We use them on TFG projects all the time.

They pass the engineering tests, and the company has been very generous in supporting various TFG activities and projects, too!

Hi,

If a metal anchor is cast or epoxied into position near to the edge of a concrete slab then damage to the edge of the slab could easily happen especially if the design of the slab reinforcing does not take this requirement into account.

I have yet to come across any medieval or post medieval timber frame buildings that are fitted with any kind of tie down device. Even a 30 ton post mill just sits on its pedestals balanced on little oak packers.

Most buildings start out their life equipped with sills. Sills have a job to do both in terms of spreading load and tying the frames together at low level. They provide the frame with alternative load paths should a major stuctural failure take place.

Frames need to be free to move such that longitudinal drying shrinkage does not unduly distort the building. For example if you have a building that is 80 feet long and this is fitted with continuous wall plates and purlins then these will shrink by approx one inch in total length as the timber dries - assuming a 0.1% long grain shrinkage. However should the timbers employed contain a significant amount of juvenille wood or reaction wood then longitudinal shrinkage could be up to 20 times greater (Hoadley, 1995). On balance a supporting wall frame sill should probably shrink by the same amount as the wall plates, especially if it is cut from the same tree, and thus would not subject the frame to undue drying stress or distortion.

How does the anchor system accomodate longitudinal shrinkage ?

Regards

Ken Hume

this is a good topic.

Daiku, thanks for the diagram for the visual learners!

Mike, would you suggest the rubber instead of the metal plate? it seems metal has different thermal properties. could possibly condensate and leave moisture to wick.? anybody scalloped the end of the post (as long as there is enough surface area for the load)

Ken, an 80' timber? do you have those? I might like one. Just kidding scarfed right? Can we trust Hoadley that if we have an 80' plate that it will shrink in the level direction 20 inches?!!! as far as longitudinal shrinkage for posts, doesn't everything just go down? the post shrinks longitudinally, the plate goes with it, the ridge the same (rafters ok). but the brace...

these pieces are going to shrink more tangentially than longitudinally so no problem right.... the brace won't hold up the plate off the post shoulder. in short, longitudinal shrinkage is minimal to all other forms, so it is negated. Right, right?

furthermore, since when did you all across the pond care about a little gap? it is structurally sound right? feel like a conversation tonight.

Happy Memorial Day.

Mo

I see two issues with concrete, it loves moisture and it is a heat sink or it is colder than the air, and water condenses on it if the temp is right. A metal base will conduct the cold from the concrete and condensation happens, leaving moisture on the surface with the wood. The old sills either had lime mortar or nothing, the lime assisted with disposing of moisture while the stone probably had enough air space to allow drying, both desirable conditions. If you leave a board of sheet of foam on the floor for a period of time and lift it up later you will see the concrete wet and damp. There is no air flow to evaporate the moisture.

The adjustable foot/post connection should relieve the situation. I have applied tar and used ice and water on the feet of post. Time will revel the evils we do.

I also wondered about the almost 20 inches of shrinkage, didn't bother with the math. I would have to add ten feet to my track to saw 80', the tree could possibly be an issue, I wouldn't rule it out entirely.

I believe Ken has some sensible points, we live in a CYA society these days. When "uplift" is spoken of do you mean in high wind areas or seismic conditions? And If you live in an area where neither are probable, do you still take the precaution? Or do you stick with the old ways of building?

Tim

Hi,

Timber framed buildings do move considerably over time, for example the ridge of Westminster Hall has now sagged over 18 inches. The scissor braced collar rafter couples in the Bishop's Camera roof at Farnham Castle have sunk by about 9 inches over the same 600 year period.

"Uplift" generally happens as a result of wind running along the axis of the building and "overturning" would decribe the same action but when a cross wind is encountered. If you don't get earthquakes then there seems little point in designining for same.

A continuous member can be made up from a number of smaller components. Trees (Douglas Fir, Sitka Spruce, Grand Fir) grow to 200 ft + high on the west coast of Scotland all in the short space of 100 years so it would not be impossible to obtain single piece 80 ft components but probably quite impractical to try and convert or transport on our narrow winding roads. I have seen some tie beams close to home that are 68 feet long, hewn out from a single tree (Norway spruce).

I suggest that you take up the longitidinal shrinkage prediction issues with Professor Hoadley. He can be contacted at UMAS. He struck me as being a fairly sensible fellow and certainly one who is very much in touch with wood.

Regards

Ken Hume

Building with sills does seem preferable as opposed to not including them. Ken, your comments remind me of the barns I've seen that have no top plates. Both sills and plates provide "alternative load paths" in case of failures, but seems history has built w/out them.

I wonder if the sill has it's origins in more of a foundation role or in a "tying together" role? Perhaps sills are less used these days b/c of modern foundations.

As far as Tim's points on concrete and moisture, maybe this is another reason why sills are omited today: they'd rot much more quickly laying on cement w/ little drying opportunities. Of course, economics probaly plays a role too. Less timber=less cost.

A barn restorer I know hates concrete as a building material, calls it "crap," says it's only good for about 30 years. I have to say I agree w/ him.

So when it was time to build his own home (a converted barn, of course) he refused to put it on a concrete foundation, even though it has a full basement. Instead, he painstakingly dry-laid large granite boulders w/ a backhoe up against the earth.

Which has me wondering, what about putting a piece of granite (or another stone) at post locations when slabs are poured? Kind of a "stone-capped footer" and have them stick proud an inch or so for an airspace. Stone does not "wick" like porous concrete (I'm guessing).

Probably be a headache for the concrete guys smoothing a floor, however.

I'd like to see a building that has shrunk by 20". Does it come off of one end? Or does it equalize throughout?

I would buy the argument if you had a naked frame of 19 bays with 12-15" or wider fresh sawn oak posts with plates and girts intterupted between... then I certainly could see gaining 20" over the length of 80 feet. And this is not guaranteed - as in some cases sever checking could make the timber larger on some faces as it opens and twists.

But I can't quite buy the idea of an 80' stick shrinking 20" over its length. It is true that Mr. Hoadley knows his stuff and that there may be a material out there that theoretically can do this. Today you transport and can scarf together 2 40' sticks to get your 80' - but I just don't see 20" happening over the length.

And, if this is not an open frame - other building components will certainly work against any shrinkage. Sheathing, wall panels, foundation ties, bracing, etc. etc. etc. all work against each other as things move and settle and age and dry. Predicting exactly where something may shrink, check, open, etc. seems fruitless. We make best practice choices on this - and if we are worried about 80' plates shrinking 20" we'd better worry about 40' trusses (we build some of those every year) shrinking 10" and falling off their supports.

I don't doubt the numbers Ken quotes for sag / shrinkage over 600 years. That is a long long time for repeated loading and unloading of a member... that may have been undersized to start.

Wood moves. It was once a tree and will always want to be a tree. It equalizes with its surroundings after some time... we certainly can plan for it. But I don't think we need to be hysterical about it (with the exception being stacking fresh oak on fresh oak on fresh oak).

And yes, sills do a good job. But we are building in a different era from middle age mills. Sometimes a reinforced slab is what makes sense for the base... and tie downs, stone bases, and what not are what works for the building. Proper detailing will ensure that the slab design accommodates the cast in place strap or pin or plate ... and proper detailing of the post end condition will ensure a long life of non wicking.

And - how does sheathing, plaster, windows and doors, and etc. etc. account for longitudinal shrinkage? I don't know any of these systems that are flexible enough to take up 20" of slack...

yes, you can do this, but i would detail this with a reveal to the slab... make the stone 2-4 to 6-12 inches taller off the slab.

i'm not sure what you mean by the airspace though...

Hi Mike,

I like your opening thoughts. You appear to be beginning to examine the prospect of how such high rates of longitidinal drying shrinkage would manifest itself in practice and therefore you are now better able to asses the suitability or otherwise of employing fixed metal anchors embedded in fairly rigid reinforced concrete. You now "know what you don't know" and can start taking steps to confirm or deny this possibility or at least mitigate the risks stemming from same.

You need to read Professor Hoadley's words very carefully. He makes specific reference to juvenille wood and reaction wood being vulnerable to high degrees of longitudinal shrinkage, whereas mature straight grained timber should shrink maybe only 1" over 80 feet.

So is my post of any relevance ?

Well recently I observed that old elm purlins were made from very young (25 - 30 years old) boxed heart timbers that were subject to more than their fair share of waving spiral grain and this got me to thinking about whether waving spiral could be classed as reaction wood and also whether this in combination with a largely juvenille wood cross section (15 - 20 years) would leave this type of component subject to large amounts of longitidinal shrinkage. If this were true then scarf joints would tend to get pulled apart and yes indeed I have observed this in practice but not by the amounts mentioned by Professor Hoadley.

Regarding using stones under timber frames I have observed on many occasions recently that large sarcen stones have been placed directly underneath main post and long / cross sill connection points. Some say that this has mystical significance (Stonehenge is made from sarcen stones) but I tend to think that this is based on good sound practice where large quantities of local hard stone are not readily available. This is also a common feature that can be seen in New England where large granite boulders have been dragged off the fields to be used to support barns.

I accept that it is easy to cock a snoot at the relevance about historical observations being made today but equally one could not argue that ploughing ahead with employing the techniques of "sans sills" design is completely free from technical risk and long term performance issues. After all, the practice has only been employed for 30 years maximum and certainly not 300 so how can you be so sure about the suitability of this approach ?

Please keep your mind open to possibilities, apply analytical and critical thought processes and just maybe we might all learn something.

Regards

Ken Hume

Yes, lets try to do this. Please.

...and please - tell me more of what I now 'know what I don't know'...