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Suspension Bridge 

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123456789Determination of Hi(K) and Ai(K) should generally be
based on the sectional wind tunnel test, or recently,
by numerical simulation on computer. Both of the
methods physically apply one of the following
procedures, [97,2]:

1. Vibration tests where a fixed girder section is given
an initial vertical and torsional displacement. The
flutter derivatives are based on the transient behavior
that occurs when the section girder is released.

2. A forced oscillation technique that involves forcing
the model through a prescribed motion and measuring
the aerodynamic forces on the model. The
aerodynamic forces may be determined using pressure
measurements at a number of pressure taps on the
model.


3. Buffeting tests, where the behavior of girder section
is analyzed for different wind velocities in the tunnel.
The behavior observed in the simulated natural wind is
analyzed in accordance with the differential modal
equations for vertical and torsional motion.

Flutter derivatives primarily depend on the girder
configuration, see Figure 4.2, for the main kind of the
streamlined girder and the truss girder. During the
preliminary computation of the suspension bridge flutter
(Chapter 5), when the final girder is not yet
determined, an equivalent flat plate aerodynamics
model can be used as a test input with the structural
mass, natural frequencies and damping ratios
corresponded to the actual bridge. The predicted
critical wind velocity of the actual bridge when
choosing the streamlined girder will be about 80% to
90% of the value of the flat plate model, [97,2].

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4.3 Buffeting wind loads

The buffeting wind loads occur because of the wind
turbulence as described in the section about wind.
Buffeting wind load acts on the bridge girder cross
section can be commonly expressed in terms of a
vertical lift force Lb, a moment Mb and a horizontal
force Db, all are per unit span as shown in Figure 4.4.

CL , CM , and CD , respectively, the non-dimensional
lift, moment and drag coefficient, determined
experimentally in the wind tunnel as a function of the
girder rotation rb in flow (angle of attack)

The buffeting forces given above assume that there
are no interaction between the aeroelastic and the
buffeting forces. This formulation is only approximate
since it bases unsteady, time-dependent results upon
a quasi-steady formulation [92,3]. That means the time
dependency of buffeting forces only depend on the
oncoming turbulence and not definitely upon the
signature turbulence (produced by the structure itself
inthe flow, even if the incoming flow is perfectly smooth,
i.e. u = w = 0). I.e. Lb and Mb requires the state of the
air updating, but does not depend on the turbulence
factor due to the girder motion.

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ICDAS  •  Hans Erik Nielsens Vej 3  •  DK-3650 Ølstykke  •   E-mail: th@icdas.dk   •  Tel.: +45 29 90 92 96  •  CVR no.: 34436169