2.2 Basic Rigging Hitches: Vertical, Choker, Basket & Bridles

Basic Rigging Hitches: Vertical, Choker, Basket & Bridles

A rigging hitch defines the physical arrangement and geometry in which a sling connects a load to a lifting hook. The choice of hitch directly dictates the Working Load Limit (WLL) of the sling assembly, the degree of rotational control, and the overall stability of the suspended object. Under ASME B30.9, all rigging configurations are derived from four fundamental hitches:

1. Vertical (Straight) Hitch
2. Choker Hitch
3. Basket Hitch
4. Bridle Hitch (Multi-Leg)

      VERTICAL               CHOKER                BASKET                BRIDLE (2-LEG)
        |                      |                    /    \                   /    \
        |                      |                   /      \                 /      \
        |                      |                  /        \               /        \
      [LOAD]                 (Choke)             |  [LOAD]  |             /          \
                             [LOAD]              +----------+          [Point 1]    [Point 2]
                                                                           +--------------+
                                                                           |     LOAD     |
                                                                           +--------------+
    WLL = 1.0 x Vert      WLL = 0.75 x Vert      WLL = 2.0 x Vert*      WLL = 2 x Vert x sin(θ)

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1. Vertical (Straight) Hitch

A vertical hitch consists of a single sling leg connecting the crane hook directly to a single lifting attachment point on the load.

Operational Characteristics

• Capacity Factor: 1.0 (100% of tagged single-leg WLL).
• Rotational Control: Zero. The load is completely free to spin, which can induce twisting stresses, unlay hand-tucked wire rope splices, or unscrew threaded attachment hardware.
• Pick Point Alignment: The single attachment point must be positioned directly above the Center of Gravity (CG). If the pick point is offset from the CG, the load will immediately tilt until the CG swings directly below the crane hook, creating severe lateral shifting.
• Typical Application: Used primarily with lifting beams, spreader bars, engineered vertical pad eyes, or for hoisting symmetrical, balanced components with a dedicated center lift lug.

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2. Choker Hitch

A choker hitch is formed by passing the sling body around the load and feeding one eye (or end fitting) through the opposite eye or through sliding choker hardware.

Operational Characteristics

• Capacity Factor: Standard rated capacity is 0.75 to 0.80 (75% to 80% of single-leg vertical WLL) when the natural choke angle is 120° or greater.
• Load Locking & Friction: The tension produced by the hoist pulls the choking eye tight against the load body, creating radial clamping friction that holds loose or cylindrical items together.
• Single vs. Double Choker: A single choker hitch does not prevent the load from spinning or slipping out laterally during transit. When hoisting long structural steel beams, pipe, or rebar, riggers must use two choker hitches spaced evenly on opposite sides of the CG, often combined with a spreader beam.
• Crucial Rule on Setting the Choke: Riggers must NEVER drive, pound, or force the choking eye down against the load with a hammer or mallet. Forcing the eye induces severe acute bending stress across the sling body and damages the eye splice. The choke must be permitted to assume its natural position when hoisted.

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3. Basket Hitch

A basket hitch is formed by passing the sling underneath the load and attaching both eyes (or end fittings) directly to the crane hook or an overhead master link.

Operational Characteristics

• Capacity Factor: Achieves 2.0 (200% of single-leg vertical WLL) ONLY when both sling legs are completely vertical and parallel (forming a 90° horizontal angle to the load).

• Angular Reduction: If the legs converge toward a single hook at an angle less than 90°, the basket capacity drops significantly according to the horizontal sling angle formula:

  WLL_basket = 2 × WLL_vert × sin(θ)

• Load Balance Requirement: A single basket hitch provides no clamping action. The load simply rests in the cradle of the sling. If the load shifts or the CG is off-center, the load will slide through the sling belly and tip out. Double-wrap basket hitches or dual slings positioned on either side of the CG are required for unattached loads.

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4. Bridle Hitches (2-Leg, 3-Leg & 4-Leg)

Bridle hitches utilize two, three, or four sling legs connected to a common master link or shackle at the crane hook, with each individual leg terminating at a separate pick point on the load.

2-Leg Bridles

• Spreads the load over two attachment points, providing excellent stability along one horizontal axis.

• Capacity is calculated by multiplying the sum of the leg capacities by the sine of the horizontal sling angle:

  WLL_bridle = 2 × WLL_leg × sin(θ)

3-Leg and 4-Leg Bridles: Rigid vs. Flexible Structures

A critical concept tested on NCCER Basic and Advanced exams is the mechanical behavior of rigid versus flexible structures in 3-leg and 4-leg bridle assemblies:

          RIGID LOAD BEHAVIOR (4-LEG BRIDLE)
                    [ Crane Hook ]
                        /  |  \
                       /   |   \
                      /    |    \
                     /     |     \
          [Leg 1]   /  [Leg 2]    \   [Leg 3]   [Leg 4: SLACK/UNLOADED]
         (TENSION) /  (TENSION)    \ (TENSION)  (Carries ~0% Load)
                  /        |        \
                 v         v         v
              +-------------------------+
              |   RIGID STEEL CHASSIS   |  <-- Three points define a plane;
              +-------------------------+      the 4th leg carries almost nothing!

1. Statistically Indeterminate Loading: In a 4-leg bridle lifting a rigid, non-deflecting structure (e.g., heavy steel machinery skid, reinforced precast concrete panel, thick machine base), geometric tolerance differences of mere fractions of an inch between sling lengths or lug heights will cause only two diagonally opposed legs (or at most three legs) to support virtually the entire weight.

2. The Golden Rule for 4-Leg Bridles on Rigid Loads: When calculating sling sizing for a 4-leg bridle lifting a rigid object, always assume that only 2 legs carry the entire load weight:

   Required Leg WLL = Total Load Weight / (2 × sin(θ))

   The remaining two legs serve strictly to balance the load and maintain level orientation.

3. Flexible Loads: For flexible loads (e.g., thin steel plates, wire mesh baskets, flexible tanks) that can deflect and equalize sling tension, all 4 legs may be considered to share load proportionally.

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Comparative Hitch Matrix

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Load Stability: Center of Gravity & Pick Point Geometry

Load stability depends on the spatial relationship between the attachment pick points and the Center of Gravity (CG):

• Stable Equilibrium: Pick points are located ABOVE the center of gravity. If the load is tilted by wind or dynamic crane movement, gravity acts as a restoring force, swinging the load back to level.
• Unstable Equilibrium (Top-Heavy Inversion): If pick points are located BELOW the center of gravity (e.g., choking the bottom flange of a tall, heavy electrical transformer), the load is top-heavy. As soon as the load is hoisted, any disturbance will cause the top-heavy mass to rapidly flip 180° upside down, resulting in lost rigging control, severed slings, or catastrophic impact.