There has been a sharp rise in the number of buildings damaged by storms in recent years, in some cases even as a result of hurricanes. One of the most badly affected areas of a building is the roof. To make sure tiles are kept securely in place, even when the wind reaches storm force, one preventative measure is to attach storm clips to hold them firmly in position. Wurst Metal Technology, based in Höchenschwand in the Black Forest, has been working with two Steinbeis Transfer Centers (STCs) – the STC for Innovation and Implementation and the STC for Process Development – to develop an effective storm fixture that can be clipped on without needing to use tools. The project was backed by the state of Baden-Württemberg as part of the innovation voucher program (voucher A and B).
In Germany, roofs are typically covered with tiles or slate. These are spaced carefully and hooked over roof battens. The hump on each tile fits neatly into the groove on the next tile to form overlapping rows of tiles that can cover an entire roof without gaps. Usually the combined inherent weight of the tiles provides enough downward force to keep everything in place.
But if there is an extremely strong wind, reaching gale force, the weight of the tiles is not enough because the combined speed and direction of the wind creates uplift. In physical terms, wind pressure or suction is created in the slipstream on the downwind side of the roof. To prevent the roof tiles from taking off, storm clips can be fitted between each tile and the horizontal battens. The functional principle is similar for all kinds of storm fixtures: a mechanical part is used to affix the tile to the batten underneath. The actual nature of the fixture can be different, however, with everything in use from screws to wires, plates, brackets, or clamps. Sometimes the fixtures are quite difficult to attach so roofers prefer clipping devices because they can simply be attached without the use of tools and they are easy to take off again.
The storm clips made by Wurst have been in use for many years and although they do the job in technical terms, they do not adhere to failure criteria under standard EN 14437. This is especially the case with respect to uplift resistance requirements, which until now the clips could not meet. This was where the Steinbeis managers Georg Villinger and Prof. Karl Schekulin came in, bringing many years of technical experience with them. To analyze the geometrical and physical properties of the storm clips, they set up a full-scale (1:1) laboratory installation. The storm clips are basically the shape of a clamping spring which is attached between the roof batten and the tile itself. The experts soon discovered that there was not enough spring in an area along the connecting piece between the upper arm and the lower arm of the clip. They then doubled the depth of reinforcement beads along the whole side, lengthened it by another 50% and made it 20% wider. In experiments, the Steinbeis team ascertained that the breadth of the metal strip had little impact on stiffness, so it would be possible to reduce the width of the metal by 15%, which also made a noticeable difference in terms of material savings in production.
Schekulin and Villinger also improved performance decisively by changing the type of steel used to a low-alloy construction steel with a 0.2% yield strength of at least 320 N/mm2 and a tensile strength of at least 390 N/mm2. The area around the reinforcement beads is highly deformable offering partial hardening, with metallurgical benefits in terms of spring capacity. Laboratory testing revealed that the clamping effect was up to 5 times the inherent weight of the tiles. For Wurst Metal Technology, this meant that the clip had no problems fulfilling failure criteria according to EN standards.
To validate laboratory testing in a real environment, another test installation was set up with a variety of roof tiles. These were used to cover a sample test roof on a scale of 1:1 in order to determine uplift resistance in keeping with the EN standards. The test installation can also simulate the functional performance of a variety of roof coverings in combination with clamping devices in order to determine the ideal positioning and the number of storm clips needed.
The test installation was a fine example of a development strategy based on intelligent low technology (ILT). All functions and forces can be tried out using simple mechanical parts without any need for pneumatics, hydraulics, or electronics. For example, the lean on the roof can be set to any angle using a rack and pinion jack, the spaces between roof battens can be set to any width using sliding blocks, and tension can be placed on the roof tiles by using a cable winch. One particularly interesting feature is the hoisting frame, which runs at exactly 90° to the tensioning device and uses a ball and roller bearing. This is to ensure that no sideway forces are exerted on the tiles as these would affect readings. Thanks to the success of the project, Wurst is now certain that the simplicity and high quality of the identified solutions will also go down well in the market.
Prof. Karl Schekulin
Steinbeis Transfer Center Process Development (Reutlingen)
karl.schekulin@stw.de
Georg Villinger
Steinbeis Transfer Center Innovation and Implementation (Höchenschwand)
georg.villinger@stw.de