structural considerations for a bath

[psychonaut]

afternoon everyone, thanks in advance for your time.

i am having a go at redoing my bathroom ....

EDIT**** i have an 1800 x 850 bath which i would imagine weighs quite a lot with 2 people and water in it.

i have 8" x 2" joists at 13" centres running parallel to the long edge of the bath. Max length of them is 4m before they sit on a wall. I also have a 2 preexisting breeze block (or something similar, thermalite maybe ) walls which sit along where one of the long and one of the short edges of the bath will be - there is no supporting wall directly underneath them, they just sit on the floorboards. they have been there for at least 30 years (built late 80's i believe) though and nothing has collapsed underneath...but with the weight of the new bath as well i am concerned about possible structural issues.

i will build a plinth / pattress for the bath anyway as i want to have a step up to it

have a look at the video here it will make more sense

do i need to worry about the load on the floor?

 

[tony1851]

Not really; timber joists can take a considerable load before failing, particularly for short-term loading such as a bath with water and two people in it.

 

[psychonaut]

EDIT*** just realised that my bath is not 600 wide it is 850. ive changed calcs below

thanks tony. maybe you can help me understand though. this has always foxed me

the tables i have read say max load of 1.5kn/m2 for those joists (well, actually, should be able to get more than that as the span and centre spacing are smaller than the smallest on the tables, but lets take it as 1.kn/m2), and in fact, this should be true of most joists in the first floor of a house.

thats 150kg per square meter. the bath, being 1800 x 850 mm gives a footprint of about 1.5 square m.

neither me or my missus are particularly heavy, say 75k each. so if i put me and my missus in the bath we are already at 1kn / m2. thats without water.

clearly i am not calculating the area properly, otherwise any time two slighlty heavier people than us stand next to each other the floor would collapse. this doesn't take into account any other loads on the floor. which area am i supposed to be putting into the equation? the whole floor area ?

dealing with water now instead of an empty bath, if the whole volume of the bath was water (1800 x 850 x 500) then it would weigh 765kg. i imagine that its nowhere near a half of the full rectangular volume in reality, so lets assume half ish so thats 350kg for the bath full to the brim, with people in it (assume people are roughly same density as water displacing their own volume so ignore their weight).

thats 2.33kn/m2. which would say to me that the floor would collapse.

I would appreciate you telling me whats what!

 

[^woody^]

Tell me more about this two people thing.

Meanwhile, a floor will hold up a bath.

 

[psychonaut]

yes, but what kind of floor and what kind of bath?? if i took my bath as being my old 8' x 2.5' x 2.5' fishtank, im not sure your statement is true...

 

[^woody^]

Any domestic floor will take any domestic bath you can get up the stairs. It's not aquatic science.

 

[psychonaut]

That's correct it's not aquatic science..its physics. If it was a cast iron bath the answer might be different. The definition of "domestic bath" is meaningless unless you can define it in terms of a maximum load, and domestic floor carrying capabilities similar. A house from 1910 (which mine is)may not have been built to the same regs as a new build.

I do appreciate your input but to put my mind at rest and to satisfy my curiosity I'd like to know how the load propagates out from the footprint area of the bath (presumably through the floorboards) otherwise the 1.5kn/m2 limits dont make sense, for anything, on an upper floor.

 

[^woody^]

built late 80's i believe

A house from 1910 (which mine is)

 

[tony1851]

Your bath will have adjustable feet underneath. It's unlikely that any of the feet will land directly over a joist - more likely offset and supported just off the floorboards; unless you know the distance of the feet from each joist, you can't calculate the load the joist(s) will be supporting.

The only suggestion concerning the Building Regs side is that "joists should be doubled up" where under the bath (Approved Document A).
In practice, you're not going to have a problem. The joists won't 'collapse' (timber can be very forgiving under high short-term loads), but it may increase the deflection of the joists. Whether that increased deflection is sufficient to crack the plaster ceiling below is anyone's guess.

 

[tony1851]

the 1.5kn/m2 limits dont make sense, for anything, on an upper floor.

That limit is the standard live load requirement for domestic loading (30 lbs/sq foot in old money). It's quite a high figure and in practice most people don't load their floors up to anywhere near that loading in practice.
It would mean that a 10ft x 8 ft bedroom would need to be capable of supporting over a ton, which you're not going to get with a wardrobe, bed, chest of drawers, carpet, some clothes and a human.
Particularly for loft conversions, I've often thought that a figure of 1.0kN/m² would be more realistic when calculating the beams.

 

[psychonaut]

woody - almost mate, but not quite - the main house is 1910's...it was extended twice, in the 60's and 80's...the bathroom is actually split across the bit where they extended the old 1910 house in the 80's. the old back wall is in the middle of it. you can see it in the video. i thank you for your time, and i appreciate you are probably correct in that it will be fine with the bath i have chosen, but i didn't ask the question to just be told "yes its fine" i actually want to know why, and want to know why what i have calculated is wrong. i am happy to be wrong, in fact i want to be wrong on this, i do however want to know why i am wrong, because it will educate me.

If you can help with that i would appreciate it.

trying to catch me out though, i see what you were thinking, that i had lied to try and make my argument stronger....its not true, and you didn't do so well, lets say an f minus - maybe apply yourself to the actual problem rather than try and sleight the person asking.

tony - thanks for your replies.

i'm going to build a frame under the bath anyway so the feet will not be in contact with the floorboards, but onto the timbers i place between the bath feet and the floorboards. bath is 850mm wide. joists are 350mm apart give or take. so there will be 2 joists under it.

i cant see the load being more than a tonne in that room, even with the 2 walls, and the joist spacing and size and span are better than what is quoted for on the load tables, even if the wood is c16 rather than c24. so all worst case scenarios still look to be ok.

thanks very much. do you know of any decent sites where i can see in simple terms how to calculate these kind of loads ?

 

[tony1851]

Personally I've not come across any sites which explain a step-by-step way of doing the calculations for loadings on timber joists.
Here's why it's not easy if you're not used to it:

You would first need to calculate how the loads are applied to the joists if all the feet are offset and built on a frame.

Once you know the position and type of loading, you then work out the maximum 'bending moment' on the beam(s); this is best understood as the internal forces inside the beam trying to resist being bent - like the beam 'fighting back' as it were, against the load.

This figure, along with the 'section modulus' (a property of the width and depth of the cross-section) then enables you to work out the
maximum bending stress in the beam. If it is below the allowable limit for - in your case C16 - you're OK, and you can then move on to the next check, which is deflection...............................
and then; check shear stress at the bearings.........................
and then; check compressive stress at the bearings........

Most SE's could probably do this in 30 minutes or so, but they took several years to learn how

 

[endecotp]

If it’s OK for two people to stand close next to each other on the floor,
and it’s OK for them to lie next to each other in a bed on the floor,
then it’s OK for them to share a bath.

(Both morally and structurally.)

 

[psychonaut]

"(Both morally and structurally.)" -lol - depends who it is...the wife might have a different opinion!

tony - i appreciate that massively, i have a degree in physics from a long time ago. i think you can tell i didn't do applied physics (engineering)...and i also appreciate that people spend a long time learning to do things that would, given the answer, seem very simple, and for which, actually, the process is not simple at all and requires a lot of learning. which is why i ask the questions, and want to know why i am (so often) wrong.

i built a double bunk bed for my son (so a suspended double bed sized bunk. it is suspended on 8 x 2 inch beams. that's how much i am paranoid. i wouldn't even let my kids go on it till me and my missus had jumped up and down on it (ooh err). unsurprisingly, it is still standing 3 years later...but i would rather over engineer something than not

 

[psychonaut]

and actually, i made it from 8 x 2 simply because of the load tables.....