Tag Archives: building diaphragm

Knee Braces in Post-Frame Buildings

Knee Braces in Post-Frame Buildings With Diaphragm-Action Design

Long time readers will recall several articles by me in regards to knee bracing, including this one: https://www.hansenpolebuildings.com/2012/01/post-frame-construction-knee-braces/

May 2023’s Frame Builder magazine (official NFBA magazine) published an article by Dimitry Reznik, P.E. and Dr. Kifle Gebremedhin specifically regarding knee braces in post-frame buildings with diaphragm action (full text can be read here: http://associationdatabase.co/NFBA/May2023/?page=16).

Here are their conclusions and recommendations:

“In the 5 buildings analyzed in this study, knee braces produced inconsistent results. Knee braces may increase or decrease horizontal eave deflection, load demand on the diaphragm, and stress unity in the posts.

Modeling knee bracing within a post-frame building is complex. The complexity extends to the roof-truss design because the truss design must include the knee brace reaction forces, The building designer is responsible for reviewing truss drawings and verify that knee brace loads are applied correctly, and that correct governing load combinations are applied while the truss designer must incorporate the load impact of the knee brace into the component design.

In all buildings with knee braces, posts were subjected to additional bending stresses under gravity loads. This behavior was more pronounced in buildings with long truss spans.

Knee braces should not be added to a building if knee braces are not specified in the design documents. Knee braces should not be specified in the design documents unless their effects on the building are considered by structural analysis.

Knee braces may benefit buildings with certain geometrical configurations and loading conditions where diaphragm action is not enough. It is recommended that the building designer first check the need for knee bracing when diaphragm action is included in the design. In the building analyzed herein, however, knee braces did not produce a consistent advantage in any of the relevant metrics of design. The stiffer frames did not consistently translate to a stronger or more efficient design. The results were mixed and highly dependent on relative stiffness of the primary frame and diaphragm.”

In layperson’s terms – knee braces should only be incorporated in fully engineered post-frame buildings where truss designs also incorporate forces imposed by said knee braces.

Let’s Talk Building Diaphragms

New Pole BuildingWhen it comes to pole buildings, a diaphragm is a structural assembly – including the timber framing (truss chords and purlins), structural sheathing (e.g. plywood, metal cladding), fasteners and fastening patterns – capable of transferring in-plane shear forces through the cladding and framing members.

Diaphragm action is the lateral resistance to racking of the building provided by the roof and wall coverings. The design relies upon the roof to act as a deep beam supported by the endwalls. This deep beam supports the tops of the sidewall columns when they are laterally loaded by wind pressure.

When a pole building is designed using diaphragm design the strength and stiffness of the system are utilized to transfer applied horizontal loads to the ground. The system includes the roof trusses, sidewall columns, endwalls, shear connectors, chord splices and ground anchorages.

The term diaphragm is usually applied to roofs, ceilings and floors. A shear wall, is just a vertical diaphragm. Shear walls supply support for the roof and floor diaphragms transmitting forces into the foundation. A diaphragm structure results when a series of diaphragms are properly tied together to form a unit. Diaphragms and shear walls used for the lateral design of a building for a box system.

(Find helpful reading on the “box” here: https://www.hansenpolebuildings.com/blog/2011/12/lateral-wind-loads/)

The under design of any one of these (and other) crucial elements can result in the applied loads being unable to adequately transfer along the load path to the ground. Too great of a load with inadequate resistance can result in tragic results.

Designed and constructed correctly, the principles of diaphragm design result in smaller sized columns and lesser embedment requirements (think less digging and less concrete).

My friend, Dr. Kifle Gebremedhin of Cornell University, did extensive and advanced full-scale testing on a pole (post-frame) building, over a period of 16 years. His results showed post-frame buildings to be much stiffer than designers had previously believed – in other words, the design procedures being used underestimate the capacity of the buildings!

Proper design of a diaphragm system is not something the average lumberyard employee or building contractor is capable of understanding, with even a lesser chance of it being able to be done correctly.

Need a pole barn, pole or post-frame building? Rely upon experts who understand diaphragm design – the savings in materials and labor will easily outweigh the investment in paying for the expertise!