Bridging the Mote: A Simple Method of Structurally Splicing Vigas and Beams
Wooden structural members are almost always an integral component in earthen buildings. They range from the wooden lintels over windows and doors, to the vigas, beams, joists and decking that comprise roofs and floors. In the American Southwest, our oldest extant earthen buildings are those dating from pre-Columbian and early contact times that are invariably characterized by flat earthen roofs supported by massive vigas, or peeled logs, with aspen, pine or cottonwood latillas, or split cedar rajas spanning the spaces between. As the buildings age and the wooden members weaken through moisture invasion, drying, heat hardening or an increased load, they sometimes fail. Failure typically occurs at the weakest point, the center of the span.
Several years ago I was called upon to assist a homeowner whose adobe living room reminded her "of the trash compactor in Star Wars;" the walls were closing in and the roof was descending. The two pathologies were interrelated and caused primarily by the long-term build-up of both roofs and moisture. I took a core sample from the top and found multiple re-roofings, one of which was comprised of nine to 12 inches of pumice which, in the style of the times, had been used to grade the overlaying asphalt system to drain to the canales and gutters. The asphalt roof had failed and the pumice, being sponge-like, was completely saturated. The former owner had chosen not to strip the roof but to install another over it. The multiple roofings (which were in places 17 inches thick) and the moisture content (which exceeded 26%) provided localized loads well in excess of the capacity of the vigas. Oddly enough, only one viga failed during the first snowstorm of the new owner's occupancy.
After the roofing problem had been dealt with and the walls dried out, I contemplated the failed viga. The solution that I implemented was the embedding of a steel plate that was (1) structural, (2) concealed and (3) eliminated the need to replace the viga which would have meant massive interventions affecting bearing walls.
First, the 13-inch diameter viga was supported at approximately 1/3 of its span inward from both walls using adjustable pole shores. Because the roof had deflected downwards I chose to lift the load 1/2 inch beyond what I calculated as its original placement. I anticipated some movement after the repair when the shores were removed. Next, using a 16-inch circular beam saw, I cut a slot seven inches deep and just over six feet in length along the bottom of the viga spanning the failed portion. The slot was the width of the saw blade, about one-quarter inch. (In subsequent installations we have used a chain saw).
After the slot had been cleaned, a 3/16-inch thick by four-inch wide by six-foot long steel plate was inserted into the cut and retained with shims. Half-inch pilot holes were then drilled through the side of the viga until the bit made contact with and marked the steel. They were spaced every 8 inches for the length of the plate. The plate was then removed and 9/16-inch holes drilled at the marks. (I chose to drill the steel outside of the slot because of the difficulty of getting a forcible bite with the electric drill motor in the limited space between vigas.) The steel was then re-inserted, the holes aligned and the pilot hole drilled through to the other side of the viga. One-inch holes to accommodate a countersunk washer and the head and nut of the bolt were then drilled on both ends of each hole. Lubricated 1/2-inch bolts were driven into the 1/2-inch holes and the nuts tightened to the point that the crushing of wood fiber was audible.
To my surprise, when the splice was finished and the pole shores removed, there was no detectable deflection in the repaired viga. Gluing a wooden fillet into the slot, filling the countersink holes with dowels, sanding and rubbing with soot until the repairs were barely noticeable, finished the job.