Section 5 Vibration control
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Special Service Craft, July 2022 - Part 7 Hull Construction in Aluminium - Chapter 7 Failure Modes Control - Section 5 Vibration control

Section 5 Vibration control

Parent topic: Chapter 7 Failure Modes Control

5.1 General

5.1.1 Natural frequencies are to be investigated for local unstiffened and stiffened panels expected to be exposed to excessive structural vibrations being induced from machinery, propulsion unit or other potential excitation sources.

5.1.2 Where the structural configurations are such that basic structural elements may be modelled individually the natural frequencies may be derived in accordance with Pt 7, Ch 7, 5.3 Natural frequency of plate, Pt 7, Ch 7, 5.4 Natural frequency of plate stiffener and Pt 7, Ch 7, 5.5 Effect of submergence, as appropriate. Under other circumstances finite element analysis is to be employed to evaluate the vibration characteristics of the structure considered.

5.2 Frequency band

5.2.1 The natural frequency of panels is generally not to lie within a band of ±20 per cent of a significant excitation frequency.

5.3 Natural frequency of plate

5.3.1 The natural frequency of a clamped plate in air is given by the following:

where
a = panel length, in metres
b = panel breadth, in metres
= panel thickness, in mm

5.4 Natural frequency of plate stiffener

5.4.1 The natural frequency of a plate stiffener in air is given by the following:

where
E = flexural rigidity of plate stiffener combination, in Nm2
GA = shear rigidity of plate stiffener combination, in N
L b = beam length, in metres
m = mass per unit length of the stiffener and associated plating, in kg/m
K i = constant where i refers to the mode of vibration as given in Table 7.5.1 Vibration mode constant K i

Table 7.5.1 Vibration mode constant K i

Mode 1 2 3 4 5
K i 22,40 61,70 121,0 200,0 299,0

5.5 Effect of submergence

5.5.1 To obtain the frequency, f water, of a plate with one side exposed to air and the other side exposed to a liquid, the frequency calculated in air, f air, may be modified by the following formula:

where
=
ρl = density of the liquid, in kg/m3
ρp = density of the plate, in kg/m3
κp =

where a, b and t p are as defined in Pt 7, Ch 7, 5.3 Natural frequency of plate 5.3.1.

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