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Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

  • 1.  Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-22-2019 17:09
      |   view attached
    I am working on a 70 year old historic home of the Monterey Style, which has a five foot deep cantilevered balcony across the front as a signature element.  One of the nominal 4x10 members was repaired a couple of decades ago by a carpenter using a 'zig zag' joint about 3 feet from the supporting wall, and not surprisingly the end of that member has sagged considerably.  Does anyone have experience trying to repair a member using expoxy and fiberglass wrap to create a 'moment connection'?  I've used the Abatron line for major structural repairs in the past with success, but not when moment resistance was required.

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    Michael Malinowski FAIA
    Sacramento CA
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  • 2.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-24-2019 01:18
    It appears you have little depth to work with. Consider a stainless steel knife plate beginning close to the wall and repair the wood as you have suggested as needed.

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    Rod Merrick AIA
    Merrick Architecture and Planning
    Portland OR

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  • 3.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-24-2019 17:06
    Thanks Rod.  It strikes me that one advantage of your suggestion: with a knife plate the detailing, analysis/calculations would be fairly straightforward for the engineer.  So far I'm not finding any 'structural data' for the epoxy/fiberglass approach.

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    Michael Malinowski FAIA
    Sacramento CA
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  • 4.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-24-2019 09:01
    My first thought was Abatron's reinforcing rods: https://www.abatron.com/product/fiberglass-reinforcing-rod/

    We've had excellent results boring into the end of a beam and "gluing" rods into the beam to add structural integrity. I'm not sure if that is an option with your situation.

    I have found the folks at Abatron to be an excellent resource, though. I'd contact them with your situation and see what recommendations they may have.

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    Thomas Bank AIA
    Principal Architect
    Simply Stated Architecture, P.C.
    Lemoyne PA
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  • 5.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-24-2019 17:01
    Thanks for the suggestion Thomas - I have been in touch with them and was leaning in that direction as a result.  One missing element is some data that an engineer could use to determine performance.  In your use, did an engineer run numbers; or was it more about 'reasonable integrity' rather than ability to support a certain load?

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    Michael Malinowski FAIA
    Sacramento CA
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  • 6.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-26-2019 08:44
    Yes, a zigzag scarf joint would not be good for a moment connection although they are frequently used in timber framing. We have done epoxy repairs of wood members using fiberglass rods or stainless steel rebar to provide tensile strength. The West System was popular in the 1970's and has suggested details.

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    Ward Bucher AIA
    Principal
    Encore Sustainable Design LLC
    Baltimore MD
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  • 7.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-27-2019 17:52

    Michael 

    John Lee and I under the tutelage of the internationally renowned engineer Tim Macfarlane did a great deal of work about 10 years ago on this sort of problem. We were looking at attaching a glass prosthesis to a conserved structural member that had lost length among other things. 


    Let's go back to basics. You know empirically that the 4x10 of original construction adequately carried the load for a number of years until it deteriorated. Therefore creating a component equal to or exceeding the original will suffice. Second a cantilever requires tension across the top and compression at the bottom. Therefore fancy joints are unnecessary if you connect the top to provide tension and the bottom abuts in a way that transfers the compressive load from the new to the old. Therefore a butt joint between a sound or consolidated square cut end of the original 4x10 and a new piece of matching dimensions and grain orientation will suffice with a tension connect at or near the top. 


    So how accessible is the top and how much remains beyond the wall of the original member?  If you have 6 plus inches of original member and full top access you can do some elegant carbon fiber connections otherwise a post-tension threaded rod connection is probably best. I can give you some rule of thumb dimensions we used but if you have an engineer who is not afraid of existing buildings especially one that has deteriorated you should have them calculate the necessary embedment and diameter of the threaded rod or rods. As I remember we determined that this approximate condition would take a 5/8" diameter threaded 3+ inches into sound material. I like redundancy and would use two rods set about 6" into the original. We also found that pullout for a threaded rod in a snug hole (not treaded) that was flushed out with a penetrating epoxy and the the rod coated in a full bodied epoxy and inserted before either epoxy set would fail somewhere in the wood beyond the epoxy penetration, so threading in is unnecessary. I would place the center of the rods at the 1/3rd points relative to the width and that same distance down from the top. The least complicated arrangement would have the rods run out the end of the attachment and be countersunk. I would use a 1" washer and snug the system together but not try to compress the fibers. The space around rods should be filled with epoxy so it is distributing the load throughout its length. I would also have an alignment pin toward the bottom of the joint. Epoxy would not be inappropriate in the butt joint both sealing the joint and providing full compression transfer but not structurally necessary. In fact the joint can be loose at the bottom. For aesthetics and keeping moisture out plug the countersunk rods. Also consider flashing the top. 


    If you have experience to work out an IKEA style connection that can be done about 6 to 8 inches back from the joint and not run the rods all the way out. Also it is not necessary to square cut the end if you want to preserve as much of the original as possible the new piece can be fit to whatever remains of the original. 


    If you have 6+inches of the original to work with I'd be glad to walk you through using carbon fiber. 


    One last thought. It is almost never a good idea to wrap exterior wood in fiberglass. One usually ends up trapping moisture and accelerating rot and failure. The same with epoxy. It should not be a coating for wood. It is a great consolidant, adhesive, and casting resin but wood moves too much for epoxy to be used as a coating unless you customize it to be flexible. 

    image1.png
    Above blocks glued to carbon fiber tape but not to each other. 
    image2.png
    A similar band bridging from table to chair would support a 250# person. Would a 2x2 normally do that?

    image3.png
    Top is the loose block band. Middle is 1-1/2" fiberglass tape folded and epoxied into a 3/4" deep kerf cut. Bottom is a solid 2x2 with carbon fiber tape on one face. They all restrained 1000# without rupture. 

    image4.png
    Some of our test rigs. 


    Good luck

    Charles A Phillips, AIA, AIC-pa

    433 Phillips Rd
    Nacogdoches, Tx 75964 


    640 Brookstown Ave
    Winston Salem, NC 27101


    336-918-3668

    From iPhone





  • 8.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-28-2019 19:22
    Thanks for this detailed reply and outline of your investigations; fascinating examples that put basic principles to work effectively!

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    Michael Malinowski FAIA
    Sacramento CA
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  • 9.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-28-2019 15:07

    I don't know anything about Mad Dog Primer. But I think its adhesion will only be as good as the bond between the old paint and the substrate. 


    Edward R. Acker | Emeritus AIA LEED AP 

    VP Education, Preservation of Historic Winchester, Inc.
    106 Clevenger Court
    Winchester VA 22601
    703.635.8968
    EdAcker39@comcast.net






  • 10.  RE: Epoxy/Fiberglass Wrap for repair of Cantilevered Wood member?

    Posted 08-26-2019 19:39
    Looking for answers that require less disassembly - I have seen good results with hidden embedded steel shapes like Tee's and Angles sized by an engineer. We drilled holes in a pattern for lags or structural screws. Jacked up the beam slightly crowned, set the steel and fasteners, then released the jacks.
    A Flitch plate is another more visible repair.
    Curious about the scarf joint. There is an art to constructing a good one. Many would consider a good Dutchman repair contributing fabric.

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    Gene Greene AIA
    Principal, Historic Architect
    Studio Greene, LLC
    Littleton CO
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