WATERPROOFING | INSIGHT
However, the fixing manufacturer will use published
standards, such as EN 1992-4 (Eurocode 2 Part 4: Design of fastenings for use in concrete), to specify the performance of their product in situ. While these values are often made accessible by the fixing manufacturer in a convenient tabulated format this does not create a responsibility for the performance of the products in situ for the manufacturer. This responsibility is shared between the constructor and the designer to interpret both the design code and the application rules, as specified by the manufacturer of the product. For segment accessories, there typically is no
design code to interpret. Parameters supplied by manufacturers are assured for the underlying series of tests and associated boundary conditions, which might differ from conditions in situ. Care is necessary to ensure assumptions made in the design are in line with the performance demonstrated in the manufacturer’s tests. The missing link here is the lack of straightforward
options for the designer to demonstrate due professional skill and care which they normally can by relying on codes or guidance, leading to a potential break in the link of assurance.
PRODUCING ADEQUATELY SAFE DESIGNS At this point, it is useful to recap the safety concept generally underlying our designs. Design codes such as ACI, AASHTO and the Eurocodes
ultimately derive the factors applied to loads and resistances from calibration to a long experience of building tradition to ensure the design attains a code specified reliability, expressed through the Reliability Index (β) linked to the Survival Probability (PS
for Precast Fibre Reinforced Concrete Segments – Vol 1 Design Aspects) for which the author led the ITAtech Activity Group. Further guidance on setting appropriate target values for tests in an ACI/AASHTO based design can be derived from ACI 214R-11 (Guide to Evaluation of Strength test Results of Concrete). Following this approach makes it possible to
introduce non-standard elements into a fully code compliant design. This approach is suitable for both ULS and SLS design.
As specific full-scale testing is, however, relatively costly and time consuming it is often replaced or supported by numerical modelling. Care must be taken that all aspects of the numerical model are calibrated against specific tests, accurately representing the modelled parameter, in the correct time range. Where the mechanical or chemical behaviour of
products is time and/or load dependent, a prediction of the parameters over extended time periods can be difficult to make. In this case testing for the full time and load range the product is expected to work in is recommended. This typically restricts the use of such products to relatively short durations, unless verified material models for ageing can be accepted. And, finally, it is important to remain conscious of the
). These
are notional values, defined for the anticipated design life of the structure. To the knowledge of the author this value is ultimately derived on the basis of national experience, and its origin is not further detailed outside specialist literature. Closer examination reveals that different code families such as ACI, AASHTO and the Eurocodes largely agree in the selection of the Reliability Index. ‘Calibration against a long experience’ is typically
not available for the relatively novel products used for segment accessories. An alternative way to demonstrate code compliant reliability is ‘design by testing’, with the test setup replicating the product in its installed configuration, in combination with a load representative in magnitude as well as speed of application and duration to the load the product is supposed to sustain as a minimum. The resistance of the configuration is established from
the average value of tests and the coefficient of variation. Clearly it is often beneficial to maximise the number of tests to reduce the statistical ‘jitter’. This process is described in more detail in EN 1990 Annex D and represented in ITAtech Report 7 (2016), (Guidance
fact that all elements of a tunnel need to act together and the reliability requirements in consequence apply to all contributing elements to the same extent. Where it is not possible to verify the performance of a product the consequences of its failure should be assessed. A typical example would be shear elements in circle joints designed to inhibit the deformation of the lining. Where their long term effect can’t be assured the effect of their absence on gasket offset and opening should be reviewed.
FURTHER GUIDANCE The performance of segment accessories has been subject to much attention over the last years. The STUVA (2019) specification has been a milestone for the standardised testing of segment gaskets, however its practical use has revealed some specific shortcomings. Other segment accessories, such as bicones and
dowels, remain largely unregulated. This situation is expected to improve with the release of the 4th
Edition
of the BTS Specification for Tunnelling (December 2023) which contains further guidance on specifying performance requirements for these important aspects of segment design, developed in collaboration with the supply chain and subjected to multiple rounds of industry-wide peer reviews. The updated BTS Specification calls for more specific
testing of segment accessories including gaskets, dowels, bicones and packers which will, in time, lead to an improved understanding of the behaviour of these elements and support data-driven design of accessories to a jointly accepted reliability level.
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