A truss is one of the most efficient structural systems used to carry loads across a span.

At first glance, a truss appears to be simply a collection of straight members connected together. However, its triangular geometry allows loads to be transferred primarily through axial forces — tension and compression — rather than bending.

This makes trusses particularly useful when engineers need to achieve long spans while keeping structural weight relatively low.

1. What Is a Truss?

A truss is a structural framework made up of straight members connected at joints, commonly arranged into a series of triangles.

In an idealised truss:

• Members are connected at joints
• Loads are applied at the joints
• Members carry primarily axial forces
• Each member is either in tension or compression
• Bending moments within the idealised members are neglected

The triangular arrangement is important because a triangle provides geometric stability without changing the length of its members.

2. Why Are Triangles Used?

Consider a rectangular frame.

If a horizontal force is applied, the rectangle can deform into a parallelogram unless its joints or members provide sufficient bending resistance.

A triangle behaves differently.

Once the lengths of its three sides are fixed, its geometry is stable. This makes triangular arrangements extremely effective for creating lightweight structural frameworks.

By combining multiple triangles, engineers can create a truss capable of carrying substantial loads across large distances.

3. How Does a Truss Carry Load?

The fundamental idea is simple:

External load → joints → axial forces in members → supports

When a load is applied to a truss, the force is distributed through the interconnected members.

Some members are pulled apart and therefore experience tension.

Others are pushed together and experience compression.

The combination of these forces allows the entire truss to act as a structural system.

For example, in a typical roof truss:

• The top chord may experience compression
• The bottom chord may experience tension
• The web members transfer forces between the chords
• The supports transfer the resulting reactions to the foundations

The exact force distribution depends on the truss geometry, loading and support conditions.

4. Main Components of a Truss

A typical truss contains several important components.

Top Chord

The upper longitudinal members of the truss.

Depending on the loading and configuration, the top chord commonly carries compression.

Bottom Chord

The lower longitudinal members.

In many conventional truss arrangements, the bottom chord primarily carries tension.

Web Members

The diagonal and vertical members connecting the top and bottom chords.

Their purpose is to transfer forces between the chords and divide the structure into triangular panels.

Joints

The points where members connect.

For ideal truss analysis, these joints are treated as pin connections so that members primarily carry axial forces.

Supports

The supports provide the external reactions required to maintain equilibrium.

5. Tension vs. Compression

One of the most important concepts in truss engineering is understanding whether a member is in tension or compression.

Tension

A tension member is being pulled.

The forces at its ends act away from the member, tending to elongate it.

Compression

A compression member is being pushed.

The forces act toward the member, tending to shorten it.

This distinction is particularly important during design because compression members can be susceptible to buckling, while tension members are generally governed by strength, connection and net-section considerations.

6. How Are Trusses Analysed?

Before selecting member sizes, engineers need to determine the forces acting in the truss members.

Two classical methods are widely used for analysing statically determinate trusses.

Method of Joints

The Method of Joints considers each joint individually.

The engineer:

1. Determines the support reactions.
2. Isolates a joint.
3. Draws the free-body diagram.
4. Applies equilibrium equations.
5. Determines the unknown member forces.
6. Moves to the next joint.

For a plane truss, the primary equilibrium equations are:

ΣFx = 0

ΣFy = 0

The process continues until the required member forces are obtained.

Method of Sections

The Method of Sections is particularly useful when engineers only need the force in selected members.

Instead of analysing every joint, an imaginary section is passed through the truss.

The engineer then analyses one side of the cut using equilibrium equations.

This can be much faster when only a few member forces are required.

7. Static Determinacy

Trusses can also be classified according to their stability and degree of static determinacy.

For a plane truss, a commonly used relationship is:

m + r = 2j

where:

• m = number of members
• r = number of support reaction components
• j = number of joints

When the relationship is satisfied, the truss may be statically determinate, provided it is also stable and properly configured. Other relationships indicate potential static indeterminacy or instability.

However, the equation alone is not enough to guarantee stability; the actual geometry and connectivity of the structure must also be considered.

8. Common Types of Trusses

Engineers use many different truss configurations depending on the application.

Pratt Truss

A common configuration used in bridges and other structures where the arrangement of diagonal members can efficiently carry the expected loading.

Warren Truss

Uses a series of approximately equilateral or triangular panels.

It is widely used in bridge and structural applications.

Howe Truss

Similar in overall form to the Pratt truss but with a different arrangement of diagonal and vertical members.

Roof Trusses

Used extensively in buildings, warehouses, sports facilities and industrial structures to create large column-free spaces.

Different configurations are selected depending on span, loading, architectural requirements and construction considerations.

9. Where Are Trusses Used?

Trusses can be found throughout structural engineering.

Bridges

Truss bridges use triangulated structural systems to transfer loads across spans.

Buildings

Roof trusses can provide large column-free spaces for warehouses, factories, halls and stadiums.

Towers

Lattice towers use triangulated members to provide strength and stiffness while controlling structural weight.

Industrial Structures

Trusses are commonly used for large-span roofs, pipe-support structures and other industrial applications.

Temporary Structures

Lightweight truss systems can also be used for temporary structures where efficient assembly and disassembly are important.

10. Why Are Trusses Efficient?

The main advantage of a truss is that it converts much of the structural action into axial forces.

Instead of relying on a large solid beam to resist bending across a long span, a truss places material where it can work efficiently through tension and compression.

This can provide:

• Efficient use of structural material
• High strength-to-weight performance
• Large clear spans
• Reduced structural weight
• Flexible structural configurations

However, trusses also introduce additional members and connections, which means fabrication, erection, connection design and maintenance must be carefully considered.

11. Ideal Truss vs. Real Truss

The behaviour of a real truss is more complicated than the simplified model used in basic structural analysis.

Ideal truss analysis assumes:

• Pin-connected joints
• Straight members
• Loads applied at joints
• Members carrying axial forces only

In real structures, connections may be bolted or welded, members may be continuous, and loads may act between joints.

These conditions can introduce additional bending moments, shear forces and secondary stresses.

Therefore, engineers must distinguish between the idealised analytical model and the actual structural behaviour.

12. The Engineering Principle Behind the Truss

The concept behind a truss is remarkably simple:

Geometry controls load paths.

By arranging members into stable triangular configurations, engineers can create structures capable of carrying significant loads while using material efficiently.

The important question is therefore not simply:

“How strong is each member?”

It is:

“How does the entire system transfer the load from where it is applied to the supports?”

Understanding this load path is fundamental to structural analysis and design.

Conclusion

A truss is much more than a collection of interconnected steel or timber members.

Its triangular geometry allows loads to be distributed through a network of members primarily carrying tension and compression.

From bridges and industrial buildings to stadium roofs and towers, trusses remain one of the most important structural forms for achieving efficient long-span construction.

The next time you see a truss, don't just look at the shape — look at the load path.