Pier blocks for a shed on 4x4 posts

[FordTruckWench]

I'm building a shed that will need to be on short stilts so that the floor level matches other structures. A long time ago I bought pier blocks with embedded metal straps. The plan was to use the straps to attach 4x4 posts to the pier blocks. However, close examination of these pier blocks shows that the straps are just a single Simpson strap bent into a U and embedded at most 1" into the concrete. Further, using these blocks means the end grain of the 4x4's is sitting directly on concrete.

I'm considering using Simpson PBS44A post bases* and casting my own pier blocks. The PBS44A would give a much stronger connection between block and post. Further, the 1" tall steel standoff would elevate the 4x4 above the concrete.

The question: What size should the concrete portion be? The commercial pier blocks are 10" by 10" by about 9" tall. I'm thinking of using 12" Sonotube cut into 9 1/2" tall pieces. Other height options without waste would be 12", 16", 24", and (ugh) 48". FYI, frost depth here is ... zero. Further, the pier blocks and stilts are temporary, but keep in mind "temporary" often turns into nearly "permanent".

Here's how I plan to cast the blocks: Attach the PBS44A upside down to a horizontal 4x piece. Cut a 3 1/2" square hole in a 1" thick piece of plywood. Drop the plywood over the PBS44A. The "top" of the plywood should be even with the "bottom" edge of the standoff. Set the Sonotube piece on the plywood. Then fill with concrete - casting the pier block upside down.

* The PBS44A is a strap bent into a V-bottomed U that reaches decently deep into concrete. A steel standoff platform slips over the strap ends.

 

[mike93lx]

What do you gain by so securely fastening the shed to small pieces of concrete when the concrete isn't anchored?

Do you really need it to be temporary? Or is this to comply with a "temporary/movable" restriction?

 

[finn]

I would use the blocks you have, and if concerned with the post end grain resting directly on the concrete, cut up a couple of asphalt shingles to separate the post from the concrete.

 

[FordTruckWench]

What do you gain by so securely fastening the shed to small pieces of concrete when the concrete isn't anchored?

In case of earthquakes, the "small pieces of concrete" need to be connected to the posts - it would not be good if the posts jump off the concrete. If the shed jumps, I want it to bring its feet with it. The straps on the commercially made pier blocks do actually suffice for this.

Do you really need it to be temporary?

Future plans require a real foundation under the shed. Unfortunately, a real foundation here is not a simple project.

 

[billconner]

I'd use the blocks you have, with the asphalt Finn suggests, and tie it to the ground with a couple if auger style anchors, pre-set just inside building perimeter. My two cents.

If doing your own blocks, use 16" sonotube but I'd use a 12" piece, and bury it a little.

I would a little calculation to be sure either blocks are big enough. 50 psf for shed divide by 4 is load on each. Unless it's really soft probably safe to assume at least 2000 psf, or maybe you can get a better value. Assuming your 12x12, that means shed can be 160 sf using 12" round only 125sf, and do on. Your probably good with the 12" round.

 

[FordTruckWench]

I would a little calculation to be sure either blocks are big enough. 50 psf for shed divide by 4 is load on each. Unless it's really soft probably safe to assume at least 2000 psf, or maybe you can get a better value. Assuming your 12x12, that means shed can be 160 sf using 12" round only 125sf, and do on. Your probably good with the 12" round.

There will be 10 posts and pier blocks. 120sf with another 8sf of "porch". Ground is loamy sand where the loam percentage drops the deeper one goes. There should be fairly solid rock not too far down. (I've already scraped off the sandy loam top layer.)

12" round is the biggest I can get without finding some concrete supply house.

 

[billconner]

128 sf on 10 posts. I'll be a little more detailed but let's say 8x16 (with porch) that's about 640 sf of surface (floor, walls (8' all around), and roof at 5 psf is 3200 lbs. 128 of storage is 5120 lbs, so 8320 total divide by 10 posts is only 832 per post. Let's downgrade soil to 1500 pdf since it's loamy. A little more than half a sf for each pad. you'd probably get away with 10" round but I'd go 12" or use the pier blocks you have and tie down with anchors. All easy to remove when you do permanent foundation. Seems like you could get away with fewer piers.

I still like your existing blocks because those Simpson things are pricey - and 10 of them. I just put one Simpson CPT88 in - $50 - but then I'm trying to see how inexpensively I can build this garage. Maybe 4x4 are not so bad, but still seems overkill for these loads.

My real concern would be uninvited animals nesting under your shed before permanent foundation.

 

[FordTruckWench]

... so 8320 total divide by 10 posts is only 832 per post. ...

Thanks for the calc.

My real concern would be uninvited animals nesting under your shed before permanent foundation.

The understructure will be tall enough that small animals (i.e rodents) will think of it as open space. Larger animals ... will be chased away by mountain lions and/or coyotes. Those in turn would not sleep near homes.

 

[billconner]

If I oversimplified my apologies. And you should be sure my estimates make sense. And anything better than swamp should support 1500 psf. BUT I'm not an architect or engineer so don't send a check.

 

[Wolley]

You can get 18 inch round precast pads at home depot around here for pretty cheap. I'd go with those and drill and bolt the post bases to that.

Or if you want to cast your own just built the form any size you want out of wood then you can embed your anchor into the concrete.

 

[FordTruckWench]

Went with Sonotube cut to 9 1/2" lengths. Figured that there is no additional benefit with 12" or 16" tall piers. (A 48" tube from the store minus four saw kerfs yielded five pieces of exactly 9 1/2".)

Built a little jig to cast two blocks at a time. The jig holds the Simpson metal plumb and exactly in the center. The blocks get cast upside down. A pile of Jenga blocks provides a stable base and clearance for the metal straps.

An 80# bag of concrete mix almost exactly fills the 12" diameter by 9 1/2" tall form tubes. At first it looks like there will be a small surcharge of concrete above the tube, but when settled it drops to even or slightly below the top of the form. I could really have used a concrete ******** here - found I had to slap the sides of the forms for several minutes to get the concrete to settle.

When the concrete settles below the form top, I can make use of the extra space to come back 12 hours later and "pond" water on top of the concrete. This lets the chemical reaction keep going, thus increasing the strength.

I'm finding that every bag of concrete produces a slightly different result. The bags must have variance in mix quantity and mix moisture. Every block is going to end up a slightly different height. Some end up with barely enough water to finish a smooth surface. Others end up dripping lots of water - even though in the mix tub it looks like wet gravel, not like mud. (I'm measuring the exact same amount of water for each bag.)