Advice on foundation for a Cabin on stilts

[Gillilands]

Hi,

I've been reading though the form over the last week or so and I'm hoping that I might get some advice on design of a raised foundation for a cabin.  As background, I have a 20'x24' foot cabin that is located on a island in the St. Lawrence River, Quebec.  The island floods most springs, and we may have as much as 8' of water under the cabin on the highest floods.  The cabin was built on a foundation of and previous cabin that was 16' wide.  Traditionally, these cabins were built using telephone poles for posts.  Mine use 9' telephone pole posts that were set on 6"x8" hemlock sleepers that were laid on the ground. The next couple of pictures show the cabin foundation dry and in flood.



 



 

As a first-time builder I did a few things wrong. 

1)  I think the biggest mistake was leaving the cabin on sleepers.  The cabin is pretty live and rises and falls a little each winter when the soil freezes.  I am considering using helical piles (see http://www.pieuvistech.com) to support the cabin.  This company has experience in the area, and proposed to use 32, 5" dia. piles driven between 7' and 14' deep.  They can drive these easily under the cabin.   I had hoped to use telephone poles for posts, but the companies hardware works better with lumber and I'm proposing to use 6"x8" hemlock posts instead.

2) The second issue is the supporting beam.  As I mentioned I used the existing foundation which was 16' wide.  I extended 6"x6"  beams to the necessary width using a half lap joint over the posts.  As I have read in Sobon and Schroeder (1984), and several post on this forum, I should not have done this and I would like to correct my mistakes.  FYI, the beams were extended 4' under half of the cabin, and by 10' under the other half to support a 6' deck along the side of the cabin.

I would like some advice on the design of: a) the joint between the beam and post, and b) the scarf joint on the beam.

a)  Post to beam connection:  I am proposing to used 6x8 posts.  I considered using a mortise and tenon on a 6x6 post to join the post to the beam, but I am concerned the placement of the piles may not be precise, and it maybe difficult to line up the joint exactly. Also, I'm not sure about having enough height to get the tenon inserted into the mortice then up onto the piles connector.  So I am considering notching 6 inches out a 6x8 and connecting the post to the beam with bolts  (I've tried to use Sketchup to show this joint).  I based this joint on that suggested in a Fema document on Wood-Pile-to-Beam Connections and I am considering using a 1/4" steel plate to reinforce the joint (see: (http://www.fema.gov/media-library-data/20130726-1536-20490-3614/fema499_3_3.pdf). 

I'm sure I could make a mortise and tenon joint work - does anyone have any advise on this joint?



 

b). Scarf Joint:  I have easy access to 20', 6"x8" hemlock beams -- maybe I can get 24 footers.  However I need four beams of 26', and three of 20'.  For the 26 footers, I am considering a scarf joint supported by a pillow beam as suggested by Jim Rogers in this post (https://forestryforum.com/board/index.php?topic=6586.0).  I"ve started a Sketchup drawing that shows the some of the ideas in the foundation.  I have not yet drawn the scarf joint or all the braces.

Again does anyone have suggestions on this joint?

Thanks
Scott



 

[Thehardway]

Based on application, ease of installation/alignment, wet location, etc.  I think the best solution would be to use galvanized "T" plates and through bolt them with some galvanized bolts or lag them with hot dip galv. lag screws.

I like traditional timber framing but there are some applications where it might not yield dividends.  The mortice would likely weaken your structure more than some lags/plates.  The potential exists here for a lot of lateral stress as well as uplift and you really can't get enough meat and relish in that small a member for a mortice/tenon to deal with the potential loads you have.  Bigger members and more ability to align and I might give it a go. 6X8 is not a lot to work with, especially in pine.  The scarf and pillow should be fine.  That's a cool cabin.  Fishing off the front porch! 8)

[Gillilands]

Galvanized Ts -- makes sense.  I've had a local metal worker who does some restoration work on historic buildings make up some drawings for these parts.   As you say, it would make it a lot easier to line up.  From your post I guessing that you suggest that I keep the scarfs and pillow beams to make up the 26 footers, and connect the all the posts and beams with metal T's?

Thanks
Scott

PS: yes fishing of the deck in the spring and duck hunt'n in the fall ;-)

[dustyjay]

I just built in a hurry and used non galvanized 1/4" steel T braces. It looks and feels very rugged. I'd recommend adding strong knee braces as well.
 

 

The holes are 5/8" diameter, but I found the most convenient way to install was to drill 7/8' (or 13/16) holes from both sides of my posts and beams. This made it A LOT easier to line up all hardware and secure plates. Some folks here might have more to say on how to get tighter fit with the bolts, but this seems pretty tough. you'd want better hardware for your aquatic environment.

It'll be cheaper to make the Ts from two parts welded together, as that saves a lot of material.

[beenthere]

As a suggestion, I'd not put bolts in line along the same grain as stress (if it comes to that) could result in splitting along that grain. Stagger the bolt holes so they are not in line and instead of four per location, I'd use just two. Allowing for some timber movement while changing moisture conditions is better with fewer in-line fasteners.
Same is true for nails and screws when fastening to wood.

[Gillilands]

This all helps.  If I use the pillow beams with the scarfs, I'm just wondering if the T's are necessary with the rest of the beam-post connections, or would a metal plate be adequate?

Based on your advice I'll be doing up some new drawings, and sending them to an engineer for approval.

Thanks again


Scott

[Rockn H]

We have a lot of elevated camps here, but our soil doesn't freeze like yours does.  These suggestions may be useless.  Could you use temporary supports and remove one pole at a time and augur a hole below frost depth and fill with concrete.  Then put the pole back using a plate to connect it to the concrete pillar/footer.  That way you won't have any wood in contact with the ground and if you do have rot it'll be easier to replace it.  On the supporting beam, could you use a knee brace and connect it with plates?