11.3 Climbing Systems (SRS/MRS), Rigging Physics, Vector Forces & Controlled Removal

11.3 Climbing Systems (SRS/MRS), Rigging Physics, Vector Forces & Controlled Removal

Arboricultural climbing, complex rigging, and technical tree dismantling represent the most physically demanding and high-liability operational domains within modern arboriculture. Every ascent, cut, and lowered limb alters structural load paths and subjects biological wood fibers to severe dynamic stresses. For the Board Certified Master Arborist (BCMA), operational mastery requires combining biological structural assessment with the rigorous physics of vector mechanics, material science, and ANSI Z133 Safety Requirements for Arboricultural Operations.

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Professional Tree Climbing Systems: SRS vs. MRS

Modern arborists access the canopy using two fundamental rope-based access configurations, each possessing distinct biomechanical advantages, friction dynamics, and anchor force distributions.

ROPE CLIMBING SYSTEM ARCHITECTURES

STATIONARY ROPE SYSTEM (SRS / SRT)       MOVING ROPE SYSTEM (MRS / DdRT)

          ( Overhead Canopy TIP )                ( Friction Saver / TIP )
                     o                                      o
                    / \                                    / \
                   /   \                                  /   \
                  /     \                                /     \
    Canopy Line  /       \ Basal Anchor                 /       \
   (Stationary) /         \ (Tied to Trunk)            /         \
               /           \                          /           \
              /             \                        /             \
         [Climber]           \                  [Climber]           \
   (Ascends 1:1 Directly)     \               (Tied to Hitch &      (Falling Leg;
                               o               Saddle Bridge)        Pulled 2:1)
                          [Basal Anchor]

1. Stationary Rope System (SRS / SRT)

In a Stationary Rope System (SRS)—historically termed Single Rope Technique (SRT)—the climbing line remains completely stationary relative to the tree. The line is anchored either at the base of the trunk (basal anchor) after passing over an overhead canopy tie-in point (TIP), or directly in the canopy using a specialized canopy anchor.

• Ascent and Work Positioning Mechanics: The climber ascends directly up the single static line using mechanical ascent/positioning devices (e.g., ISC Rope Wrench with a friction hitch, Rock Exotica Akimbo, or Petzl Zigzag paired with a Chicane). Ascent efficiency is high (1:1 direct travel); every meter of rope climbed corresponds to one meter of vertical elevation gained.
• Friction and Redirects: Because the rope does not move through the canopy union, canopy friction is effectively eliminated. Arborists can establish complex, multi-point redirects throughout the crown without increasing rope drag, enabling rapid lateral work positioning in wide-spreading crowns.
• Basal Anchor Force Physics: A critical engineering factor of a basally anchored SRS is that it creates a 2:1 force multiplier on the overhead canopy tie-in point. The downward tension exerted by the climber's body weight (W) on the climbing leg is matched by an equal downward tension (W) on the ground anchor leg. Thus, the overhead branch must support: $F_{\text{overhead TIP}} = 2 \times W_{\text{climber}}$ The arborist must select an overhead TIP significantly more robust than that required for a traditional canopy anchor.

2. Moving Rope System (MRS / DdRT)

In a Moving Rope System (MRS)—traditionally known as Doubled Rope Technique (DdRT)—the climbing line passes over an overhead anchor point (ideally through a ring-and-ring friction saver, pulley saver, or natural crotch) and returns to the climber's saddle.

• Mechanical Advantage: One end of the rope is attached to the saddle bridge via a hitch cord or mechanical prusik (e.g., Petzl Zigzag), while the falling leg passes through the friction device. When the climber pulls downward on the falling leg, a 2:1 mechanical advantage is achieved, halving the muscular force required to lift body weight (F = W / 2), though twice as much rope must be pulled through the system.
• Canopy Friction and Cambial Protection: Because the rope moves dynamically across the anchor with every movement, natural crotch rigging generates severe friction that burns the vascular cambium and accelerates rope sheath wear. ANSI A300 and Z133 strongly encourage the use of friction savers (cambium savers) to protect tree tissues and ensure predictable hitch friction.

Work-Positioning Lanyards and the Two-Point Attachment Rule

• Arborist Saddles: Feature floating waist bridges constructed of high-strength hollow-braid rope or webbing that allow multi-axis pelvic rotation during lateral branch walking.
• Fliplines / Lanyards: Work positioning lanyards allow the arborist to stabilize their stance on vertical trunks or scaffolds. When operating a chainsaw aloft, ANSI Z133 mandates the use of a steel-core (wire-core) flipline to provide cut resistance against accidental, momentary chainsaw chain contact.
• The Two-Point Attachment Mandate (ANSI Z133): Arborists must be secured with at least two independent points of attachment (e.g., primary climbing system plus a work-positioning lanyard) whenever operating a chainsaw aloft in a tree. A single point of attachment is permitted only during initial ascent or when the arborist can demonstrate that a second point creates a greater operational hazard (e.g., severe entanglement risk during emergency felling).

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Rigging Physics, Vector Forces & Dynamic Shock Loading

Tree rigging is the controlled lowering of severed tree sections using ropes, blocks, friction devices, and anchor trees. Because green wood is extremely dense and heavy, rigging operations generate massive dynamic forces that easily exceed structural safety thresholds if not meticulously calculated.

VECTOR FORCES ON A RIGGING BLOCK ANCHOR

                    [Overhead Rigging Point]
                              (O)
                             / | \
                            /  |  \  Angle Between Lines (theta)
             Load Line     /   |   \    Lead Line to Lowering Device
            (To Severed   /    |    \   (To Port-a-Wrap at Base)
               Log)      /     |     \
                        v      v      v
                    [Load]          [Ground Anchor]

         Anchor Force Formula: F_anchor = 2 * F_load * cos(theta / 2)
         When lines are parallel (theta = 0 deg): F_anchor = 200% of F_load!

The Rigging Vector Force Multiplier

When a rigging line passes through an overhead arborist block (pulley) to lower a log, the total downward force exerted on the rigging anchor point (F(anchor)) is the vector sum of the load line tension (F(load)) and the lead line tension (F(lead)). Assuming friction in the pulley is negligible, F(lead) ≈ F(load).

The resultant anchor force is governed by the vector angle formula: $F_{\text{anchor}} = 2 \times F_{\text{load}} \times \cos\left(\frac{\theta}{2}\right)$ Where θ is the angle between the load line entering the block and the lead line exiting to the ground lowering device.

• Parallel Lines (θ = 0°): In standard butt-hitching where the line drops vertically to a trunk-mounted lowering device at the base of the tree, θ ≈ 0°. Because cos(0°) = 1: $F_{\text{anchor}} = 2 \times F_{\text{load}} \times 1 = 200\% \text{ of } F_{\text{load}}$ The anchor point experiences twice the dynamic weight of the falling piece! A 1,000-lb dynamic load exerts a crushing 2,000-lb downward force on the rigging anchor union.

Dynamic Shock Loading: Kinetic Energy Conversion

When a severed branch or trunk section free-falls before the rigging line catches it, gravitational potential energy is converted into kinetic energy: $E_p = m \cdot g \cdot h$ Where m is the mass of the wood, g is gravitational acceleration, and h is the vertical fall distance before the rope becomes taut.

When the rope arrests this falling mass, the kinetic energy must be dissipated through work done by the elongation (stretch) of the rigging line, the elastic bending of the tree trunk, and friction across lowering devices: $F_{\text{dynamic peak}} = \frac{m \cdot g \cdot h}{\Delta L} + m \cdot g$ Where ΔL is the total stopping distance (rope stretch plus tree deflection).

• The Catastrophe of the Static Catch: If a non-elastic line is used and the ground crew locks the line rigidly against a bollard without allowing slippage, ΔL approaches zero. A 500-lb log falling just 4 feet can generate an instantaneous dynamic shock load exceeding 6,000 to 8,000 lbs (12 to 16 Gs), easily snapping rigging blocks, breaking synthetic lines, or catastrophically snapping the top of the tree.
• Dynamic Friction Management: Ground crew must manage trunk-mounted friction devices (e.g., Port-a-Wrap, GRCS) to let the piece run—allowing a controlled length of line to slip during deceleration. This extends the stopping distance (increasing ΔL), smoothly dissipating kinetic energy as thermal energy across the friction cylinder and preventing shock-load spikes.
• Rope Material Science: Rigging lines utilize high-tenacity polyester (low stretch, high abrasion resistance) or polyester-nylon double braids. High-stretch nylon absorbs dynamic shock well but creates unpredictable rebound, while static ultra-high-molecular-weight polyethylene (Dyneema) possesses near-zero stretch and must never be shock-loaded without dynamic lowering devices.

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Sectional Dismantling & Technical Felling Mechanics

Controlled removal requires specialized cutting techniques to steer, orient, and drop heavy timber away from targets.

Advanced Rigging Techniques

1. Butt-Hitching: The rigging line is tied at the butt (cut face) of a vertical trunk section, passing through an overhead block mounted just below the cut. The severed log drops vertically past the block, generating high dynamic shock loads that require skilled ground friction control.
2. Tip-Tying: The line is tied at the distal tip of a heavy lateral branch. As the cut progresses, the butt drops while the tip is suspended, inverting the limb into a vertical orientation to thread between tight obstacles.
3. Balancing: A balancing sling (spider leg) is attached to the branch at its center of gravity. When severed, the limb remains perfectly horizontal in mid-air, allowing the piece to be floated away from delicate rooflines or landscaping.
4. Speedlining: A tensioned highline cable or synthetic line runs from the upper canopy down to a distant ground anchor. Severed branches are attached via trolleys or carabiners and slid down the line directly to the chipper, completely bypassing obstacles beneath the tree.

TECHNICAL FELLING NOTCH ARCHITECTURE

CONVENTIONAL NOTCH (45 deg)           OPEN-FACE NOTCH (70-90 deg)

         /|                                  /|
        / | <--- Angled Cut                 / | <--- Angled Top Cut (45-50 deg)
       /  |                                /  |
      +---+ <--- Flat Base Cut            +   | <--- Apex (Hinge forms here)
      |   |                                \  |
      | H | <--- Hinge (~10% Diameter)      \ | <--- Angled Bottom Cut (30-40 deg)
      |   |                                  \|
      +---+ <--- Back Cut (1-2 in Above)     -+-- <--- Back Cut (Level with Apex)

[Hinge Closes at 45 deg Fall]          [Hinge Guides Fall Completely to Ground]

Felling Cuts: Hinge Mechanics and Notch Profiles

Directing a falling tree or trunk section requires creating an engineered mechanical wooden axle known as the hinge.

1. The Hinge Dimensions:
   • Thickness: Approximately 10% of the trunk diameter (0.10 × D).
   • Width (Length): Approximately 80% of the trunk diameter (0.80 × D).
   • Strict Mandate: Never cut through the hinge wood! The hinge controls the direction, speed, and lateral stability of the falling tree. Severing the hinge releases all mechanical control, predisposing the tree to unpredictable twisting, barber-chairing, or fatal kickback.
2. Notch Comparisons:
   • Conventional Notch: Formed by a horizontal lower cut and a 45-degree downward top cut. The back cut is placed 1 to 2 inches (2.5 to 5 cm) above the horizontal notch apex. Limitation: When the tree falls through a 45-degree arc, the notch faces close together, snapping the hinge while the tree is still halfway in the air.
   • Humboldt Notch: Formed by a horizontal top cut and an upward-angled 45-degree lower cut. Commonly used in timber harvesting to save log volume; back cut placed 1 to 2 inches above the flat cut.
   • Open-Face Notch (70° to 90° Notch): The modern arboricultural gold standard. Cut with a 45-degree downward top cut and a 45-degree upward bottom cut, creating a wide 70° to 90° opening. The back cut is placed level with the notch apex. Crucial Advantage: Because the notch opening is wide, the hinge remains intact and functional through almost the entire 90-degree fall, guiding the tree precisely all the way until it hits the ground.
3. Bore Cutting and Barber-Chair Prevention:
   • When felling heavy forward-leaning trees, the wood along the back of the trunk is under severe tensile stress. If an arborist makes a standard back cut from the rear forward, the tensile stress causes the trunk to fracture vertically up the middle before the cut is completed—an explosive, lethal trunk failure known as a barber-chair.
   • The Bore Cut (Plunge Cut): The chainsaw bar tip is plunged directly through the center of the trunk behind the intended hinge. The arborist carves the hinge to its precise 10% thickness from the inside out, leaving an intact holding strap of wood at the rear of the trunk. Felling wedges are driven into the cut behind the bar. Finally, the rear holding strap is severed from the outside, releasing the tree cleanly and eliminating barber-chair risk.

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Crane-Assisted Tree Removal (ANSI Z133 Requirements)

Using mobile cranes to dismantle hazardous trees eliminates dynamic shock loading and minimizes property impact, but it introduces extreme rigging and hoisting hazards governed by ANSI Z133.

CRANE RIGGING AND CENTER OF GRAVITY (CG)

                    [Crane Hook]
                         |
                         v
                        (O)
                       /   \
                      /     \ <--- Synthetic Choker Slings
                     /       \
          +---------+---------+---------+
          |         |  (CG)   |         | <--- Center of Gravity
          |    *    +---------+    *    |
          +-----------------------------+
                         |
                         v  <--- Clean Cut (Zero Drop / Zero Shock Load)
          ===============================
          Trunk / Parent Stem

1. Shock Loading Prohibition: Dynamic shock-loading of a crane is strictly prohibited by ANSI Z133. The crane must be attached to the tree section, line tension established, and green wood weight fully accounted for before the arborist completes the final cut. There must be zero free-fall; the severed piece must float cleanly away from the cut face without dropping.
2. Calculating Green Log Weight: The arborist must calculate the exact green weight (W) of every pick to verify it remains well within the crane's load chart capacity at the specific working radius: $W = V \times \rho$ Where V is the volume of the cylinder (V = π · r² · L) and ρ is the green wood density.
   • Hardwoods (e.g., Oak, Maple, Beech): ρ ≈ 55 to 65 lbs/ft³ (880 to 1,040 kg/m³).
   • Softwoods (e.g., White Pine, Fir): ρ ≈ 40 to 50 lbs/ft³ (640 to 800 kg/m³).
   • Example: A 30-inch diameter (2.5 ft diameter, r = 1.25 ft) white oak log measuring 12 feet long has a volume of V = π × (1.25)² × 12 ≈ 58.9 ft³. At 60 lbs/ft³, the pick weighs 58.9 × 60 = 3,534 lbs (plus foliar end-weight).
3. Rigging Relative to the Center of Gravity (CG):
   • Slings must always be attached above the center of gravity of the section.
   • If slings are rigged below the center of gravity, the piece will violently flip upside-down the instant the cut is completed, inducing catastrophic lateral shock loads on the crane boom and crushing the climber against the trunk.

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Technical Rigging & Dismantling Comparative Matrix (ANSI Z133)