The Mechanics Of Roof Trusses

While their aesthetic appeal is obvious, the mechanics of roof trusses are the real reason for their use in building design. The sole purpose of a truss is to carry heavy loads and transfer them to supporting structures such as the walls of a building. Whether supporting a roof or a bridge, truss design is based on the same simple geometric and physical principles. At the core of this design is the inherent rigidity of triangles and their ability to distribute loads equally throughout their structure.

The simplest way to visualise this is to imagine standing the wooden triangle from a pool table upright and pressing down on it. However hard you push, it would be impossible to collapse the triangle by physical force alone. In contrast, imagine the same experiment using a wooden picture frame constructed from wood of the same thickness. With relatively little downward force the top edge would snap in the middle, or the frame would bend sideways and break at the joints. This change in shape when loads are applied is known as deflection and triangles are virtually immune to its effects.

What forces act on a roof truss?

As the loads supported by a truss are mainly applied to the joints, they only act along the axis of each individual piece, or member. This subjects the structure to two axial forces, compression and tension. As axial loads are carried equally by all parts of the member, weight bearing is as high as possible. While there are several ‘secondary stresses’ also acting on the truss, their effects are minor. Certain elements of truss design also work to counter these stresses.

Compression

As the name would suggest, compression occurs when the particles of a material are pushed together. In the case of roof trusses, the weight of the roof they support exerts a downward force on the upper members. This causes compression in the ‘top chord’ (the upper part of the truss). This includes any diagonal struts in addition to the rafters. In oak trusses, the hardness of the timber itself reduces the effect of this compression in addition to the truss design. Without the connected bottom chord of the truss, the rafters could push out at the base, toppling the walls below.

Tension

this is caused by force stretching the ends of the member apart along its axis. As the top chord is compressed, the joints with the bottom chord (the lower beam) are drawn outward. This places the beam under tension, along with any vertical posts incorporated into the design. However, as the lower joints are drawn apart, this forces the uppermost joint tighter together. Along with the interaction of opposing forces along the posts and struts, this creates equilibrium, balancing out the loads. Once again, in oak trusses the timber itself is resistant to tension, increasing overall strength.

Shearing and Bending

The main difference between a truss and a frame is the use of pin joints in truss engineering. These allow a certain amount of flexing at the joints. As a result, where application of external loads would otherwise result in shearing or bending, they are distributed into axial forces. Also, truss engineering ensures loads are only applied to the joints to further avoid this. The only possible effect of shearing on roof trusses is if a force causes them to tip forward or back. In a truss roof system they will be secured by a ridge beam and purlins, making this impossible.

Queen Post Truss - Hardwoods Group

How to make oak trusses

The word truss derives from the French ‘trousse’ meaning a group of items or things bound together. In architectural terms, a truss is a support formed from a web of triangles designed to evenly distribute weight. The simplest form of truss is a basic triangle formed from a beam and two rafters. Although strong, this basic design has a lower load bearing capacity than other truss styles, lacking braces. Regardless of design, there are several features common to all oak roof trusses, each contributing to their strength and durability

Joints

All roof truss designs consist of straight members connected at joints or nodes. Formed of tenon and mortice joints and hardwood pegs, these can flex and move to avoid shearing or bending. In some modern oak timber structures, web members can be joined with connector plates, allowing similar flexibility

Triangular design 

As described above, the weight distribution derived from a triangular design is the key to truss strength. Even with more complex designs, the basic shape is subdivided into smaller triangular units to form a web. Each of these increases the load bearing capabilities of the structure. Even in a relatively open plan hammerbeam truss, the strength is still derived from a series of small stepped triangles. The internal bracing also helps to resist shear forces.

Use of materials

Although oak is naturally a heavy material, trusses use relatively little timber to support large loads. This is further improved by the natural strength of oak compared to other materials. As a result structural weight is significantly reduced, making them a far more efficient choice than large, solid beams. The timber itself is also naturally resistant to compression and tension. For a premium product like oak, this also reduces the cost of materials. Oak is famed for its longevity and oak roof trusses should not need to be replaced in the owner’s lifetime.

At Hardwoods group we offer a range of oak roof trusses designed for character and functionality. We also offer a bespoke design and manufacture service to ensure that whatever your needs, we can help. Our oak trusses can be supplied in kit form or assembled ready for installation. Contact us to discuss your requirements, we will be happy to help.

 

Helpful Links

Ten Amazing Architectural Truss Designs

Types of Trusses

Roof Truss Design Guide

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