Challenges in Identification of the Humber Bridge Modal Parameters based on an Ambient Vibration Test

In July 2008, an ambient vibration test was performed at the Humber Bridge, in the UK, which is a suspension bridge with a main span of 1410m that is in the fifth position of the ranking of the world longest bridge spans. This paper is focused on the application either of simple and fast identification tools used to check on site the quality of the acquired data or of more sophisticated routines employed in the subsequent processing of the achieved database, using computational tools previously developed at FEUP, with the goal of accomplishing more complete and accurate results. The size of the database enforces the use of identification techniques that are based on the processing of several datasets together or on the use of algorithms for the automatic interpretation of stabilization diagrams. Both approaches are successfully applied together with the Covariance driven Stochastic Subspace Identification method. 4 IOMAC'09 – 3 International Operational Modal Analysis Conference the goal of accomplishing more complete and accurate results. The size of the database enforces the use of identification techniques that are based on the processing of several datasets together or on the use of algorithms for the automatic interpretation of stabilization diagrams, instead of the more traditional procedures based on the tedious task of manually analysing one stabilization diagram for each dataset. Figure 1 : Bridge view from the South side. 2 BRIDGE BRIEF DESCRIPTION The Humber bridge, with a span of 1.410 meters, had from the end of construction, at 1981, to 1998 the world longest span. Nowadays, it is at 5 position of the ranking of world longest spans. The bridge superstructure was erected from 1976 to 1981 by a partnership of several British companies (British Bridge Builders) following the design developed by Freeman Fox and Partners. The bridge deck is divided in three segments with lengths of 280m (North side span), 1410m (main span) and 530m (South side span). Its section is a trapezoidal hollow box 22m wide and 4.5m deep with 3m wide panels cantilevering from each side of the box to carry the walkways. It is built up from welded stiffened steel plate panels, generally 18.1m long. The connections between the three decks and the towers or abutments is provided by pairs of steel A-frame rockers that were designed to permit only longitudinal motion and rotation about lateral and vertical axes. The two towers consist of two hollow reinforced concrete legs, 155.5m high, braced by four horizontal beams; the lowest one accommodates the supports for the bridge decks (Figure 1). The Barton tower (south side) is founded on clays. Therefore, it is supported by two 24m diameter concrete caissons sunk 8m into the clay. The Hessel tower is founded on chalk, 8m below the ground level, by a massive reinforced concrete slab 44m long, 16m wide and 11.5m high. The main cables are made of 14 948 parallel galvanised draw wires 5mm in diameter which, for ease of erection control and anchoring, were divided into 37 strands. The deck is suspended in the two main cables by high tensile steel ropes, which are inclined to help damp out any oscillation of the deck. 3 AMBIENT VIBRATION TEST 3.1 Test procedure The test was conducted using 10 triaxial Geosig seismographs, model GSR-24, which were previously tested in the lab (Fig. 3) to ensure that the different types of used acceleration sensors (force balance sensors from different manufactures: Ref Tek, Guralp and QA) were well calibrated and that the time synchronization between measuring unit, achieved with external GPS sensors, was accurate. During the test of the main span, two pairs of seismographs were permanently located at two reference sections (R1 and R2, marked with red squares in Fig. 3), while the other 6 worked as moving sensors, three in each side of the bridge, covering the remaining measurement points (blue circles in Fig 3) in different test setups. Each measurement section was instrumented at