Hi All,

First time poster, so be gentle

Ive been scouring over this site and its search function, read a heap of books and visited all the websites i can find, and i still can't find any recommendations for a solid post to slab connection method.

I read with interest the issue of concrete and timber being a bad mix due to the rot that follows, and that clay/lime and timber is ok due to the wicking of moisture.

Is it as simple as cutting a notch up the base of the post and securing to the slab with an upside down T, or is a more complete steel foot used?

Any books/websites/pics you could point me to would be greatly appreciated

thanks in advance,

bumping for the new guy.

If there are perimeter walls, then place a Simpson Strong Tie Hold Down (STHD14) on the outside of each perimeter post, which will be hidden by the enclosure. CB.

what is your application?
do you need a specific calculated value for hold down?
in many cases you won't need a hold down depending on how your walls are anchored.

to keep the post off masonry use a stainless steel plate cut 1" smaller then the post, or even in some cases 1/4" ABS plastic. you are looking for something that acts as a barrier to keep things from wicking.


the image daiku points to is fine if you are out near the end of the wall. these need to go in with care as the amount of concrete above and behind can be critical.


depending on use i've used plenty of the PAHD/HPAHD hold downs. length up the post can be as critical as the detail down into the concrete depending on uplift and wind loading.

Mike:

There are so many varieties of these straps! The one that I indicated was one that an engineer once specified for me. What's the difference between the one I listed, and the one you listed? CB.

our PEng, speced a couple of simple L shaped steel pcs lag bolted to post/concrete for all four of our cruck posts. Our basement posts simply rest on concrete footings separtated by pcs of roofing shingles, just as Mike described with steel/plastic.

When we need to resist uplift and there is no wall to hide straps, we usually epoxy allthread into the slab, drill the bottom of the post, bore an access hole in the side of the post for the washer, nut, and wrench, and plug the access hole after installation. Don't forget to use a standoff like Mike mentioned. Of course, details vary and should be approved by a PE.

Clark -

There are many varieties! Seems the PE we work with usually uses one of the HPA or PA models. Not sure why - but I'll ask next time I talk to him. I think the HPA models are thicker guage than the ST models... and it looks like the design values differ depending on application.

We only use these when we are in a wind load situation and need to tie directly to the foundation (or are doing commercial work). Most residential work (with walls / shear capacity in the walls) can work by smartly tying down the walls and tying the walls to the frame.

-Mike

Daiku, i'll be sure to have a look around Adelaide for the connector you mentioned, looks like a good type of solution

BPmike, thankfully i don't need to engineer it to be held down, it was more of a "how to tackle" question than one of engineering.

the good thing about Adelaide is practically no live loading (rarely see rain or hail let alone snow!), we are only very moderately seismic here, and no heavy winds to worry about, but i'll have the connection to slab over-engineered anyway for peace of mind.

thanks again all hope to pick your brains again soon!

Timberlinx

I've used these a few times now, I like them alot, they will level your post bottoms....

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'...

Here are a few recent post bases where traditional sill framing were not an option.

4 post clusters sitting on a core filled and reinforced block column (to match the main walls - this is in tornado country) with local limestone caps and facing. Steel T plate in the center bolts down to a cast in connection and keeps the posts off the masonry. Floor system is steel and concrete which also included tubing for radiant. The straps have yet to be applied but will look similar to the detail of the cross tie.

Examples #2 - A large project on the water (RI coast) with high wind loads - this section of the building is 42' x 80' +/-. Center bents are 40' +/- clear span. The frame was required to do all the lateral loading as we couldn't count on shear values for the wall with the amount of glass, etc.

The floor system is a commercial style steel framing and concrete pan deck (to hold cars and large assemblies of up to 300 people) that includes radiant tubing and will be covered in brick pavers - it also includes a full basement. Post base is a moment connection - the spacing off the floor is tall enough to include a slight reveal, mortar bed, and the brick. These were set on steel plates that were set into piers. The post base was welded to the steel plate after the frame was racked, pegged, plumb, and square. There is a gusset hidden inside the post - we cut an angled slot to slide these in.

Sweet!
I don't think I've ever seen that much scaffold

Regarding longitudinal shrinkage .001-.002 of the length is typical, so 1-2" in that 80' length. Juvenile or reaction wood can indeed shrink many many times that amount. I've had decking shrink right off the joist it was on, 3/4" in about 12', shearing the screws in the process. The entire bole is not going to be juvenile or reaction wood though. The top of the stem will contain enough juvenile wood to cause some problems if you work close enough to meristematic tissue..the growing tip. In both of these what is going on is not necessarily the grain you see but the grain that makes up the secondary wall, or lamella, of the invidual cells.. the microfibril angle, MFA. In young supple trees that need to be able to bend with the forces of nature the microfibers that make up the thickest portion of the cell wall, and thus determine shrinkage direction, are wound at an angle to the axis of the tree, up to a 45* angle. As the water that is bound to these microfibrils leaves the cell wall the individual elements move closer together.. shrinkage. If they are laid at an angle the timber shrinks lengthwise. As the tree matures the MFA straightens coming into line with the axis of the tree.

Reaction wood also has MFA's on the bias. Interestingly as well a softwood builds tissue on the underside of a lean and injects huge amounts of the tree form of concrete, lignin, into those tissues. Its' strategy is to address support from the compression face, supporting the lean on compression wood. Hardwoods grow a specialized layer of rubbery gelatinous tissue on the inside of the cells called the G layer. This tissue is above the neutral axis of the tree and acts as cables or rubber bands in tension, trying to right the lean from the topside and forming tension wood. When you try to sand tension wood and keep getting the "fuzzies" this is that G layer of rubbery tissue hanging out of the cell lumens. Bowmakers sought out tension wood for their bows long before "smart" folks came along .

Hi Don,

Now I know what I didn't know !

The thought that was going through my mind about spiral grain is that because the grain has deviated from being parallel to the axis of the tree then it would also follow that shrinkage rates would then also start to deviate from pure longitudinal to cross grain shrinkage rates and hence it might become necessary to start to take longitidinal shrinkage into account in design and construction.

I maybe made my point rather clumsily but in elm the juvenille wood does not tend to spiral for the first 15 years or so then as subsequent wood is laid down then it starts to misbehave and what's more it can and does change its mind every 6 inches or so along the length of the trunk going from clockwise to anti clockwise to form wavy spiral grain.

I see lots of spiral grain timbers in buildings and have oft pondered why the carpenters chose to use it. They must have realised the downsides to this problematic timber.

Thank you for that informative explanation. Seems that Prof Hoadley got it right after all though we are in now in danger of deviating "off topic".

Regards

Ken Hume

They shrink wrapped the whole building so they could warm it up and set stone and be comfy all winter with no lost days to weather. 60 foot trusses with sheathing on them about 10 feet above the TF peak, scaffolding all anchored and guyed out. They had a gent from a local marina, who is used to wrapping expensive and large boats come by... then they turned on the heat and went to town.

Now we are off topic, for sure... here is a shot from the week before last - they have ripped the seams and the building will see the light of day... our frame is in there, just beyond the porta-johns...

A five-John job? Speechless...

Add 3 job trailers and another 5 up the hill...

Man, my first photo-posting experience. Sure hope the process gets faster. Anyhow, here is how I've done the post-to-foundation connection on a couple of occations. The steel base in this case is 4" high only because it was discovered the posts were 3" too short after they had been cut to length, usually 2"x2"square tubing is used. I have welded re-bar to the base and set it in the wet concrete but it was just too damn stressful getting all of them lined up and level before the concrete set up. In this case I just drilled holes and used anchors and lags after the forms had been removed. Of course there is a short stub tenon on the post bottoms to prevent lateral movement.

Did it work this time?

My friend Don used granite plinths for his barn. I will ask him what the connection is.

WOW - i was only after post to slab connections, and ended up getting schooled in selecting tension wood, shrinkage and the best (by far) scaffolding ive ever seen

absolutely love it, deviate from topic as much as desired, its great!!

thanks to those who posted steel "feet" for the posts, that was what i was imagining would be used by those in the know. to my thinking (scary thought, i know!) a bottom plate/sill doesnt seem logical when using a monolithic concrete slab foundation as is necessary in Adelaides reactive clay soils (individual post footings/piers tend to move differently, tearing poorly built homes apart). does a timber bottom plate/sill rot with all that concrete contact over in north america?

also, as was mentioned in one of the posts, the economics of running a large timber around the "foot" of the building doesnt add up if each post is securely attached to the slab.

if you wanted to avoid ALL concrete + wood from touching each other, is there a limestone/sandstone/fired mudbrick or natural material available to use as a first course that would be structurally sound enough to take the load of the posts? infill is straw bale, as we dont really have SIPs here, so adding a course or 2 to get the posts and straw bales off the concrete is advantageous to the buling overall in this instance.

thanks again for your input! i, too, am knowing more things that i didnt know.

those are handsome. i'm curious as to the connection. do tell if you find out.

you can certainly use sandstone... in this case i added a timber sill plate and a pt sill plate underneath. just wanted something between the stone and the pine. i imagine you could run a full course or two to get the bales up off the slab...

these are sitting on a footer down below frost line. the slab was poured well after the frame was up. the owner (a doc) set the sandstone foundation himself. was one of the most accurate foundation work i've worked with.





in your case - would the posts be inboard of the bales? you could still send the posts to the slab on a steel plate / pin connection and then set the stone around the perimeter. you'd have to add some insulation to the inside - but this might work out if your stone was narrower than the bales.

Looks like you were there just in time too, nice press. We borrowed a homemade one with an electric grinder this year. Darn thing had an uncanny aim with the occasional apple.

I used this hinged foot on my shop. I wised up and cut some holes in the bottom for lag installation clearance on the next ones. This uses lags in withdrawal. It could be a strap up the side if you wanted to be in shear. I welded some hooks on the underside of the plate embedded in the concrete then welded the hinge in place after it had set. Once it was tipped up I welded the whole base down. It could be shorter for work on top of a slab, I was making room for a slab pour, its on a pier there.


Ken, you are quite right if I'm understanding your drift. A tree with "straight" microfibril angles and spiral grain is going to shrink lengthwise too. My understanding of interlocked grain is that the tree grows for several years spiralling one direction then spirals the other way for a few, back and forth through life. One other place "abnormal" grain really shows up shrinkagewise is around knots. Knots stand proud in a timber that is surfaced green and then dries since the longitudinal grain is poking out of the timber and is surrounded by tangential grain. In stick framing a floor or roof I like to put knots up towards the top of the joist or rafter if possible. But, when you crown the timber the grain curving around the knot almost always draws that side shorter forcing me to put the knot down... darn the bad luck.

Mike,

I regards to what I meant about airspace, this could have been worded more clearly, but I meant a stone-capped footer sticking proud of the surface would elevate a sill, hence providing an airspace.

Was thinking the stone-capped footer could be used with or w/out a sill.


Nice hinge idea, Don P.

I just came from Don's house, and forgot to ask. We were talking engines and corn shellers, and forges, so I was distracted. I will ask him soon, I see him once or twice a week.

Don P, that was from the first year we ran the cider press. Last year we made a big day of it, and will again this year. Anyone around in the fall can stop in for some fresh squeezed cider. We were also talking about line-shafting for the open bays of the barns basement. He has corn shellers, grinders, bone-meal grinders etc. that he will be running as well.

just wanted to say that this thread has some great info on it!

Thanks!