It is usual, during our work as researchers at CARTIF, we have to model and solve (with the help of advanced software) complex mechanical systems. Their behaviour is affected by the interaction effects, with different levels of coupling, among several physical phenomena of differing nature (structural deformation, heat transfer, electromagnetic fields, etc). These cases are known as multiphysics problems and are solved using computational multiphysics, a new discipline which sets out theoretical and numerical challenges. Mathematically, multiphysics problems are defined by a set of strongly coupled partial differential equations that require the development of strong algorithms to be solved in an efficient way.
In the past, because of the lack of computing power, the effects of the connection between the different physical fields could only be considered in a rough way or be completely ignored. Nowadays, the improvement of software and hardware makes possible to solve most of this problems using multipurpose calculus commercial codes, e.g., ANSYS or ABAQUS. The possibility of including connection effects leads to a better understanding of the causes and the consequences of the involved natural phenomena. From an engineering perspective, it is possible to approach problems from a more general perspective, making feasible to obtain a closer estimation of the actual performance of each of the different proposals for any prototype. Products obtained by this method are safer and more cost-effective, meeting customer’s needs in a better way.
“Elephant´s foot” buckling localized at the tank base
The most important multiphysic simulation method in structural engineering is the Fluid-Structure Interaction (FSI), this method is the one with more practical uses at industrial level and the most developed of all. It consists on analysing the interaction produced between a deformable solid and the fluid (liquid or gas) surrounding it or circulating inside of it. This interaction happens when the pressure applied by the fluid over the solid produces a deformation of the structure that modifies the boundary conditions of the fluid flux. This modification changes the pressure applied over the solid and so on, when this happens it is said that the structure and the fluid are coupled and therefore they cannot be analysed separately (with the exception of weak-coupled systems). FSI method is widely used at many industries, such as automotive (airbag deployment), aerospace (sustentation surfaces fluttering), biomechanics (aneurysms), energy (combustion at boilers), etc.
The figure shows the multiphysic simulation of the dimpling produced at the bottom of an open tank when is under seismic action, phenomena known as “elephant’s foot”.
In recent years, being the instrumental techniques cheaper and cheaper and the computational algorithms more accesible (even open source) several researchers and consultancy companies are developing new 3D abilities. Laser scanning or photogrammetry techniques are applied to mechanical or structural systems in order to collect some geometric specifications, which may be not available for different reasons.
Although direct engineering process will usually have the technical reports and drawings of the product prior to its building or manufacturing, it is usual that the old factories or buildings are not documented or, if they are, it is quite common that the drawings do not match to project. And even so, the time may have caused differences in the material behavior (chemical attacks, damage, settlements of supports or other common structural pathologies).
Footbridge stadium Balearic (Mallorca, Spain)
Often the collected data are focused on geometric dimensions and surface characteristics such as roughness and color. One of the most obvious applications is the three-dimensional reconstruction of architectonic buildings, either for rehabilitation or development of augmented drawings (BIM) or for historical or industrial heritage.
Being very useful the geometric data collected, in structural engineering it is necessary to add more information about the characteristics of different building materials, the joints between them and their possible interaction with the supports and the ground.
Fortunately, other enabling technologies to extract some additional information are also becoming more widely available. In this post we will see how using simple acceleration records and identification algorithms together with computational model updating techniques can complete the geometric information so that all technical specifications, necessary to estimate the dynamic behavior of the structure under study, can be obtained. These procedures do not require destructive testing and, even though these tests were viable, they did not provide the required information despite their higher cost.
First it should be noted that the geometric data collectedusing 3D techniques, irrespective of dimensional accuracy, refer to a particular state of load on the structure (at least due to the gravitational action) and corresponds to a particular ambient temperature. Both conditions can affect in a significant way when dealing with slender structures such as bridges and pylons. Furthermore these constructs generally experience unavoidable deformations due to environmental actions that can also affect dimensional accuracy of the 3D model.
Second it is interesting to note that in structural engineering and building is usual to use commercial components (proper cross-sections, formworks, pipes, lamps, …) of known discrete dimensions. This enables the possibility of carrying out adaptive scaling for improving the dimensional accuracy or for local refinement. So, it is not necessary comprehensive dimensional records and low cost systems (both instrumental as compact cameras and computer software) can be good enougth.
Considering the above and assuming certain skills for computational modeling, it is posible to create a preliminary model of the structure. On this model, using the finite element method, it is easy to estimate the incremental deformation due to certain loads or thermal actions and through appropriate correlations begin to estimate certain internal parameters (effective density, stiffness, damage, etc.). However, the methodology is especially important when the above information is combined with modal data.
To do this, first thing is to have the experimental eigenmodes (identified through operational modal analysis by post-processing acceleration records under environmental loading) and then select certain parameters of the computational model to be modified. Now it is the turn to adjust the value of these parameters (through optimization routines and depending on the sensitivity of each parameter and its range of reasonable values) to match with the experimental modes to the numerical ones (calculated via FEM). This process should take into account not only the most representative mode shapes but also their modal frequencies and damping.
Once proper values for these parameters are determined, the computer model can be used not only to generate the corresponding technical documentation of the as-built structure but also to estimate their vulnerability to accidental loads, or to evaluate the life-span or to estimate the performance of conservation jobs, among other applications. Those tasks are known as “structural re-engineering”, whose advantages can be matter for other post.
How to reduce structural conservation task expenses by implementing monitoring systems?
The structures are not everlasting. They are projected to play a role for a certain number of years. Thus, a wind turbine mast lifespan is about 20 years while in the case of a bridge it depends on the type and the material used. According to Guy Grattesat, metal bridge will have a lifespan of 40 years, 100 years for a reinforced concrete, between 15 and 20 years for those made of wood and about 200 years for masonry. Nevertheless, exceeding the life expectancy does not necessarily mean dismantling the structure. Generally, what we do is a more comprehensive monitoring effort and implementing conservation works if necessary.
Wire and accelerations sensor installation process inside the handrail
The European bridge stock is catching its lifespan and it is showing signs of fatigue. According to Eva Lantsoght, Professor of engineering at the University of San Francisco de Quito, “European bridges are old, but their replacement involves a great investment. Only in the Netherlands, there are about 3,000 bridges that could cause problems, being their refurbishment cost around one million euros each”. To mention another example, the first wind turbine parks (1984), whose technological emergence came in 2002 (according to the GWEC, Global Wind Statistics), have great needs for maintenance and the increase of these needs will have the same exponential curve, I can imagine, that its development has had in the past.
18 triaxial MENS accelerometers (sensors developed by CARTIF) installed equidistant along the span inside the handrail.
Having this background in mind, I would like to highlight the importance of implementing new maintenance strategies to reduce structural conservation task expenses. State-of-the-art monitoring systems, developed in recent years at increasingly low costs can be the solution. Usually in this type of structures, the procedure consists on the installation of a network of accelerometers to record the ambient vibration response of the structure. Using identification techniques, the modal characteristics can be estimated and be used to evaluate the structural integrity.
The idea is to know the structural behavior in operation conditions and to determine the valid rangeby controlling only a few parameters (frequency, mode and damping), being these parameters easy to evaluate and to record their trends along the time. Although the meaning of the word ‘monitoring’ refers to the capacity of the acquisition system to record certain values, it is also important to add the ability to process the data and to generate alarm codes, if it would be necessary.
In this aim, researchers require a deep understanding of the technical matter involved as well as a big experience in experimental techniques and data processing. OMA (Operational Modal Analysis) identification techniques are based just on the acceleration records but if some loading signal applied on the structure could be also record, additional information can be obtained using EMA (Experimental Modal Analysis “EMA”). Both approaches should gradually replace, or at least complement, the traditional visual inspections and static loading tests. Monitoring systems could be an emergent business for technology-based companies in cooperation with new maintenance standards or strategies by the infrastructure’s authorities.
In this regard, I can mention the success of low-cost monitoring system of Pedro Gómez Bosque footbridge (built in Valladolid) operated by CARTIF team since 2012. This stress-ribbon structure undergoes no linear effects. The collected data has been useful so far to understand the behavior under dense pedestrian loading, at different met conditions (summer or winter, strong wind…), etc. Analyzing this information, we know what it is normal and what is not, so we can establish an optimal operational range and determine when anomalies appear and why. In these terms, the structure is an ideal benchmark not only for structural monitoring but also for understanding mechanical problems, pedestrian loading and other engineering issues.
