Ductile and fatigue properties of steel
Construction steel is the material for the backbone of a country's economic development. A nation's state of economic development can be gauged from the annual per capita consumption of steel. The table below shows at a glance the steel consumption of selected countries of the world. Countries with a strong manufacturing base, such as Germany and Japan, have much higher per capita steel consumption.
The figures in the graph below speak of the immense growth potential of the steel industry in the country. Our country has just started its journey towards urbanisation and industrialisation, the two power houses of economic development. Both the power houses are fuelled by steel consumption. Steel production is a core industry in every country and is considered 'strategic' to national development. In the last five years, the Bangladesh steel industry has undergone a qualitative change with the bulk of its constructional steel now being produced in the high strength 500 MPa category. While high strength ensures economy in design and construction by reducing steel consumption it also raises the danger of brittle failure if proper attention is not paid to Quality Assurance in the steel production process chain. This article will try to highlight the importance of two important properties of steel in ensuring safe construction.
In recent years the Bangladesh construction industry has also undergone a qualitative change. Hitherto, the most common structure being designed and built was a reinforced concrete building frame for commercial and residential purposes. In recent years multi-storey industrial buildings, mostly in the garments industry, have been built. Flyovers in the urban centres are yet another new development. Flyovers and multi-storey industrial buildings are all highly steel intensive construction. A typical flyover requires 2,500 tonnes of reinforcing steel per kilometre. It also requires another 300 tonnes of pre-stressing steel tendons per kilometre. A 10-storey industrial building needs around 12-15kg steel reinforcement per square feet of built up area. In contrast, a residential building with 10 storeys would need around 4kg of steel per square feet.
Industrial buildings are constantly subjected to vibrations from operating machinery. In a flyover the deck is subjected to the rolling load of moving vehicles where the bending stresses reverses rapidly. These load reversals will persist during the service life of the flyover. Further, moving vehicles below the flyover impart vibratory impact loads to the piers of the flyover. In both the cases these are dynamic loads. Earthquake ground movement imparts stresses beyond the yield strength of steel and sections of the building frame or flyover may undergo considerable 'plastic rotation' and form a 'hinge'. This plastic rotation absorbs most of the seismic energy and prevents a catastrophic collapse of the building or flyover. In a 'plastic hinge' type failure the building is damaged beyond repair, short of collapse. It should be mentioned that in forming a plastic hinge the load carrying capacity of the steel increases. Therefore, the ductile property of steel is the only and most important requirement for protection against catastrophic collapse.
The American Concrete Institute (ACI) code and our own Bangladesh National Building Code (BNBC) requires Ductility of steel measured as a ratio of Tensile strength to Yield Strength as a basis for ensuring safety in Earthquake design of structures. The European 'Euro Code-2' and the Australia-New Zealand ANZ Code prescribe Elongation at Maximum Force (EMF) as the measure for safe earthquake design. In reality both the approaches are similar. While the ACI and BNBC approach is 'stress' based the Euro Code 2 and ANZ approach is 'strain' based.
Industrial buildings and transportation infrastructure can fail, sometimes quite catastrophically, at the end of their useful lifetimes due to fatigue failure of the steel and concrete. Fatigue occurs due to cyclic loading of the structure. An industrial building or flyover can be considered to have a useful life between 40 to 50 years. Fatigue failure can also be quite sudden and without warning and the consequences can be very disastrous. The International Standards Organisation (ISO) has laid down detailed specifications for the fatigue testing of constructional steel. The prescribed limit for the successful test of fatigue strength of constructional steel is 5 million load reversals on an axially loaded steel specimen.
The Bangladesh University of Engineering & Technology has excellent facilities for the testing of ductile and complete tensile properties of constructional steel as per international standards and codes. It can conduct tests on 8mm to 60mm reinforcing steel bars. Unfortunately there is no fully fledged facility in the country to test the fatigue properties of constructional steel.
In conclusion, the 'ductile' property of steel remains inherently latent and 'unused' over the lifetime of the structure. If no major earthquake occurs during the life span of the structure the 'ductility' of the steel remains unchanged. On the other hand the 'fatigue' property of steel is 'used up' gradually over the service life of the structure. Both the steel and concrete become more vulnerable to failure at the end of the service life of the structure. This is one of the reasons, old buildings and bridges become more risky to use after its useful service life.
The writer is the head of marketing and product development at BSRM Group and can be reached at [email protected].