Tuesday, June 17, 2014

Gateway (X)

Back to site yesterday with another fairly long day to move the foundation prep work along. Over the past few days I had put together some plywood forms based on measurements taken at site. The forms turned out to be a good fit:


After a bit of searching, I obtained some epoxy coated 1/2" rebar. The epoxy coating makes them far more rust resistant. I had the 90˚ end bends done by the rebar company, and then completed the rest of the bending on site. These three rods reinforce the 6" wide concrete beam which spans between the posts:


As you can see, these forms resemble a dog bone in shape.

A fair bit of time was consumed by chipping out loose concrete debris from last weeks cutting and jack-hammering session, followed by cleaning. Then I bored holes to accept the rebar:


I used Simpson Strong-Tie anchoring epoxy to fasten the rebar into the concrete:


My plan to to attach new concrete to old using a combination of mechanical and chemical means. The epoxied-in rebar forms the mechanical portion of the connection.

The form pictured above was leveled carefully and then fastened to the existing concrete using Tapcon concrete screws. I placed the top of the forms 2.5" lower than the top of the existing flagstones, though the stones themselves turned out to be of irregular thicknesses. Some flagstones were 2.125" thick, others 1.75" and some only 1.25" thick. By keeping the forms 2.5" below the top of the existing flagstone height, I can accommodate the placement of flagstones of any of the thicknesses around the concrete. Better to expect worst case scenario sometimes, and since I won't be laying the flagstones, I want to forestall any potential problems.

At the end of the day I elected to do more jack-hammering, knocking off the remaining concrete from the metal shoes so that the metal can be recycled:


That was two hours of my life I'll not be getting back, if you know what I mean. Those shoes are heavy suckers too - the biggest ones are, I would guess, 150lbs each.

The remaining forms were left at site and I have done the preliminary work to fit them, however I won't fasten them in place until later this week when I can bring a builder's level to site and set them relative to the height of the one already placed. That will require my helper Matt, who has kindly agreed to come to site later this week to keep me on the straight and narrow.

Today I went to the drying facility to check up on the Port Orford Cedar, which has been sitting in dehumidification for a couple of months. To my surprise, quite a bit of the material was already dry, down to between 8% and 14%:


All the material in the above stack is now dry and has been removed from the kiln. Again, I was surprised so much of it was ready so soon. The degrade was minimal, though the stock was not totally devoid of checks.  Even though all the stock is free from heart center, I still had a little bit of checking here and there on the 8"x9" rear posts, so I am wishing I had put relief kerfs in those sticks. I didn't think that their rift grain orientation would precipitate checks, but this was not 100% true.

View of the end of the stack:


The stock marked '10' on the ends isn't for this project. It's sugar pine, and has ZERO checking.

The biggest timbers, those for the main posts and main crossbeam, will remain in dehumidification for a few months yet - they are down to 20% M.C. or thereabouts, so they are definitely getting closer than when they stated. It's looking like I won't need to do any vacuum kilning after all, so that saves a bit of money. It's nice to be able to bring the wood down in moisture content by a slower and gentler means, though vacuum kilning does work incredibly well for larger sections.

The relief kerfs on the chunkier stock have been doing their job by and large:


On this one though, the kerf didn't open up too much at all, at least not so far:


Some of the bonus 'mistake' pieces, where I received several 48" long chunks of beam instead of a 17' long beam, were drying well:


Not sure what there are going to grow up to be.

Well, a little end checking here and there, and little face checking here and there, but overall the material is looking good. Now I'll have to make room in my shop somehow for this material. I expect to move it over to the shop over the next couple of days.

All for now, over and out. On to post 11

5 comments:

  1. Hi Chris, are there ever any situations where you'd be concerned in cutting significant kerfs to manage drying (ie strength of the piece)? Or is this always a good practice when working with thick pieces of wood?

    Thanks!

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    1. Siavosh,

      thanks for the questions.

      I have no concern about the affect of kerfing on the strength of the timber, whether it is to be used as a beam or a column. I think it is always good practice when working with large sections.

      Imagine you have a 10"x10" timber, and rip a kerf down one face. Stood as a column one would be concerned about are slenderness ratio (tendency to buckle), and kerfing the timber does not significantly affect that. In most applications, so long as the timber's slenderness ratio is not extreme, resistance of the column to compression loads is more than adequate. Hoadley mentioned in his book Understanding Wood that a chair leg just 1/4" thick would be strong enough to resist compression if only you could control its tendency to buckle. Columns are generally more than adequately strong, and the strongest column form, I might add, is a cylindrical column.

      Now, as far as beams go, which are most optimally made as a rectangle in section, we have bending loads (modulus of elasticity) and deflection to be concerned about. If you had X span and Y load, and used a 4"x10" joist of a given species and a given grade, it would either be stiff enough to resist deflection (1/360 of span for floors, 1/240 span for rafters) or it wouldn't be. If it wasn't stiff enough, then either you deepen the section, say to a 4"x12", change to a stronger species, or you sister another timber to the side of it to gain strength. Let's say you put two 4"x10" beams together to achieve your result. How would an 8"x10" beam with a kerf ripped in it be significantly weaker than the sistered 4"x10"s? I would say it would be about the same, if not slightly stronger given the integrity of the fiber connection over one half of the beam depth.

      And if one oriented a beam with a kerf ripped in it so that the kerf was in a horizontal plane instead of a vertical one, then that kerf is located at the neutral axis of the timber where bending loads are neutral, so it shouldn't have a pronounced effect in that case.

      A couple of final points:

      -the kerf can be patched with glued-in wedge-shaped insert strip, so this regains some what is lost by kerfing.
      -in the case of this gate, what governs the sizes of the pieces has much more to do with aesthetics than it does with obtaining an efficient structure with the minimum of material. The timbers in this gate are significantly over-sized in terms of the loads they bear, so any potential strength loss from kerfing is something I feel I can safely ignore.

      If you have a framing situation where bending loads are significant challenges to the wood species employed (usually due to designing around long spans), and resultant deflections unacceptable, the solutions to solve for this with solid timber often run into issues with obtaining material in such larger sections, not to mention increased problems with proper drying and in-service wood shrinkage with those sections, loss of ceiling height, and objections that may arise to the aesthetics of such large sections. The usual solutions are to replace the timber with metal, to use LVL's or glue-lams, or I-joists.

      All the above to say that solid timber has inherent limitations, and that where used within its limitations, I think the benefits of kerfing far outweigh any potential strength loss from the small amount of removed material.

      And if you didn't kerf, then when the timber dries out it will crack and check all the more - what about the strength losses from that? I've seen many timber frames out here with shrinkage-induced large cracks running right though joints. Such cracks are certainly not going to improve the strength of the connections.

      ~C

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    2. Thanks Chris, always educational visiting your blog.

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  2. Hi Chris,

    Just wanted to clarify something you mentioned in your comment on kerfing.

    If you had a kerf in a beam oriented vertically, you're right that it won't significantly affect the beam strength of that timber. However, a horizontal kerf in the side face of a beam would impact the strength, because while bending stresses are low at the neutral axis, shear stresses are at their maximum. If the shear strength of the 8x10 timber was exceeded, the top and bottom halves might slip, and you would have two 8x5 beams, which certainly don't have the strength or stiffness of the original beam.

    Now in practice, maybe the deflection criteria result in a beam size for which shear strength is never a concern, but it's certainly something to consider.

    -Daniel G

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    Replies
    1. Daniel,

      that's a good point to add, and you are partly correct that shear stresses are maxed out along the neutral axis, however in point of fact those stresses are lowest in the middle of the beam - zero in fact - and get larger as we move to the ends of the beam.

      In this case I have a 16.5" tall beam, supported below with another 5" tall section, spanning only 9', so I am not too concerned about such matters as shear loads as the timber is sufficiently large in section to resist any loads it may incur. I do not anticipate any significant deflection of the main beam under its dead weight only, and there is no live load to be considered, so I've pretty much put the matter of shear to one side and considered the virtue of producing timber with as few defects as possible as being of paramount importance. If the beam were to be loaded by something else, then the consideration of shear stress, as you note, would be definitely important.

      ~C

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