Enter Truss Data
Use metres, kilonewtons, gigapascals, and square millimetres. Choose an even panel count for the symmetric centre joint.
Example Data Table
| Input | Example value | Purpose |
|---|---|---|
| Overall span | 24 m | Sets the supported truss length. |
| Panels | 6 | Creates a 4 m panel length. |
| Truss height | 4 m | Sets the chord separation. |
| Uniform load | 10 kN/m | Represents gravity loading across the roof line. |
| Elastic modulus | 200 GPa | Estimates movement for a steel-like material. |
| Chord area | 6000 mm² | Used for the upper and lower chord members. |
Formula Used
The calculator applies the direct stiffness method to a two-dimensional pin-jointed truss. Every member is treated as an axial element.
Here, L is the overall span and n is the number of panels. An even panel count provides a central top node for the optional point load.
Interior top joints receive one panel length. The two end top joints receive half a panel length.
E is elastic modulus, A is cross-sectional area, and l is member length. The program assembles all element matrices into the global system.
The restrained support movements are zero. Solved joint displacements produce reactions and member extensions.
Positive axial force is shown as tension. Negative axial force is shown as compression. Stress is reported in MPa when force is in kN and area is in mm².
How to Use This Calculator
- Enter the support-to-support span and an even number of panels.
- Enter the vertical truss height between the chord centre lines.
- Enter downward uniform loading and any central top-joint load.
- Choose a load factor appropriate for your study case.
- Enter material modulus, chord area, web area, and an allowable stress.
- Select Calculate Truss Forces to display reactions, forces, stresses, and movement.
- Review the largest force, stress, and vertical deflection first.
- Use the CSV button for the member table or print the page to save a PDF.
Pratt Truss Force Basics
Recognizing the Structural Form
A Pratt truss is a triangulated framework. Its top and bottom chords resist global bending. Vertical members connect matching panel points. Diagonals slope toward the center of the span. Under downward gravity loading, the diagonals commonly work in tension. The upper chord commonly carries compression. The lower chord commonly carries tension. These patterns make the form useful for bridges, roofs, and long supported structures. Actual force signs still depend on loading, supports, geometry, and joint idealization.
Support and Load Assumptions
The calculator uses a parallel chord Pratt layout. Both supports sit at bottom chord ends. The left support restrains horizontal and vertical movement. The right support restrains vertical movement only. Loads act downward at top joints. A uniform line load is converted into equivalent joint loads. End joints receive one half panel contribution. Interior joints receive one full panel contribution. An optional center load is applied at the center top joint. This configuration provides a clear idealized structural model.
How Forces Are Solved
Member forces come from two dimensional truss analysis. Every joint is treated as a pin. Each member carries axial force only. Bending, shear, joint eccentricity, and self weight are not included. The program assembles a stiffness matrix for all members. It then solves joint displacements. Axial extensions produce member forces. Positive values indicate tension. Negative values indicate compression. This method also estimates vertical deflection. Material modulus and member areas mainly influence predicted movement in this determinate idealization.
Geometry and Force Paths
Geometry strongly affects force paths. A greater truss height usually reduces chord force for the same span and load. Very shallow trusses can create large chord forces and noticeable movement. Panel count changes member lengths and connection locations. More panels can shorten individual members. However, it also creates more joints and fabrication work. Keep panel widths practical. Use a height that suits clearance, roof slope, transport limits, and design rules. Always model the real support spacing carefully.
Stress Does Not Finish the Design
Stress is calculated from axial force divided by cross sectional area. The displayed utilization compares this stress with the entered allowable stress. It is only a screening value. Compression members can fail by buckling before material stress reaches the limit. Connection capacity can also control the design. Check slenderness, effective length, gusset plates, welds, bolts, bearing, and fatigue where relevant. Use factored loads and design-code resistance checks for final engineering work. Treat this page as a learning and preliminary assessment tool.
Reviewing the Output
Review the reaction values before trusting the member table. The vertical reactions should closely equal total applied vertical load. Small differences may result from rounding. Examine the largest force and largest stress first. Then inspect compression members for buckling risk. Compare displacement with project serviceability limits. Change one input at a time when studying alternatives. Keep units consistent. Enter metres, kilonewtons, gigapascals, and square millimetres as labelled. For unusual loads, wind effects, moving loads, or nonstandard supports, use a complete structural model. Independent professional review remains essential before construction begins.
Frequently Asked Questions
What type of Pratt truss does this page model?
It models a symmetric, parallel-chord Pratt truss. The supports are at the bottom-chord ends. Vertical members join matching top and bottom nodes. Diagonals lean toward the centre of the span.
Which loads can I enter?
You can enter a uniform downward line load across the top chord and a downward point load at the centre top joint. Both values are multiplied by the selected load factor.
Why must the panel count be even?
An even count creates a central panel point. This keeps the geometry symmetric and provides a clear location for the optional centre point load.
What does a positive member force mean?
A positive force means the member is in tension. A negative force means compression. Very small values are labelled near zero to reduce confusion from numerical rounding.
Why do elastic modulus and area affect deflection?
They define axial stiffness. Stiffer members extend or shorten less under load. In this ideal determinate truss, force distribution is mainly controlled by geometry and load placement.
Does utilization check member buckling?
No. Utilization compares axial material stress with your entered allowable stress. Compression members also need separate buckling checks using effective length, end conditions, slenderness, and lateral restraint.
Are connection forces included?
The member table gives axial member force. Design of gusset plates, bolts, welds, bearing, block shear, and local connection effects requires a separate connection analysis.
Why might reactions differ slightly from the total load?
The reactions should balance the applied vertical load. Small apparent differences usually come from rounded values shown on the screen, not from the underlying calculation.
Can this calculate wind or asymmetric loading?
This version is intended for symmetric vertical gravity loading. Wind, uplift, horizontal loads, unsymmetrical point loads, and moving loads need a broader structural model.
Why can a taller truss reduce chord force?
A taller truss creates a larger internal lever arm. The chords can then develop the moment-resisting force couple with less axial force for similar span and loading.
Can I use these values for final construction design?
No. Use them for education, preliminary sizing, and comparison studies. Final work needs verified loads, member design, buckling checks, connections, lateral bracing, code combinations, and qualified engineering review.