i have few questions, and and suggestions.(a photo or two really would help).
you say 4x8? joist? are you sure? that would be (by 70's standards) 3 1/2 x 7 1/4"! most likely quarter sawn oak right?
quarter sawn oak will have a very ruff finish as opposed to a smooth sanded finish! like an SPF joist!
and at 48"oc i can understand the use of 5 quarter ply!! OSB(Oriented Strand Board) or RTD plywood?
crawl space: is it an unfinished floor with fill and stone gravel? or poured concrete?
the posts at the 6 foot intervals:
4x4 or lally column? if it's posted in the middle their must be, i'm guessing. a footer under them?
be it stone or standard 4" concrete floor!
i'd not be so concerned about the bounce(flex), as i would be about the shear of the static load!
150 gal: thats what 6foot long? so to be over three joist you'ed need a 8foot+ base to sit over the end joist
and only have one in the middle!(if you can hit that symmetry).if your only building a 72-74" stand the two joist your sitting over,
would be at best in the middle or off set and not realy supporting the perimeter.
that would very much concern me!!!
oh, and the Filled Weight of a 150 gal is 1800#s not 1250# and thats water only!
after stand/substrait/any rocks, i'd bet your at around 2000#s+!!!
if it was me, i'd put a hung (simpson strong-tie), dbl 2x8 header between all the 48' oc joist the tank sits on ( be no more then 12in
from the posted 6' point loads you have now) and hang (use a joist hanger) two single joist 16" oc between them to your sill plate!
run a continuous bead of Construction adhesive on top of the 2x you put in to lessen floor squeaks.
that'll save you cost of lumber, useing 12 footers cut in half seeing your only spaning from sill to header, not the full 12 foot.
in my opinion that shoud be fine, for that load.
"my plan was to run the plywood as the bottom base and then span the 2x4s on top of the plywood".
what do you think of that? My hope was that the plywood would distribute weight over a larger area"
as opposed to directly under the spans. No idea if that theory will hold water though."
the ply won't distribute any weight! the 2x sitting perpendicular of the joist is what thats for, go with 2x6 for the base.(on edge)
and top perimeter of the frame.
then consider what your using to cover the frame your making, if it's non structural material you'll need to point load the from the top of the frame down to the joist!
when you build the box. (i would do that anyways) with 2x4s. again put them so the 2x4s sit full on the 2x6 top and bottom perimeter frame!
sorry to be so long winded(lol)
i really do know what i'm talking about structurally.
this is the last house i built (i've been building over 35 years)
11.900 squ ft, 14ft first floor walls, 12ft 2nd, all 2x6, full span TGI floors
this it the stand i built for a(my) 75
this is what i had to do to take the shear force of the load!
e-mail me ([email protected]
) and i'd be more than happy to help you anyway i can getting it right!!
take everybody's advice , don't just put that filled tank on that floor just yet!!