Material degradation and fatigue are primary concerns. This involves assessing the long-term performance of concrete, steel, and composites under cyclic loading and environmental stress.

• Corrosion potential in steel reinforcements and cables, especially in coastal or de-icing salt environments.
• Concrete cracking and spalling due to freeze-thaw cycles or alkali-silica reaction.
• High-strength material brittleness leading to sudden failure without warning.
• Use case: The premature deterioration of many 1970s-era bridges highlights the cost of underestimating material durability.

Uncertain subsurface conditions pose a fundamental threat. This category evaluates the stability of the ground supporting the bridge's abutments and piers.

• Settlement or liquefaction of soil during seismic events or under heavy loads.
• Scour erosion around piers from river currents, a leading cause of bridge failure.
• Slope instability at approach embankments or in hilly terrain.
• Real example: The 2009 collapse of the I-35W bridge in Minnesota was linked in part to undersized bearing pads and unanticipated load distributions, a foundation design flaw.

Extreme hydrological events challenge a bridge's resilience. This involves modeling water forces, debris impact, and long-term environmental wear.

• Flood and tsunami loading exceeding original design water levels and velocities.
• Debris accumulation at piers, creating additional drag and damming effects.
• Climate change impacts, such as increased precipitation intensity and sea-level rise altering flood plains.
• Use case: Bridges in hurricane-prone regions require specific analysis for storm surge and wave forces not typically in standard codes.

Earthquake-induced ground motion can cause catastrophic failure. This assesses the bridge's behavior under sudden, multidirectional shaking and potential soil-structure interaction.

• Resonance and amplification of seismic waves through certain soil types.
• Pounding between adjacent spans due to insufficient separation gaps.
• Liquefaction of supporting soils, causing loss of bearing capacity.
• Real example: The 1995 Kobe earthquake demonstrated the vulnerability of older rigid-frame bridges versus modern designs with seismic isolation bearings.

Errors during the build phase can introduce latent defects. This covers risks from methodology, sequencing, quality control, and unforeseen site conditions.

• Temporary works failure, such as falsework or shoring collapse during erection.
• Welding or post-tensioning defects that are difficult to detect post-construction.
• Schedule and budget overruns due to complex erection techniques in difficult locations.
• Use case: The use of innovative incremental launching or balanced cantilever methods requires meticulous stage-by-stage analysis to prevent progressive collapse.

Evolving use and overload scenarios threaten long-term serviceability. This evaluates demands from traffic, accidents, and future capacity needs not in the original design.

• Increased legal load limits from modern freight vehicles versus historical design trucks.
• Fatigue from high-volume, high-speed traffic causing cumulative damage.
• Impact loads from vehicle collisions with piers or superstructure.
• Why it matters: A bridge designed for 20-ton vehicles may face premature failure if routinely subjected to 40-ton loads, requiring rigorous load rating and monitoring.

Inadequate load capacity is a primary concern, often stemming from miscalculations or cost-cutting.

• Use of substandard materials or undersized support members that cannot handle projected traffic and environmental stresses.
• Example: The 2007 collapse of the I-35W Mississippi River bridge was linked to a design flaw in gusset plates.
• This matters as it directly threatens catastrophic failure under normal or extreme conditions, risking lives and infrastructure.

Unstable substructure due to poor soil analysis or water scour can compromise the entire bridge.

• Evidence of settling, tilting piers, or erosion around abutments shortly after construction.
• Example: The 2018 Genoa bridge collapse was partly attributed to degradation of the pier foundations.
• This is critical because foundation failures are often hidden and can lead to sudden, progressive collapse without obvious warning signs.

Poor workmanship and non-compliance during the build phase introduce latent defects.

• Improper concrete curing, inadequate welding, or failure to follow engineered specifications for bolt torque and alignment.
• Use case: Discovering honeycombed concrete or corroded rebar during early inspections.
• This matters because construction flaws weaken the bridge from day one, accelerating deterioration and reducing its intended lifespan.

Water infiltration and freeze-thaw damage are accelerated by flawed drainage design.

• Clogged or absent weep holes, poor deck sealing, and inadequate slope leading to ponding.
• Example: Rapid concrete spalling and rebar corrosion in bridge decks in northern climates.
• This is a watchlist item because chronic water exposure is a leading cause of corrosion and concrete degradation, demanding early intervention.

Restrained thermal movement can induce catastrophic stress if expansion joints fail or are omitted.

• Joints that are blocked, seized, or designed with insufficient range for local temperature extremes.
• Use case: A bridge deck buckling or cracking during a heatwave due to locked joints.
• This matters as it can cause sudden, severe damage to the deck and supports, requiring immediate closure and repair.

Non-redundant structural systems lack alternative load paths if a single component fails.

• Designs relying on a few critical, fracture-critical members without backups.
• Example: Some older two-girder bridges where one girder failure causes total collapse.
• This is a critical red flag because it eliminates a margin of safety, making the structure vulnerable to unexpected events or localized damage.