Civil Engineering Reference
In-Depth Information
The natural frequencies of the beam are influenced by the flexural stiffness
EI(x)
and the mass per unit length
m(x)
.
Generally, the larger local flexural stiffness or mass per unit length will lead to the increment or descent of the natural
frequencies, respectively. Furthermore, any changes at nodal points for specific mode shape have little influence on the
corresponding frequency and this characteristic can be utilized in the design of engineering structures.
15.3 Modal Testing on Beam-Like Structure
Firstly, a beam-like structure consisting of four steel parts connected with bolts is introduced, and then the experimental set-
up for the modal testing is introduced. Finally, the modal testing for this beam-like structure with different bolted
connections is conducted to study the effects of the connection location and the bolt preload on its modal parameters.
15.3.1 Beam-Like Structure
The beam-like structure is composed of four steel parts and three joints can be found on it. The dimensions of the beam-like
structure are 2,000
60
10 mm (L
W
H) with each part of dimensions 500
60
10 mm (L
W
H). Different
parts are connected with each other through eight bolts, as shown in Fig.
15.4
.
In order to allow testing this structure, there will always exist a connection between the structure and the environment.
The free-free condition means no connections with environment should exist. In practice this condition is approximated by
hanging the test structure on very soft springs [
1
]. To minimize the influence of the suspension, the beam-like structure is
suspended in its lateral direction so that the bending modes will appear in a plane which is perpendicular to the suspension
direction, as shown in Fig.
15.4
.
15.3.2 Modal Testing Set-Up
Modal testing set-up consists of test structure, exciter systems, force and motion transducers, measurement and analysis
systems, and suspension systems. The hammer impact is selected as the vibration force to be applied to the test structure,
which is PCB
TM
D086C03 excitation hammer (sensitivity 2.25 mV/N, resonance frequency
22 kHz). The motion
Fig. 15.4
Schematic diagram
of the beam-like structure
