The ability to analysis structures to demonstrate their resistance to extreme loading is becoming more important. By Extreme Loading CREA refers to naturally occurring loads, such as wind, wave, temperature and earthquake. Man-made loads such as fire and explosion are classed as hazard loads.
All structures need to be designed to resist all reasonably predictable loading; these are usually defined as having an annual probability of occurrence of 0.02, or once in 50 years. These loads are well referenced in design codes internationally.
However, novel structures can yield unexpected responses to normal environmental loads, such as vortex shedding in the case of wind. Vortex shedding can be studied using codified hand methods, or by using dynamic analysis. The use of dynamic analysis allows a more rigorous assessment of the response frequencies and durations, and hence a better estimation of fatigue damage. CREA can study vortex shedding and if necessary advise on mitigation measures to remove or reduce detrimental responses.
Sensitive structures, such as those housing chemical or nuclear processes are usually designed to resist events with a probability of occurrence of 10-4 or once in 10,000 years. In these cases the loads are very much more severe, and it is often necessary to consider the so called "cliff edge" effect. The design rules for the 10-4 event will often allow energy absorption and therefore residual damage, with the proviso that there is no loss of containment. Engineers at CREA have over 20 years experience in dealing with the analysis of structures to resist such extreme loads.
CREA offers an engineering consultancy service for the design of structures and components subjected to extreme environmental, hazard or unusual loads (extreme loads):
When considering response to these types of loading the application of standard design codes of practice is either modified or not applicable. CREA's engineers have over 20 years experience in designing to these conditions. The trend is now for operational safety cases to be required to obtain regulatory approval to operate plant and systems that pose a danger to the general public and workers alike. These safety cases now often require the demonstration that designs satisfy the ALARP principle. The ALARP principle is the demonstration that the risk of severe injury or death is As Low As Reasonably Practicable. This requires that the designers demonstrate that the design has used "state of the art methods", and that all reasonably foreseeable loads have been considered. Reasonably foreseeable loading can often mean looking at loads with a return period of 1:10,000 years, compared to the normal design consideration of 1:50 years.
In designing for extreme loads it is normally acceptable to exceed normal code allowables. CREA's engineers have experience in applying design codes from many different countries to design against extreme loading.
In designing to resist extreme loading it is often possible to design to mitigate effects. For instance, structures can be designed to "ride" earthquakes, or resist fire and explosion.
Another common approach, more normally applied to fire resistance, is to allow a structure to collapse, but to control the collapse. The process is for the safety case to define a reasonable safe evacuation time, then for the fire protection to provide stability for the evacuation period. This technique allows a cost effective protection system to be designed.
CREA offers its advanced design consultancy in several ways:
CREA also has experience of working for one of the UK Government Regulators in relation to nuclear safety. This gives us an insight into the level of work required to satisfy regulators.
The techniques necessary to design for extreme loads also lend themselves to trouble-shooting designs where there has been a failure to perform to specification, where a specification has changed significantly, or where there has been an actual failure in operation.