TECNI® Catenary Load, Sag & Tension Calculator
Estimate cable tension and anchor forces for overhead wire rope spans. Select your cable construction and diameter to automatically estimate cable self-weight, then enter your suspension loads and required sag.
1. Cable & Span Details
Not all diameter and construction combinations are necessarily stocked by TECNI.
Automatically estimated from construction and diameter. These are theoretical weights, not verified TECNI product specifications. You may enter the actual manufacturer's weight manually.
Vertical drop from the anchor level to the lowest point of the loaded cable.
Additional weight spread evenly along the horizontal span. Excludes cable self-weight.
2. Suspension Loads
Enter each load's weight and horizontal distance from the left anchor. Maximum 20 suspension points.
3. Calculation Results
Anchor tension is the resultant force acting at each cable end. It is not the same as the total suspended weight.
Reducing cable sag generally increases tension considerably. Results are preliminary static estimates, not certified working load limits.
4. Cable Span & Suspension Diagram
Vertical scale exaggerated for clarity. Assumes two level anchors and static vertical loading.
How to Use the Catenary Calculator
- Measure your span. Enter the horizontal distance between the two anchor points in metres.
- Select your cable. Choose the material, construction and diameter. The calculator automatically estimates the cable's weight per metre. Replace this with the manufacturer's actual weight when available.
- Determine your sag. Enter the maximum vertical drop permitted below the anchor level. Remember that smaller sag creates greater tension.
- Add distributed loads. Include additional weight spread evenly along the span, excluding the cable's own weight.
- Select your suspension arrangement. Choose a single load or multiple suspension points, then enter the weight and position of each item.
- Review the results. Check the estimated anchor tensions, cable angles and cable length. Ensure that all components and structural supports are properly assessed before installation.
Understanding Catenary Cable Systems
The Zero-Sag Myth
One of the most common misconceptions about catenary systems is that a wire rope can be pulled perfectly straight between two anchor points while supporting a suspended load.
A flexible cable supporting a downward load cannot remain perfectly horizontal under finite tension.
As sag decreases, the cable angle becomes shallower and the horizontal tension increases significantly. Even relatively small loads can generate substantial forces at the anchor points when a cable is pulled very tight.
For a single point load suspended at the centre of a span, the simplified horizontal tension equation is:
H = W × L / (4 × S)
Where H is horizontal tension, W is the downward point load, L is the horizontal span and S is the centre sag. Force and length units must be consistent.
As sag approaches zero, the required tension increases without limit.
Why Does Cable Sag Matter?
Sag is one of the most important considerations in catenary design. A relatively small increase in permitted sag can substantially reduce cable tension and the forces transferred to the anchor points.
The tightest-looking installation is not necessarily the safest or most practical design.
Understanding Cable Stretch
Wire rope can extend under load through elastic stretch and the bedding-in of its construction. The amount of extension depends on cable material, construction, tension and installation conditions.
Initial stretch, temperature changes and long-term movement can affect the finished installation. Where appropriate, cable tension should be checked after installation.
Multiple Suspension Points
When several objects are suspended from one continuous cable, each load contributes to the cable profile and anchor reactions. Unevenly positioned loads can produce different tensions at the left and right anchors.
This calculator allows individual suspension weights and positions to be entered for a preliminary assessment.
Choosing the Correct Wire Rope
Consider the cable's minimum breaking load, applicable working load limit, construction, corrosion resistance, end terminations and installation environment.
TECNI supplies stainless steel and galvanised wire rope, fittings, tensioning components and custom cable assemblies for architectural, commercial and industrial applications.
Frequently Asked Questions
Can a catenary cable have zero sag?
No. A flexible cable carrying a downward load requires some sag to develop the vertical components of tension necessary to support the load. Approaching zero sag produces extremely high tension.
Why is cable tension much higher than the suspended weight?
When the cable angle is shallow, only a small portion of its tension acts vertically. Much greater tension is therefore required to support the suspended weight.
Does the calculator automatically include cable self-weight?
Yes. Selecting a cable construction and diameter generates an estimated weight in kg/m, which is multiplied by the span length. The estimate can be replaced with the manufacturer's actual weight.
Are the cable weights exact TECNI product specifications?
No. The automatic weights are theoretical estimates based on nominal cable diameter and construction. They are not measured or verified TECNI product specifications. Always use confirmed manufacturer data for engineering design.
What is the difference between cable sag and stretch?
Sag is the vertical drop in the suspended cable profile. Stretch is the increase in cable length under tension. Cable stretch can change the final installed sag and tension.
Can I calculate multiple suspension points?
Yes. Select Multiple Point Suspension and enter the weight and horizontal position of each suspended item. Up to 20 individual loads can be entered.
Why are the left and right anchor tensions different?
Loads positioned away from the centre of the span create unequal vertical reactions at the anchors. The resultant tension at each end can therefore differ.
Does cable diameter affect the calculation?
Cable diameter influences estimated self-weight, but diameter alone does not determine cable strength or allowable working load. Material, construction and manufacturer specifications must also be considered.
Does the calculator include wind loading?
No. This version estimates static vertical loading only. Wind, particularly on banners, signs and decorations, can create significant additional forces and must be assessed separately.
Can I use this calculator for intermediate supports?
No. This model assumes one continuous cable between two end anchors. Intermediate structural supports and multiple spans require a separate engineering analysis.
Can I use the results to select anchor fixings?
Results can assist preliminary planning, but anchor selection must also account for substrate strength, fixing orientation, edge distances, safety factors and other applicable design requirements.
Can TECNI help with catenary installations?
TECNI supplies wire rope, cable fittings, tensioning components and custom cable assemblies. Contact our team on 01278 773510 or email sales@tecni.uk for product enquiries.
This calculator provides preliminary estimates using an idealised parabolic cable model with static vertical loads and level anchors.
It does not account for cable elasticity, temperature, wind, dynamic effects, uneven anchor heights, anchor movement, structural deflection or three-dimensional loading.
Estimated cable weights are not verified product specifications. Calculated forces are not certified working load limits. The calculation method and complete installation must be independently assessed by a suitably qualified engineer before being relied upon for real installations, particularly those over public areas.