Enter Truss Inputs
Use metric units in meters, kN/m², GPa, and mm². The form stays in one page, while the calculator fields adapt to large, medium, and mobile screens.
Formula Used
1) Service area load: qs = D + L + SW
2) Service line load: ws = qs × spacing
3) Factored area load: qu = (D + SW) × dead factor + L × live factor
4) Factored line load: wu = qu × spacing
5) Net uplift line load: wup = max[0, (W × wind factor − D − SW) × spacing]
6) Support reaction: R = wL / 2
7) Maximum moment: M = wL² / 8
8) Maximum shear: V = wL / 2
9) Approximate chord force: F ≈ M / depth
10) Equivalent inertia: I ≈ A × depth² / 2
11) Service deflection: δ = 5wL⁴ / (384EI)
This approach treats the truss as a simply supported beam with equivalent stiffness. It is useful for estimating loads, reactions, and serviceability during early planning.
How to Use This Calculator
- Enter the flat roof truss span and spacing.
- Add service loads for dead, live, wind uplift, and self weight.
- Enter truss depth and number of panels.
- Set material stiffness and approximate chord area.
- Adjust load factors to match your preliminary design approach.
- Click the calculate button to show results above the form.
- Review reactions, moments, uplift, chord force, and deflection.
- Use the CSV or PDF buttons to save the result summary.
Example Data Table
| Example Input or Output | Value | Unit |
|---|---|---|
| Span Length | 12.00 | m |
| Truss Spacing | 4.00 | m |
| Dead Load | 0.60 | kN/m² |
| Live Load | 0.75 | kN/m² |
| Wind Uplift | 1.10 | kN/m² |
| Truss Depth | 1.20 | m |
| Factored Reaction per Support | 50.40 | kN |
| Factored Maximum Moment | 151.20 | kN·m |
| Service Deflection | 2.96 | mm |
| Deflection Check | Within limit | - |
FAQs
1) What does this flat roof truss calculator estimate?
It estimates tributary load, line load, support reactions, shear, bending moment, uplift reaction, approximate chord force, and service deflection for a preliminary flat roof truss layout.
2) Is this suitable for final structural design?
No. It is a screening and planning tool only. Final truss sizing, connection design, load combinations, bracing, and code checks should be completed by a qualified structural engineer.
3) Why are loads entered in kN/m²?
Roof loads are commonly defined as area loads. The calculator multiplies them by truss spacing to convert them into line loads carried by one truss.
4) What is the purpose of truss depth?
Depth strongly affects stiffness and chord force. A deeper truss usually reduces chord force demand for the same bending moment and improves deflection performance.
5) Why is deflection based on equivalent inertia?
The tool uses a beam-equivalent stiffness model to estimate service deflection quickly. This is practical for early studies, but it is still an approximation.
6) How is uplift handled here?
The calculator compares wind uplift against dead load plus truss self weight. Any positive remainder becomes net uplift line load and uplift reaction.
7) What does panel count affect?
Panel count defines panel length and helps estimate an average end web force. It also guides early truss layout studies and detailing decisions.
8) Can I use different load factors?
Yes. The form includes editable dead, live, and wind factors so you can compare preliminary scenarios or align the study with your preferred design basis.