The Forestry Forum

Hey All!

I'm working up the designs for a 12x16 deck off the south side of my house.  I want to use PT 4x4's, double stacked, on top of 12" dia. concrete piers placed at 4'.  As joists, in the 2nd row, I'll place 4x4x16's half-lapped into the 12' side top 4x4.  These joists will also be supported by 12" dia. concrete piers that are 3.5" taller than the ones around the rim.  As boards I plan on using 2x6x12' PT boards.  I'm going this route because the eventual goal is to enclose the deck (also using barn/timber framing) and make a green house out of it. 

My question is this (at least the first of what I presume are going to be many):
I want to eventually install an aquaponics system (fish tank based hydroponics re-circulation system) using between 500-600 gallons of water per system (2).  As we all know, water weighs 8.33 lbs./gal. which means I'm looking at each system weighing somewhere between 4,165 lbs. and 4,998 lbs.  The systems would be along the 12' sides with the middle open.  Is my flooring going to be strong enough to support that kind of weight?  Keep in mind the equipment beyond the water could weigh another 500-800 lbs.

I can attach pictures if anyone needs more information so that my question is clearer.

Thanks in advance for the friendly advice!
~Thom

Here's a quick sketch to show you what I'm talking about.
(https://forestryforum.com/gallery/albums/userpics/53876/layout.jpg?easyrotate_cache=1557165871)
 


The bottom one is correct, the top one has the 4x4x12 improperly placed at the top of the pic on the right side.



(https://forestryforum.com/gallery/albums/userpics/53876/layout-1.jpg?easyrotate_cache=1557169414)
 

Stacking members is no different than placing them side by side. When you stack two 4x4's you are not creating a 4x8 (section modulus 30.66"³, moment of inertia 111.1"⁴) it is still just 2 4x4's(combined section modulus 14.292^"3,moment of inertia 25.02^"4), less strong in bending and immensely less stiff.

Never notch a beam more than 1/4 depth and even then it is not a good idea on the tension side if there is any way it can be avoided.

Just as a personal prejudice, 4x4's are typically lower grade cores made up of mostly juvenile wood. Their best and highest use is holding up a mailbox or several strands of barbed wire.

Start with the entire plan and calculate your loads, we know there are significant floor point loads, you might want to consider direct foundation support under those tanks. The weight of future greenhouse equipment, should this be a slab instead of a wood floor. There is also the snow and dead loading from the roof.  Then go down to the toolbox on the lower left column and start playing with the beam calcs under the construction calcs there. Holler back when you've played around with it a little more or if you get stuck.

You have a PM, Don P.