A scissor truss is a type of roof truss that provides a vaulted ceiling while still supporting the roof loads. It consists of two opposing diagonal members that cross each other, forming a "scissor" shape. The geometry of the truss determines the length of the diagonal members and the axial forces they must resist.
Why Use a Scissor Truss Calculator?
Designing a scissor truss requires accurate sizing of the diagonal members to ensure they can safely carry the imposed loads. The calculator below helps engineers and architects quickly determine:
The length of each diagonal member based on the span and rise (height).
The axial force in the diagonal members caused by a uniform roof load.
How the Calculator Works
The calculator uses simple static equilibrium and geometry:
Diagonal Length: Each diagonal forms a right‑triangle with half the span as the base and the rise as the height. The length is calculated with the Pythagorean theorem:
L = √[(span/2)² + height²]
Axial Force in Diagonal: Assuming a uniformly distributed load (w) over the entire span, the total vertical load is w × span. By resolving forces at the truss centre, the axial force (F) in each diagonal can be approximated as:
F = (w × span × span) / (4 × height)
This formula derives from balancing the vertical component of the diagonal forces with the total load.
Using the Calculator
Enter the span, rise, and uniform roof load in the fields above and click Calculate. The result will display the diagonal member length (in feet) and the axial force (in pounds). Ensure all inputs are positive numbers; otherwise, the calculator will prompt you to correct them.
Design Considerations
While the calculator provides a quick estimate, a complete structural design should also consider:
Material properties (e.g., allowable stress for wood or steel).
Additional loads such as snow, wind, or point loads.
Connection design and support conditions.
Code requirements and safety factors.
Consult a licensed structural engineer for detailed design and verification.
function calculate(){
var span = parseFloat(document.getElementById('span').value);
var height = parseFloat(document.getElementById('height').value);
var load = parseFloat(document.getElementById('load').value);
var resultDiv = document.getElementById('result');
if(isNaN(span) || span<=0){
resultDiv.innerHTML = 'Please enter a valid positive number for Span.';
return;
}
if(isNaN(height) || height<=0){
resultDiv.innerHTML = 'Please enter a valid positive number for Height.';
return;
}
if(isNaN(load) || load<=0){
resultDiv.innerHTML = 'Please enter a valid positive number for Uniform Roof Load.';
return;
}
// Diagonal length calculation
var halfSpan = span/2;
var diagonal = Math.sqrt(halfSpan*halfSpan + height*height);
diagonal = Math.round(diagonal*100)/100; // round to 2 decimals
// Axial force calculation
var axialForce = (load * span * span) / (4 * height);
axialForce = Math.round(axialForce*100)/100; // round to 2 decimals
resultDiv.innerHTML = 'Diagonal Member Length: ' + diagonal + ' ftAxial Force in Diagonal: ' + axialForce + ' lb';
}