Understanding Engineering Forces in Post and Beam Construction

“With carefully engineered connectors from Connext Post and Beam, you can be confident that your structure will stand strong for generations.”

Post and beam construction is a robust and visually striking building method that has been used for centuries. The framework, consisting of vertical posts and horizontal beams, creates a structure known for its strength, durability, and timeless appeal.

Fire Tower Engineering Timber, has worked closely with Connext Post and Beam to calculate the design load capacities for their connectors, ensuring that they meet the highest standards of safety and performance. In this blog, we will provide a very simple explanation of the fundamental engineering forces that play a crucial role in post and beam construction.

What is Post and Beam Construction?

Post and beam construction involves using large vertical posts and horizontal beams to form the primary structure of a building. These elements are typically made from timber, although modern variations might include steel or aluminum. The beams can either rest on top of the posts or be framed into the sides. The connections of the members were historically accomplished by carving the wood to fit pieces together, however, modern methods often employ steel or screws at those locations to simplify the carpentry required and often gain higher capacities. The frame that is created supports the weight of the building, including the roof, walls, and upper floors.

The Key Engineering Forces

When designing and constructing a post and beam structure, it is important to understand the engineering forces that act on the different components of the building. These forces include compression, tension, shear, and flexure, all of which must be carefully managed to ensure the structure’s safety and longevity.

Compression

Compression is the force that pushes materials together. It is typically considered to act axially but with stacked members can compress across the cross section as well. In a post and beam structure, the vertical posts typically experience compression as they support the weight of the beams and everything above them. This load is transferred directly down through the posts and into the foundation. The ability of the posts to handle compression while not buckling is crucial to the overall stability of the building. Just as important as just whether the material crushes under the load is keeping it from bowing. This involves making sure that tall posts are not too skinny as to invite unintentional flexure into the system (bending discussed later). Think of taking a ruler and pushing inward on the ends, causing it to bow. This is resolved by making posts larger cross sections or adding bracing.

Tension

Tension is the force that pulls materials apart and acts axially in the member. In post and beam construction, tension typically affects the beams and braces, especially when there are trusses involved or high lateral forces without a building envelope to resist the load. The connectors engineered by Connext Post and Beam are designed to handle these tension forces, ensuring that the beams remain securely attached to the posts, even under significant loads.   

Shear

Shear forces occur when two opposing forces act on a material, causing it to slide or tear, typically acting perpendicular to the member faces. In post and beam structures, under uniform loading conditions, shear forces are highest at the ends of the members which can become a concern at the joints where the beams meet the posts. This is typically manifested in a load that visually makes a beam wants to slide down the post face. So, in the knife plate connectors, the dowels in the beam and the screws in the post, keep the system in place. Shear can also happen perpendicular to the knife which wants to pry the timber apart at the knife. Shear capacity in this direction is weak as it induces tension perpendicular to grain which is very very weak in wood and is typically designed out of structures to avoid this issue. Connext Post and Beam connectors have design values to resist these forces in the two different directions.  Wood, in particular, also has an internal horizontal shear that is directly proportional to the vertical shear applied and needs to be considered when the member is designed. Essentially the wood grain can slip past each other and shear along the grain compared to shear across the grain as mentioned above. This type of shear is not resisted by Connext connectors and comes into place when sizing the member for strength.

Flexure

Flexure occurs when a beam or other horizontal member is subjected to a load perpendicular to the member axis that tries to bend the member, creating tension on one face and compression on the other. Flexure is an internal force within the beam that puts the highest stresses on the outermost grain of the wood. Under uniform loading conditions, this load is highest in the middle of the beam and lowest at the ends. The force in the beam is also sometimes called a moment force. Connext connectors do not actively resist flexural forces within the beam as a knife plate connection is generally considered a shear connector. 

The Importance of Proper Engineering

The collaboration between Connext Post and Beam and Fire Tower Engineering Timber provides specifications for every connector and is designed to give your engineer the information to allow your structure to handle member forces effectively. It is important to hire an engineer for your project to review the work and determine the loads that the system will see. The capacities are determined for the connectors but the other half of the equation is knowing the forces that members are resisting. 

Engineering Specifications

Engineering design values can be found under the Design and Specs at the top of the website. Each of the connectors has the load directions show and corresponding load values in a table. The load diagrams will be very similar to and be some combination of the four different forces that can be imparted on a beam (tension, compression, bending, shear). For the most common connectors, "T" resolves the tension in a beam, "V" resolves the vertical shear in a beam, and "V2" resolves the horizontal shear in the beam. These load directions are in relation to the connector and apply even if the connector is rotated to a post base configuration. Extra considerations and engineering considerations may still need to be applied, but at their basic level, these capacities are what the connectors can handle directly.

Conclusion

Understanding the engineering forces of compression, tension, shear, and bending is essential for anyone involved in post and beam construction. With carefully engineered connectors from Connext Post and Beam, you can be confident that your structure will stand strong for generations. Whether you are a contractor or a homeowner, these insights help ensure your project’s success and longevity.