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Engineering a solution: A structural perspective of Macdonell-Williamson House

New and old timber sections prepared for insertion of reinforcing steel plates

By

Michael Csiki, Majula Koita and Chantel Godin

Buildings and architecture

Published Date: Dec 05, 2014

Photo: New and old timber sections prepared for insertion of reinforcing steel plates

The structural rehabilitation of Macdonell-Williamson House has provided a great opportunity for the engineers at Quinn Dressel Associates to contribute to the preservation of a piece of Canadian history.

Over the years, the house was altered by its various owners who added certain features and demolished others. For a period of time, the house also stood vacant, exposed to the elements. As a result of both change and neglect, the original structural load path changed from the time it was first constructed. (The load path is the natural path in which the weight of the floors, snow, roof, walls and occupants travels through the joists, beams, columns and walls and eventually makes its way down into the foundation and diffuses into the earth.)

The two-storey house consists of solid, stone exterior walls and foundations, stone and brick chimneys and large timber beams spanning up to 13 meters (43 feet) in length. When the Williamsons owned the house, the west fireplace – which supported the second floor and a section of the roof – was removed. Over time, this caused the second floor to sag, resulting in diagonal shear cracks in the plaster wall that remain visible today.

With the removal of the fireplace, the loads changed paths and travelled an alternate route down a partition wall and onto the ground floor joists, which are framed into a header beam supported by the foundation wall. This header beam, which was not intended to carry the loads from the second floor or roof, was under such stress that the deflection and rotation of the timber became clearly noticeable.

In 1994, Restoration Engineering of Brockville advised that new steel beams and columns be installed to support the sagging second floor, but the damage to the critical header beam below had already been done. In 2012, temporary walls recommended by James Knight & Associates Professional Engineers were installed to compensate for deficient support of certain sections of the ground floor due to the overloaded header beam.

A number of solutions were considered to address additional loadings imposed on the first-floor timber beams. Ideally, replacing the original deteriorated timber beams with new timber of appropriate size and section would have been the simplest solution for restoring the building to its full structural capacity. But the desire to preserve original building fabric precluded this type of intervention. Another common rehabilitation strategy involves bolting steel plates on the sides of the timber beams. Macdonell-Williamson House, however, has a unique feature on the underside of its first-floor timbers, which are exposed in the basement: they have a hand-carved beaded edge. In order to preserve this unique heritage feature, an alternate method was selected – the Wood Epoxy Reinforcement (WER) system.

The WER system allows us to conceal reinforcement material into the body of the existing wood to create a composite structural member of sufficient strength. Different reinforcement materials can be used – including carbon-fibre, fibreglass or, in this case, steel. In order to install the concealed steel, the top surface of the old beam was exposed by removing the floor boards above. While the old beam was being reinforced, it was temporarily supported from below. A precise, narrow slot was cut into the top of the old beam using an electric chain router. Then, a custom-made steel reinforcing plate was inserted into that slot, accompanied by a liquid epoxy.

The WER system can also be used to extend the length of an existing timber by using the reinforcement to join two separate lengths of beam. An added benefit of this system is that it will, in most cases, prevent any further deterioration of the wood caused by fungi, insects or other agents.

Liquid epoxy injected into the wood also has the capability of easily filling voids and irregular cavities. Epoxy resin binds the steel plate to the wood, as well as serving as a filler to replace decayed or missing parts of the wood. Ultimately, the process adds to the strength of the new assembly.

Macdonell-Williamson House has seen some truly state-of-the-art restoration work over the years. Steel-framing support now compensates for a missing chimney and the concealed WER reinforcement re-channels the load path and increases load capacity, while retaining as much of the original timbers as possible. This work will extend the lifespan of the house, and will preserve the heritage building for generations to come.