calculating lbs per sq ft. to support soaker tub

everguard

my son wants to install a concrete soaker tub in a house he just bought. He says it will weigh 750Lbs unfilled. The joist standard in most newer homes is 40psf but he says: "For example, if our bathroom is 10 x 10 that puts the max weight for the bathroom 4 tons? No way. There has to be a max load.

Plus, the standard weight of 1cubic ft of water is 62lbs. A standard 50 gallon tub would hold over 413 lbs of water. The water alone overloads the floor standard of 40 lbs/sq ft if your tub is 5ft x 2ft. Forget about the weight of the tub, the hardware and possibly 2 people.

That means that by default, every bath tub in the world is already past the max floor weight when full."

Anyone know how to determine if he can calculate if he can install a soaker tub in his bathroom. thanks. Ron

Daniel Holzman

The 40 psf live load for a typical floor is assumed to be a distributed load. When computing the stress on a joist, you perform the calculations based on the live load plus the dead load, which is also a distributed load, typically about 10 psf, and includes the weight of the flooring and self weight of the joists.

A tub such as you described is typically modelled as a point load. The stress on the joists is computed based on the combination of live load (distributed), dead load (distributed), and the point load of the tub. Describing the mathematics of computing combined dead, live and point loads is a bit too much for this post, but you may be able to get a lumber yard or perhaps the supplier of the tub to perform the computations.

In general, you compute the maximum stress on the joists, compare that against the allowable stress on the joists, and that tells you whether you are OK for strength. Similarly, you compute the deflection of the joists caused by the combination of loads, and use the computed deflection to computed the l/D ratio, which tells you if the flooring is likely to crack (this is mostly a problem with stone or tile). Computing deflection is more complex than computing strength, and is normally done using computer software.

everguard

Daniel:
Thanks for the reply.
Ron