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FEM example: Calculation of a pedestrian bridge

This practice related FE example for the static calculation of a pedestrian bridge was issued as a non-profit project in cooperation with the company Kirchgässner Elektrotechnik GmbH, with whom we have a long-standing partnership. The Kirchgässner Elektrotechnik GmbH is a strongly expanding family business with headquarters in Binau and the right contact for the realization of complex automation solutions, control engineering, fire fighting technology and special machine construction.

The old pedestrian bridge on the popular hiking trail between Apfelgarten and Dauchstein Castle had been inaccessible since a longer period of time. So the managing director and local councillor Jürgen Kirchgässner decided without further ado to donate a new bridge to his home town of Binau.

As a partner and specialist for finite element simulations, we were happy to contribute the strength verification for the bridge construction. This practical example is intended to give the interested reader an insight into the static design of the bridge construction up to the installation of the bridge in a spectacular action by means of a cargo helicopter.

fem example calculation pedestrian bridge
Old pedestrian bridge between Apfelgarten and Dauchstein Castle.

Assumed loads for the FE simulation

For the computational strength verification and the adequate estimation of the fatigue strength of the aluminium bridge, we must first make some considerations regarding the expected loads. It is not sufficient to examine the bridge structure for its ultimate load alone. Rather, additional loads act on a bridge due to temperature differences in summer and winter, which lead to significant changes in length. Therefore, the FEM calculation of the pedestrian bridge is carried out at low as well as at high temperatures for the required bearing load.

Boundary conditions for the FEM simulation of the bridge

In addition to the ultimate load and the thermal length change, the correct support of the bridge also plays a decisive role for the strength. In practice, it is often observed (e.g. on a motorway bridge) that the end of a bridge is flexibly supported on a steel roller. This allows the bridge to expand or contract accordingly, especially in the longitudinal direction, at different external temperatures.

fem calculation example pedestrian bridge
FEM calculation example pedestrian bridge: Material data, boundary conditions and loads

FE- simulation results and security of the pedestrian bridge

The results of the calculations must clearly demonstrate that the maximum load can be carried by the bridge at any ambient temperature. No overstressing of individual parts of the bridge structure may occur. Furthermore, the deformations that normally occur under load should not give passers-by a feeling of insecurity when crossing. The bridge must therefore be sufficiently rigid. The following animation illustrates how the bridge behaves under load at different outside temperatures:

It can be clearly seen how the bridge not only deflects under the load, but also expands or contracts due to the effect of different ambient temperatures.

As expected, maximum component stresses occur near the foundation of the bridge. However, these stresses are not critical and the bridge can therefore be rated as fatigue-resistant.

fe example simulation pedestrian bridge
Maximum stresses near the bridge foundation (non-critical)
Delivery and installation of the new bridge

The site where the new bridge is to be installed is difficult to access. Therefore, the finished bridge shell is first delivered to the installation site via the nearby access road as far as possible.

fem example pedestrian bridge new according to calculation
New bridge after delivery via nearby access road

By attaching ropes for transport and positioning to the bridge structure, insertion is easily carried out by means of a cargo helicopter.

The bridge carcass to be positioned on the forest road is delivered by air and positioned on the new foundations by helpers on the ground after brief prior coordination. Volunteers have already dimensioned and erected the new bridge foundations for this (Not part of this simulation example) months before. Thanks to the good preparation, positioning the bridge only takes a good six and a half minutes and in this case is far cheaper than positioning with a truck-mounted crane.

FE simulation example pedestrian bridge assembly
Positioning of the new bridge by a cargo helicopter

Finally, the new bridge is in place and can now be planked before it is opened to hiking traffic.

fe simulation example calculation pedestrian bridge positioning
Positioning the bridge and setting it down on the new foundation

This example project for an FE simulation clearly shows that calculations and simulations are playing an increasingly important role even for everyday products. In this way, the correct design can save costs and material and at the same time prove that the new bridge will still bring pleasure to hikers in the surrounding area many years from now.

Sources: Newspaper report of the Rhein- Neckar- Zeitung and pictures of the Kirchgässner company.

Further example projects for FE- calculations of component strength:

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Data protection
We, the GOEBEL ENGINEERING GmbH (Registered business address: Germany), would like to process personal information with external services. This is not necessary for the use of the website, but allows us to interact even more closely with them. If desired, please make a choice: