Territorial Authority has identified as having the
potential to impede strategic transport routes after an
earthquake
• Priority Buildings have shorter timeframes for action.
The most significant changes to the Building Act include:
• Time Frame for identifying Earthquake-prone buildings:
- High Seismic Risk Areas:
+ High Priority buildings – 2.5 years
+ All other buildings – 5 years
- Medium Seismic Risk Areas:
+ High Priority buildings – 5 years
+ All other buildings – 10 years
- Low Seismic Risk Areas:
+ All buildings – 15 years
•Following identification of a potentially earthquake-prone
building, the owner is required to provide and engineering
assessment of the building within 12 months.
The most significant changes to the Building Act include:
• Time Frame for strengthening Earthquake-prone buildings:
- High Seismic Risk Areas:
15 years
- Medium Seismic Risk Areas: 25 years
- Low Seismic Risk Areas:
35 years
• Category I Heritage buildings would be eligible to apply for up
to a 10 year extension to complete strengthening work.
The most significant changes to the Building Act include:
• Building Alterations:
- Strengthening will be allowed even if after the alterations
the building does not comply with the provisions of the
Building Code that relate to:
+ means of escape from fire
+ disabled access
- The Territorial Authority may require the owner to carry out
strengthening works in addition to other substantial
alterations.
Other provisions of the Building Bill covered in the
first draft:
•
Use an assessment methodology to be specified by MBIE.
•
Earthquake prone can apply to parts of a buildings as well
as the building as a whole.
•
Level of work required is only such that building or part is
no longer earthquake prone.
•
MBIE to provide monitoring of TA performance, direction
and guidance.
• An earthquake-prone building register is to be maintained by
the Territorial Authority.
- The public will have access to information about earthquakeprone buildings.
- The register to include only those buildings determined as
earthquake-prone.
Proposed changes to the Building Act
Essentially:
• The trigger level (33%NBS) for earthquake prone buildings
has not changed.
• The time frame for assessment and completion of any
required strengthening is related to the seismicity of the
area.
(is there going to be sufficient resources in some areas ?)
• Strengthening does not trigger other Building Code upgrade.
• MBIE will have a much greater role in setting procedure,
monitoring performance of TAs, providing direction and
guidance.
Current status of NZSEE Guidelines Review
“Assessment and Improvement of the Structural
Performance of Buildings in Earthquakes”
published by NZSEE in June 2006
available from:
http://www.nzsee.org.nz/publications/assessment-and-improvement-ofthe-structural-performance-of-buildings-in-earthquake/
•Developed to assist Territorial Authorities meet their
obligations under the 2004 Building Act to identify Earthquakeprone Buildings (EQP).
•Provide guidance for engineers with assessment of buildings
and structural strengthening of various types of buildings.
Current status of NZSEE Guidelines Review
Changing expectations:
•The Canterbury Earthquake Sequence has completely changed
the national environment for the management of earthquake
prone and risk buildings.
•Better understanding and new procedures are required to
meet society’s expectations for building safety and minimization
of damage in a significant earthquake.
•More extensive guidance required for engineers to achieve a
greater consistence of outcomes.
Current status of NZSEE Guidelines Review
Review of Initial Evaluation Procedure
(IEP, Section 3 of Guidelines)
•Update guidelines published and seminar presentation
November 2013:
- Based on Christchurch experience with international input.
- Detailed guidance on the assessment process for various types
of buildings.
include develop a qualitative understanding of the building and its response
- Identification of Critical Structural Weaknesses (CSW).
- The effect of CSW on possible building response to earthquakes.
- The requirement to identify “building resilience”
- The geotechnical requirements for the IEP
Current status of NZSEE Guidelines Review
Review of Initial Evaluation Procedure
(IEP, Section 3 of Guidelines)
•Training workshops for engineers held March-April 2015
- groups of 5-engineers worked though ‘case studies’
•Objective: to obtain greater consistency in building
assessments (%NBS)
Current status of NZSEE Guidelines Review
Review of Detailed Seismic Assessment
procedures, including guidance on:
• Modelling the earthquake.
• Geotechnical considerations.
• Procedures for Force and Displacement Based analysis.
• New developments in structural strengthening
materials and techniques.
Current status of NZSEE Guidelines Review
Review of Detailed Seismic Assessment
procedures, including guidance on:
• Reinforced Concrete Structures
(Frame, Shear Wall, Dual Frame-Wall)
• Steel Structures
• Moment Resisting Frames with Masonry Infill Panels
• Unreinforced Masonry Buildings
• Timber Structures
• Improvement of Structural Performance
A major project funded by MBIE and EQC that draws on NZ and
international research and practice. Completion before mid-2016
Structural Assessment and Strengthening:
Engineering Objectives
What is the engineer looking for ?
What is the process ?
• Quantify the mass of the building and its natural period.
• Determine Seismic Loading.
• Identify the load path between the various building masses and
ground.
• Calculate the loads in the various members of the load path.
• Determine the capacity of the members and their joints to carry
these loads.
Structural Assessment and Strengthening:
Engineering Objectives
• Assess where failure is likely to occur, and the type of failure.
Will this failure cause minor disruption or lead to catastrophic collapse ?
Does the building have “resilience” ?
• Assess the %NBS and consequential effects.
• Identify options for structural strengthening.
• Obtain costing and review benefits
• Agree the preferred solution.
What to look for in an existing Building ?
• Good materials.
• Good workmanship.
• Good form:
– Square or rectangular plan subdivided into small spaces.
– Masonry wall span to thickness ratios less than 16.
– Masonry wall height to width ratio less than 4, with at least
two sections of wall in each direction =< 1.
– Complete and consistent load paths.
• Good maintenance.
Christchurch Girls’ High School, Montreal Street
What do we need to strengthen ?
Masonry elements:
• Chimneys:
-
Strengthen stack:
• Galvanised steel tube grouted into flue,
• Helifix hoop ties in every third horizontal mortar joint.
-
Fix stack back into diaphragm support.
• Parapets.
• Bay window heads and spandrels.
• Piers between window/doors.
What do we need to strengthen ?
• Inhibit out-of-plane failure of masonry walls:
- Tie veneers to walls,
- Strengthen walls to span between supports.
- Fix gable-end walls to roof framing.
- Fix walls to floor and ceiling diaphragms.
• Tie ‘L’ & ‘T’ junctions back into body of wall.
• Fix roof framing to top of walls.
• Fix floor framing to foundation piles and
perimeter foundation walls
What elements of the buildings should be
considered for strengthening ?
Christchurch Arts Centre,
Worcester Street
Old Fire Station, Lyttelton
Old Fire Station, Lyttelton
Avon House, Wellington
Case Study
Avon House, Wellington: Floor Plan
Avon House, Wellington
Application of carbon fibre strips
into brickwork
Fairlie Library Café, Retrofit
Avon House, Wellington
Test Setup
Avon House, Wellington
Face Loading-Deflection Graph
Avon House, Wellington
Face Loading Test Results
• Compression Strength:
Brick - 10.1 MPa with CoV 0.15
Mortar - 3.3 MPa with CoV 0.37
Plaster - 1.4 MPa with CoV 0.39
• Bed Joint Shear Strength:
0.041 MPa
with CoV 0.29
Wall Strength increase: 345%
to 112% NBS
Chimneys:
Chimneys:
• Plywood ceiling
diaphragm.
• Galvanised steel
strap around
chimney and nail or
screw fixed down
through plywood
into timber frame
and blocking
under diaphragm.
Chimneys:
• Plywood floor
diaphragm.
• Galvanised steel
strap around
chimney and nail or
screw fixed down
through plywood
into timber frame
and blocking
under diaphragm.
• Packing and carpet
over plywood.
Chimneys:
• Galvanised steel tube
grouted into flue.
• Helifix bars mortared
30mm into every
third bedding joint
to form a confining
hoop.
Chimneys:
• 4 No. Helifix bars
each bent into an
angle.
7 Simple Steps to consider when
strengthening a building:
• Obtain good engineering advice.
• Determine the earthquake resistant capacity of
the building; is the building earthquake-prone ?
• Determine the ‘Critical Structural Weakness’.
• Design and document strengthening scheme(s)
that enhances the capacity of the building to
resist earthquake effects.
7 Simple Steps to consider (continued):
• Cost these strengthening schemes.
• Prepare a programme of works that is
affordable.
Everything does not need to be done at once,
and a staged programme may work around
tenant requirements.
• Reach agreement with the local Territorial
Authority for the proposed programme of
works.
Summary:
• Most unreinforced masonry buildings can be
strengthened to an acceptable level of
earthquake resistance.
• After the earthquake is too late.
• You require good advice now.
You need to do it now !
Electric Substation, Clyde Rd, Christchurch
L’Aquila
Italy
Thank you