ArticleIn this article, Professional Engineer and Woodworking Expert, Les Winter, P.E. discusses the causes of wood chair collapses which commonly lead to fall injuries.
The experts at Robson Forensic have investigated many incidents of chair failures to understand the cause of failure and the condition of the chair leading up to the incident.
Wood Chair Failure Investigations
Wood chair collapses generally occur due to a failure of the chair’s joinery. In most chairs, the four vertical legs are connected to one another by rails and stretchers. The rails connect the legs together at their tops, and support the seat. The stretchers provide additional bracing to the legs lower down to prevent the legs from moving horizontally relative to each other.
The connections between the legs and the rails and stretchers are made using joinery. Typically, the joinery employed is the variety known as mortise and tenon. A mortise is a rectangular hole drilled into a side of the leg. The tenon is a peg, of the same shape as the mortise, which extends out from each end of the rails and stretchers.
In order for the mortise and tenon joint to remain rigidly connected, and prevent chair collapse, the manufacturer must meet two conditions:
- First the tenon and mortise must fit together very snugly.
- Second, adhesive (glue) must be applied so that the mating surfaces of the mortise and tenon are thoroughly wetted.
If the mating components are not fitted sufficiently snugly, the joint will have excessive “slop” and most adhesives will not be able to adequately bridge the gap. The tenon will eventually pull out of the mortise. If the adhesive is not sufficient in quantity and uniformly applied, the joint will be “starved” of adhesive and a weak connection will result. The tenon will, again, eventually pull out of the mortise.
Expert examination of the snugness of the fit, and the adequacy of the adhesive spread, can usually lead to a determination of the cause of a chair’s collapse.
Notice of Impending Chair Failure
In most instances there are recognizable signs of weakness before the eventual failure and collapse of a wooden chair. Early signs of a problem include a tendency to wobble, which indicates that the joints have lost their integrity. Visual inspection of a compromised chair may show cracks in the paint along joints or the exposure of unfinished surfaces that have recently pulled out.
Chairs exhibiting any of these traits should be removed from service until they can be properly repaired or disposed of. Repairs should only be attempted by a skilled furniture repairer/restorer or the chair’s manufacturer. If no skilled repairer or manufacturer is available to repair the chair, once the components begin to loosen, and the chair develops sway, it should be discarded.
Wood Chair Failure Investigations
Visual inspection of a failed chair will often provide definitive evidence of the cause of failure and condition of the chair leading up to the time of collapse. Examination of the snugness of fit between mortises and tenons, uniformity of adhesive applications, and other visual cues provide telltale signs regarding the adequacy of design, quality of craftsmanship, and sufficiency of any repairs performed throughout the life of the chair.
For more information submit an inquiry through our website or contact Les Winter directly.
Skilled Woodworker & Woodworking Machinery Expert
Les Winter is a professional engineer and accomplished woodworker who specializes in forensic casework involving woodworking machinery and wood joinery failures. He is regularly retained in casework involving equipment operations, maintenance, and guarding. His joinery casework examines the design, construction and repair of chairs, benches, and other wooden furniture.