12.2 Likelihood of Failure, Likelihood of Impact & ISA Risk Matrix Rating

12.2 Likelihood of Failure, Likelihood of Impact & ISA Risk Matrix Rating

The ISA Tree Risk Assessment Qualified (TRAQ) methodology eliminates arbitrary numerical point-scoring systems in favor of an objective, standardized two-matrix qualitative rating system. By separating the likelihood of an event from the magnitude of its consequences, the TRAQ framework allows the Board Certified Master Arborist (BCMA) to systematically evaluate structural defects, aerodynamic wind loads, and target exposure. Mastering this system requires deep familiarity with the definitions of likelihood, consequence tiers, matrix lookup mechanics, and the underlying biomechanical failure criteria of woody stems.

---

The ISA Qualitative Risk Framework

THE TWO-TIERED ISA TRAQ RISK MATRIX FLOW

  +------------------------+      +------------------------+
  |  Likelihood of Failure |      |  Likelihood of Impact  |
  | (Improbable, Possible, |  x   |  (Very Low, Low,       |
  |   Probable, Imminent)  |      |   Medium, High)        |
  +------------------------+      +------------------------+
               |                              |
               +--------------+---------------+ 
                              |
                              v
                 ============================
                 MATRIX 1: LIKELIHOOD MATRIX
                 Yields: Likelihood of Failure & Impact
                 (Unlikely, Somewhat Likely, Likely, Very Likely)
                 ============================
                              |
               +--------------+---------------+ 
               |                              |
               v                              v
  +-------------------------+    +-------------------------+
  |  Likelihood of Failure  |    |  Consequences of Fail   |
  |        & Impact         | x  | (Negligible, Minor,     |
  | (from Matrix 1)         |    |   Significant, Severe)  |
  +-------------------------+    +-------------------------+
               |                              |
               +--------------+---------------+ 
                              |
                              v
                 ============================
                 MATRIX 2: RISK RATING MATRIX
                 Yields: FINAL RISK RATING
                 (Low, Moderate, High, Extreme)
                 ============================

The Standard Assessment Time Frame

Under ISA TRAQ standards, risk ratings are evaluated against a standard specified time frame—typically 1 year (12 months) unless explicitly stated otherwise. Likelihood of failure cannot be judged indefinitely because trees continually produce compensatory wood (reaction wood and woundwood), compartmentalize decay, or suffer progressive structural degradation. All likelihood ratings assume normal, expected regional weather conditions within that time frame, excluding catastrophic, design-exceeding phenomena like F4/F5 tornadoes or Category 4/5 hurricanes unless specific storm-hardening standards are under study.

---

Categorizing Likelihood & Consequences

Likelihood of Failure Categories

1. Improbable: Failure is not expected under normal, ambient weather conditions within the specified time frame (e.g., sound trunk, healthy branch union, minor superficial wound).
2. Possible: Failure could occur under normal weather conditions, but it is unlikely during the specified time frame (e.g., moderate cavity with adequate sound shell thickness, stable codominant stems with slight included bark).
3. Probable: Failure is expected under normal, anticipated weather conditions within the specified time frame (e.g., extensive basal rot exceeding failure thresholds, major dead scaffold branch overextended over a high-use area, active longitudinal shear crack).
4. Imminent: Failure has started, is actively occurring, or is expected momentarily under ambient weather conditions (e.g., freshly heaved root plate with snapped tension roots, stem fractured halfway through, split crotch actively opening and closing in the wind). Requires immediate emergency target exclusion or tree felling.

Likelihood of Impact Categories

Target impact likelihood depends on the occupancy rate and target location relative to the tree part's fall zone:

1. Very Low: The target zone is rarely or never occupied; targets are seldom present, or the tree part would fall into an unoccupied brush area or remote woodland.
2. Low: Infrequent or temporary occupancy (e.g., secondary maintenance trail, rarely visited corner of a residential backyard, access road used once weekly).
3. Medium: Intermittent or recurring occupancy (e.g., suburban driveway, park walking path, picnic table with predictable daily or weekend use, semi-frequented parking stalls).
4. High: Constant, continuous, or predictable high-density occupancy (e.g., primary occupied residence, day care facility, elementary school classroom, heavily traveled multi-lane arterial highway, primary rail line).

Matrix 1: Likelihood Matrix Lookup

Matrix 1 synthesizes the Likelihood of Failure and the Likelihood of Impact to establish the combined Likelihood of Failure & Impact:

Consequences of Failure Categories

Consequences are based on the value of property, the potential for personal injury, and the disruption of critical societal infrastructure:

1. Negligible: Minor, low-cost property damage that is easily repaired or replaced (e.g., damaged turf, broken landscape shrub, broken clay flowerpot); no personal injury.
2. Minor: Low-to-moderate repairable property damage (e.g., crushed chain-link fence, dented garden shed roof, cracked windshield); minor personal injury requiring basic first aid on scene.
3. Significant: Substantial structural property damage (e.g., crushed vehicle, structural penetration of residential roof, damaged outbuilding); serious personal injury requiring emergency medical hospitalization; disruption of secondary public roads or localized utility distribution lines.
4. Severe: Catastrophic structural collapse of an occupied primary residence, school, or commercial building; permanent disabling personal injury or loss of human life; prolonged disruption of critical lifeline utilities (e.g., municipal water supply mains, regional high-voltage transmission lines, hospital power feed).

Matrix 2: Risk Rating Matrix Lookup

Matrix 2 pairs the Likelihood of Failure & Impact (derived from Matrix 1) with the Consequences of Failure to generate the final qualitative Risk Rating:

Understanding the Final Risk Ratings

• Low Risk: Low failure likelihood or negligible consequences. Minor defects that require no immediate intervention; standard routine re-inspection cycle.
• Moderate Risk: Mitigation actions are typically recommended, but work can be scheduled as part of standard planned maintenance budgets.
• High Risk: Serious defects with high target occupancy. Requires prompt mitigation (pruning, target management, cabling, or removal) within days to weeks.
• Extreme Risk: Imminent failure threatening high-value targets or human life with severe consequences. Demands immediate emergency intervention (evacuation, street closure, target exclusion, or immediate felling).

---

Biomechanical Assessment of Key Structural Defects

A qualitative risk rating must be grounded in precise biomechanical analysis of wood strength, geometry, and loading dynamics.

HOLLOW CYLINDER & T/R BUCKLING THRESHOLD

           +-----------------------------+ <--- Outer Radius (R)
           |     SOUND WOOD SHELL        |
           |   +---------------------+   | <--- Residual Wall Thickness (t)
           |   |                     |   |
           |   |   INTERNAL CAVITY   |   |
           |   |    (DECAY COLUMN)   |   |
           |   |                     |   |
           |   +---------------------+   |
           |      t / R >= 0.30 - 0.33   |
           +-----------------------------+
  [t/R < 0.30]: Thin shell undergoes tangential buckling (flattening)
  [Open Cavity > 30% Circumference]: Torsional shear failure occurs

1. Internal Wood Decay & Stem Hollows: The t/R Ratio

Trees are optimized composite load-bearing columns. When internal heartwood decays, the trunk functions mechanically as a hollow tube or cylinder. In classic engineering beam theory, the outer fibers of a cylinder resist the vast majority of bending moments, while the central core experiences minimal stress.

• Mattheck's t/R Criterion: German biomechanicist Claus Mattheck established the foundational rule of thumb for hollow stem stability: $\frac{t}{R} \ge 0.30\text{ to }0.33$ where t is the residual sound wood shell thickness and R is the radius of the stem (or t/D ≥ 0.15, where D is diameter).
• Mechanisms of Shell Buckling: As long as the sound wood shell thickness is at least 30% to 33% of the trunk radius, the trunk retains substantial bending strength. However, when decay progresses and t/R falls below 0.30, the outer shell becomes vulnerable to transverse shell buckling (flattening/kinking). Under wind loads, compressive forces on the leeward side cause the thin shell to buckle outward or collapse inward like a crushed aluminum can, leading to catastrophic snap-off.
• Cavity Openings and Circumferential Disruption: The t/R ratio assumes a complete, closed cylinder of sound wood. If the cavity is open to the exterior (an open cavity or "C-shaped" cross-section):
  • The 30% Opening Rule: If the circumferential width of the cavity opening (C(cavity)) exceeds 30% of the total trunk circumference (C(cavity) / C(trunk) > 0.30, or an opening arc > 120°), the cross-section loses its closed-tube structural efficiency.
  • Torsional Shear Failure: Under swirling, turbulent winds, trees experience intense torsional twisting. An open cavity cannot transmit shear stresses circumferentially; the edges of the opening curl inward or slide past each other, leading to rapid longitudinal splitting and collapse.
• Longitudinal versus Circumferential Decay: Fungi that form vertical, cylindrical decay columns within heartwood (e.g., Laetiporus, Phaeolus) leave outer sapwood rings intact, maintaining functional beam capacity. In contrast, fungi or defects that spread circumferentially or cause ring shake (delamination between annual rings) destroy the composite shear transfer between wood layers, multiplying failure risk.

2. Cracks and Mechanical Fissures

Cracks represent planar mechanical separations of wood fibers and are among the most dangerous structural defects:

STRUCTURAL CRACK TYPES & FAILURE SIGNATURES

  [Transverse Tensile Crack]          [Ribbed Shear Crack (Longitudinal)]
       (Horizontally Across Grain)             (Vertically Along Grain)
             |                                      |
             v                                      v
  - Xylem fibers pulled apart            - Formed by torsional twisting
  - Ultimate tensile yield exceeded      - Tree forms longitudinal woundwood ribs
  - Failure IMMINENT under wind          - "Ram's horns" if curled into cavity

• Transverse Tensile Cracks: Cracks that run horizontally across the wood grain on the tension side of a leaning stem or bend. These indicate that the wood fibers have exceeded their ultimate tensile yield strength and have physically torn apart. A transverse tensile crack indicates active, imminent mechanical failure; the remaining section cannot sustain additional wind load.
• Ribbed Longitudinal Cracks (Shear Cracks): Vertical cracks running along the grain, frequently associated with internal decay or frost action. Under wind-induced twisting (torsion), the opposing sides of the crack slide back and forth. The tree attempts to stabilize the shear crack by forming pronounced longitudinal ridges of dense woundwood (woundwood ribs or "ram's horns" if the woundwood curls inward into an internal hollow).
• Frost Cracks: Radial longitudinal splits initiated from internal wood wounds during rapid temperature drops in sub-zero winter conditions. While aesthetically alarming, frost cracks that form sound external woundwood ribs without internal decay rarely cause whole-tree failure.

3. Codominant Stems with Included Bark

Codominant stems occur when two or more stems of similar diameter arise from a single junction without a dominant leader:

• Anatomy of the Union: In a structurally sound U-shaped branch junction, wood fibers from the branch and trunk grow together and interlock across the branch bark ridge, forming a strong mechanical collar.
• The Included Bark Mechanism: In a narrow, V-shaped codominant junction, the stems press tightly against each other as they expand radially. Instead of interlocking xylem, the bark is pinched and folded inward between the stems (included bark).
• The Compressive Wedge: The trapped bark acts as a continuous internal mechanical wedge. With every millimeter of annual radial growth, the expanding stems push each other apart, generating high internal tensile stresses across the apex of the crotch.
• Asynchronous Sway & Dynamic Cleavage: During storm events, wind causes the two codominant canopies to sway asynchronously (out of phase). The stems pull apart, subjecting the unjoined crotch to massive cleavage forces. The junction is held together only by a thin band of outer sapwood along the lateral perimeter, resulting in high probability of catastrophic splitting.

CODOMINANT UNION BIOMECHANICS

      U-Shaped Sound Union                 V-Shaped with Included Bark
          \         /                              \       /
           \       /                                \     /
            \ BBR /  <-- Interlocking Xylem          \ ! /   <-- Trapped Bark Wedge
             \   /        Branch Bark Ridge           \ /        (No Fiber Union)
              | |                                      V  <----- Compressive Cleavage
              | |                                     / \ 

4. Leaning Trees and Root Plate Stability

Distinguishing between benign, adaptive leaning trees and unstable, failing leaning trees is one of the most critical diagnostic challenges for the BCMA: