Carbon Fiber Repair Options for Historic Buildings, Woodwork – Part 2

We were looking at every fragment of Menokin from the standpoint of being able to return it 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 gone before; to go for more elegant and reversible solutions.

We were calculating the loads and stresses of the various intersections in the building and making prostheses to test what their size and dimensions would need to be to support these loads. 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 it,  much like surgeons in a war zone, it functions as a laboratory to solve problems that could have applications in other arenas, adding to our skill and our toolbox of solutions. It certainly might make sense in many buildings to, for example,  support the basement side of a sagging floor joist rather than cut it and sister or put up lolly columns to carry the floors or completely replace the joist, cutting the walls to fit in the new. Now we might have more discreet repair options for many building challenges.

Continuing in my explanations to Tim Macfarlane in December of 2007, I wrote:

i-beam analogy.033“Following on our earlier discussions with you about the paucity of statistics regarding strength of old timbers versus 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.

The latter showed a large increase in stiffness.

testing

cf blocks 246

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.

flex

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

jack-loads-blocks

rigid

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Click here to see how it responds to pressure.

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Continuing my explanation to Tim: “If a piece of CF tape could strengthen a 2×2, why not a rafter? I calculated the loads on the joist ends for the second floor of the NE quadrant to see what we really need.

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; and timbers with their cores eaten away by termites leaving strong exterior shells and often with large mortises and original tooling intact.”

floor loads and needs

Pounds per square inch was not great (I estimated around 100), but I realized each joist end was actually bearing on only 12 square inches of contact surface in the mortise so 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

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Here I was trying to demonstrate the ease of recreating the volume of missing elements, but I was also working how would you create these repairs if you were scaling them (which always has to be part of the planning).

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In my mind while working out these techniques on the bench is how you would scale them up. It is one thing to cast in an 8” section of beam; another to do it in a beam 13’ long. Here I used saran wrap as the mold release. So it was just a little thing, but gave me important information towards scaling up to bigger timbers.

A

B1

B2

trace01   trace02   trace03

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.

C

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Another structural solution that needed to be developed:

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

cast-end-01 cast-end-02

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.

cast-end-03   cast-end-04

cast-end-05

Here’s how this would look in the house


menokin-cad-07 menokin-cad-06 menokin-cad-05  menokin-cad-08  menokin-cad-03

Click on any image to enlarge

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 specifically the methods to reintroduce damaged timbers back into the building can find that information here.

menokin-at-dusk

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We had mainly been dealing with timber elements, but now we started to think about how these techniques could be used to strengthen the masonry of the building…coming up in the next post.

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