SEISMIC EVALUATION AND STRENGTHENING

THE 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• Ductility• 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

• ULS, SLS, CLS• 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 Risk

• 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.

Ductility

• Quick Summary• Do I really need to know about it?• What is it simply?• How can a change in ductility effect

my structure?• 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– Location– 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 performance

The Detailed Assessment (DA) Close Up

• Has had many names; DSA, DAP, DEE....• The DEE is only used on earthquake damaged

structures.• 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

• 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

Percentage of New Building Standard (%NBS)

Relative Risk (Approximate)

Grade Description

>100 <1 time A Low Risk

80-100 1-2 times A Low Risk

67-80 2-5 times B Moderate Risk

33-67 5-10 times C Moderate Risk

20-33 10-25 times D High Risk

<20 >25 times E High Risk

So What’s the Difference?

ISA• Qualitative• Initial, broad tool• Quick and Inexpensive• Screen-out Structures• Used to identify critical

structures• Limited, Not thorough• Industry Accepted• Should not be used as a

basis for Go, No Go Actions

DSA• Quantitative• Detailed, focussed tool• Looks at structures

under the magnifying glass

• Predicts failure mechanisms

• More Expensive Takes time.

• 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 Assessment

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 $$$

Strengthening

• 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 staged.

The Full ProcessDesktop

Study

• Commonly Completed by TAs• Quick, very crude studies.

Initial

Seismic

Assessmen

t

• Often in the form of the IEP.• $1000-$3000 per building.

Detailed

Assessmen

t

• Detailed Assessment• Cost varies based on building and complexity.• $5,000-$30,000 per building.

Concept

Strengthening

• 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

• 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.