figuring proper post size

[ckprivette]

greetings y'all.

I have some black locust logs that I want to use for my upcoming TF shed. I can get an 8"x8"x12' post out of each of them, some maybe 8x10. But before I get too far into this, I'd like to know if these posts would be big enough for my plans. What factors of my shed determine minimum post size?

fyi: my shed plans are similar to the famous Sobon shed, only mine will be 12x40, instead of 12x16.

board feet of thanks!
chris

[Dave Shepard]

The posts in the Sobon shed are 8"x8" white pine. Your locust will be much stronger. How many bents are you going to extend the frame by? Work that locust fresh!

[ckprivette]

i'm planning on 4 bents. i need to be able to drive a tractor between all posts.

[Jim_Rogers]

To figure the size of a post needed, you need to understand at least two things that will effect the size. One is the load the top of the post will be carrying down from the plate and rafter/roof system.

To figure the load you need to identify the amount of area that this post will support.

So let's look at the standard 12' x16' garden shed and see what a post would support.

In a top down or plan view we see that the post, for this example we'll use the middle bent, will support this area:



 

This picture shows the area a center post would hold up. It is 46 sqft and with the snow load for NC of 25 lbs per sqft and a dead load of 10 lbs for the roofing materials we get a combined load of 35 lbs per sqft. Times the 46 sqft we have a total load of 1610 lbs coming down the post from the roof system.
Next we need to figure the area of the post.
The standard post in a 12'x16' garden shed is a 8x8. But it is reduced on one side to a smaller size to comply with a "general frame rule" of 1/2" under. So that mean the post end surface that is supporting the plate is 7 1/2" x 8" or 60 sq inches.
Next we should remove the tenon as the tenon it self doesn't normally bottom out in the mortise. The tenon on this post is 2" x 7 1/2" or 15 sq inches. So 60-15 = 45 sq inches.

Now we need to understand how many pounds per sq inch can this post hold up.
To understand that we have to look at the type of wood we're going to be using.
Chris has told us he'd like to use Locust.
Referring to my NDS book (national design specifications) I see that Locust isn't listed in the hardwood section as it isn't usually a commercially harvested hardwood that would be readily available to the average consumer.
So we need to pick a wood that would most equal locust that is listed.
For the point of this exercise, I'm going to use white oak. I'm not sure if they are equal or not, but that's what I'm going to use.
Looking now, at my NDS book I see that the value for white oak in "compression parallel with the grain" (Fc) is 400 lbs per sq inch for grade #2 post and timber.
We have 45 sq inches of post.
400 x 45 =  18,000 lbs
That means this post can hold up more than 10 times the load being it's being asked to support.

No problem there.

The second part of a post size decision is whether or not there is enough post for all the joinery that will connect to it.

Chris has sent me the sketchup model of the frame he'd like to build. And there are several things attaching to an interior bent post. One is the tie beam, and the brace between the tie beam and the post. And then there are two braces going from the post up to the plate. One brace on each side, left and right as you face the post from the outside. These opposing braces can at time be a problem as the brace pocket mortises most of the time connect. And at times if the brace tenon is long could collide.
Normal brace pocket layout would be 3" deep plus a reduction housing of 1/2" which gives the mortise total depth of 3 1/2" x 2 as there is one on each side. That means about 7" of this 8x8 post will be removed. Leaving only 1" of space between them. If these mortises were cut too deep, then they usually connect and that 1" of wood between them is removed. Sometimes on purpose, sometimes by accident.
In reviewing this frame design I have noticed that there are a lot of braces in this frame. And some of them maybe able to be deleted as they may not be needed.
This frame is suppose to be 40' long and with it being that long most likely will need to have at least one scarf joint cut in the plates. Most likely two so that there can be some overlapping of the scarf halves. Scarfs should be located properly and they should be supported by a brace to insure that the will not deflect under any load.

This means some of the braces will be needed, but maybe not all of them.

Personally, I feel that the 8x8 post is ok for this frame.

Jim Rogers