CONSTRUCTION FIXINGS


in a resin anchor stresses are developed in the base material, in that case parallel with the resin bond. These stresses gradually dissipate into the surrounding material and as they dissipate the tension in the bolt declines proportionately and along with it the clamping force. This effect starts quite quickly when the stresses are at their highest and, as they decline, so it slows down - as you would imagine. If we consider the life expectancy of a safety critical fixing in a building to be 50 years then over this period the clamping force will have declined, due to this effect, by something in the order of 50 to 60 percent. To take this into account the manufacturer will need to set the original clamping force at around twice the recommended tensile load. Before confirming this clamping force the manufacturer will check that this level of induced load will not take the bolt material near to yield and, in the case of a resin anchor, that it will not over stress the resin bond. This means that a carefully chosen tightening torque will do two things for us - it will ensure the clamping force for the life of the fixture and protect the fixing from being overloaded. So I hope you can understand why manufacturers place so


much emphasis on anchors being correctly torqued. For the switched on fixings distributor this represents not so much a problem as an opportunity – an opportunity to sell torque wrenches! But let’s move on the to idea of using this feature of fixings to


test them. For a start we cannot


use torque checks to determine allowable loads on fixings where we have no recommended load data, this can only be done using a calibrated piece of test equipment. There are, however, some instances when an engineer will


want reassurance that anchors have been correctly installed. Of course if the fixings being used have an ETA and are installed by a trained operator working under supervision then there should be no more reassurance required. But if he or she insists on some checks being carried out then maybe torque checking will provide a cheaper alternative to load testing? After all it’s easier to carry out, anyone can do it, the equipment is much cheaper and sometimes there is insufficient access for test equipment anyway. Or the fixing may not lend itself to being tested without being disturbed e.g. anchors with hexagon bolt heads. So what are the drawbacks? Well the first problem is just the inaccuracy. As I stated above the relationship between the torque and the clamping force is not a very accurate one, there can be a large discrepancy, so if we want accurate checks then a calibrated piece of test equipment is required. This inaccuracy will only be worsened by the passage of time with the possible ingress of dirt or dust (or - heaven forbid – rust!) between the mating surfaces. The next problem is this wretched load relaxation business. If we install a fixing today using the recommended torque of, say 100Nm, within a week it may be down to 80Nm, and within a month around 70Nm; 60Nm after a year; 50Nm after 10 and as little as 40Nm after 50 years. This is a very rough approximation of the phenomenon but you get the idea. What it means is that


if we check the torque after a few days we cannot expect to see the original installation torque. If we do then it will suggest that the fixing was originally over tightened. But we should at least be able to recover the original torque within less than a turn. An engineer carrying out load tests with calibrated equipment may well find a quick check of the residual torque quite illuminating before carrying out the actual test. A dial indicating torque wrench will be needed for this – see below.


In conclusion we can say: Torque checking cannot replace a test meter for accurate load


testing and determining allowable loads. It may be useful for giving a rough indication of installation


quality if access is poor or the fixing has a hexagon bolt head as long as we bear in mind the inherent inaccuracy and are mindful of load relaxation.


Types of torque wrench If you’re very posh, like me, and you’ve got a thing about


tightening torque, you might have a torque wrench with a dial on it, which actually shows the torque as it increases. These are very useful for investigating the condition of anchors before load testing but are pretty expensive and not what one would use for every day work on a construction site where the traditional “break back” type, which clicks on reaching the set torque, is more appropriate.


Typical “break back” torque wrench – photo courtesy of Norbar Torque Tools Ltd.


Typical dial indicating torque wrench – photo courtesy of MHH Engineering Ltd


A couple of points to watch Although all fixing manufacturers quote tightening torques in


N.m most torque wrenches still carry calibration in lbf. ft as well. If the wrench is set to X lbf. ft instead of X N.m the fixing will be over tightened by about a third so encourage your customers to check the scale before setting the torque. When a “break back” wrench clicks it simply indicates that the required torque has been reached, it does not prevent a higher torque being applied if the user keeps pulling on the wrench. For fixings with projecting studs always use a deep reach


socket as a standard socket may not reach the nut. Torque wrenches, like most measuring equipment, need careful use and calibrating once a year.


68 Fastener + Fixing Magazine • Issue 67 January 2011


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188