Carbon Fiber Repair for Historic Buildings, Woodwork, Part 2

We were looking at every fragment of Menokin from the standpoint of being able to return nearly if not all pieces to the building. Over the last 40 years we had developed many consolidation and repair techniques, but as stated earlier, this was an opportunity to improve the art; to go where we hadn’t tread before; to achieve more elegant and reversible solutions.

We were calculating the loads and stresses of the various intersections in the building to determine the load-bearing needs and capacities of each location (tensile and compression) and began imaging making prostheses to extend the capacity of weakened timbers.

In some places we could do invisible mending, but in many cases where there has been no solution before, we began to wonder whether a visible piece of carbon fiber tape support would be a reasonable response. It is honest and it highlights the original as is without compromising its integrity but while augmenting/supporting so that it can return to its place in the building. This way we would not have done anything to re-write history.

Menokin is a worst case, almost nightmarish situation.

But in repairing Menokin — much like surgeons in a war zone — being forced to work with what we had functions as a laboratory to solve problems that could have applications in other arenas, thereby adding to our skill and our toolbox of solutions. 

What was devised has applications for many other building conditions.

For example, adding carbon fiber tape to support the basement-side of a sagging floor joist would evade more invasive or unattractive approaches:

  • cutting off the end to sister on a new piece or and sister ,or
  • putting up lolly columns to carry the floors, or
  • completely replacing the joist, cutting the walls to fit in the new timber.

Now we have more discreet repair options for many building challenges.

Modern dimensional lumber versus old-growth timber, we needed to begin thinking about what our existing timbers needed and how weakened timbers could be augmented, both for compression and tensile forces.
Modern dimensional lumber versus old-growth timber, we needed to begin thinking about what our existing timbers needed and how weakened timbers could be augmented, both for compression and tensile forces.

Following on our earlier discussions about the paucity of statistics regarding strength of old timbers with the greater numbers of rings per inch far exceeding the strength of modern construction lumber and how to determine the remaining strength of weakened original timbers and joint intersections, we carried out some simple experiments to measure the force required to bend an unsupported dimensional piece of wood and then that same piece with one layer of CF bonded to a side.

testing

These are 2″ x 2″ yellow pine segments adhered on one edge only of a strip of CF tape. You can see how flexible it is.

cf blocks 246
Two inch blocks to simulate a discontinuous or fractured beam bound together by a single ply of carbon fiber tape on one side.

But tensioned the blocks perform as a unit.

Yet when aligned horizontally so the blocks touch, it supports itself without deflection. See video here.

If a piece of CF tape could strengthen a 2×2, why not a rafter? It was time to calculate the loads on the joist ends for the second floor of Menokin’s NE quadrant to see what we really need.

floor loads and needs

Working through typical timber intersections in the structural joinery of the Menokin framing, I focused on several problems:

  • need to stiffen a weakened timber that might have missing areas where it crossed other timbers and fit into a mortise;
  • weakened, eroded, or partially missing tenoned ends and mortises;
  • timbers with their cores eaten away by termites leaving strong exterior shells and often with large mortises and original tooling intact.

Pounds per square inch of load on the floor itself was not great (I estimated around 100 psi), but I realized each joist end was actually bearing on only 12 square inches of contact surface in the mortise. This put a lot of weight on the lower edge of a relatively short cross-section of the girder.

What if we could increase the square inches supporting that weight, thus lessening the possibility of shear failure? This lead to the CF mortise insert (see drawing), with the idea being that the CF insert conforms to the faceted end of the tenon and pockets into the mortise in a strong bedding medium, maybe a silicone rubber or urethane. In this way the weight would be transferred to a larger area.

epoxy-fill_withCF-tape

Here I was trying to demonstrate the ease of recreating the volume of missing elements with an epoxy pour to the profile. In this first little test I used cling wrap as the mold release.

It is one thing to cast epoxy in an 8” section of beam; quite another to do it in a beam 13’ long. So it was just a little thing, but gave me important information towards scaling up to bigger timbers. And an opportunity to begin sketching out how a large timber repair might work.

Working out these techniques on the bench is how you work to scale them up.

Remember those mittens you had as a kid that were attached with the string that ran through your coat so you couldn’t loose them? We’ve begun to imagine a joist with each end resting in a CF mortise with CF tape connecting the two ends adhered along the bottom of the joist or folded and set in a narrow groove.

Next challenge: where we have discontinuous elements, could we complete that missing section of timber to meet the glass envelope planned for Menokin?

In the shop I took an old pine timber and cut it on the bandsaw to replicate a typical eroded joist end. The idea was to make a casting of the end like your dentist does in capping your tooth. By creating an acrylic, glass or laminated composite that fit to the timber and had a structural tenon on the other end, we could bridge the gap.

Here’s how these composite joists and rafters would complete the structure.

We now started to think about how the techniques we had been developing for Menokin woodwork could be used to strengthen the masonry of the building …coming up in the next post.

menokin-at-dusk

For those interested in seeing promotional videos and books developed by the conservation team with assistance from Malcolm Dax to explain their Menokin Glass House concept and the methods to reintroduce damaged timbers back into the building can find that information here.