Seismic Evaluation and Strengthening ABCs

Today’s Discussion
• Introduction
• Discussion of Terms
• Assessment Options and Tools
– The Initial Seismic Assessment
– The Detailed Assessment
• Reading Between the Lines
• Moving Forward From the Assessment
• Closing Comments
Discussion of Terms
Simply what does each term mean
and what do all the acronyms
stand for?
Common Terms Used
Percentage of New Building Standard (% NBS)
Initial Seismic Assessment (ISA)
Detailed Assessment (DA)
Ultimate Limit State (ULS)
Serviceability Limit State (SLS)
Collapse Limit State (CLS)
Earthquake Prone
Earthquake Risk
Critical Structural Weakness
Collapse Mechanism
Importance Levels
Unreinforced Masonry
Percentage of New Building
Standard (% NBS)
• Most commonly used term in Seismic Assessment.
– Also the most misconstrued, misunderstood term.
• A performance comparison to:
Similar Structure
Similar Materials
Constructed on the same site
Designed to current loading standards and codes
Constructed using current industry standard
• Established Performance Benchmarks
– Common Benchmarks are 33% NBS and 67% NBS.
– Policy is often based around these terms
– Does not provide the full picture.
Initial Seismic Assessment
Often referred to as the ISA.
New but same old methodology.
It is an improved version of the IEP.
Includes the IEP, plus a bit more.
Qualitative Assessment Methodology
– We will discuss the ISA and IEP a bit later on.
Detailed Assessment
• Abbreviated to the DA
• Previously called the Detailed Seismic
Assessment or DSA
• Quantitative Assessment Methodology
• Engineers put pen to paper and do the
numbers for the building.
– We will discuss the Detailed Assessment a bit
later on
The Limit States
• What do they mean?
• Ultimate Limit States – Structure
Standing but Badly Damaged
• Serviceability Limit State – Structure has
minor damage and is still functional
• Collapse Limit State – The absolute point
of collapse (No safety left)
Earthquake Prone vs Earthquake
What does each term mean?
Earthquake Prone:
Earthquake Risk:
What is the difference:
– Legally? Practically?
• What if my structure is Earthquake Prone?
– Don’t Panic!
– Talk to your Engineer
– What are your options
Critical Structural Weakness
• Critical Structural Weakness (CSWs)
• What are they? Examples
• What is the Effect?
– Legally? Practically?
• How do I manage a structure that has one?
– Don’t Panic!
– Talk to your Engineer, Discuss the Risk.
Collapse Mechanism
• A Specific identified Mechanism that
shows how the building will fail.
• Generally Prematurely to full structural
capacity being developed or in a sudden or
brittle manner.
• Also includes situations where there is no
clear alternative load path.
Importance Levels (IL 1-5)
• Related to the use or function of the structure the
occupancy and the consequences of failure.
• IL 1 – Isolated, low risk structures;
• Tractor Shed, Masts, Swimming Pools
• IL 2 – Normal Structures;
• Offices, Shops, Houses, Low rise, warehouses
• IL 3 – Larger Crowds, Contents of High Value
• Prisons, Shopping Malls, Schools, High Rise Buildings
• IL 4 – Post Disaster Function Structures
• Fire stations, Hospitals, Airports, Emergency Utilities
• IL 5 – Special Structures
• Hydro- Dams, Nuclear Power Stations, Extreme Hazard
facilities, Probably the Beehive.
Quick Summary
Do I really need to know about it?
What is it simply?
How can a change in ductility effect my
• Engineers Judgement.
Unreinforced Masonry
• Often Referred to as URM
• What is URM?
• Different Types of URM
– Blocks, Bricks, Single/Double/Triple Brick, Cavity Brick,
Rubble Cavity Brick, Stone
– The Bricks and Mortar
– Cavity Ties
• Why is it such a risk?
– In-plane
– Out of plane
• Can URM be strengthened?
– FRP, Helical Cavity ties, Shotcrete, Monolithic solutions
Assessment Options
What are the tools the Engineer
has available, how are they used
and what should I ask for?
The Initial Seismic Assessment
(ISA) Close Up
• Initial screening tool
– Typically includes an IEP and report
• Looks at typical features
Age of Construction
Structure Type
Importance Level
• Based heavily on age of construction
• Results based around engineering judgement
• Results may vary between engineers, %NBS
ISA Results
• May be used to identify buildings
warranting a detailed assessment of their
structural performance
• Provide a preliminary score for a
comparative risk grading of buildings
• Provide a means of determining priorities
for improvement of structural
The Detailed Assessment (DA)
Close Up
• Has had many names; DSA, DAP, DEE....
• The DEE is only used on earthquake damaged
• Generally, undertaken after a structure is Identified by
the ISA.
• Involves both quantitative and qualitative assessment.
• Procedure based around NZSEE, SESOC guidelines
and loading standards.
• Often uses high level analysis; Time History, Push
Over Analysis, 3D Finite Element.
• Identifies; performance, weaknesses, failure
mechanisms and risks.
DA Results
• Will determine the level of risk posed by the building within
an acceptable level of accuracy
Percentage of New Building
Standard (%NBS)
Relative Risk
<1 time
Low Risk
1-2 times
Low Risk
2-5 times
Moderate Risk
5-10 times
Moderate Risk
10-25 times
High Risk
>25 times
High Risk
• Determine whether or not it meets the requirements of the
Building Act for it to perform satisfactorily in a moderate
earthquake (i.e. above 33%NBS)
• Identify Critical Structural Weaknesses
• Inform a strengthening scheme
So What’s the Difference?
Initial, broad tool
Quick and Inexpensive
Screen-out Structures
Used to identify critical
• Limited, Not thorough
• Industry Accepted
• Should not be used as a
basis for Go, No Go Actions
• Quantitative
• Detailed, focussed tool
• Looks at structures under
the magnifying glass
• Predicts failure
• More Expensive Takes
• Gives a finite answer.
When should Each Tool be Used?
How many structures do you have?
How much time do you have?
What is your budget?
What Level of Service do you need?
How urgent is the assessment?
What are my Priorities, Do I know yet?
What are the risks?
Do I have a Defensible position?
Reading Between the Lines
From Scoping the Assessment to
Interpreting the Report.
Scoping the Assessment
• What type of Assessment do I need?
• What are my limitations?
– Time, Money, Tenants, Insurance, Life Safety?
• Where do I want to go with this?
• What is My level of service?
• Discuss with the Engineer.
– Ask questions
– Discuss the Issues
– Help us, Help you.
Interpreting the Report
• What is the Engineer trying to tell me?
• Where should I go from here?
• What are my Options?
– Less than 33% NBS
– Less than 67% NBS
• What is the real risk?
– Don’t just focus on the %NBS, CSWs, Escape
Routes, Post Earthquake Function, Liquefaction
• Have a discussion with the Engineer.
– Get the complexity out of the report.
Moving Forward From the
What’s Next?
Further Investigation
A Second Opinion
Risk Assessment
Intrusive Investigation
Cost/Benefit Assessment
More Detailed Assessment
– All sorts of levels of assessment
• Simple (Static Analysis)
• Complex (Time History)
• All about the $$$
Don’t Panic!
Discuss the Options
What level am I trying to achieve?
Typical Strengthening Levels
– 100% NBS,
– 67% NBS, (Advised, NZSEE Recommended)
– 34% NBS, (Legal Requirement)
• However what can be practically achieved?
• For large or several buildings may have to be
The Full Process
Desktop Study
Initial Seismic
• Commonly Completed by TAs
• Quick, very crude studies.
• Often in the form of the IEP.
• $1000-$3000 per building.
• Detailed Assessment
• Cost varies based on building and complexity.
• $5,000-$30,000 per building.
• Concept Strengthening design to achieve the desired level of service
• Often includes various options for different levels of service.
• Cost based on complexity. $10,000 to $50,000 per building.
• Detailed Strengthening Design
• Includes all details, drawings, Technical Specification etc.
• Cost based on Complexity. $20,000 to $80,000 per building.
Closing Comments
• Its all about Discussion,
– Local Engineers, NZSEE, MBIE, SESOC, IPENZ
• Knee-Jerk Reactions are not the answer.
• Develop a Defensible Position
– Have a plan
– Work towards a goal
– Prioritize
• Don’t be bullied into making rash decisions.
• Think about what the long term goal is.

similar documents