🔹 What is LY and LZ?
In STAAD.Pro:
- LY = Effective length of member about local Y-axis
- LZ = Effective length about local Z-axis
These define buckling length of steel members.
LY & LZ Assignment – Detailed Explanation (Based on Floor Plan with Bracing)
🔷 1. Understanding the Model in Your Image
From your diagram:
- Grid spacing = 6m × 6m
- Columns at all intersections (A–D, 1–5)
- Beams in both X & Y directions
- X-bracing provided in selected bays
- One interior bay has irregular bracing (central bay)
👉 This directly controls effective length (LY, LZ) of columns.
🔷 2. What Exactly LY & LZ Represent
| Parameter | Meaning | Controls |
|---|---|---|
| LY | Buckling length about local Y-axis | Weak axis buckling |
| LZ | Buckling length about local Z-axis | Strong axis buckling |
👉 Important:
- Columns buckle in both directions
- Bracing reduces length only in that direction
🔷 3. Key Rule from Your Plan
👉 Effective length = distance between lateral restraints (bracing points)
So:
- If NO bracing → full height considered
- If bracing exists → reduced length
🔷 4. COLUMN-WISE LY & LZ EXPLANATION
🔹 (A) Corner Columns (A1, A5, D1, D5)
Condition:
- Bracing connected through adjacent bays
- Restrained in both X & Z directions
Result:
- LY = 6m
- LZ = 6m
👉 Why?
- Bracing panels are directly connected → column cannot sway freely
🔹 (B) Edge Columns (Example: A2, B1, C5)
Condition:
- Bracing only in one adjacent bay
Behavior:
- One direction → restrained
- Other direction → partially free
👉 But in your plan:
- Since beam-column joints connect to braced bays
→ stiffness transfers
Practical STAAD assumption:
- LY = 6m
- LZ = 6m
👉 Conservative & standard practice for sheds
🔹 (C) Interior Columns in Braced Bays (A3, B2, C4, etc.)
Condition:
- Fully surrounded by braced panels
Behavior:
- Restrained in both directions
Result:
- LY = 6m
- LZ = 6m
👉 These are most stable columns
🔹 (D) Interior Column in Irregular Bracing (Center Bay)
Condition:
- Only one diagonal / non-uniform bracing
Behavior:
- Unequal stiffness
- One direction slightly flexible
👉 Still in practical modeling:
- LY = 6m
- LZ = 6m
But:
⚠️ Advanced design → reduce only in braced direction
🔷 5. WHY ALL VALUES ARE 6m IN YOUR IMAGE?
This is the most important concept 👇
👉 Your bracing is provided in plan (horizontal stability)
👉 But LY/LZ for columns depend on:
✔ Vertical restraint (storey-wise)
NOT just plan bracing
🔥 Critical Insight:
In your model:
- No intermediate bracing along column height
- Only one floor level
👉 Therefore:
Effective length = full column height = 6m
🔷 6. WHEN WILL LY / LZ REDUCE IN THIS PLAN?
If you modify model:
Case 1: Add mid-height bracing (say at 3m)
Then:
| Direction | Value |
|---|---|
| LY | 3m |
| LZ | 3m |
Case 2: Bracing only in X direction
| Direction | Value |
|---|---|
| LY | 6m |
| LZ | 3m |
🔷 7. BEAM LY & LZ IN YOUR PLAN
🔹 Beams in X-direction
- Span = 6m
- Connected column to column
👉 LY = 6m
👉 LZ = 6m
🔹 If purlins / slab exist
Then:
| Parameter | Value |
|---|---|
| LY | Reduced (spacing of purlins) |
| LZ | Full span |
🔷 8. STAAD INPUT (FOR YOUR MODEL)
You can include this in your blog:
PARAMETER 1
LY 6 ALL
LZ 6 ALL👉 Or member-wise assignment:
LY 6 MEMB 1 TO 50
LZ 6 MEMB 1 TO 50🔷 9. VISUAL LOGIC
Without Vertical Bracing:
- Column behaves like:
Free to buckle entire height
→ LY = Full height
→ LZ = Full height
With Vertical Bracing:
Column split into segments
→ Buckling length reduces🔷 10. COMMON MISUNDERSTANDING (VERY IMPORTANT)
❌ “Bracing in plan reduces LY/LZ”
✔ WRONG
✅ Only bracing along column height reduces LY/LZ
🔷 11. PROFESSIONAL DESIGN TIP
For industrial structures:
- Always check:
- Local axis orientation
- Weak axis buckling
- Actual restraint condition
👉 In Structural Engineering:
Weak axis (LY) usually governs design
🔷 12. BLOG SUMMARY
In STAAD, LY and LZ represent the effective buckling length of columns in local Y and Z directions. In the given floor plan with bracing, although bracing is present in plan, it does not reduce the effective length unless intermediate vertical restraints are provided along the column height. Hence, LY and LZ are taken as full storey height (6m) for all columns.
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