Understanding Vertical Share vs. Tension

🎯 The Two Critical Values

When calculating rigging loads, you must understand the difference between vertical share and sling tension. These are NOT the same thing!

📊 Vertical Share (Vertical Reaction)

What it is: The portion of the total load weight that a pick point must support vertically.

Think of it this way: If you could magically place a scale under each pick point, the vertical share is what that scale would read.

For a centered load on a 2-leg bridle:

Vertical Share (each leg) = Total Weight ÷ 2

Example: 10,000 lb load, centered

• Left leg vertical share: 5,000 lbs
• Right leg vertical share: 5,000 lbs
• Both scales would read 5,000 lbs

---

⚡ Sling Tension (Actual Force in the Leg)

What it is: The actual force inside the sling leg material. This is ALWAYS higher than the vertical share when the sling is at an angle!

Why it's higher: Because the sling is pulling at an angle, it must generate extra force to support the vertical load.

Formula:

Tension = Vertical Share × Load Angle Factor
Load Angle Factor = 1 ÷ sin(angle from horizontal)

---

🔍 Visual Example

Imagine a 10,000 lb load with centered CG, span = 120", hook height = 60" above pick points.

        🪝 Hook (at 60" height)
       /   \
      /     \
     /       \
    /  60°    \
   /           \
  📍-----------📍  (120" span)
  Pick 1    Pick 2
     10,000 lbs

Calculations:

Step 1: Vertical Share

• Load is centered, so each pick point carries half
• Vertical Share (each) = 10,000 ÷ 2 = 5,000 lbs

Step 2: Find the Angle

• Horizontal distance from pick to hook = 60"
• Vertical height = 60"
• Angle from horizontal = arctan(60/60) = 45°

Step 3: Load Angle Factor

• Factor = 1 ÷ sin(45°) = 1 ÷ 0.707 = 1.414

Step 4: Sling Tension

• Tension = 5,000 × 1.414 = 7,071 lbs

---

🚨 Critical Insight

The vertical share was 5,000 lbs, but the sling tension is 7,071 lbs!

This is why shallow angles are dangerous — as the angle decreases, the tension multiplier increases dramatically:

| Angle | Factor | Tension (if vertical = 5,000 lbs) | |-------|--------|----------------------------------| | 90° | 1.000 | 5,000 lbs | | 60° | 1.155 | 5,775 lbs | | 45° | 1.414 | 7,071 lbs | | 30° | 2.000 | 10,000 lbs (2× the vertical!) | | 15° | 3.864 | 19,319 lbs (nearly 4× !) |

---

✅ Key Takeaways

1. Vertical Share = What the pick point supports vertically (from the load weight distribution)
2. Sling Tension = The actual force in the sling (always higher when at an angle)
3. Load Angle Factor = The multiplier from angle geometry
4. Never use vertical share to select slings — you MUST calculate tension!
5. Steeper angles (closer to 90°) = lower tension
6. Shallow angles (below 30°) = dangerously high tension

---

🎓 Remember

If someone asks "what's the load in that leg?" — they probably mean TENSION, not vertical share!

Always specify which value you're discussing to avoid confusion and dangerous mistakes.