Case Study

Italian consultancy service provider trusts LMS testing solutions for accurate operational modal analysis

Essebi

ESSEBI helps its customers with LMS Test.Lab and LMS SCADAS Mobile from Siemens PLM Software through structural dynamics investigations in civil engineering

With a large number of ancient monuments and buildings, Italy is fertile ground for engineers to perform structural diagnosis and health monitoring through structural dynamics tests. These help to gain better knowledge and understanding of such structures. Years ago, knowledge in structural design as we use it today didn’t exist and such assessments were not possible. Without these, most of the structures dating back more than 50 years ago are now slowly reaching the end of their design lifetime. Because most of the bridges and viaducts, for instance, extend over the Italian highway network, investigations are necessary on the basis of modern technologies. These allow engineers to assess whether the current state is reliable or whether structural rehabilitation is required to extend the lifetime.

The results of the assessments help engineers and infrastructure departments to make informed decisions to avoid unexpected failure and plan the necessary investment for structural rehabilitation.

Based in Rome, but active throughout the country, Essebi S.r.l. (Essebi) offers measurement and monitoring services as a freelance and consultancy service provider specializing in civil engineering. The company helps its customers to improve their knowledge through tests conducted on a variety of structures. The company specializes in monitoring services, structural diagnostics for masonry, concrete and steel constructions, as well as vibration measurements with particular attention to experimental modal analysis and the dynamic characterization of structures. The company also performs tests on building floors to evaluate the level of comfort based on data analysis and the perception of the inhabitants according to the standards of International Organization for Standardization (ISO) 2631 regarding human exposure to whole body vibrations. Finally, Essebi uses laser scanner technology to capture specific structures to preserve the artistic aspect and to create computer aided design (CAD) models for the structures that have none.

Essebi is familiar with the LMSTM testing solutions included in the Simcenter™ portfolio, from which it uses LMS Test.Lab™ software and LMS SCADAS™ hardware for vibration measurements. For civil engineering applications, the company works with operational modal analysis (OMA) technology. The reason for this is that the structures to be tested are too large to generate a measurable excitation that could build enough energy on the entire construction in a sustainable, economic manner. OMA overcomes this limitation because it uses the natural excitation by traffic or wind as an input force.

Understanding the dynamics of Devil’s Bridge

The experimental dynamic investigations and analysis conducted by Essebi today enable the future conservation and maintenance of structural integrity across a number of infrastructure and building concerns. Using OMA, a dedicated team of engineers works on defining the modal parameters as accurately as possible, so that in the coming years an essential comparison can be made that can give clear indications of the health of the structures. LMS testing solutions enable Essebi to reach its targets by providing the right equipment to achieve optimal measurement results.

One of the most important investigations Essebi conducted involves Badia Bridge (in Italian, Ponte della Badia), also known locally as Devil’s Bridge to reflect the audacity of the construction. This impressive attraction is located 30 meters above the river Fiora and dates from the third century B.C. Built by the Etruscans and rebuilt in Roman and medieval times, the bridge is located in the archaeological park of Vulci in Lazio, Italy. It includes a castle built later in the 12th century.

Following a significant rise of the river level in November 2012, the right stack of the central arch was badly damaged, threatening the overall stability of the bridge. It underwent reconstruction work thereafter. Using the strong wind that constantly blows through the gorge as the source of excitation, Essebi conducted an investigation with the LMS testing solutions to demonstrate the effectiveness of the static rehabilitation of the bridge. This investigation also served as input for the structural finite element modeling (FEM) for future validation.

Lorenzo Lepori, a structural engineer specializing in structural dynamic testing at Essebi, notes. ”The vibration level on the bridge is very low, so the measurement equipment needs to be very sensitive to capture that. LMS SCADAS has a high dynamic range; we can measure very high to very low vibrations with the same equipment while maintaining high accuracy. I know that when I go in the field with closed eyes and this hardware, the results will be accurate for post processing and my customers will be happy.”

Furthermore, the company decided to use LMS Test.Lab for dynamic characterization such as OMA and experimental modal analysis (EMA). “The software is intuitive to use, comes with a user manual that explains all the features clearly and supports easy postprocessing,” says Giorgio Sforza, mechanical engineer and CEO at Essebi. “The system performs the analysis quickly and interactively. It is easy and practical to create a spreadsheet to compare results.”

LMS testing solutions for various applications

Since Essebi is involved in various types of measurements, in different types of structures and in various environments, the company relies on the exceptional flexibility of the Siemens PLM Software testing products. The pressure and the daily challenge is to satisfy every customer, regardless of the site or the individual requirements. This is why Essebi appreciates the great adaptability of LMS testing solutions that cover many of its extensive needs.

“LMS SCADAS Recorder with mobile technology has a memory card where you can store the time data without using any other interface or PC,” says Lepori. “For us, the tool is extremely reliable, especially when we find ourselves in rather difficult circumstances to acquire the data we need. A while back, we had to perform vibration measurements inside a gallery and there was no electricity to help us work in a comfortable, easy way. We were able to use the internal battery independent of an external power supply to carry out the necessary measurements quickly and smoothly. We were grateful that we were able to do our job efficiently without losing time and without impairment by the limitations of the work environment. Small details like these matter and help save a lot of effort.”

For its customers, Essebi mostly requires 16 channels for its data acquisition, but occasionally the company performs analysis with up to 50 channels. LMS Test.Lab allows the company to manage multiple channels at the same time. In certain cases, the engineers must gradually work to cover very large structures, since they cannot be measured completely in one take. Therefore, many fractioned measurements must be collected in LMS Test.Lab to reassemble the overall data and find the final modal shape. LMS Test.Lab has no problem analyzing large data blocks, whether they come from one or more data files.

With the use of LMS Test.Lab Impact Testing, Essebi makes a correlation between the dynamic stiffness calculated by the dynamic test and that calculated by the traditional static test on the floor. Certain applications require both approaches to be performed simultaneously. In a school environment, for example, Essebi users performed two static and seven dynamic tests, which also considered the first flexural frequency. This allowed them to compare both in a quick and cost-efficient way and better understand the integrity of the school’s floor.

Is OMA the future?

Italy is an earthquake-prone country with a big need for these types of tests for both buildings and infrastructure, especially for those where the finite element CAD model is missing or incomplete. An experimental characterization of the dynamic behavior of a structure helps engineers to validate and update the finite element model. This allows engineers to perform the structural verifications as defined in the Italian and European seismic regulations and standards, such as Technical Standards for Construction (NTC) 2008. Essebi is a firm believer in the advantages of using modal analysis and does its best to offer this service to its customers.

“In the field of vibration measurement, our company is working in the modal testing area. Our applications are oriented towards civil structures and we generally run operational modal analysis because we find it more suitable for real scales and large structures. For this, we use LMS Test.Lab with impact testing, modal analysis and operational modal analysis modules. Last but not least, we rely on LMS SCADAS Recorder to collect the data,” says Lepori.

One of the main advantages of OMA is that the normal activities surrounding the structure to be tested can continue as normal during measurements. Be it a highway or a school on a busy day, everyone can continue doing what they are doing during the Essebi analysis. “My wish,” explains Lepori, “is that OMA becomes the most important technique for vulnerability evaluation of structures combined with numerical simulation, especially when it comes to earthquakesensitive areas.”

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