Hire a structural engineer... I don't mean to be rude, but an account that has ONE question, and refuses appropriate advice, is NOT likely to be an Engineer.
This site is not meant to replace appropriate consulting. You should find a local engineer qualified to do the work and pay for the specialist knowledge you need, don't go trawling the internet. If you do manage to get a peicemeal answer, you'll likely get insuficient and incomplete help, and your project may very well fail.
Good luck,
YS
B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!
I do take your point. It is for a planning, cost allocation only. The thickness does affect much to the cost. I do believe someone is out there for a reasonable indication.
Okay, so you're being very nice and honest about why you want this, SO I'll actually have a crack at it for you. Three caveats: I've NEVER sized glass for this purpose before, and wouldn't even be sure what product to you and the approach I'm applying is NOT what I would do if actually designing this element, but a gross simplification. Two, I am not covering any of the myriad of issues to which I previously alluded, and Three: Like all advice on this forum, this is offered completely without responsibility.
The water pressure on the glass is the same irrespective of what width the tank is, except for a relatively small circular tank where hoop stresses govern. So the stress on your glass will be:
Forcing Load = 0.5 * depth^2 * unit weight of water
= 0.5 * (2m)^2 * 9.81 kN/m^3
= 19.62 kN/m
This has to be carried by the glass accross to the supports as a flexural element either with three edges supported, or with four edges supported. I'm simplifying this down to being two edges supported to make the calculations easier...
Moment within glass = (Forcing Load * Width^2)/8
= (19.62 kN/m * 9m^2)/8
= 22.1 kN·m
The ultimate stress of a sheet of annealled glass is commonly taken as being 4ksi, or roughly 27.5 MPa. Assuming an old fasioned factor of safety of 4 gives us an allowable peak stress of 1ksi, or roughly 6.9 MPa.
The stress in our glass is equal to the Moment divided by something called the section modulus. There are both elastic and plastic section modulii, however with something brittle like glass ONLY the elastic is available to us.
Rectangular Elastic Section Modulus = bt^2/6
*Assumption: The load is carried by a 0.5m strip of glass.
Elastic Section Modulus = 0.5t^2/6
Thus we get:
Allowable Stress = M/S
Rearranging:
S = M/Allowable Stress
= 22.1 kN·m /6.9MN/m^2
subbing in we get:
0.5t^2/6 = 22.1 kN·m /6.9MN/m^2
t^2 = 265.2 kN·m / 6900kN/m^2
t^2 = 0.03843 m
t >= 0.196 m (or 200mm thick, a totally unreasonable answer)
Thus, taking advantage of three FIXED supporting sides (a condition I don't think would be easily acheived, but would certainly help) we get a required moment capacity of:
M = 7.03 kN·m
resulting in a thickness of:
t >= 62.3 mm
And say you could get really high stregth glass (expensive) and the manufacturer can justify a lower safety factor (NOTE: This is not really your engineer's call; They are constrained by the physical properties and failure modes of the materials... DO NOT harrass them about something they can't control),
Allowable stress = 9 MPa
t >= 47.8 mm
Say 50mm glass.
This feels about right to me... Like demayeng I would be very suprised if someone could make 30mm glass work without intermediate stiffeners for this depth of water.
Good luck,
YS
B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!
