Durability, evaluation and remediation (1)
26-05-2022 | Posted by Joaquín Martí
Sustainability improves when things last longer. Structures undergo many challenges, some foreseen in the design, others less so. Those challenges must be anticipated, so that the structure is prepared to withstand them. Some typical problems are fires, cracking and fatigue, the vagaries of the ground, vibrations, noise, and the chemical expansion of concrete. We review here the first half and leave the others for another post.
Fires can be a threat to the operation of our facilities and the integrity of our structures. They entail changes in the materials, as when they burn or melt, and the increased temperatures cause thermal dilation and changes in the state of stress.
Sometimes we analyse structures to ensure that they perform adequately in a postulated fire. This is the case of many storage tanks for liquefied natural gas, nuclear structures, even a bridge with piers located in wooded areas. We also participated in an international exercise to predict and interpret the effects of fire on composite steel-concrete structural members.
Other times we are called in after the fire occurred, whether to investigate its causes or to analyse its consequences and propose remedial measures. Among the former, we studied fires at several wind turbines; and we investigated the catastrophic failure of a glass furnace, a facility that operates at about 1500ºC. We also examined a factory that had experienced both a fire and the collapse of an electricity pylon to determine the causal relationship. .
In the second line of work we studied a large concrete structure where a fire consumed all the wooden formwork, subjecting the exposed surface to high temperatures for several hours. We conducted the site inspections, carried out thermal and mechanical analyses, organised the coring and testing of samples, and designed the necessary remedial measures.
Many materials undergo cracking, whether by monotonic loading or as a result of repetitive demands. Tensile cracking of concrete and fatigue cracking of steel are frequent examples.
Principia has analysed concrete cracks at various wind turbines, most caused by poor designs of the tower-footing connection. We also investigated the cracks caused by the full-scale testing of a concrete tower, their influence on its dynamic behaviour, and its subsequent failure. We also studied cracks in other concrete structures, like bridges, to determine their causes and suggest remedial measures. And dams, to prevent cracking caused by the thermal expansion associated with the concrete curing.
Steels may also undergo cracking, especially by a fatigue or embrittlement process. We have studied an injection block, railway wheels, brake discs, wind turbines, an electricity pylon that suffered liquid-metal embrittlement. And, apart from metals, we also investigated cracking of graphite blocks embrittled by radiation.
Repetitive loading generates fatigue problems in many materials. Wind turbines experience them frequently: drive train, multiplier, orientation and yaw mechanisms, even the main rack in the nacelle, we have studied all those problems. The same applies to railways, where we have analysed rail supports, wheels, brakes, etc. And electrical generators, steam generator tubes in nuclear plants, mixers for plastic flows, bedplates, tanks, brakes, etc.
Improving the life of equipment and structures has many advantages and it is worth doing our best to achieve it.