It is not the pressure-it is how far from the support that the force (from the pressure) is applied. So for a shorter piece of glass, the support is closer to the center of the glass sheet and can better resist the outward force. When the support is farther from the center, the glass can deflect (flex) under the force.
Here's some proof. Have you ever used a 1" X 4" board as a bridge? If the length is short (2 ft or less) and you walk across it, it does not bend much and can hold your weight. Now trying using the same 1" X4" board but with a length of 6' and walk across. It will bend more or may break but it is not subjected to more pressure. Your weight and the size of your feet have not changed- pressure is the force of your weight divided by the area of the bottom of your feet.
The force of the weight of the water on the area of the glass gives you the pressure. The reason the pressure does not change as tanks get larger if the height stays the same is that the area the pressure is acting on gets larger,so the greater force due to weight is divided by a larger area. I really should do the calculations and put up an excel file or something for people to find out how much pressure their DIY tanks will place on the bottom and sides. This way you can choose your materials based on how they will handle the pressure.
Here's some proof. Have you ever used a 1" X 4" board as a bridge? If the length is short (2 ft or less) and you walk across it, it does not bend much and can hold your weight. Now trying using the same 1" X4" board but with a length of 6' and walk across. It will bend more or may break but it is not subjected to more pressure. Your weight and the size of your feet have not changed- pressure is the force of your weight divided by the area of the bottom of your feet.
The force of the weight of the water on the area of the glass gives you the pressure. The reason the pressure does not change as tanks get larger if the height stays the same is that the area the pressure is acting on gets larger,so the greater force due to weight is divided by a larger area. I really should do the calculations and put up an excel file or something for people to find out how much pressure their DIY tanks will place on the bottom and sides. This way you can choose your materials based on how they will handle the pressure.
