earthquake risks

[ben]

You don't read much about earthquake risks to tanks on the American forums, so I thought I'd take this opportunity to see what Kiwis do to minimise loss in an earthquake. I used to like earthquakes, but since I've been serious about keeping fish, I sh*t myself when I think about them! (excuse the French)

I look forward to the day I have a house with a basement, that's got to be the safest place for your tanks right? aahhh, my very own fishroom can't wait!

Ben

[Warren]

There are 2 ways to protect against earthquake.

For both, the first thing you need to find out is the maximum G-Force an earthquake in your area will subject the objects in your house to. This is a bit of a grey area, because all you can use for this information is the maximum measured force during the History of the area. This does not of course guarantee there won't be a bigger earthquake in the future.

I my area, the largest recorded earth quake measured 7.2 The recommended G-Force for the area is 0.7 I have doubled this to be safe. This means an acceleration force of 1.4G's in any direction.

Protection methods:

1. Flexible Stand:

Allows the tank to move on its stand, ie allows the stand to flex. This method serves to reduce the forces on the tank. It works very well provided the extremities of the stands ability to flex are not reached. If they are the stand will break. I do not use this method as I do not yet know how to calculate the stand requirements. It also better suits smaller tanks.

2. Rigid Stand:

The stand in this case is strong enough to support the tank rigidly. All the forces from the earthquake are transferred into the tank in this circumstance. The tank design must therefore be able to withstand 1.4 x the normal forces it would usually take. Glass is a quite magic substance as it can naturally take very short impulse loads up to approx 200%. No extra design is usually needed in the tank design to allow for earthquakes if the tank has been designed properly from the start (see my glass thickness article - from the FNZAS home-page).

The stand must be strong enough to take the full weight of the tank + stand x 1.4 with the force coming from any direction. The tank will need a rim all the way round its perimeter to stop it sliding off the stand. The rim needs to be able to support 1.4 x the weight of the tank. The tank must be strapped down to the stand so it cannot jump off. You guessed it, the straps must be able to support 1.4x the weight of the tank. The stand must be able to support 1.4 x the weight of the tank + the weight of the stand. The stand will need to be bolted down to the floor to stop the whole lot (tank + stand) from jumping round.

The floor needs to be able to support 1.4x the weight of the tank + stand. If it is a wooden floor, you may have to strengthen it by adding new piles. You will also need to check the lateral stability of your foundation to make sure it is rated to carry the extra side forces from the tank + stand.

Two Examples:

Warrens Current Tank #1:

Tank is 2400 x 820 x 680mm and hold 1200kg of water (8 x 3.25 x 2.2 feet - 300Gal 2640lb water).

The stand weighs 400kg (880lb). The total weight for the system is 2400kg (5280lb). The maximum force I have allowed for is 33600N (Newtons) or 3360kg (7392lb) of force in any direction.

The ring foundation that supports my wooden floor is capable of withstanding the lateral load, but the floor isn't. I added 6 new piles under the floor directly under the tank set in a 600mm (2 foot) square concrete pad.

Warrens New Tank:

Tank is 3100 x 1000 x 1200mm and holds 2970kg of water. The stand is built as a steel plinth 200mm tall and forms the base of the tank. The total weight is approx 4500kg (9900lb). The design loading for this tank is 63000N or 6300kg. This is 6.3 tonnes, about 4 x that of the average family car! This tank will sit on the same floor as the previous tank. The floor will not support the weight and the ring foundation will not support the extra lateral force in an earthquake.

This time I have to add a welded steel frame under the floor set in 8 x 600mm concrete pads. The frame will be diagonally braced to support 100% of the lateral load. This will decouple all the forces from an earthquake and transfer them directly into the ground.

These measures may seem a little extreme to some people. I however value my fish and take the appropriate action to ensure their survival through a large earthquake. With some luck they will come through it better than me. Not everyone has tanks as large as these, but some people have larger ones.

The biggest problem faced by most people is just not being sure what to do to protect their tank. The forces involved in an earthquake are often very hard to imagine. Unfortunately the calculations required to work out just what is required are very specialised. The people who know how to do it are highly qualified. Just the maths alone is beyond most of us. The only solution open to most is to get a structural engineer to go the calculations for us. The first time I ever made a big tank properly I used a structural engineer. It cost $400.00 for the analysis, but it saved me more than that in materials when I went to build it. In the process, I also learned from the structural engineers how to do the calculations. My maths is reasonably good so I was lucky to be able to understand it. However, this is because I've got an engineering background to start with. It may be beyond some people to understand and others just won't want to put in the effort to learn. For those people, they will have to pay someone to do the calcs for them.

I hope this helps.

[Caryl]

Quite frankly, I don't think most fishkeepers in NZ give earthquakes much thought when it relates to their tanks. It is a case of - fingers crossed. Besides, when the big one hits I guess we figure a cracked tank is going to be the least of our problems!

I have a 1.2m tank built into a wall unit so it is enclosed on 3 sides with a raised lip along the bottom front edge. It also sits on 3cm thick polystyrene. We are on a faultline which runs up the valley so an earthquake would most likely move my tank forwards and backwards, not side to side (although the solidly built, well braced, wall unit would stop the sideways movement). Because of the way it is built into the unit, any earthquake big enough to manage to tip it forward would also do serious damage to the house itself.

Having the glass break would be most likely and indeed, I have had that happen after a very minor quake caused a stress fracture. The tank at the time was 250 litres approx. and the water travelled through almost 7m (22ft) of carpeting, across 8m (26ft) of vinyl and was making its way up the hall carpet when we got up in the morning and thought - gee the floor looks shiny! Splash splash.

The fish were lying on their sides, still wet but no water left. Luckily we had a second tank all set up (the one in the wall until which has almost 350 litres in it I think) as we were waiting for it to stablise before transfeerring the fish. We grabbed them and threw them into the second tank figuring not-quite-stable water conditions were better than no water at all Only one fish died. They were all big fish 15cm or over, a mixture of silver dollars, a silver shark and some acaras from memory. They all had severe scale damage on the side lying on the gravel but recovered well (except the dead one )

I feel guilty every time I see the current Earthquake Commission adverts telling us to Fix Fasten and Forget. I know the bookcases should be bolted to the wall but my newest one is not (and it has all my fish books in it!!).

I have been in a big earthquake and they are no fun!! The one I remember moved our house 10cm (4") off its foundations and I can still hear the sound of the chimneys crashing down around us and the pictures and ornaments smashing at my feet. We didn't have a fish tank in those days

[Caryl]

Warren's reply was still in transit when I read the orginal post so I didn't read it until after I had posted my reply above.

My wall unit is attached to the wall but I doubt it is good enough judging by Warren's information. We already had extra piles under our wooden floor so it is braced from below. The wall unit itself is 3.6m (12ft) wide and goes from floor to ceiling as it covers the whole end wall of the small end of the lounge.

I do not have any chairs near the tank so no-one should get injured from it in the event of a quake. It also has a solid wooden lid covering the glass lid beneath so when all my souvenir glasses from the wine and beer festivals and the Ansonia clock and books come crashing down, they should bounce off the lid and miss the tank below.

Here's hoping the polystyrene absorbs a lot of movement!

[Benny]

hey warren, how good were you at physics when you were at school?

[Warren]

Hi Benny,

I got 96 in school C and 97 in UE. I've also done tertiary mechanics + physics. I've never done structural engineering however. It is not too difficult to grasp the basics if you have a reasonable understanding of physics and mechanics to start with. The maths isn't too hard. Its about the same as 6th form calculus.

Most of it is common sense. If you have a big tank sitting on a stand and you push against and it moves at all, then it is going to break in a big earthquake. The rim that runs around most people tank is usually not strong enough. If you think you can rip it off if you grab it in your hands and really heave on it, then its not strong enough. Try to put things in perspective. If the tiny amount of force a person can exert on a stand can move it, this is nothing compared to what it will see in a decent earthquake. It does not cost much extra to build a stand to hold a tank in place properly. In most cases its only a couple of dollars of extra screws.

I've seen Caryl's setup and it looks really strong. The only bit I'd be concerned about is the front strip across the bottom of the tank. It may not have enough screws in it to stop it breaking off in a 'Big One'. It should have at least 10 screws evenly distributed across its length. Each screw is good for approx 50kg, so 10 will give nearly 500kg of protection if the tank moves all at once and not one end first. If the tank is 350L it is probably at least 500kg all up in weight. The unit will support it no problem as it is built into the structure of the house.

At the end of the day, you can only do so much. The idea is to do the best you practically can. There is always the chance of an earthquake that is so big that nothing will save the tank. All you really get by trying to earthquake proof your tanks is piece of mind. They will probably survive if you've taken the necessary precautions but there is no guarantee. The fingers crossed method seems to work pretty well for most of us.

[Caryl]

Once again we got off topic from the original thread Perhaps we should ask Cees to move these earthquake posts to a separate thread as I am sure the information Warren has given will be interesting and useful to many but they might not realise it is here since it is under the heading Fibreglass Tanks.

[cees]

As you probably noticed, this topic was moved to Technical Forum

[Warren]

Cool, thanks Cees. I'm pretty good at getting carried away once I get started on something. I tend not to notice what originally set me off on the path...