Hello; Tanks can weigh from 10 to 12 pounds per gallon depending on the amount of gravel and equipment used. Water is around 8-1/2 pounds per gallon and to this the weight of the tank itself, the stand and all other things must be added.
(OLD RECYCLEDPOST some words will run together, this happens when I copy and paste to the forum)) Here are some web addresses about floor capacity. The first address is a new one from a post by another Monster Fish forum member a while back. Here are the addresses again with the first being new to the list. Floor capacity depends on a lot of criteria, there is not a simple answer to the question.
This web address is added to the list
http://www.cichlid-forum.com/articles/aquarium_weight.php It is perhaps the easiest to follow.
http://www.ehow.com/how_7485788_calculate-floor-load-capacity.html
The web address above is a site about calculating load capacities of floor joists. They use a formula for the strength of a beam: maximum load in pounds = FBd^2 / 9L.
The width of the wood joist in inches is B (1.5 inches for standard 2 by dimensional lumber.)
The depth in inches is d (7.25 in for a standard 2x8.)(9.25 in for a 2x10)
The distance (span) in feet is L (Unsupported distance)
F is something called the fiber stress when wood bends. It varies with the type of wood but graded lumber will be at least a 1,000 number, some are higher.
They use an example of 2x10 floor joists with a span of 14 feet on 16 inch centers and come up with 1,019 pd for each joist. They figure the area supported by each joist as 18.7 sq ft. They get 54.5 pd per square foot of floor area. I think this includes the weight of the flooring and anything else that is supported by the joists.
I looked at some other sites (Addresses listed below) and found a discussion of static loads and live loads. Two sites describe how to measure the deflection of flooring due to weight loads.
http://www.johnbridge.com/vbulletin/deflecto.pl
http://www.awc.org/calculators/span...d=10&submit=Calculate+Maximum+Horizontal+Span
I found these sites interesting but hard to pin down for a general rule of thumb. Too many variables involved; Type of wood, condition of wood, dimensional lumber or engineered truss, age of wood and on and on.
http://www.eng-tips.com/viewthread.cfm?qid=62845&page=10
I just guessed when I put a 450g tank I my wood framed floor