Authored by: Rimkus Forensics Marketing Team
Published 4/17/2026
On January 28, 2022, the Fern Hollow Bridge in Pittsburgh collapsed while a bus was crossing it. The National Transportation Safety Board (NTSB) later determined in HIR-24-02 that a fracture-critical tie plate had failed due to corrosion and section loss, and that documented repair recommendations had gone unaddressed for years. No one was killed, which was unusual; the underlying pattern of unaddressed distress was not.
For claims professionals and litigation attorneys, structural distress is often where the forensic analysis begins. Classification, mechanism, and progression each may influence coverage evaluations, causation analyses, and repair recommendations.
This article covers the distress-damage-deterioration distinction, serviceability and strength considerations, common investigative methods, characteristic distress patterns, and the role timing may play in a forensic investigation.
Key takeaways: What claims professionals need to know about structural distress
When structural distress appears in a claim or dispute, how it is classified may shape which policy provisions apply, how causation is framed, and whether the investigation holds up.
What matters most
- Distress, damage, and deterioration are distinct mechanisms; which applies may determine code thresholds and policy triggers
- Serviceability and strength limit states may carry different code responses
- Visible distress rarely marks the initiating cause; surface conditions can lag the underlying problem by months or years
How investigations typically proceed
- Document review helps establish what was known and when, often central in coverage disputes
- Visual inspection and NDT may establish the hypothesis; coring and petrography are generally needed to opine on mechanism
- Once evidence is altered, removed, or repaired, opportunities to document original conditions may be significantly reduced
Rimkus supports claims professionals and attorneys with independent forensic investigations; contact us to discuss specific requirements.
What is structural distress and why does classification matter?
Structural distress generally describes load-induced or unanticipated impairment in a structural member before it has lost load-carrying capacity. Distress sits alongside deterioration and damage as three conditions forensic investigators commonly need to distinguish, and the terms are not interchangeable.
- Deterioration is typically time-dependent: corrosion through a slab, freeze-thaw degrading masonry mortar, moisture causing section loss in steel connections.
- Damage is generally event-initiated: impact, overload, flood.
- Distress commonly precedes failure and may result from either mechanism, often both operating together.
Which mechanism applies is only part of the picture. Severity also often turns on which limit state the condition has crossed: serviceability or strength. Serviceability limit states, addressed in the American Society of Civil Engineers (ASCE) Standard 7, involve conditions where building function is impaired without yet implicating structural safety.
The International Existing Building Code (IEBC) Section 202 defines substantial structural damage as a reduction greater than 33% in story lateral capacity, or as a reduction greater than 20% in the capacity of a vertical gravity component supporting more than 30% of the total floor and roof area, with the remaining capacity falling below 75% of current code requirements.
Identifying the sequence of contributing factors, and the limit state the condition has crossed, is often the most consequential part of the forensic investigation.
How do forensic engineers investigate structural distress?
Investigation typically moves from document review and field preparation, through visual and nondestructive screening, to core extraction and laboratory analysis. Each step generally informs the next: the documentary record shapes field work, field work shapes testing, and testing grounds any opinion on mechanism.
Document review and field preparation
Document review generally precedes site work because the as-found condition may be difficult to evaluate without knowing what was designed. Drawings, shop drawings, RFI logs, inspection reports, and maintenance records may help establish design intent, loading history, and condition over time. A maintenance log showing recurring distress reports without corrective action may be evaluated as evidence of notice; field modifications not reviewed by the engineer of record have frequently been cited as contributing factors in connection failures.
Visual inspection and NDT screening
With the documentary record in hand, the investigation moves to the structure itself. Visual documentation, including photographs, crack pattern mapping, deformation measurements, and connection condition, may help establish the hypothesis and identify where further testing is warranted. Ground-penetrating radar (GPR), commonly referenced through ASTM International (ASTM) D6432-19 and ACI 228.2R, can locate embedded reinforcement, identify internal voids, and detect delamination without opening the structure, and may be useful in punching shear investigations and post-tensioned systems where tendon locations need confirmation before coring. Ultrasonic pulse velocity (UPV) per ASTM C597-22 provides a relative measure of concrete condition. The rebound hammer per ASTM C805 functions as a surface screening tool and does not measure compressive strength directly; presenting rebound results as strength data may be treated as a methodology error opposing experts can challenge.
Where screening tools identify concern, half-cell potential surveys per ASTM C876-22 offer a quantitative screen for active corrosion in reinforced concrete. Against a copper/copper sulfate reference electrode, potentials more negative than −350 mV indicate a high probability of active corrosion; potentials more positive than −200 mV indicate a low probability. The survey measures electrochemical activity, not section loss; core extraction and petrographic analysis is generally necessary to quantify the deterioration.
Core extraction and laboratory analysis
Core extraction per ASTM C42/C42M-20 moves the investigation from surface screening to verified material data. Cores taken at distressed, undistressed, and transitional locations may allow investigators to map where deterioration begins, where it intensifies, and the in-place properties. Petrographic examination per ASTM C856/C856M-20 addresses materials science questions the field alone may not resolve: alkali-silica reaction, delayed ettringite formation, freeze-thaw damage, carbonation depth, and microcracking that may indicate mechanism and approximate timing. An investigator who opines on a mechanism without petrographic data may be relying on observations, testing, engineering analysis, and other available evidence. Whether petrographic analysis is warranted depends on the materials involved, the suspected mechanism, and the questions being investigated.
What patterns investigators look for in concrete, steel, masonry, and foundations
Methods and standards guide how an investigator documents and tests; what they look at depends on the material. Distress may present with characteristic patterns across concrete, steel, masonry, and foundation systems, and careful evaluation of those patterns can help investigators develop and test hypotheses regarding the underlying mechanism .
Concrete structures
Concrete communicates distress through crack geometry, which may map to mechanisms. Flexural cracks typically appear near midspan on the tension face, perpendicular to the longitudinal axis; fine stable ones are generally a serviceability concern. Shear cracks appear differently: diagonal tension cracks at approximately 45 degrees near the supports, where a widening hairline crack may itself be an investigative finding. Splitting cracks along reinforcement are commonly associated with corrosion-induced expansion of embedded steel; when rust staining and spalling follow, exposed reinforcing is typically treated as a strength-level indicator.
Steel structures
Steel distress often manifests at the microscopic level before it is visible, making it easy to miss and hard to document after the fact. Fatigue cracking commonly develops from cyclic loading at stress concentrations: connections, welds, and geometric discontinuities. Fatigue cracks are often identifiable under magnification by beachmarks, the striation pattern left by successive load cycles. Brittle fracture surfaces tend to be flat and granular and may propagate without the plastic deformation that might otherwise provide visible warning. Connection distress, including gusset plate distortion, rust bleed at a weld toe, and out-of-plane bowing, is frequently documented in inspection photographs years before a failure, which may make the pre-failure documentary record as relevant to forensic structural analysis as the physical evidence collected afterward.
Masonry
Masonry distress patterns may be diagnostic of mechanism. In-plane diagonal shear cracking is commonly associated with lateral load demand exceeding wall capacity. Stair-step cracking along mortar joints at window and door corners often reflects differential foundation settlement. Horizontal cracking at a bond course typically reflects differential thermal or moisture movement between wythes, often a different mechanism from vertical cracking at pier centers.
Foundation movement
Settlement is among the more commonly misclassified distress conditions in insurance and litigation contexts, often associated with the crack patterns above. Differential settlement, where different parts of the structure move at different rates, may produce racking of window and door frames, diagonal cracking at re-entrant corners, and floors going out of level. The forensic question is often whether the rate and pattern are consistent with long-term consolidation, a triggering event, or active ongoing movement.
When is forensic investigation warranted, and why timing matters
Recognizing these patterns is one thing. Deciding which ones warrant formal forensic investigation is another, and the difference can be significant because admissibility-grade documentation is generally not available retroactively once evidence is altered or lost. Forensic engagement is often appropriate when a routine inspection may not resolve ambiguity about cause, mechanism, or timing: a crack pattern inconsistent with normal loading, deterioration progressing faster than exposure would predict, or any distress where litigation or coverage determination is anticipated.
Once engagement is warranted, the next question is how soon, because unaddressed distress may not remain static. A condition stable for years may become unstable under an additional demand: a heavy snow load, a pipe leak saturating a floor system, or corrosion advancing past a threshold the reduced section may no longer sustain. In well-documented collapses, the forensic question is often the same: what was known, when, and what response followed.
NTSB Report HAR-19-02 documented that cracking in the Florida International University pedestrian bridge had been observed by project engineers before the March 2018 collapse that killed six people, and that multiple parties with authority to stop work did not act. The probable cause was design calculation errors, but the collapse followed visible distress that went unanswered, illustrating how the notice-and-response question may arise regardless of distress origin.
That same question appears central to the National Institute of Standards and Technology (NIST) investigation into Champlain Towers South. NIST preliminary findings from September 2025 identified that the pool deck began collapsing at least seven minutes before the tower fell, and that distress was visible in the weeks before the June 2021 collapse. Cases of this scale may raise questions about ASCE 7’s concept of progressive collapse, defined as the spread of an initial local failure from element to element resulting in the collapse of an entire structure or a disproportionately large part of it. Where that pattern is alleged, the evidentiary record of pre-collapse distress often becomes central to coverage and liability determinations.
From observation to defensible analysis
The gap between what was known and what was documented may affect the reliability of subsequent analyses. Findings are generally strongest when supported by documented field observations, testing, engineering analysis, and, where appropriate, laboratory data. Evidence preservation sits beneath all of that: stabilization, demolition, and repair work can destroy the physical record before it is documented, which is why early forensic engagement is often one of the more protective steps a claims professional or attorney can take.
For structural matters where early documentation, expert witness testimony, or root cause analysis may be valuable, contact us to discuss the investigation.
Frequently asked questions
How do forensic engineers evaluate structural distress when multiple causes may be involved?
Investigators evaluate the sequence of contributing factors and the relative role each may have played in the observed condition. This may include assessing whether load-induced distress, deterioration-driven capacity loss, environmental exposure, or an impact event contributed to the outcome.
What is a half-cell potential survey, and what information does it provide?
A half-cell potential survey measures the electrochemical activity of embedded reinforcing steel, providing a probabilistic indicator of active corrosion, not a measure of section loss or remaining structural capacity. Presenting half-cell data as evidence of structural compromise without coring or physical examination to quantify section loss may overstate what the method establishes.
Why is early documentation important in a structural distress investigation?
Physical evidence in structural distress cases can change quickly: deformation conditions may be altered by emergency repairs, and material samples collected after remediation may reflect modified rather than as-found conditions. Early engagement may allow a forensic team to document and preserve evidence before conditions change, reducing the need to reconstruct original conditions from records, photographs, or later observations.
This article is intended to provide general information and insights into prevailing industry practices. It is not intended to constitute, and should not be relied upon as, legal, technical, or professional advice. The content does not replace consultation with a qualified expert or professional regarding the specific facts and circumstances of any particular matter.