Structural analysis of GRP tank covers

Structural analysis of GRP tank covers

Background

GRP covers are widely used in the water-treatment industry to help control odours. The covers need to be able to withstand the local conditions of wind and snow and, depending upon the specific location, may also be required to handle personnel loads, impact loads etc.

Our brief

Our client approached us with outline designs for three conical tank covers of different diameters. In each case, the client wanted to establish whether the covers met the requirements set out across the Water Industry Mechanical and Electrical Specification (WIMES) and EN 1991.

Our approach

Understanding the key requirements

Clearly, the client was looking to ensure that their proposed structures could withstand the various load cases stipulated in the WIMES standards. However, due to the competitive nature of the industry, the real challenge for many fabricators is to try and achieve this in as cost-effective a way as possible. This invariably means they are looking to optimise GRP laminate thickness, in order to minimise material costs.

Identifying the load cases

WIMES specifies a number of in-service loading conditions which must be considered.

Loading ConditionComments
Self-weightFor the in-service self weight loading, the GRP covers were assumed to be fixed to an underlying steel tank.
Personnel loadsDistributed and concentrated personnel loads were considered.
Wind and snow loadsWorst-case scenario wind and snow loadings, for the majority of the UK, Ireland and Denmark were chosen. Site altitude, surrounding-building height and terrain category were similarly chosen in order to cover a wide range of installation sites.
Impact loadsHard- and soft-body impacts were considered.
Hydraulic loadsThe pressure applied by the fluid within the tank.
Thermal gradient loadsRepresenting a temperature variation of up to 25 °C between the inside and outside of the tank.

Identifying the design factors

Property- and load-design factors were also taken directly from WIMES. These were combined and subsequently used to calculate factored, allowable stresses for the various materials of construction.

Modelling the covers

The tanks were to be constructed from varying layups of chopped strand mat, unidirectional tape and foam core, as shown in Figure XX and table YY.

Various thicknesses of laminate used in the construction of the largest tank cover
Thickness, TLaminate construction
T22 x layers of chopped strand mat (CSM)
T33 x layers of CSM
T44 x layers of CSM
T66 x layers of CSM
T2 + 6UD + T22 x layers of CSM
6 layers of unidirectional fabric
2 x layers of CSM
S-S and T-T represent beam sections of differing dimensions, both constructed using T2 laminate skins around polyurethane foam cores.

Laminate constructions

FEA model of the largest tank cover

Carrying out the analysis

The structures were analysed using finite element analysis software. This considered a combination of all six load cases. Unfactored loads were used and then compared with the factored allowable stresses for the various materials of construction.The design stresses were obtained as an average value for all the Gauss point stresses of every finite element.

Deflections in the covers were measured, and compared with the allowable limit of (span/150).

Linear buckling analysis was conducted, investigating seven buckling modes on various load-critical combinations.

Vertical deflections in the largest tank cover
von Mises stresses in the various laminates and structures in the T3, T4 and T6 laminates
von Mises stresses in the various laminates and structures in the T2 + 6UD + T2 laminate
von Mises stresses in the various laminates and structures in the T-T and S-S beams

The outcomes

Minor modifications to the originally proposed layup were found to be required, in order to provide additional reinforcement at certain critical locations. With this change in place, all three cover designs were shown to be suitable for the specified conditions.


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Finite Element Analysis (FEA)

A computational tool for simulating and analysing the response of a structure to applied mechanical or thermal loads - used for design verification and optimisation


Industries

Construction

Their low weight, durability and mouldability are the key drivers for using composites in infrastructure applications. Lightweight composite bridges, tanks and pipes can be easily installed, whilst allowing designers to achieve freeform shapes.  Our consultants can help you assess the fitness for purpose of your composite structures.


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