Vibration acceptance criteria

Aspec Engineering’s Adam Mayers outlines the details of the codes/standards defining the acceptance criteria for vibration, particularly for vibration of industrial structures.

The effects of vibrating loads on structures can often be significantly greater than the effects of static loads of equal or greater magnitude. These effects can include: fatigue damage such as cracking and breakage, damage to operating equipment due to excessive ambient vibration and human discomfort, among others.

Traditionally, structures subjected to dynamic loads have been designed by trying to ensure that the major natural frequencies of the structure are not close to the frequency of the applied forces to avoid resonance. While the calculation and study of the structure’s natural frequencies presents a guide to the behaviour of the structure, it does not give the complete picture. Generally, the overall response of the structure to applied vibration forces should be determined and compared to defined acceptance criteria. Similarly, where vibration may be causing problems on existing structures, the overall response should be measured with appropriate equipment, and compared to acceptance criteria.

Sources of vibration

Vibration in structures is typically characterised as either transient or forced vibration.

Transient vibration: Transient vibration is caused by a release of energy and may last for several periods of vibration of the system. Mechanical shocks and impacts will cause transient vibrations, and the structure will vibrate at its natural frequency, with the amplitude of vibration decaying over time from an initial value, due to damping.

Forced vibration: Forced vibration is produced in a structure when it is affected by an external source of vibrational energy, such as vibrating machinery. The structure will then vibrate at the same frequency as that of the source of vibration (the forcing frequency or operating frequency) and not at its natural frequency. Where multiple sources of forced vibration at slightly different frequencies are present, ‘beating’ can be observed, whereby these sources come in and out of synchronisation, acting together or against one another, to vary the overall vibration amplitude. The movement (or response) of a structure affected by vibration will depend on the relationship between the forcing frequency and the natural frequency and also on the degree of damping present. Extreme structural vibration is often a consequence of resonance, which can be an extremely damaging phenomenon. Resonance occurs when the external source of vibration is at the same, or similar frequency, as a natural frequency of the structure. As the frequency of a vibrating source approaches the natural frequency of the structure, the amplitude of vibrations within the structure will be amplified, which can result in major damage to the structure of equipment. The level of amplification is often defined as a ‘dynamic amplification factor’ which is a function of the ratio of the forcing frequency and natural frequency of the structure.

Structural integrity

The German standard DIN 4150 Part 3 provides vibration velocity guidelines for use in evaluating the effect of vibration on structural integrity (see Table 1). The guideline limits presented in the standard are based on experience and are defined as ‘safe limits’ up to which no damage due to vibration effects has been observed for a particular class of building. ‘Damage’ is defined by DIN4150 to included even minor non-structural damage. For continuous long-term vibration, 10 mm/s peak vibration velocity is seen as a safe limit for structural integrity in industrial buildings.

Although these limits are defined in the code as being for vibration in the horizontal direction at the top floor of a building, ASPEC’s experience with vibration studies at several Australian industrial plants has shown that these limits are effective for local vibration levels as well. Structural damage has been observed on members whereby the vibration velocity exceeds approximately 20-40 mm/s.

These guidelines are very useful as a first pass to determine the suitability of vibration levels. Where there are significant structural integrity concerns however, a detailed vibration and fatigue analysis, often using Finite Element methods (FEA), may be required. Again, this analysis should consider the full response of the structure to the applied vibration forces, and the resulting loads and stresses developed in the structure.

Equipment serviceability

High levels of vibration can adversely affect the life of operating equipment and can increase the frequency of breakdowns, leading to excessive maintenance. A great deal of work is often conducted to measure the balance and vibration of individual pieces of equipment, but often the level of ambient vibration transmitted through the supporting structure can have a significant effect on the life of equipment. To address this issue, it is often necessary for additional vibration isolation features to be included in the design (e.g. vibration isolation mounts, spring / dampers, separation of support structures etc.).

Equipment manufacturers will typically provide information on acceptable levels of ambient vibration. In the absence of this however, there are only limited sources of useful guidelines on acceptance criteria. The now withdrawn standards AS2625.1 and AS2625.2 provided guidelines for evaluating the effect of ambient vibration on the life of equipment. While these standards are withdrawn, the simple guidelines they provided are still of use to engineers working with dynamic structures.

Depending on the class of the equipment (e.g. small, medium, large etc), AS2625 defined RMS vibration velocity limits corresponding to different qualitative evaluation zones. The four zones are defined in the standard as:

GOOD (Zone A): The vibration of newly commissioned machines would normally fall within this zone.
ALLOWABLE (Zone B): Machines with vibration within this zone are normally considered acceptable for unrestricted long-term operation.
JUST TOLERABLE (Zone C): Machines with vibration within this zone are normally considered unsatisfactory for long-term continuous operation. Generally, the machine may be operated for a limited period in this condition until a suitable opportunity arises for remedial action.
NOT PERMISSIBLE (Zone D): Vibration values within this zone are normally considered to be of sufficient severity to cause damage to the machine.

These guidelines are especially useful where sensitive equipment is required to operate in the vicinity of highly vibrating equipment.

Human body perception and response

The human body can detect magnitudes of vibration lower than those which would normally cause mechanical or structural problems. The discomfort or annoyance produced by whole body vibration is a very influential factor and may be the one of the limiting parameters in the design of the structure.

Data on human exposure to vibration has been incorporated into AS2670: Evaluation of human exposure to whole-body vibration. Vibration exposure limits are given as a function of:

Direction of motion, either horizontal or vertical.
Frequency of vibration.
Acceleration of the oscillations.
Exposure time.

The method prescribed in AS2670.2 for vibration between 1 and 80 Hz, is to compare the magnitude of RMS vibration (acceleration) to established base curve levels which are approximately equal to a level of significant human annoyance and/or complaints about interference with activities. A more complicated analysis is required for vibration below 1 Hz.

Depending on the type of building these base curves are multiplied by a factor to determine an acceptable level of vibration.

Excessive vibration in structures can lead to a range of detrimental effects and should be assessed against established acceptance criteria. The three main criteria (and relevant codes) that need to be considered when evaluating the effects of vibration are: