The use of BS9295:2020 ‘guide to the structural design of pipelines’ for concrete pipelines

Mark Flavell, Technical Director at CPM Group Ltd

Prior to the publication of BS 9295:2010, the design procedure for concrete pipelineswas given in BS EN1295-1 ‘Structural design of pipelines under various conditions of loading’which detailed the nationally establishedmethod for the UK. The publication of BS 9295:2010 provided information and guidance on the application of BS EN 1295-1.

The publication of BS 9295:2020 has combined the above documents into a single guide. Additionally revisions to themethod for the design of both concrete and flexible pipelines have been included.

Principal Changes to concrete pipeline design Trenchwidth

The guide nowplaces a greater emphasis on trenchwidth as this has a major effect on pipeline loads.

In a narrowtrench design the load on a pipeline is reduced by frictional relief between the backfill and the trenchwalls. As the trench getswider, this effect declines until no relief develops and it becomes awide trench condition.

The designermust consider if the trenchwidth can be controlled during construction and can then selectwhether the NarroworWide trench method is applicable. For a narrowtrench design, itwill require an assumption of the trenchwidth to bemade or defaultwidthswhich have been used formany years.

This is consistentwith the design procedure for large diameter flexible pipelines issued in BS9295:2020,where the designer againmust select an installation class and quality. This in turn then determines the initial deflection incorporated into the design.

If trenchwidth control cannot be assumed, the guide recommends awide trench approach is used. However to date, narrowtrench design has been safely used formany yearswith no adverse issues. This is testament to the conservative designmethod and strength of concrete pipes. Narrowtrench designs can offer amore economic design but as previously stated, the designerwould have to specify themaximumtrenchwidth permissible for the design.

Bedding Factors

Recognition of the lateral support provided in awide trench or embankment condition has been considered resulting in an increase in bedding factors for concrete pipelines. The higher bedding factors are nowconsistentwith clay for diameters up to 750mm. For larger diameters, themethod used in

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the American Concrete Pipe Association ‘Concrete pipe designmanual’ has been adoptedwhere the bedding factor varies dependant on the depth.

The table detailing the Bedding Factors and Installed Depth Range has been updated.

Traffic Loading

Traffic loading is nowin accordancewith Eurocodes specifically BS EN 1991- 2 ‘Traffic loads on bridges’.

The critical case of BS EN 1991-2 Section 4 LoadModel 1 and LoadModel 2 is used to determine the surcharge at a given depth. As pipelines are short span structures, LoadModel 3 is not critical.

Limit stateDesign

An option for a Limit State design is included. Partial factors are applied to the permanent and variable loads as an alternative to a global safety factor. Both the Ultimate limit State (pipe crushing strength) and a Serviceability limit state (permissible crackwidth) are used to calculate the required bedding factor and the critical case is then considered. The traditional method has different global safety factors depending if the pipe is reinforced or unreinforcedwhere as the Limit statemethod uses the one set of partial factors. The Serviceability limit state can be applied to design for durability if this is the primary consideration.

Egg Shaped (Ovoid) Pipes

Amethod for egg-shaped pipelines has been included. This takes a different approach to circular pipelines and determines the flexural bending tensile stresswhich can be checked against the declared permissible by the manufacturer or the value given in BS EN 1992-1-1.

Design Procedure Summary

A set of design examples is nowincludedwhich fully set out the design process. Summary

BS 9295:2020 provides a single source of design reference for concrete pipelines. The revised procedures nowconsider lateral supportwith enhanced bedding factors and are consistentwith Eurocode traffic loading. A Limit State option not previously considered for pipelines is included. Limit State design philosophy is recognised as an advancement over a traditional design approach. A simplified table readily provides permissible cover depths for concrete pipelineswhereas the typical examples provide a template for a full structural analysis.

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