Guide to Design of Culway - WIM

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UDefinition MAIN ROADS Western Australia Guide to the Design of Culway.doc Appendix A Pentagon Systems Procedure for Culway MSA Strain Gauge Installation This information note has been produced to describe a technique used by Pentagon Systems for many years as a method for repeatable and proper strain gauge placement. Strain gauges are used to measure the strain on the underside roof of a culvert as a vehicle pass over. The strain transducer and matched buffer amplifier assemblies are calibrated by ARRB by applying known mechanical strains to the transducer and then adjusting the buffer amplifier gain to obtain a correct electrical output. These matched assemblies are then issued with a calibration certificate. In-field calibration of a buffer is possible (refer to ATM 22). The strain transducers are machined from mild steel. In operation a displacement measured over the 300mm length of the gauge is focused on the 30mm ring. This produces a mechanical amplification of about 10. UHandling Strain transducers or MSA’s are highly sensitive calibrated pieces of equipment that must be handled with extreme care to ensure accurate continued operation. MSA’s should always be picked up from the centre ring and never from the ends as their own weight could cause permanent deformation. Padded instrument suitcases should always be used when storing and transporting gauges to site. Gauges should never be dropped or have external pressure applied to them as damage could result. Any permanent deformation to the gauge will result in an electrical output offset that in some causes cannot be balanced out with the associated buffer amplifier. The transducer or MSA in these circumstances must be either repaired or replaced. UInstallation This technical note does not cover details in regards to placement of MSA’s as this varies with each site. This document however attempts to explain a tried and proven procedure which is repeatable for securing gauges to the roof of the culvert. Poorly mounted and secured gauges can cause a number of problems in relation to site calibration, site drift and maintaining a consistent performance from the site. Gauges which are mounted or secured with some element of deformation due to the method they were secured or the fixing points can produce inconsistent outputs when put to work. The gauges, by design have been calibrated to operate within a range. When attempts are made to operate these gauges outside this range, erroneous results can lead to the problems that have been identified previously. Before any gauge is secured to the roof of the culvert the following precautions should be made to ensure the gauge is not damaged and that it can operate in a not deformed or pre-stressed state. • The use of a gauge template (a steel bar which is dimensionally identical to the Page 12
MSA) for drilling of mounting points to the culvert roof. Template bars with alignment points should be used to exaggerate any unevenness in the culvert roof. This will assist the technician to take corrective measures to fix the problem. • Mounting studs must be perpendicular to the culvert roof. The studs must be aligned to the centre of each gauge-mounting hole. Out of alignment studs will result in MSA’s being prestressed before they are secured. • Spacers and washers with large surface areas must be used to disperse the load to the gauge as it is tightened to the culvert roof. UGauge tensioning Tensioning a gauge to the roof of a culvert is in some way similar to tensioning a cylinder head to an engine block. Extreme care must be taken to ensure the tensioning procedure produces consistent results with little or no prestressing. The gauges are much more fragile and must be secured carefully. A method which is used by a number of technicians requires the use of an old style buffer amplifier which gives the operator a visual real-time indication of what stresses are being applied to the gauge while it is being tensioned to the culvert roof. The newer style buffer amplifiers are all of the auto balance type that makes continuous changes to balance that is unsuitable for this exercise. There is no need to have a matched buffer amplifier for this exercise, as we are only interested in the relative changes to output as displayed on the analogue meter on the front of the unit. Firstly, the buffer amplifier must be zeroed with a known good working MSA prior to tensioning any gauges on site. This establishes a baseline for unknown damaged gauges. The first of the site gauges is connected to the manual buffer box before it is placed in position and which should only require a small amount of adjustment to be zeroed. This is usually carried out while the gauge is on the floor. Gauges that deviate dramatically from the zeroed point have some mount of permanent deformation that results in the electrical offset described earlier. Depending on the amount, the gauge should be returned to ARRB for checking and possible recalibration. The gauge should now be placed in position while still connected to the manual buffer amplifier and the securing nuts should be tightened by hand while at the same time the meter of the buffer amplifier is viewed to determine if any induced stresses are occurring. Little to no movement to the needle should be observed during this process. A ring spanner, torque wrench is used to tension each nut such that the needle moves no more than a 1 – 2 mm in either direction. The pattern of tensioning each bolt should occur until such time the desired tension is achieved. By the time the tensioning has finished the needle should be as close to zero as possible. If the needle fails to zero during the tensioning process a problem exists with the mounting location or mounting points. Ensure the surface is flat and the bolts are aligned centrally to the gauge holes and perpendicular to the culvert roof. A right-angle square assists with this procedure. Adopting this procedure ensures all gauges have been mounted in a manner which ensures minimal to no pre-stressing and ensures that the re-mounting of a replacement MAIN ROADS Western Australia Guide to the Design of Culway.doc Page 13
Page 1 and 2: UGUIDE TO THE DESIGN OF CULWAY Docu
Page 3 and 4: 2. Site Location The selection of a
Page 5 and 6: Figure 1. Culvert Response for a si
Page 7 and 8: 600 < H CULWAY PAVEMENT BASECOURSE
Page 9 and 10: Barrier Design. 6.2 Culway Layout T
Page 11: 8. REFERENCES R J Peters Main Roads

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