CONSTRUCTION FIXINGS Load relaxation


and anchor testing By Mark Salmon, Independent Fixings Consultants


In his last article, published in issue 67 of Fastener + Fixing Magazine in January, Mark Salmon outlined the relationship between tightening torque and clamping force and introduced the idea of “Load relaxation”. In this article he follows this up by explaining how this can affect anchor testing in general, why it’s particularly significant when testing resin anchors and why it should be born in mind when selling them.


in the anchor bolt, and hence the clamping force through the fixture, to relax. In issue 67 I showed the stresses around the expansion point of an expansion anchor. The diagrams below are my attempt to show the stresses in the base material parallel with the bond of a resin anchor plus the stresses in the resin bond itself which also relax.


L


oad relaxation – for those of you who may have missed issue 67 - is the phenomenon whereby stresses induced into the base material around the fixing gradually dissipate further into the material causing the tension


When we test fixings on site this may or may not be


Immediately after tightening – high stress around the fixing and in the resin, high tension in bolt and clamping force through fixture.


apparent. With fixings which have been tightened down we should experience no load relaxation until we have exceeded the clamping force introduced by tightening. Yes, the clamping force will be in the process of relaxing but until it relaxes to below the test load we won’t notice it. But with fixings where we have not clamped down a fixture, or a test adaptor simulating a fixture, e.g. a resin stud with no fixture applied, we may see load relaxation because all the stresses in the base material and in the resin will have been introduced by the test load and will start to relax immediately they are applied. This relaxation is seen as a small drop off in the load reading. With most fixings all we need to do when this happens is to return the load to the original figure and the subsequent relaxation will usually then be hardly apparent. But it can cause doubt in the mind of some observers, the natural reaction being “The fixing must be failing.” One recent development which emphasises this is the introduction of digital readouts on some testing equipment. We get to a load and instead of it holding steady the load is seen to be clicking back giving the appearance of a failure. Somehow with hydraulic gauges the slight reduction in load is more readily accepted. The problem we get with resin anchors in this regard is to do


with the way they fail and to understand that we need to go back to the way they work. Essentially what we do when we install a resin anchor is to


After relaxation – stresses dissipate into material and within resin, tension in bolt and clamping force through fixture both reduce.


80 Fastener + Fixing Magazine • Issue 69 May 2011


drill a hole, usually using a hammer drilling machine, part fill that with resin so that once the anchor is inserted it will be fully encased in the resin which, when cured, is very hard indeed. This hard material forms very strong interlocks between the rough shapes of the anchor on one side and the surface of the hole on the other. We can see the “roughness” of the anchor as it will usually be either the thread of an anchor rod, the knurling of an internally threaded socket or the deformed shape of reinforcing bar. But the characteristics of the hole sides are harder to appreciate. There are two aspects to this. The first is easy – the roughness of the hole sides. This roughness is generated on an almost microscopic level by the hammering action of the drill bit. It may not feel rough but it is rough enough for the resin to form a very useful interlock.


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