The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008
and (van Gosliga et al., 2006), at large dams, e.g. (Grimm-
Pitzinger and Rudig, 2005) and (Zogg and Schulz, 2007), and at
lock gates, e.g. (Schäfer et al., 2004) and (Hesse and Stramm,
2004). Furthermore, deformation monitoring for structural
deformation measurements of a concrete and timber beam is
described in (Gordon et al., 2004).
Figure 1. Felsenau viaduct, completed in 1975.
Main objectives of the deformation monitoring by TLS on the
Felsenau viaduct were on the one hand to get to know the
advantages and limits of the new measurement technology for
load test in the field of bridge monitoring, and on the other hand,
a comparison with precise levelling should point out the
possibilities of TLS with focus on the measurement accuracy
and detection of deformations.
In section 2 of this paper, the load tests are described as well as
the geodetic instruments which were used for the load tests.
Section 3 deals with processing of the TLS data and section 4
compares the results of TLS and precise levelling. Finally,
section 5 discusses the results, and conclusions are given in
section 6.
2. LOAD TESTS
For the load tests, the Felsenau viaduct was closed for traffic in
order to minimize vibrations of the bridge girder. The traffic
was diverted. Several parties were involved for the proper
accomplishment of the tests, i.e. the Office of Civil Engineering
of the Canton of Berne, the Federal Traffic Office, a local
engineering company, and the ETH Zurich. The Institute of
Structural Engineering at ETH Zurich had the technical lead.
The IGP was responsible for the geodetic measurements.
1
settlement
Figure 2. Expected deformation behaviours of the cantilever
slabs and bridge girder under loading.
2.1 Initial situation
The load tests were performed during two nights when the
weight was positioned on different sections of the viaduct. Two
tanks, each weighting approximately 54 tons, were used as a
load. The load tests were performed at several sections of the
viaduct. The descriptions and analyses below refer to the first
night when the weight was positioned in the middle section of
the viaduct. The span length and the height were 156 m and
60 m respectively.
The load was positioned on the northern outer side of the
prestressed cantilever slabs (Figure 5). Hence, deformations
were expected as a deflection of the cantilever slab and a tilting
and settlement of the bridge girder (Figure 2). The deflection of
the cantilever slabs were of main interest for the civil engineers.
The procedure of the load test was scheduled in four main steps:
initial measurement, loading PI with one tank, loading P2 with
two tanks, final measurements P3 without any load. Recovery
periods of about 30 minutes represented an important aspect.
They allowed the viaduct to relax and to minimize the
vibrations and oscillations caused by the traffic, respectively by
the tanks. This was a very important aspect for the precise
levelling due to the fact that sensitive levelling compensator
would not work under oscillating movements of the
underground. The measurements of the deformations were
performed with a tacheometer, a precise level and a terrestrial
laser scanner.
2.2 Terrestrial laser scanning (TLS)
For the measurements by TLS, the terrestrial laser scanner
Imager 5006 by Zoller+Froehlich (http://www.zf-laser.com)
was chosen (Figure 3). The choice was based on the scanning
speed of about 500’000 points per second, the measurement
accuracy and the availability of the instrument. According to
specifications by the manufacturer, the range noise is about
2.0 mm in a distance of 25 m and a target reflectivity of 20%
(dark-grey target). The scans were performed on the bridge
girder of the Felsenau viaduct. The carriageway surface was
dark-grey due to the asphalt. Furthermore, the time for a scan
was an essential factor due to a tight schedule of the load tests.
The measurements had to be planned fur a minimum closing
time of the viaduct for traffic.
Figure 3. Terrestrial laser scanner Imager 5006 by
Zoller+Froehlich (http://www.zf-laser.com).
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