There is no simple method of testing the strength of a steel structure in situ, which poses problems for civil engineers. Pressure from transport groups to increase the traffic loadings on bridges, combined with the need to ensure safety, would make a simple in situ test highly desirable. And offshore platform operators and owners of other steel structures could benefit.
Broadcast and telecoms company NTL has developed a simple, low-cost and reliable method of testing the strength of its masts and towers which it also leases to third-party service providers to maintain radio transmitters and antennae.
The tremendous growth in mobile telephony together with commercial pressures and environmental control of ‘blots on the landscape’ has forced NTL to find a safe solution to the problem of populating its masts with antennae.
In the Sigma-Tau test inventor Peter Heslop, a chartered civil engineer and NTL’s group manager for site engineering, has found a way to assess the strength of the mast supporting the antennae without having to remove a section of steel and send it to a laboratory for lengthy and expensive testing.
In Heslop’s test an operator with a hand-drill and specially designed rig takes just 20 minutes to carry out the procedure which relates steel yield stress (sigma) to torsion (tau).
Using the test Heslop says the company has been able to identify masts which proved to be stronger than shown by the original design making it possible to install more antennae from the start. But reappraisal of already populated masts is still the main aim.
Aided by a rig clamped to the mast member, an operator drills a small hole 6mm across in the steel. The depth is not critical and depends on the cross-section; some holes will go right through.
The next step is to deform the hole by driving a spline slightly larger than the original hole into it. The spline has a stellated circular cross section. Each spline cuts its own groove parallel to the axis of the hole.
A second spline, of identical shape and size to the first, is then inserted to a precise depth and forcibly rotated in the hole. The torque which develops is measured and used to assess the strength of the material.
Bushes which clamp into the jig ensure each operation is concentric with the other.
In the Sigma-Tau test there is no simple formula. Heslop says strength is deduced by calibration. Torque readings are plotted on a chart against steels of known strength. From that the actual strength of the steel can be found.
The test, developed with welding industry research body TWI in Cambridge, is causing a flurry of interest among established test houses.
Though it is used to test commercial construction grades of steel, Heslop says the applications are much wider and could include testing plastics and even hardened steels. Drill and spline tools would need to match the test material.
Corrosion could be a problem. In masts, the 6mm holes are filled with grease or a zinc-rich paste. But underwater structures that are exposed to salt would need a different approach.
There is huge potential for an on-the-spot test that is cheap, quick and non-destructive, Heslop claims.