HBM provides reliable measurement results: GRASSL relies on this for infrastructure monitoring

The transport infrastructure is exposed to immense loads and both internal and external factors that lead to fatigue day by day. To avoid serious damage and detect weak points at an early stage, structures must be regularly checked. This is done by experts, such as ones at the GRASSL engineering office.

They are looking for possible weak points through model calculations. In certain cases, the engineers recommend an evaluation of their calculation results through long-term measurements under the real conditions of the concerned structures. This often generates additional options for the client, and in many cases, they help relieve the strained finances of the public sector.

GRASSL has found a partner in HBM that provides high-performance measurement solutions for infrastructure monitoring in a complete package. The two monitoring systems collaboratively developed by the teams of GRASSL engineering office and HBM Service & Support already provide reliable measurement results.

Problem

A large part of the German transport infrastructure is outdated and needs renovation. Some structures may even have to be replaced for safety reasons. Thus, the condition of the aging transport infrastructure must be regularly checked.

Solution

HBM Service & Support offers turnkey measurement solutions for efficient infrastructure monitoring. The international GRASSL engineering office has opted for the complete measurement technology package offered by HBM along with their associated services.

Results

Individual measurement solutions from HBM Service & Support enable GRASSL to obtain reliable long-term measurements of the culturally protected sites, such as the Graskeller and Mönkedammfleet viaducts of Hamburger Hochbahn AG. Based on the results, the consultant civil engineers can make robust recommendations on these sites’ future.

The aspects that matter in infrastructure monitoring

A large part of the German transport infrastructure was built during the 1960s to 1980s. Many transport routes, especially in rail transport, are much older. Traffic volume has also significantly increased. Therefore, these aged structures must be carefully examined to detect any possible defects at an early stage.

The experts at GRASSL engineering office first carry out model calculations as per the legal regulations by using the safety specifications of the applicable guidelines for recalculation. The results allow them to make statements about a structure’s load-bearing capacity, its theoretical remaining service life, necessary upgrades, or the need to replace the structure with a new one.

Long-term measurements for validation of model calculations

Regardless of how meaningful and reliable the forecasts regarding a structure’s service life and the risk assessments based on model calculations are, in some cases, GRASSL recommends civil engineers to perform the long-term measurements of a structure stipulated in the guidelines, based on the results of model calculations, under real conditions.

The realistic test often shows that the actual remaining service life of a bridge is higher than the one previously calculated in the model, or a replacement structure that is recommended by model calculations can be disregarded since the structure can be safely used for a longer period through targeted upgrades. This will result in significantly lower costs to customers and, often, fewer annoying traffic delays.

For the recalculations of the steel superstructures of the Graskeller and Mönkedammfleet viaducts of Hamburger Hochbahn, a team of the GRASSL engineering office carries out long-term measurements for their client. HBM Service & Support provides GRASSL’s engineers with support through a customized monitoring solution and associated services.

A safe future for the protected Graskeller and Mönkedammfleet viaducts

GRASSL is testing the Graskeller and Mönkedammfleet viaducts that were built in 1911 on Hamburger Hochbahn AG’s behalf. This part of the Hanseatic city metro is a protected cultural site, and its restoration will begin from 202. The long-term measurements of the structure performed by GRASSL and HBM provide information about the required measures.

A particular challenge is that the superstructures of the viaducts that are to be monitored consist of components with different cross-sections, which were not welded but riveted. Therefore, determining the realistic mechanical stresses for the calculations is extremely complex.

Thus, GRASSL carefully determined the relevant measuring points of an extensive monitoring concept, which are necessary to measure the actual stress affecting the bridge. Based on this data, the structural health-monitoring experts from HBM selected the technology components required for measurement.

Customized upgrade based on reliable measurement data

Within just four weeks, the HBM team installed a total of 69 linear strain gauges in two measurement sections, along with a strain gauge rosette and four Pt100 temperature measuring points for correcting the temperature-induced measurement errors of the recorded strains. The measuring points were specified by GRASSL in such a way that any significant changes in the measured variables, which are caused by normal operating conditions, can be recorded and evaluated. The data can then be used to identify further damages or structural changes in the support system.

The sensors were then connected to one control cabinet per section using 1,000 meters of measuring cable. In this case, two Quantum X MX1615B strain gauge amplifiers and three rugged SomatXR MX1615B-R measuring amplifiers guarantee the precision and reliability of data acquisition. All measured data is recorded 24/7 using HBM's catman AP measurement data acquisition software and stored on a central measuring computer. The raw data is statistically processed directly on site.

The most important aggregated data, such as the Rainflow data, is then automatically transferred to the engineering office via a mobile LTE data link for analysis. This also allows remote access at any time to check the condition of the measuring system and analyze the measured data live.

Despite the tight time frame, the test measurements with defined load and load position on the Mönkedammfleet and Graskeller viaducts could be carried out as planned. Following this, the long-term measurements were initiated to measure the actual load at the relevant points.

After completing the infrastructure monitoring by the start of 2021, the experts at the GRASSL engineering office will make a forecast about the structures based on the determined strain on the steel under the actual traffic load. Based on the results determined by GRASSL, the two culturally protected sites can then be made fit for the future.

Civil engineers rely on the experience and know-how of HBM Service & Support

In the long-term measurements of the two viaducts of Hamburger Hochbahn, the qualified and certified HBM project engineers who are in charge of developing tailor-made monitoring solutions for civil engineers comprehensively convinced the consulting engineers of GRASSL engineering office. They were able to offer not only the turnkey infrastructure monitoring solutions and their professional on-site installation from a single source but also, given the tight time frame, their experience, HBM’s available personnel resources, and the fact that a competent contact person at HBM was available at all times.

About the GRASSL engineering office

GRASSL has been working in the field of engineering for over 70 years. Moreover, the services offered by its consulting engineers and approved test engineers for the inspection and evaluation of structures are in demand internationally. Today, GRASSL employs around 200 people at eight locations.