Design

Roof Trusses

1. Loads
1. Top Chord Live Load (Snow)
1. Formula from ASCE 7 to Determine Top Chord Live Load
1. Pf=0.7CeCtIPg
2. Ce=Exposure Factor
3. Ct=Thermal Factor
4. I=Importance Factor
5. Pg=Ground Snow
2. Slope Factor (Cs)
3. Unbalanced Loads (windward, leeward)
4. Step Roof Condition (if applicable)
5. Top Chord Pitch -
   Examples: 4/12, 5/12, etc.
2. Top Chord Dead Load
1. Roofing Material
2. Roof Top Equipment
3. Bottom Chord Live Load
1. Attic Storage Loads, if applicable
4. Bottom Chord Dead Load
1. Ceiling material
2. Lights
3. Mechanical Equipment
4. Ceiling Insulation
2. Additional Design Factors & Loads
1. Centers -
   Examples: 2’, 4’, 8, etc.
2. Bearing Width at Heel
3. Bottom Chord & Web Lateral Bracing Location (provided by truss fabricator)
3. Permanent Truss Bracing & Connections (designed by building designer)
1. Truss X-Bracing
2. Wind Bracing
3. Connection Requirements of Bracing (by building designer)
4. Truss Connection Requirements for Uplift (truss print will show uplift reactions)
1. Anchored to Column, Direct Bearing on Column at Bearing Wall
2. Anchored to Truss Supports (Carriers) by Using:
1. Tie Down Blocks
2. Steel Anchor Clips
3. Steel Straps
3. Bolts & Nails
1. Type –
   Examples: Hardened Bolts, Ring Shank or Spiral Shank Nails
2. Size of Fasteners
3. Quantity of Fasteners
5. Equipment Loads
1. Concentrated Loads on Top or Bottom Chords
2. Location of Equipment Loads
6. Transferring Loads
1. Transferring the Roof Loads & the Equipment Loads:
1. To the Truss Supports (Carriers)
2. To the Columns
3. To the Footings
4. To the Connections & Fasteners of Each Component