DESIGN, VALIDATION, AND IMPLEMENTATION OF
STRUCTURAL FIBRE COMPOSITE ELEMENTS IN POTABLE WATER STORAGE
Lachlan Nicol, Product Engineer, Wagners CFT Michael Kemp, Executive General Manager, Wagners CFT Hany Habib, Principal Civil & Structural Engineer, SA Water
David Jaensch, Lead Asset Engineer, SA Water Martin Bolt, Senior Project Engineer, Fulton Hogan
Paper Summary
Water storage facilities have historically included the use of traditional materials such as reinforced concrete and steel columns to support the roof structure. However, significant dilapidation due to corrosion has led to substantial strength reductions of the existing structural columns and are in need of replacement. SA Water are currently undertaking a water storage tank refurbishment program which includes the restoration of approximately 100 tanks between 2016 and 2020.
To offset the problem of material degradation, Wagners Composite Fibre Technology were engaged to develop a light weight, long lasting, corrosion resistant solution utilizing Glass Fibre Reinforced Polymer (GFRP or FRP) which has a design life of 100 years. This paper describes the design, validation testing, and implementation of the Wagners fibre composite tank columns for use as part of the SA Water refurbishment program.
The average water tank generally consists of 20 reinforced concrete columns supporting the roof structure of 300x300mm square profile and are approximately 8.5m high. The columns that have been designed and manufactured by Wagners utilize the pultrusion process with ECR type glass reinforcement and a vinyl ester resin matrix which has been tested and complies with the Australian Standard; AS/NZS 4020, for use in potable water.
The fibre composite column design described above represents a new approach in water tank materials which eliminates the risk of premature failure of structural members due to long term corrosion. The inherently low weight and high strength of the material lends itself to quick and easy installation to ensure water tanks are refurbished and back in service as quickly as possible with minimal cost implications. In conjunction with the low maintenance requirements for fibre composite, the structural components designed by Wagners for use in water storage tanks offer a low whole of life cost throughout their 100-year design life.
1.0 Introduction
Water storage facilities have historically included the use of traditional materials such as reinforced concrete and steel columns to support the roof structure. However, significant dilapidation due to corrosion has led to substantial strength reductions of the existing structural columns and are in need of replacement.
To offset the problem of material degradation, Wagners Composite Fibre Technology were engaged to develop a light weight, long lasting, corrosion resistant solution utilizing Glass Fibre Reinforced Polymer (GFRP or FRP) which has a design life of 100 years. This paper describes the design, validation testing, and implementation of the Wagners fibre composite tank columns for use as part of the SA Water refurbishment program.
2.0 Background
2.1 Existing Infrastructure
SA Water (SAW), a South Australian Government agency, owns and manages over 750 water storage tanks across the state for both potable water and raw water supply. Typical standard tank designs are 4.5ML and 9.09ML concrete tanks, however water retaining structures have been constructed up to 130ML in size in some instances. These tanks are typically constructed utilizing concrete roof support columns that have a 300x300mm profile and are approximately 8.5m high.
2.2 Design Life Observations
Each tank is subjected to an inspection regime where targeted dives or ROV inspections are undertaken on a 5-year cycle for condition assessment, risk review and water quality cleaning prioritisation. SA Water’s Asset Management and Engineering condition assessments of this asset class have identified significant deterioration of roof support concrete columns in the above water level and the wet-dry zones of the tanks due to the aggressive nature of the high temperature and high humidity environment. This aggressive environment is especially prevalent during the hotter seasonal months where ambient external shade temperatures can range up to 49°C.
The in-service concrete tank ages range nominally from construction dates between 1883 and 1980 with main water asset construction periods in the 1940’s and 1960’s. The abovementioned issues have affected the concrete tank structures to such an extent that the tanks are observing deterioration to a condition grade 5, which is effectively a de-rated structure, after approximately 60 to 80 years of service life. This is a significantly shorter lifetime compared to the expected asset life of nominally 100 to 120 years. SA Water are currently undertaking a water storage tank refurbishment program which includes the restoration of approximately 100 tanks between 2016 and 2020 to remedy the issues and extend the design life or to re-use some of the out of service tanks.
Figure 1 - Deterioration of traditional concrete (L) and steel roof support columns (R)
2.3 Advantages of FRP
The Wagners CFT fibre composite column option was included as a part of the water storage tank renewal program, in conjunction with traditional concrete rehabilitation products, for the significant advantages in construction and reduced return to service time. The installation of the pre- fabricated composite columns can save onsite construction time up to 4 weeks. Another key advantage from a construction perspective is the associated reduction of the Workplace Health and Safety risks compared to the more traditional concrete rebuilding and renewal products available such as carbon fibre wrapping or other coating techniques. Total cost impact analysis has also shown a distinct advantage for the use of FRP columns compared to the alternate rehabilitation methods which are very reliant on manual labour and have the potential to be more prone to quality assurance issues.
Fibre composite, and specifically the vinyl ester resin and E-CR (corrosion resistant) glass used in the Wagners manufacturing process has excellent resistance to corrosion in harsh environments.
3.0 Technical Development 3.1 Example Project
The Kingscote project was used as the case study for validation testing as it was determined to be representative of the most common tank designs in South Australia. The project included two 9- megalitre, 39m diameter concrete tanks with 20 roof support columns in each tank. The drinking water storage tanks are located on Kangaroo Island, in South Australia and had not been in service for over 20 years.
The columns that have been designed and manufactured by Wagners for this specific project was to utilize the pultrusion process with E-CR type glass reinforcement and a vinyl ester resin matrix which has been tested and complies with the Australian Standard; AS/NZS 4020, for use in potable water.
3.2 FRP Column Design
Based off the specific project inputs such as applicable wind loading and tank geometry, detailed finite element models were developed to fully establish the resultant forces applied to the roof support columns. From the models, ultimate design loads were established and are listed as follows:
N*c = 49.2kN (axial compression load) M*c = 1.46kNm
N*t = 58.7kN (axial tension load) M*t = 2.1kNm
The 200x200mm profile was selected for use based off its theoretical tension and compression capacities when applying the Euler and Johnson Buckling formulas. The aforementioned forces and moments were resolved into single point loads to achieve the same design action effect for testing while allowing for the effects of eccentricity. The resultant test loads were calculated by applying a factor to allow for variability of structural units of 1.5 as per AS 4100. The factor that was applied to the ultimate design load is considered conservative when compared to the more frequently used factors in AS/NZS1170.0 and given the low coefficient of variation of the Wagners pultruded material. The test loads with the variability factor applied were calculated to be 73.8kN with a 43mm offset from the centre of the column for the compression load (Pc), and 88.05kN with an eccentric loading 36mm offset for the tension load case (Pt).
3.3 FRP Column Validation Testing
compression and tension load tests. As per the certified design, the columns tested were 200x200mm SHS members made up of 4 off 100x100x5.2mm pultruded profiles. The Wagners pultruded profiles utilized E-CR type glass reinforcement and a vinyl ester resin matrix. The length of the columns was 8.5m which represented the worst-case design.
3.3.2 Instrumentation, test setup, and methodology
Figure 2 denotes the test setup and the instrumentation used for both the tension and compression load tests. The columns were secured to the test rig using brackets that were representative of the fixtures that were to be used on the Kingscote project. One bracket was positioned off-centre in relation to the hydraulic loading cylinders central axis to simulate the eccentricity as described in section 3.2. Displacement transducers were positioned at mid-height of each column as well as at the end of the test rig to establish the mid-span bow and the column height movement under the test load.
Figure 2 - Test Setup (L) and bonded compression test (R)
Prior to the commencement of each test, a bedding load was applied to the columns of approximately 20% of the ultimate design load, this bedding load ensured that the material and fixings ‘settled’ and would not unfavourably affect the recorded displacements. Once the bedding load had been applied and in turn released, the columns were taken up to the test load for both tension and compression and held for a minimum of 15 minutes.
The columns were then taken to a higher loading to further prove their strength capacities. All additional column loading was applied to at least 40% in addition to what was required in the Kingscote design.
When analysing the deflections at midspan, it can be said that the bonded columns are stiffer in bending than the bolted columns. This can be attributed to the slackness of the bolts which means the four 100x100 profiles do not act in complete cooperation. The bonding of the beams across the entire length ensures the profiles act together and allow the full utilization of the modulus of elasticity in combined section profile to act in the same direction and therefore give a smaller midpoint deflection.
The additional loading applied to the columns would lead to a recommendation that the column capacities are as per the following:
Item Compression Capacity Tension Capacity
Bonded 200x200 112.5 kN 127.5 kN
Bolted 200x200 105.2 kN 125.3 kN
It is anticipated that if the columns were tested to ultimate failure, the capacities could be further increased.
Figure 3 - Load vs time column test results 4.0 FRP Column Installation Process
The installation of new FRP columns into existing water storage tanks is straightforward and takes a substantially less amount of time than the traditionally proposed methods of repairing the existing columns. For a typical tank size, similar to the Kingscote project i.e. 9ML tank with 20 columns,
structure. The light weight nature of the columns allows the use of chain blocks and winches for installation and doesn’t require the roof structure to be removed for large crane access.
While the columns can be installed without removing the roof, it can often be a requirement of the project due to the demolition and removal of the existing concrete or steel columns as well as on occasions, the replacement of the roof structure. Figure 4 shows the Burnside Tank in South Australia emptied with the existing concrete columns ready to be cut and removed with the use of a diamond rope saw.
Once the existing columns and hold down bolts are removed, and new bolts chemically anchored to the floor, the new FRP columns can be lifted into place. The base plates and top connection brackets are pre-assembled onto the columns external to the tank, minimising the work inside the restricted space. The FRP columns are then bolted and levelled to the preinstalled hold down bolts, with temporary props attached to the columns when required to restrict the effect of high winds prior to the reinstatement of the roof structure. To ensure the columns are installed correctly within the allowable eccentric load requirements as discussed in section 3.2, each column is surveyed post installation to verify the verticality as well as determine the eccentricity based off the location of the columns in relation to the roof structure.
Figure 4 - Burnside Tank with roof removed and existing concrete columns
Figure 5 - Burnside Tank with new FRP columns installed
5.0 Conclusion
It is evident from the completed compression and tension testing that the fibre composite roof support columns met and subsequently exceeded the requirements of the design loading on the Kingscote Water Tank project. The excellent structural results achieved paired with the inherent corrosion resistance of the material ensures that the columns will meet or exceed their expected design life of 100 years. The use of fibre composite, prefabricated columns in this application not only provide whole of life cost reduction due to their low maintenance and resistance to harsh environments, but also help to reduce installation costs and downtime of the water storage assets.
6.0 References
[1] Clarke, J.(Ed) (1996), Structural design of polymer composites Eurocomp design code and handbook. E & FN Spon, London, UK.
[2] AS/NZS 1170 (2007) Structural Design Actions Set [3] AS 4100 (2016) Steel Structures
[4] AS/NZS 4020 (2005) Testing of products for use in contact with drinking water [5] 2017-18 South Australian Water Corporation Annual Report
[6] Barbero, E. J. and Tomblin, J., Euler Buckling if Thin-Walled Composite Columns. Thin-walled structures, 18, 117-131(1993)
[7] Zureick, A. and Scott, D. W., Short-Term Behaviours and Design of Fiber-Reinforced Polymer Slender Members Under Axial Compression. Journal of Composites for Construction, 1(4), 140- 149 (1997)
Author Biography Lachlan Nicol
- Over 6 years of fibre composite engineering experience
- Project managed the design, manufacture and installation of fibre composite structures throughout Australia, New Zealand, United States, and United Kingdom.
- Currently Employed by Wagners CFT as a product engineer - Bachelor of Engineering – University of Southern Queensland
Author postal address 11 Ballera Court, Wellcamp, Queensland Author email address [email protected]
Author mobile phone number 0473 733 070
