At Blender we love a good challenge, so when I needed a restraint for my new rainwater tank at home, I decided to engineer one myself. I wanted an unobtrusive restraint that was easy to fabricate and install and it needed to mount straight onto the side of the house. As it is a slimline tank, I wanted confidence that it would be secured properly and not tip over. A big motivation for this restraint was to simplify installation and avoid the need to use the large quantity of concrete required for the manufacturer’s proprietary design. A rainwater tank is a great source of water for the garden and reduces reliance on mains water which there is currently a shortage of in Auckland.
I used my knowledge of finite element analysis (FEA) and mechanical design to ensure my solution was secure by complying to the New Zealand standard for seismic design (NZS 4219:2009 Seismic performance of engineering systems in buildings).
According to the standard, Auckland is a low seismic area. Combining the various factors for component risk, performance and location, the multiplier for the component weight was only 0.30. This means that the restraint (or restraints in total) need to resist 0.30 times the maximum weight of the water tank in any horizontal direction. This ends up at only 321kg. Split between two top and two bottom restraints (all approximately equidistant from the centre of mass of a full tank) the demands on the design are very small.
The top brackets are wall mounted to position the tank close to the building – after all it is slimline so there is little sense in having it waste space by sitting further away. The way in which the tank was to be attached to these restraints is driven by the geometry of the tank, which was made to suit the manufacturer’s proprietary restraint design. A steel box section was chosen for the brackets and was bolted together with standard hardware. These are held to the tank by M16 bolts dropped in from the top. A 3D model and FEA gave me confidence that the bracket design has a high factor of safety. The brackets were mounted to wall studs using M12 lag screws which provide a convenient fixing and easy waterproofing using EPDM washers.
A concrete pad was formed above the existing path to provide a level surface and increase the concrete thickness to that specified by the tank manufacturer. Polymer reinforced concrete was chosen to avoid the need for steel mesh and to achieve a uniform strength throughout. Pryda angle brackets fixed with Ramset Ankascrews were used for simplicity to prevent the base of the tank sliding, to further comply with the standard. The Ankascrews have a lot of good technical information for application strength, are easy to install and only leave a hole when removed. A calculation was performed to confirm the strength of the Ankascrews due to their proximity to the edge.
The final result is a fully installed rainwater collection tank that isn’t in the way, as it sits discreetly alongside my house – all that is left to do is finalise the stormwater connections and install a pump. I am confident that it is safely installed and will withstand external loading in the event of seismic activity. Using some simple engineering techniques I was able to create this simple bracket for my needs. The standard is relatively straightforward and the process can be easily applied in different scenarios.
The restraint used in this article has been custom-designed to suit its unique installation. The information contained here is meant to be informative only and no guarantee or licence is given for the solutions shown. Blender Design welcomes discussion of your own engineering needs to create a tailor made solution.