Designing structures to support suspended vibrating screens

Identifying the appropriate support for the type of screen

Since there are a wide variety of vibrating screens on the market, it’s important for structural engineers to determine in advance whether the equipment is mounted to the structure using rigid or flexible supports. In most cases, the screen (whether linear, incline, grizzly, banana, gyrating or other type of screen) is supported by rigid supports connected to a structure. Several references are available to ensure that the resonance effect is avoided during dynamic analysis and that the vibration limits of the structural supports are met. These references include studies by Blake (1964), Baxter and Bernhard (1967), Richart, Hall and Woods (1970) and the ISO 10816-1 standard.

The design becomes more complicated when the screen or group of screens is in a building and hangs beneath the floor from the supports, thus allowing the equipment to swing and pivot. The HS Sifter (or Plansifter) is an example of this type of screen (see Figure 1¹).

This system is an arrangement of several large screens equipped with decreasing sizes of mesh placed one under the other from largest (at the top) to smallest (at the bottom). The material (e.g., flour, grain, graphite) strikes the upper screen first. It then passes to the others as each screen helps separate the material by size using a very rapid rotating movement created by a motor and an off-centre rotor. The hangers supporting the sifter are made up of several wooden rods (Figure 2), which allow for weak transmission of effort and absorb shocks.

Generally speaking, the equipment manufacturer provides the structural engineer with the reactions at the supports where they interface with the structure. However, with structural supports, it does not prescribe vibration limits to prevent possible damage to the equipment.

So how do we design a structure to ensure adequate performance of a suspended screen? Because the National Building Code does not prescribe vibration limits for industrial structures, which criteria should we use? How do we optimize the structure model to obtain reliable analytical results?

To shed light on this topic, BBA’s structural engineers have examined these questions.

Design references

The literature provides some possible solutions. According to ISO 10816-3 (Mechanical vibration), there are two support classes, flexible and rigid. The support conditions are determined by the relationship between the machine and its support. In cases where the natural frequency of the structural support is at least 25% higher than the equipment’s excitation frequency, the support can be considered rigid. All supports not in this category are considered flexible.

Classifying the support between the hanger and the screen chamber is up to the equipment designer, not the structural engineer. A bolted connection from the hanger to the floor beam is a rigid support for which the manufacturer will supply the quasi-static reactions from the dynamic load conditions on the machine. Although the active screen swings and pivots, the vibration limits prescribed by standards for flexible supports should not be used blindly when designing the structure. Also, once the structure’s deformations are obtained through analysis, it’s important to check that they don’t interfere with the gyration circle identified by the manufacturer at the low point of the screen (see Figure 2 example, equal to 50.8 mm).

ISO 4866 (Mechanical vibration and shock) establishes principles for vibration measurement and data processing. It deals only with measuring structural vibrations.

In fact, structural safety guidelines with proposed vibration limits are available in the German standard DIN 4150-3 (Vibration in buildings – Part 3, Effects on structures). This standard provides a method for measuring and assessing the effects of vibration on structures designed to take basically static loads. The standard also includes an evaluation of the effects under short- and long-term vibration. Limits and criteria are provided to ensure that the vibrations don’t cause damage to the structure and that the structure thus remains functional.

An optimal structural analysis model

Furthermore, when vibrating screens are suspended from the floor of a multi-storey building, it’s effective to optimize the structural model to simplify the dynamic analysis and avoid having to model the entire building. A research project has compared the results of the analysis after studying an overall model and three simplified models. Optimizing the simplified models included replacing some non-modelled structural components using springs with equivalent stiffness. On comparing the results, the simplified model with both the screen floor and the springs to simulate all upper floors turned out to be the most accurate in relation to the overall model, while the results of the model analyzing only the floor of the screens were quite different.

In fact, if we want to use a simplified model, it must contain the part of the structure subjected to the dynamic loads. For the horizontal vibrations, it’s the upper floors that are most heavily impacted, whereas for the vertical vibrations, modelling of just one floor is sufficient.

Therefore, adequate design of a building that supports suspended vibrating screens requires accurately interpretating the manufacturer’s data, choosing an optimal structural analysis model and applying the appropriate standards. This will ensure proper building structural behaviour, operator safety and correct equipment operation.

Do you have to design buildings that support suspended vibrating screens? Get in touch with us; we would be happy to help with your project.

Credits

1. Great Western Manufacturing