STRUCTURE ASSESSMENT

INTRODUCTION

RELIABILITY STRUCTURE

The probabilistic measure of assurance of performance with respect to some prescribed conditions. A condition can refer to an ultimate limit state (such as collapse) or serviceability limit state (such as excessive deflection and/or vibration)

INTRODUCTION

STRUCTURE ASSESSMENT

purpose of assessment is to know the condition and ability of the structurethe results of assessment may be used to determine necessary action to be taken, (prioritize inspection, inspection methods, repair or retrovit) assessment involving code driven inspection, experience and field survey

PHILOSOPHY OF RISK BASED INSPECTION IN STRUCTURE ASSESSMENT

Risk is a function of both probability of failure and consequences of failure.Risk based strategies use the concept of risk to focus inspection and maintenance resources to the parts of the plant where they have the greatest benefit in reducing risk.Risk based strategies only work when a feedback process is used on a regular basis to keep the risk assessment and strategy up to date.

RISK MATRIX

Risk = Probability of Failure X Consequences of Failure

PROBABILITY OF FAILURECONSEQUENCES OF FAILURELOWMEDIUMHIGH(3)(2)(1)HIGH(1)321MEDIUM(2)432LOW(3)543

PURPOSES OF STRUCTURE ASSESSMENT

Inventory the structure facilities and equipment conditionOverall reduction in risk for the facilities and equipment assessed Acceptance/understanding of the current riskPredictive maintenance

ASSESSMENT METHOD

COLLECTING DATA QUALITATIVE ASSESSMENT (VISUAL INSPECTION + NDE IF NECESSARY)PROBABILITY AND CONSEQUENCES OF STRUCTURE FAILURESTRUCTURE RISK (CRITICALLITY RANKING)QUANTITATIVE ASSESSMENT (NDT/NDE PERFORMANCE)STRUCTURE ANALYSIS/ENGINEERING ANALYSISRECOMMENDATION

ASSESSMENT METHOD

Structure classificationPrimarySecondaryTertiaryVisual inspection (qualitative)Member conditionDeformationConnection memberEnvironment and safety hazardsNDT inspection (quantitative)MT/PTUT (thickness and or scanning)Eddy currentLoad test (if applicable)

ASSESSMENT METHOD

Define Probability of FailureService conditionEnvironmentConstruction quality recordDefine Consequences of FailureStandbyFinanceLocationStructure analysis (applicable code)Actual loadDeflection limit and Stress analysis

REFERENCE

STANDARD CODE

ACI (AMERICAN CONCRETE INSTITUTE)

ACI 318 ACI 117

AISC (AMERICAN INSTITUTE of STELL CONSTRUCTION)

ASD (ALLOWABLE STRESS DESIGN) LRFD (LOAD RESISTANCE FACTOR DESIGN) CLIENT SPESIFICATION

ENGINEERING & CONSTRUCTION DATA

PERSONNEL & EQUIPMENT

Personnel in Each Plant

Structure Engineer (1 person)

Structure Inspector (2 person)

NDT/NDE Inspector (2 person)

Helper (3 person)

Inspection Equipment

Welding Gage

Measurement Gage (caliper, roll meter, etc)

Binocular and inspection mirror

Scraper, hammer & wire brush

UT Thickness

Safety Equipment

Camera with high resolution (optional)

Steel Member

Structural deficiencies affecting steel member :

Corrosion

- Minor/Light

A surface corrosion with no apparent loss of section

- Moderate

Corrosion that is loose and flaking with some pitting. The scale or exfoliation can be removed with some effort by use of a scrapper or chipping hammer. The element exhibits measurable but not significant loss of section

- Severe

Heavy, stratified corrosion or corrosion scales with extensive pitting. Removal requires exerted effort and may require mechanical means. Significant loss of section

Steel Member

Impacted Corrosion

Corrosion collected between two interfacing surfaces, usually two steel plates. Impacted corrosion can be minor, moderate, or severe as described above. Impacted corrosion can severely deform the steel member due to expansive nature of the corrosion product.

Pitting

Formation of cavities due to corrosion

CONCRETE MEMBER

Structural deficiencies affecting steel member :

Cracking (overstress, corrosion & general)

A separation into two or more parts as identified by the space between fracture surfaces in the concrete

- Hairline - crack width less than 1/32 in

- Fine crack width between 1/32 in and 1/16 in

- Medium crack width between 1/16 in and 1/18 in

- Wide crack width greater than 1/8 in

Efflorescence

A white deposit caused by crystallization of soluble salts brought to the surface by moisture leaching through the concrete

Delamination

A layered separation of the concrete. When a delaminated area of concrete is struck (sounded) with a hammer, ahollow sound will be emitted

Leaching

The dissolution and washing away of the calcium hydroxide in concrete. Moisture enters the through cracks open to the surface.

CONCRETE MEMBER

Spall

A roughly circular, oval, or elongated depression in the surface of a concrete element caused by separation of a portion of the surface concrete.

- Small (pop-out) less than 6 in in diameter and 1 in deep

- Medium between 6 in and 12 in in diameter and up to 2 in deep

- Large over 12 in in diameter and any depth

Scaling (Erosion)

The gradual loss of surface mortar and aggregates

- Light loss of surface mortar up to in deep

- Medium loss of surface mortar between in and in deep, including loss between large agregate

- Heavy loss of mortar greater than in deep significantly exposing large agregate

CONCRETE MEMBER

Hollow Area

An area of concrete that emits a hollow sound when struck with a hammer, indicating the existence of a fracture plane beneath the surface

Honeycomb

Typically small pocket voids formed by the entrapment of air during the placement of the concrete

END PRESENTATION