A Tale of Two Bridge Surveys
Hamburg, Germany, boasts over 200 bridges. That’s more than London, Venice and Amsterdam combined. The Hamburg Port Authority (HPA) is responsible for maintaining 130 of those bridges, along with 130 kilometers of roads, 300 kilometers of railways, ferry terminals and the St. Pauli Elbe Tunnel. Ten of those bridges are moving bascule, lifting and draw bridges.
Recent years have seen a number of bridge refurbishment and replacement projects at the port. The Köhlbrand Bridge is probably the best known of the bridges in the Port of Hamburg and also the second longest road bridge in Hamburg. Refurbishment of the bridge took place in 2014. At that same time, and also part of the investment program, work began on construction of the new Rethebrücke (Rethe Bridge), a bascule bridge, and the draft planning got underway for the new Kattwyk Railway Bridge project.
At Rethe, HPA built a new bascule bridge to replace the existing lift bridge. The old lift bridge was built in 1934 and had reached the end of its technical lifespan. As a main road connection and as the second crossing of the Southern Elbe towards the Autobahn 7 it is expected to handle approximately 7,000 vehicles daily. It also serves as a fully functional alternative for the Köhlbrand Bridge.
The new Rethe Bridge was built with a road and a rail side to allow train and road traffic to use the bridge together. Before construction of the new bascule bridge, the roadway had to be closed up to 40 times daily to allow trains to cross. This was in addition to periods when the bridge is raised to allow ships to pass up and down the Elbe River. There are an average of eight scheduled ship passages per day, and ship operators willing to pay a fee can make an unscheduled passage.
The Kattwyk Bridge was dedicated in 1973 as a rail bridge, but today carries both trains and road traffic. The lift bridge had to close to road traffic 30 to 50 times per day to allow trains to pass. According to Port Authority figures from two years ago, 8,200 vehicles per day used the Kattwyk Bridge; 2,200 of those vehicles were heavy goods trucks. Each of those disruptions caused a closure for 15 to 20 minutes.
On Site in Hamburg
POB met with Dr. Eng. Matthias Brunkhorst of the Hamburg Port Authority to take a closer look at the Rethe and Kattwyk Bridges, and learn more about the surveying projects supporting bridge maintenance in general and the new bridge constructions specifically. His presentation later at the Trimble Dimensions user conference focused on the Kattwyk Bridge. That presentation was a follow-up to a presentation on the Rethe Bridge at a previous Dimensions conference.
“I like surveying bridges,” Brunkhorst began as we drove to the Rethe Bridge, “it’s my favorite pastime.”
The Hamburg Port Authority’s Survey and Hydrography service is responsible for data collection, processing, hosting and visualization. This covers all survey data and harbor entities — both land and water — along with georeferencing and maintaining GIS. And, with 130 bridges under port care, this clearly keeps Brunkhorst engaged in his favorite pastime.
He points out, “Surveying bridges is always a kind of technical measurement. In the main, we are surveying the abutments regularly. Each bridge has two abutments, one at each side. We are taking a reference to discrete points on each abutment. Our task when surveying bridges is obtaining a geodetic reference between the point on the abutment to fixed points lying away from the structure in areas with no deformation. Those are the fixed points.”
Rethe Bridge Photo by Perry A. Trunick
Brunkhorst describes the Rethe Bridge as Europe’s “mightiest” bascule bridge and Kattwyk as its “mightiest” lift bridge. The Rethe Bridge was the subject of his first Dimensions presentation on surveying and measuring in tidal-dependent areas.
With a span of 104.2 meters, the Rethe Bridge is one of Europe’s largest double-span road bridges and Europe’s largest double-span rail bridge. The roadway portion of the bridge was opened to traffic in July 2016 while work continued on the rail portion. The old lift bridge continued to function for rail traffic during the construction. Once the rail side is open, the old lift bridge will be removed, allowing the width of the ship passage to expand from 44 meters to 64 meters.
At an estimated cost of 208 million euros, the Kattwyk bridge replacement is expected to be completed in 2020.
Measuring Up at Kattwyk
Both the Rethe and Kattwyk Bridges are in tidal-dependent areas. The effects of the tides can be seen in the measurements that are the subject of Dr. Brunkhorst’s presentation.
Kattwyk Bridge Photo by Perry A. Trunick
The old Kattwyk Bridge is vibrating horizontally and vertically under tidal force, he explains. Setting up the actual, realtime geodetic monitoring included the north axis (parallel to the bridge) and east axis (crosswise to the bridge). There are three different types of object points involved, Brunkhorst explains:
- Foreland points, as he terms them, lying a little away from the water,
- Abutment points associated with the bridge abutments themselves,
- Pillar heads.
Fixed points were established in three dimensions, with four precise vertical control points in areas not affected by deformation, and 11 object points.
Working in an absolute model, they observed a full round with two faces, a routine part of optical measurements for monitoring. Using total station measurement, scale corrections were made on pillars and height corrections were “compared to ibidem.”
The actual results showed all object points are vertically vibrating in sine oscillations. Brunkhorst notes in a foreland example the scale is 3 millimeters above zero and 2 to 3 millimeters below zero — an amplitude range of up to 6 millimeters.
The mean time of vibration meets the tidal-dependent induction. The time of vibration at a half day to one day and the amplitude and significance of the height vibration increase with tidal force.
The pillars are parallel to each other and under the influence of river force. With 30 minutes between points, movement of pillar end points and all object points are vertically vibrating in sine oscillations with amplitudes of 1.5 to 3 millimeters. Timing here leads to a tide-dependent induction. The amplitude and significance of the height vibration increase with the tidal force.
Brunkhorst explains that one round of metering the two fixed points delivers four single mean values for each epoch for 48 epochs per day. This has led to a conclusion the whole existing structure is vibrating horizontally and vertically under tidal force. Vertical vibrations are increasing directly as a result to 3 millimeters amplitude. Horizontal vibrations are generally reaching amplitudes of 1 millimeter.
Geodetic monitoring of the old Kattwyk Bridge is providing horizontal and vertical stability with accuracy better than 1 millimeter. There is a need for the same level of accuracy with the new bridge.
It is extremely important to note, says Brunkhorst, that solid, deep rammed and grouted surveying pillars proved themselves as data points for grounded fixed points.
Brunkhorst says the project used Trimble S8 total stations with FineLock, Trimble 4D Control software and monitoring prisms.
The findings at the Kattwyk Bridge site showed similar movement to that recorded at the Rethe Bridge. This won’t affect the plans for the new bridge, but, he says, he expects the same movements in the new bridge.
The calculations included mathematical variables for temperature, barometric pressure and humidity, but humidity has almost no influence, Brunkhorst explains. It is theoretically present, but not to any practical degree. There are, however, complex formulas for tidal influence on the measured and given distances between the west and east sides of the river. When the water comes in, there is deformation in the distance east and west, and complex corrections on distance due to tidal force had to be developed.