Existing Structures: Evaluation, Damage, and Retrofit

Introduction and Context

Earthquake engineering is not limited to the design of new buildings. A large proportion of seismic risk is carried by existing structures designed before modern seismic provisions, altered without adequate engineering review, or deteriorated through age and environmental exposure. Existing-building assessment therefore requires a different approach from new design. The engineer must infer the structural system, identify actual material and detailing, reconstruct likely load paths, interpret damage, and decide whether intervention is necessary, feasible, and proportionate to the risk.

Assessment and retrofit are decision processes under uncertainty. Drawings may be incomplete, material properties may vary, and observed cracking may have several possible causes. A safe decision must combine document review, field investigation, testing, analytical modelling, performance objectives, cost, constructability, occupancy, and regulatory requirements. The objective is not necessarily to make every existing building equivalent to a new building; rather, it is to reduce unacceptable risk to a defined performance level.

This chapter addresses rapid and detailed assessment, interpretation of earthquake damage, vulnerability evaluation, and selection of structural and nonstructural retrofit measures.

Learning Objectives

After completing this chapter, the reader should be able to:

1. Distinguish rapid, preliminary, and detailed seismic assessment.

2. Identify structural and nonstructural earthquake damage and infer probable failure mechanisms.

3. Develop a defensible assessment plan using drawings, inspection, testing, and analysis.

4. Apply basic vulnerability and demand-to-capacity concepts while recognizing their limitations.

5. Select suitable retrofit measures, including jacketing, bracing, wall addition, foundation strengthening, and nonstructural restraint.

6. Evaluate retrofit alternatives in relation to performance, constructability, cost, uncertainty, and governance.

1. The Assessment Problem

1.1 Existing-building performance objectives

A retrofit decision begins with a performance objective. Typical objectives include:

• Immediate occupancy or operational continuity: the building remains usable, with only minor repair requirements.
• Life safety: extensive damage may occur, but collapse and life-threatening falling hazards are prevented.
• Collapse prevention: the structure retains limited residual strength and stability after severe shaking.

The selected objective depends on occupancy, consequences of failure, hazard level, importance category, historical value, client requirements, and available resources. Hospitals, emergency coordination centres, schools, industrial facilities, and buildings sheltering vulnerable populations commonly require objectives more stringent than ordinary buildings.

Performance objectives should specify both hazard intensity and expected damage state. “Strengthen the building” is not an adequate objective unless the intended hazard and acceptable damage are defined.

1.2 Sources of uncertainty

Uncertainty is present in at least four forms:

• Epistemic uncertainty: incomplete knowledge of geometry, reinforcement, material properties, or prior alterations.
• Aleatory variability: natural variation in ground motion and material response.
• Model uncertainty: limitations in representing cracking, deterioration, soil interaction, and connection behaviour.
• Decision uncertainty: uncertainty concerning future use, maintenance, or acceptable loss.

A professional assessment should state important assumptions, identify information that would most reduce uncertainty, and avoid reporting highly precise numerical results unsupported by the evidence.

1.3 Load-path thinking

The basic assessment question is: how does seismic inertia travel from the mass of each floor to the ground? A complete load path normally includes diaphragms, collectors, chords, beam-column joints, frames or walls, foundations, and supporting soil. Failure of any link can cause disproportionate damage even when individual members appear strong.

Irregularities that deserve particular attention include soft or weak storeys, discontinuous walls, large openings, setbacks, torsional asymmetry, short columns, captive columns, heavy overhangs, unreinforced masonry infill, pounding gaps, and abrupt changes in stiffness or strength.

2. A Hierarchy of Assessment

2.1 Rapid visual screening

Rapid screening is a short, structured field procedure used to identify buildings requiring further investigation. It records occupancy, number of storeys, approximate structural type, apparent irregularities, site conditions, visible deterioration, falling hazards, and evidence of prior damage. It is not a proof of safety and should not be used to certify a building for a specific performance objective.

The screener should immediately identify life-safety hazards such as unstable parapets, damaged stairs, falling cladding, exposed live services, gas leakage, blocked exits, and severely damaged columns. A building may be structurally stable but unsafe because of nonstructural hazards.

2.2 Preliminary assessment

Preliminary assessment uses available drawings, a systematic inspection, approximate dimensions, limited testing, and simplified calculations. It seeks to determine whether obvious deficiencies exist and whether detailed analysis is justified. Typical outputs are:

• a verified or corrected structural-system description;
• an inventory of damage and deterioration;
• a list of critical deficiencies;
• preliminary material properties and member capacities;
• an initial estimate of demand-to-capacity relationships; and
• recommendations for monitoring, occupancy restrictions, repair, retrofit, or detailed evaluation.

2.3 Detailed assessment

Detailed assessment is appropriate for important buildings, buildings with substantial damage, disputed decisions, complex irregularity, or proposed major retrofit. It may include destructive and nondestructive testing, reinforcement mapping, foundation investigation, geotechnical studies, diaphragm and connection analysis, nonlinear static or dynamic analysis, and explicit treatment of uncertainty.

A detailed assessment should be traceable. The report should identify data sources, inspection dates, inaccessible areas, test locations, statistical treatment of results, modelling assumptions, load combinations, acceptance criteria, and sensitivity of conclusions to uncertain parameters.

3. Investigation and Evidence Collection

3.1 Desk study

The desk study should collect original drawings, calculations, construction records, occupancy and alteration permits, maintenance records, previous inspection reports, photographs, and information about the earthquake. The engineer should compare the design documents with current conditions because unauthorized modifications frequently alter the seismic system.

Useful historical questions include:

• Was the building constructed in stages?
• Has a storey been added or removed?
• Were walls removed to create commercial openings?
• Has heavy equipment or storage been introduced?
• Did the building experience previous earthquakes, fire, settlement, or flooding?
• Were repairs cosmetic, structural, or undocumented?

3.2 Field survey

Field inspection should proceed systematically by grid, storey, and structural element. Damage maps should distinguish cracks, spalls, crushing, buckling, yielding, dislocation, residual drift, corrosion, and water damage. Crack width alone is rarely sufficient to diagnose seismic performance; location, orientation, continuity, and associated deformation are more informative.

Measurements may include floor elevations, interstorey drift, out-of-plumbness, joint opening, gap closure, member dimensions, and connection slip. Photographs should include scale, orientation, location, and a reference drawing.

3.3 Testing

Common techniques include rebound hammer testing, ultrasonic pulse velocity, concrete cores, carbonation and chloride testing, cover meters, reinforcement scanning, weld inspection, bolt inspection, masonry sampling, and steel coupon testing. Each method has limitations. For example, rebound readings are influenced by surface condition and moisture and should not be treated as direct compressive strength without calibration.

Sampling must represent the building rather than only accessible or undamaged locations. Test results should be interpreted statistically and with engineering judgment. A single unusually high or low result should not automatically define the whole structure.

3.4 Safety during investigation

Investigation itself can expose workers and occupants to hazards. Temporary shoring, exclusion zones, controlled access, service isolation, fall protection, and emergency planning may be required. Removing finishes or drilling cores should not compromise a damaged member without temporary support.

4. Interpreting Earthquake Damage

4.1 Reinforced-concrete frames

Important damage patterns include:

• Beam plastic hinges: flexural cracks concentrated near beam ends may indicate desirable energy dissipation if rotation capacity and anchorage are adequate.
• Column hinging: severe column-end cracking, crushing, or longitudinal-bar buckling is generally more dangerous because it can produce storey mechanisms.
• Joint shear failure: diagonal cracking and concrete deterioration in beam-column joints may occur where transverse reinforcement is inadequate.
• Short-column failure: infill, window bands, or partial-height walls can restrict a column’s effective length, causing concentrated shear damage.
• Strong-beam/weak-column behaviour: simultaneous column damage at a storey may indicate an undesirable mechanism.

A brittle shear failure is particularly concerning because strength can be lost with little warning. The assessment must distinguish flexural yielding from shear degradation and must consider anchorage and lap-splice details.

4.2 Masonry and infilled frames

Unreinforced masonry may exhibit diagonal X-cracking, stepped bed-joint cracking, out-of-plane separation, corner failure, and wall overturning. Infill walls can increase initial stiffness and attract seismic force, but they may also create short columns, torsional irregularity, or an abrupt weak storey when removed at ground level.

The engineer should not simply ignore infill in the analytical model. If infill is present and bonded to the frame, its stiffness, strength, variability, openings, and possible brittle failure should be considered. If it is isolated or badly deteriorated, a different model may be justified.

4.3 Steel structures

Damage may occur through local flange or web buckling, fracture near welded connections, bolt slip, brace buckling, gusset-plate distortion, panel-zone yielding, or column-base failure. Fracture surfaces and deformation patterns can reveal whether failure resulted from low-cycle fatigue, poor weld quality, inadequate ductility, or excessive demand.

4.4 Foundations and soil

Foundation damage may appear as differential settlement, rocking, sliding, pile-cap cracking, uplift, or rotation. Soil liquefaction, lateral spreading, landslide movement, and bearing failure can create damage that cannot be solved by strengthening the superstructure alone. A building that is apparently undamaged above ground may still have an impaired foundation load path.

4.5 Nonstructural damage

Ceilings, partitions, façades, glazing, parapets, mechanical equipment, storage racks, pipes, tanks, elevators, and fire-protection systems can cause injury, loss of function, or major economic interruption. Nonstructural damage can also conceal structural damage or block emergency egress.

5. Vulnerability Evaluation

5.1 Screening indices and score-based methods

Score-based methods assign weights to features such as structural type, age, irregularity, deterioration, soil condition, and observed damage. They are useful for prioritizing large building inventories, especially when detailed data are unavailable. Their output is usually a relative vulnerability ranking, not a direct probability of collapse.

A score should not be interpreted outside the population, hazard, and calibration conditions for which it was developed. Applying a method developed for one construction practice or seismic region to another can produce misleading rankings.

5.2 Demand-to-capacity evaluation

For a component or mechanism, a basic ratio may be written as

\[

\mathrm{DCR}=\frac{D}{C}

\]

where \(D\) is the design or analysis demand and \(C\) is the corresponding available capacity, expressed in consistent units and defined for the same failure mode. A ratio above unity indicates that the assumed demand exceeds the assumed capacity; it is not, by itself, a precise probability of failure.

Capacity must reflect existing material, deterioration, detailing, load history, confinement, anchorage, interaction between axial force and bending, and the applicable acceptance criterion. For ductile components, deformation or rotation capacity may be more informative than force capacity. A model that checks only member flexural strength may miss joint shear, splice failure, diaphragm discontinuity, or foundation instability.

5.3 Global analysis

Linear analysis can be appropriate for regular buildings with moderate expected inelastic response and reliable force-distribution assumptions. Nonlinear static analysis can investigate sequence of yielding, strength degradation, and mechanism formation, but results depend on load pattern, hinge models, torsion treatment, and residual strength assumptions. Nonlinear dynamic analysis can represent cyclic response and record-to-record variability more directly, but it requires greater expertise and does not remove uncertainty in the structural model.

The selected analysis method should match the decision. Excessively sophisticated analysis with poor input data can create false confidence. Conversely, a simple model is inadequate when brittle failure, torsion, soil interaction, or major irregularity governs.

6. Retrofit Strategy and Selection

Retrofit should follow a hierarchy:

1. Define performance objectives and constraints.

2. Remove or reduce hazards where practical.

3. Correct the governing deficiency rather than indiscriminately strengthening all members.

4. Ensure a continuous load path and compatible deformation capacity.

5. Check the modified building, foundations, connections, and nonstructural systems.

6. Plan construction sequencing, temporary stability, inspection, and quality assurance.

A retrofit can inadvertently increase stiffness and attract force, shift damage to an unstrengthened storey, overload foundations, or create torsional imbalance. Local strengthening is therefore not automatically safer than global intervention.

6.1 Reinforced-concrete jacketing

Concrete, reinforced-concrete, steel, or fibre-reinforced polymer jackets can increase column or beam strength, stiffness, confinement, and ductility. Concrete jacketing is useful where substantial capacity and fire resistance are required, but it increases member dimensions, weight, and construction disruption. Steel jackets can be installed rapidly and provide confinement, though corrosion protection, fire protection, and connection detailing are essential. Fibre-reinforced polymer jackets are lightweight and minimally invasive, but they require sound substrate, careful surface preparation, environmental protection, and suitable anchorage; they are not a universal solution for shear, fire, or load-transfer deficiencies.

Jacketing must address the member’s complete mechanism. Strengthening a column without checking its footing, beam-column joint, lap splice, and adjacent beams may merely relocate the failure.

6.2 Steel bracing

New concentric or eccentric braces can increase lateral strength and reduce drift. Braced frames require reliable collectors, gusset plates, beam-column connections, and foundations. Concentrically braced systems may experience brace buckling and connection demands; eccentrically braced systems can provide ductility through controlled link yielding, but link and connection detailing is critical. Bracing may obstruct doors, windows, circulation, or services and should be coordinated architecturally.

6.3 Addition of structural walls

Reinforced-concrete or steel plate shear walls can substantially increase lateral stiffness and strength. They are effective when arranged to control torsion and connected continuously to diaphragms and foundations. New walls introduce large overturning and shear forces; existing slabs, collectors, wall boundaries, and foundations must be checked. Openings, coupling beams, construction joints, and interface shear transfer require careful design.

6.4 Diaphragm and connection strengthening

A strong vertical system is ineffective if floors cannot transfer inertia forces. Measures include slab overlays, added reinforcement, collector beams, drag struts, chord strengthening, improved wall-to-diaphragm anchors, and replacement or strengthening of weak connections. Diaphragm retrofit is especially important in buildings with flexible or discontinuous floors and in structures with heavy perimeter walls.

6.5 Foundation strengthening

Foundation intervention may include enlarging footings, adding reinforcement and shear keys, underpinning, micropiles, grade beams, tie beams, pile-cap strengthening, or improving soil conditions. The selected method depends on bearing pressure, uplift, sliding, overturning, settlement, access, groundwater, and adjacent structures. Underpinning must be sequenced so that temporary load redistribution does not destabilize the building.

6.6 Nonstructural restraint

Nonstructural retrofit includes anchoring equipment, bracing ceilings and partitions, securing tanks and cylinders, restraining shelves and racks, improving façade connections, installing flexible utility connections, and protecting stairs and egress routes. Restraint design must consider mass, centre of gravity, attachment capacity, expected acceleration, clearance, and possible impact. Fire, electrical, mechanical, accessibility, and infection-control requirements may govern installation in occupied buildings.

7. Worked Application: Preliminary Evaluation of a Soft-Storey Building

Consider an illustrative four-storey reinforced-concrete apartment building. The ground floor has an open parking area, while upper floors contain masonry infill. Site inspection identifies limited transverse reinforcement in ground-storey columns, diagonal cracking in several infill panels, and residual drift concentrated at the ground floor. The original drawings are incomplete.

The preliminary hypothesis is a soft-storey mechanism: the ground floor has substantially lower lateral stiffness and strength than the upper floors, and the columns may be vulnerable to shear or combined axial-flexural failure.

The investigation should first verify column dimensions, reinforcement, splice locations, joint details, diaphragm continuity, infill distribution, foundation type, and actual ground-floor openings. Cover-meter scanning and selected cores may reduce uncertainty. A rapid calculation may compare estimated ground-storey demand with available column and joint capacities, but the decision cannot rely on a single ratio.

Suppose a preliminary model estimates a lateral shear demand of 1,200 kN for the ground storey. The sum of reliably established shear capacities of the critical columns is estimated as 900 kN after accounting for deterioration and conservative detailing assumptions. The corresponding preliminary shear DCR is

\[

\mathrm{DCR}=\frac{1,200}{900}=1.33.

\]

This indicates an apparent deficiency under the stated assumptions. It does not prove collapse, because force redistribution, actual infill behaviour, cyclic degradation, and foundation response remain uncertain. The result justifies detailed assessment and interim measures such as restricting occupancy or installing shoring where necessary.

Potential retrofit alternatives are: (a) column jacketing with joint and foundation checks; (b) new reinforced-concrete walls placed symmetrically and connected to the diaphragm; or (c) steel braced frames at selected bays. The preferred solution should be selected by comparing achieved drift and mechanism control, construction access, parking impact, foundation demand, fire protection, cost, and verification requirements. Adding a wall at only one façade could reduce ground-storey drift but create torsion; therefore, global response must be checked.

8. Illustrative Case Study: Retrofit of a School Building

Background

An illustrative two-storey reinforced-concrete school was constructed in the 1970s. It has a flexible ground floor used as an assembly and circulation space, masonry infill on the upper floor, and heavy unanchored mechanical equipment on the roof. The building is in a moderate-to-high seismic region and must remain available after an earthquake.

Problem

Screening identifies a weak ground storey, inadequate anchorage of roof equipment, discontinuous infill, and uncertain beam-column joint reinforcement. A previous minor earthquake caused diagonal cracks in several upper-storey masonry panels but no apparent global distress.

Analysis

Detailed inspection confirms that several ground-storey columns have inadequate transverse reinforcement. Nonlinear analysis indicates concentration of drift at the ground floor and early shear degradation in two exterior columns. The roof equipment can slide and impact adjacent services. The existing foundations have limited reserve against the overturning forces associated with a new wall system.

Approach and solution

The selected strategy combines two balanced reinforced-concrete walls near the stair and opposite perimeter line, local steel jacketing of critical columns, diaphragm collector strengthening, foundation enlargement with new tie beams, and anchored equipment frames. Masonry infill is either positively connected and included in the assessment or separated using a designed seismic gap where interaction would be harmful. Construction is sequenced so that temporary bracing remains effective while walls and foundations are connected.

Outcome

The retrofit reduces ground-storey drift, distributes lateral resistance more evenly, improves the load path, and reduces nonstructural falling and operational hazards. The school’s architectural layout is modified only where necessary. Post-construction inspection verifies reinforcement, welds, anchors, concrete quality, wall-to-diaphragm connections, and equipment restraints.

Learning points

The case demonstrates that a retrofit is a system intervention. Column strengthening alone would not have corrected diaphragm transfer, foundation demand, or equipment hazards. Balanced wall placement was required to control torsion, and nonstructural measures were essential to the continuity objective.

9. Governance, Regulation, and Professional Judgment

Assessment and retrofit must comply with the governing building code, existing-building provisions, material standards, occupational-safety rules, fire regulations, accessibility requirements, and local approval procedures. Where codes permit engineering judgment, the basis should be documented rather than hidden in unexplained safety factors.

Engineers have a duty to communicate uncertainty and limitations clearly. A report should distinguish observed facts, inferred conditions, analytical assumptions, and recommendations. Owners should be informed of immediate hazards even when a complete assessment is pending. Conflicts of interest, pressure to minimize cost, and incentives to declare a building safe without adequate evidence are serious ethical risks.

Retrofitting occupied buildings requires coordination with occupants, contractors, authorities, insurers, and facility managers. A technically adequate design can fail in practice if access, evacuation, temporary works, inspection, or maintenance is neglected.

10. Contemporary Tools and Developments

Digital photographs, mobile inspection forms, laser scanning, photogrammetry, drones, reinforcement imaging, and building information models can improve documentation. Sensors may monitor vibration, crack movement, tilt, temperature, and post-earthquake residual drift. Digital twins and probabilistic models can support portfolio prioritization, but they are only as reliable as their data and calibration.

Artificial intelligence can assist with image classification, damage mapping, anomaly detection, and inventory screening. It should be treated as a decision-support tool, not an autonomous structural assessor. Images may be incomplete, lighting-dependent, or misleading; safety-critical conclusions require qualified human review and traceable evidence. Data governance must address privacy, cybersecurity, ownership, and retention.

11. Best-Practice Workflow

A robust professional workflow is:

1. Establish occupancy, hazard, performance objective, and decision deadline.

2. Secure immediate hazards and control access.

3. Gather documents and reconstruct the actual building.

4. Map structural and nonstructural systems and their load paths.

5. Inspect systematically and record uncertainty.

6. Test representative materials and details where results affect decisions.

7. Identify likely governing mechanisms, including brittle and foundation failures.

8. Select analysis complexity consistent with the problem and data quality.

9. Compare retrofit alternatives using performance, constructability, cost, disruption, and maintainability.

10. Design interfaces, temporary works, quality assurance, and inspection requirements.

11. Verify the completed retrofit and establish a monitoring and maintenance plan.

Challenges and Limitations

Existing-building assessment is limited by hidden conditions, uncertain construction quality, inaccessible elements, changing occupancy, incomplete geotechnical information, and imperfect analytical models. Retrofitting may increase mass, stiffness, or force demand and can create new irregularities. Numerical results are sensitive to assumed ductility, deterioration, connection behaviour, and soil response. Consequently, engineering judgment remains essential, and conclusions should be updated when new evidence becomes available.

Chapter Summary

Existing-structure evaluation is a staged process beginning with rapid screening and progressing, where justified, to preliminary and detailed assessment. The central task is to understand the complete seismic load path and identify the governing failure mechanism. Damage interpretation requires attention to location, deformation, detailing, cyclic degradation, and interaction between structural and nonstructural components. Vulnerability methods range from score-based screening to component demand-to-capacity checks and nonlinear global analysis; each has a defined domain of applicability.

Effective retrofit is performance-based and system-oriented. Jacketing, bracing, wall addition, diaphragm and connection strengthening, foundation intervention, and nonstructural restraint must be selected in relation to load transfer, torsion, drift, constructability, and uncertainty. Retrofit decisions also require regulatory compliance, ethical communication, quality assurance, and post-construction verification. Modern digital and AI tools can improve evidence collection but cannot replace qualified engineering judgment.

Key Terms

Rapid visual screening: A structured initial survey used to prioritize buildings for further evaluation.

Detailed assessment: Investigation and analysis using verified data and advanced methods appropriate to the decision.

Performance objective: Defined seismic hazard and acceptable building performance.

Load path: Continuous route by which seismic inertia forces travel to the ground.

Soft storey: A storey with significantly lower lateral stiffness than adjacent storeys.

Weak storey: A storey with significantly lower lateral strength than adjacent storeys.

DCR: Demand-to-capacity ratio for a specified failure mode.

Plastic hinge: A localized region intended or capable of undergoing inelastic rotation.

Joint shear failure: Brittle deterioration of a beam-column connection under shear demand.

Short-column effect: Concentration of shear demand caused by reduced effective column length.

Diaphragm: Floor or roof system that transfers horizontal inertia forces to vertical resisting elements.

Collector: Element that gathers and transfers diaphragm force to frames or walls.

Jacketing: Addition of concrete, steel, or composite material around an existing member.

Braced frame: Lateral system using diagonal members to resist horizontal actions.

Shear wall: Wall designed to resist substantial in-plane lateral force and overturning.

Underpinning: Strengthening or replacement of foundation support beneath an existing structure.

Nonstructural component: Building element not part of the primary gravity or seismic-resisting system.

Epistemic uncertainty: Uncertainty caused by incomplete knowledge.

Aleatory variability: Natural randomness in loads, materials, or response.

Residual drift: Permanent lateral displacement remaining after earthquake shaking.

Review Questions

Short Answer Questions

1. What is the primary purpose of rapid visual screening?

2. Why must nonstructural hazards be included in post-earthquake assessment?

3. Define demand-to-capacity ratio and state one limitation of its use.

4. What is the difference between a soft storey and a weak storey?

5. Identify two reasons why a new shear wall may require foundation strengthening.

Long Answer Questions

1. Explain a complete workflow for detailed seismic assessment of an existing reinforced-concrete building.

2. Discuss how damage patterns in reinforced-concrete frames can reveal brittle or ductile failure mechanisms.

3. Compare concrete jacketing, steel bracing, and reinforced-concrete wall addition as retrofit strategies.

4. Explain why diaphragm and collector design is important even when the vertical lateral-force-resisting system is strong.

5. Discuss the ethical, regulatory, and construction-management responsibilities associated with retrofit of an occupied building.

Analytical / Application Questions

1. A building has an estimated seismic demand of 1,500 kN and a reliably established capacity of 1,200 kN for a governing mechanism. Calculate the DCR and explain what additional evidence is required before selecting a retrofit.

2. A three-storey building has severe drift concentrated in its open ground floor, while upper floors contain discontinuous masonry infill. Identify likely mechanisms and propose a coordinated assessment and retrofit strategy.

3. A proposed shear wall reduces calculated drift but produces high overturning forces and substantial torsion. Explain how you would evaluate whether the wall is an appropriate retrofit and what alternative measures might be considered.

Multiple Choice Questions with Answers

Question 1

What is the primary purpose of rapid visual screening?

A. To provide final structural certification

B. To prioritize buildings for further investigation

C. To determine exact concrete strength

D. To replace nonlinear analysis

Correct Answer: B. To prioritize buildings for further investigation

Question 2

In the expression DCR = D/C, what does C represent?

A. Crack width

B. Construction cost

C. Available capacity for the specified failure mode

D. Column height

Correct Answer: C. Available capacity for the specified failure mode

Question 3

Which condition is generally most dangerous in a reinforced-concrete frame?

A. Minor beam flexural cracking

B. Column shear failure

C. Small nonstructural partition cracks

D. Paint delamination

Correct Answer: B. Column shear failure

Question 4

Why can adding a wall at only one side of a building be problematic?

A. It always decreases strength

B. It may create torsional imbalance

C. It eliminates foundation demand

D. It prevents diaphragm action by definition

Correct Answer: B. It may create torsional imbalance

Question 5

Which retrofit is primarily intended to confine and strengthen an existing column?

A. Column jacketing

B. Roof waterproofing

C. Window replacement

D. Floor finishing

Correct Answer: A. Column jacketing

Question 6

What is a key limitation of score-based vulnerability methods?

A. They cannot rank buildings

B. They always require nonlinear dynamic analysis

C. Their calibration and applicability may be region-specific

D. They measure only concrete strength

Correct Answer: C. Their calibration and applicability may be region-specific

Question 7

A demand of 1,200 kN acts on a capacity of 900 kN. What is the DCR?

A. 0.75

B. 1.00

C. 1.33

D. 2.10

Correct Answer: C. 1.33

Question 8

Which element transfers seismic inertia forces within a floor system toward walls or frames?

A. Diaphragm

B. Pile cap only

C. Parapet finish

D. Expansion joint sealant

Correct Answer: A. Diaphragm

Question 9

Why may strengthening a column alone be inadequate?

A. Columns never participate in seismic resistance

B. Failure may shift to joints, foundations, splices, or adjacent members

C. Strengthening always reduces capacity

D. Columns do not carry axial force

Correct Answer: B. Failure may shift to joints, foundations, splices, or adjacent members

Question 10

What is the appropriate role of AI-based damage image classification?

A. Unreviewed final safety certification

B. Decision support subject to qualified human verification

C. Replacement for field inspection in all cases

D. Automatic approval of retrofit permits

Correct Answer: B. Decision support subject to qualified human verification