Calculator Form
Example Data Table
| Case | Span m | Rise m | Spacing m | Service Load kN/m² | Load CV | Steel Fy MPa |
|---|---|---|---|---|---|---|
| Small warehouse roof | 12 | 2.2 | 3 | 1.55 | 8% | 250 |
| Medium shed roof | 18 | 3 | 4 | 1.89 | 10% | 250 |
| Wide light industrial roof | 24 | 4.5 | 5 | 2.10 | 12% | 345 |
Formula Used
Half span: span / 2
Rafter length: √((span / 2)² + rise²)
Service gravity load: dead load + live load + snow load + self weight
Load standard deviation: service gravity load × coefficient of variation
Characteristic load: service gravity load + z factor × standard deviation
Factored area load: load factor × characteristic load
Line load: factored area load × truss spacing
Reaction each support: line load × span / 2
Equivalent moment: line load × span² / 8
Chord force: equivalent moment / truss rise
Web force: support shear / sin(web angle)
Allowable stress: yield strength / safety factor
Stress utilization: calculated stress / allowable stress
Deflection: 5wL⁴ / 384EI
Deflection utilization: calculated deflection / allowed deflection
Anchors each support: uplift reaction / anchor capacity
How to Use This Calculator
Enter the roof span, rise, spacing, and panel count.
Add roof loads in kN per square meter.
Enter the statistical variation and confidence z factor.
Add steel strength and section properties.
Select a load factor and safety factor.
Press Calculate to show the result above the form.
Review utilization values. Values above one need redesign.
Download CSV or PDF for project notes.
Metal Roof Truss Design Article
Metal Roof Truss Design Overview
A metal roof truss carries roof loads through connected members. The top chord usually works in compression. The bottom chord usually works in tension. Web members transfer shear between both chords. This calculator gives a sample design check. It does not replace a licensed engineer.
Why Load Statistics Matter
Roof loads are not perfectly fixed. Snow, wind, maintenance loads, and material weights vary. A statistical allowance helps add margin. The tool uses a coefficient of variation and a z factor. These values raise the service load to a characteristic load. A higher confidence factor gives a more conservative sample result.
Main Design Checks
The calculator estimates tributary load from truss spacing and span. It then converts area load into line load. Reactions are found from simple beam equilibrium. The sample bending moment is used to estimate chord force. Web force is estimated from support shear and diagonal angle. Steel stress is compared with allowable stress. Deflection is checked against the selected span limit.
Geometry and Serviceability
Truss depth has a strong effect on chord force. A shallow truss creates larger axial demand. Greater rise usually lowers chord demand. But it can change roof slope, fabrication length, and bracing needs. Service deflection is also important. A roof that is safe can still feel weak if it moves too much. The deflection ratio helps review comfort and finish protection.
Reading the Results
Use the utilization ratios as warning signs. A value below one suggests the sample member passes that check. A value above one suggests redesign is needed. Increase member area, section modulus, truss depth, or spacing control. Check uplift separately. Wind uplift can require more anchors, even when gravity design looks safe.
Practical Notes
Real truss design needs connection checks. It also needs buckling lengths, lateral bracing, local codes, load combinations, and fabrication tolerances. Metal roofs can be light, so uplift may govern. Always confirm loads from project drawings. Use this calculator for learning, early estimates, and discussion only. Final sizing should be approved by a qualified structural professional.
Before You Decide
Compare several spacing options. Review each load source. Save the output. Share the assumptions with your engineer. Clear assumptions reduce mistakes during design review.
FAQs
What does this calculator estimate?
It estimates truss geometry, load demand, support reactions, chord force, web force, stress ratios, deflection, uplift reaction, and anchor count for a sample metal roof truss.
Is this a final structural design?
No. It is a sample calculation tool. Final metal truss design needs local code checks, connection design, bracing checks, buckling review, and approval by a qualified structural engineer.
Why does the calculator use a z factor?
The z factor adds a statistical load allowance. It increases service load based on the selected coefficient of variation. Higher z values create more conservative sample loads.
What does utilization mean?
Utilization compares calculated demand with allowable capacity. A value below one suggests the selected member passes that check. A value above one suggests revision is needed.
Why is truss rise important?
Truss rise controls the effective truss depth. More depth usually reduces chord force. Very shallow trusses often need stronger chords and better deflection control.
How is deflection checked?
The calculator uses a simple beam deflection formula. It compares calculated service deflection with a span-based limit, such as L/240 or L/360.
Why is uplift checked separately?
Wind can lift a light metal roof. Gravity checks may pass while uplift still controls anchors, bearings, and hold-down details.
Can I change steel properties?
Yes. You can enter yield strength, member area, section modulus, inertia, elastic modulus, and safety factor to test different sample member options.