Hussey Seatway Ltd Deflection and Vibration Analysis of Auditorium Seating

Company Profile

Hussey Seatway Ltd. is a British company producing telescopic and fixed seating for auditoria, stadia, arenas and lecture theatres. Their expertise in fixed and telescopic bleachers and retractable seating solutions has enabled them to provide a range of seating products to accommodate various audience seating requirements conforming to various worldwide standards including British Standards, and Fire Retardancy specifications.


Background

Hussey Seatway are retractable seating experts and offer several types of retractable seating solutions. Retractable seating is proving to be very popular as the benefits include increasing the versatility and maximising the use of available space. As such retractable seating is an ideal choice for use in multi-purpose spaces. IDAC were required to analyse a bespoke seating system designed by Hussey Seatway for a Synagogue that made use of a Telescopic Platform (TP System) at the front and with two roll-away units. The TP System provides a space saving retractable ‘space frame’ seating platform, which can be used to mount seating. Whilst the roll-away structure provides a mobile seating solution.

Analysis

IDAC were required to analyse the structural integrity of the seating system in accordance with the deflections and vibrations criteria as specified by the National Building Code of Canada 1985. The 3D geometry of the seating system, comprising of the units mentioned above, was supplied to idac in Autodesk Inventor format. The geometry was simplified to remove any small features that were deemed unnecessary for the analysis. Sliding contacts (without friction) were used between the nylon skids and the corresponding supports. The units were simply supported on a concrete floor, without any longitudinal constraints. However, in order to avoid rigid movements in the longitudinal direction during analysis, fully fixed constraints were used at some supporting pads. A check of the horizontal reactions at the constraints showed that they were lower than the calculated forces due to friction. This verified that the frictional forces would be able to oppose the external forces acting in the longitudinal direction.

Stress analyses of the assemblies and the rails were carried out as well as a modal analysis of the assemblies, and an assessment of the slender parts’ deflections. The loads applied to the assemblies simulated the movement of people in both horizontal directions along the platforms.

The linear stress analysis results highlighted areas where the stresses were too close to the material limit. The graphic to the left shows an area of a high stress concentration. These areas had to be re-designed and then re-analysed. Analysis of the re-designed components showed that the stresses experienced were acceptable.

A deflection assessment of slender parts was also carried out in accordance with the National Building Code of Canada 1985 to check for buckling collapse. All deflections were found to be within the limits specified within the code. The graphic below left shows a contour plot of the deformation experienced by the seating assembly under load.

A modal analysis using FEA was carried out to determine the fundamental vibration mode shapes and the corresponding frequencies. It was useful to know the modal frequencies of the structure to ensure that the frequency of the applied periodic loading would not coincide with a modal frequency and hence cause resonance, which could lead to large oscillations and possibly failure of the structure. The results obtained showed that resonance would not be an issue. The graphics to the right show the first and second mode shapes obtained from the modal analysis.


Design Benefit

Hussey Seatway was able to validate the product without vigorous and expensive testing. It also enabled them to design a structure to permit the maximum number of individuals. IDAC were able to identify the areas of high stresses so that Hussey Seatway was able to improve on the design. The analysis of vibration modes was also a critical component of design, which is often overlooked. Structural elements can be prone to perceptible vibration, which in this case could irritate the audience but worse still, could put them in danger.