SCI has commenced destructive tests on another Blind Bolt product – the Heavy Duty bolt. This is the latest product to be tested to destruction in a laboratory – previously SCI managed the testing of BlindBolts and Thin Wall bolts. All three bolts are “blind” fixings, used when access to install and tighten the bolt is from one side only.
The Heavy Duty (or “HD”) bolt has a central pin running the full length of the bolt, meaning the main body of the fixing is a tube. After the bolt is installed in a hole, the pin is struck which forces cut segments of the tube to splay out on the “blind” side to act as the anchor when the bolt is tightened. HD bolts are very slightly smaller than the drilled hole diameter, so would be described as “close tolerance”, meaning there is minimal slip between joined elements.
SCI’s responsibility has been to design the test arrangement and specify the test procedure. This may sound simple, but thoughtful arrangements are needed to ensure the tests can be carried out efficiently in the available test equipment. In addition to determining the design resistances for use in the UK and other European countries, the goal is to gain accreditation for these fixings in North America, which is similar to CE Marking. The test arrangements and procedures must therefore also satisfy the American requirements.
For use in the UK, the design resistances are calculated in accordance with Annex D of BS EN 1990. This statistical approach determines the design resistance as a certain number of standard deviations below the mean – fewer samples mean a larger reduction. SCI generally recommend five samples as a reasonable balance between testing costs and the impact on the design value. Annex D of BS EN 1990 has an almost identical process which can be used the determine the characteristic value, the only difference being a less onerous reduction below the mean value. If the characteristic value is calculated, a partial safety factor yM2 would be required in order to calculate the design value – but no partial factors for innovative fasteners exist. SCI therefore recommend calculating the design value directly.
Having tested very many fixings over a number of years, one observation is that for carbon steel the tested shear resistance is almost always significantly greater than the resistance calculated in accordance with the code. The results for stainless steel are closer, but follow the same pattern. The design shear resistance is always taken as the lower of the code-compliant calculated value and the resistance determined from the tests.
Once testing is complete, the design resistances will be prepared and an SCI Assessed report completed for the client. A final responsibility is to determine the resistances in accordance with the American accreditation document.