Civil Engineering Reference
In-Depth Information
Chapter 2
Cable-Stayed Footbridge: Investigation into Superstructure
and Cable Dynamics
Reto Cantieni
Abstract
A 35 year's old cable-stayed footbridge was investigated into analytically and experimentally to becoming able to
rate it's state of health. The paper presents the dynamic tests performed on this twin-32-m-span prestressed concrete
structure and the respective results. Besides the main girder the bridge exhibits an A-shaped pylon as well as eight cables
consisting of posttensioned parallel wire tendons. On the one hand, the tests covered experimental modal analyses of the
whole structure including main girder, pylon and cables as well as of two of the cables as isolated structures under ambient
excitation conditions. In addition, the structure was loaded with a jogger crossing the bridge and the maximum cable
response to manual excitation was determined. The latter is not discussed here. The bridge state of health could be rated
“satisfying”.
Keywords
Ambient excitation Dynamic behavior Footbridge Jogger excitation Modal analysis
2.1
Introduction
Considering the fact of a cable-stayed footbridge cable having failed in the northern part of Switzerland, the owner of
Oberwies Footbridge, the Swiss Federal Roads Office, FEDRO, decided to check the state of health of this 35 years old
bridge (Fig.
2.1
). The reason for being concerned is straightforward: Oberwies Footbridge is crossing Motorway A1, one of
the main motorway arteries in Switzerland, in the Z¨rich neighborhood. A collapse of Oberwies Footbridge would close all
eight lanes of this motorway. Already today, traffic jams occur in both directions on a regular basis, every morning and
afternoon/evening.
Visual inspection of the bridge did not reveal any significant damage. This also included close-up inspection of the
tendons in the anchoring points vicinity. In addition, analytically checking the bridge's load carrying capacity under static
loads showed one minor weak point only (pylon cross girder carrying the main girder). However, visual inspection also
revealed that the bridge is vibrating significantly when trucks pass underneath at a speed of between 60 km/h and the allowed
maximum, 100 km/h. Also, passing joggers excited the structure to perceptible vibrations. It was therefore decided to
perform an experimental investigation to becoming able to rate the bridge dynamic behavior.
2.2 The Bridge
Oberwies Footbridge consists of a cast in-situ and posttensioned main girder, a reinforced concrete pylon and eight cables
(Fig.
2.2
). The cables are of the BBRV parallel wire type with the fixed anchor being covered by a concrete slab at the main
girder and the mobile anchor being located at the pylon. The long cables consist of 55 wires 7 mm, the short ones of 36 wires
7 mm. The cable free length is some 24 and 11 m respectively. The pylon and the two abutments are supported by two large
piles each.
