This article walks through the design of box culverts by giving an overview of the applicable design standards used for box culverts to Eurocodes. The importance of adhering to these standards is emphasized to ensure structural integrity, durability and safety.
A box culvert is a type of structure used in civil engineering and construction to convey water or other fluids beneath roads, railways, or other obstacles. It is typically rectangular or square in shape and consists of a bottom slab, two side walls, and a top slab. This form allows them to provide a passage for water flow while also supporting the load of the overlying transportation infrastructure. Thus, designing a box culvert involves two primary aspects: hydraulic design – dealing with the efficient passage of water and structural design – dealing with stability of the culvert to support traffic and other loads. These two broad aspects of box culvert design will be comprehensively reviewed in subsequent articles.
To design a box culvert, there are well established standards that must adhered to in order to ensure structural integrity, durability, and safety. In Europe, and other parts of the world, the Eurocodes set the benchmark for design standards, offering a comprehensive framework for the design and construction of civil engineering structures. This article explores the key design standards used for box culverts to Eurocodes, emphasizing the importance of adherence to these guidelines.
Most of these standards apply mostly to the structural design aspects. Hydraulic design aspects of culverts are usually conducted using standard procedures provided in hydraulic design references.
The relevant Eurocodes and associated National Annexes have been considered as the primary documents to be adopted for reinforced concrete design. However, there is still a degree of apprehension to fully embrace their use, and this is no doubt attributed to their complexity, presentation and inter-relationship, compared to previous British Standards. Additionally, the application of Eurocodes to specific reinforced concrete elements is not as straightforward as the design of regular beams, slabs and columns. This is especially true, for structural items like box culvert which requires interactive design of slabs and walls under several loading conditions.
Designing a of box culverts has always required reference and understanding of traffic loading on bridges, and this remains a fundamental requirement when undertaking a box culvert design. In addition to the Eurocodes ECO, ECI and EC2, and NAs, there is now a specific Eurocode, BS EN 14844:2006 Precast concrete Products — Box culverts, dealing with aspects of design, manufacture and installation. This document in turn, cross references BS EN 13369:2001 Common rules for precast concrete products. Additionally, there is non-contradictory complementary information in the form of PD 6694-1:2011 Recommendations for the design of structures subject to traffic loading to BS EN 19971:2004, and PD 6687-2.•2008 Recommendations for the design of structures to BS EN 1992 2:2005. These documents are a ‘must have’ for designers who want to undertake box culvert designs to the Eurocode.
In broad terms, the Eurocodes follow similar principles to British Standards for reinforced concrete design in terms of ultimate limit state (ULS) bending and shear. Although there are minor variations in allowable shear stresses, these have negligible impacts on the design of box culvert units. However, the Eurocodes mandate higher loadings for box culverts, compared to previous British Standards, necessitating the need to provide enhanced structural capacity.
Summary of Most Relevant Documents
It is critical that Client requirements are clearly defined, at the inception and this demands a recognition and understanding of all aspects, including the loading parameters to be incorporated into the culvert design. Below is a summary of the relevant documents in the design of box culverts.
BS EN 1991-2:2003 – Eurocode 1: Actions on Structures – Part 2: Traffic Loads on Bridges
This document sets out the loading classes for road bridges and defines a series of Load Models (LM1-LM3),
- Load Model 1 — Concentrated and Distributed Load
- Load Model 2 — Single axle loads
- Load Model 3 – Loads for Special vehicles.
- Load Model 4 — Crowd Loading
Each Load Model describes a configuration of wheel loading to represent different wheeled vehicles. It is significant that the wheel loads within these models is now 150kN (LMI) and 200kN (LM2) & 165kN (LM3). This is compared to the previous wheel load of 301-1B at 75kN.
The design is progressed, generally adopting the worst-case effects of these Load Models. The loads are applied to the culvert structure by considering the zones of influence as described in PD 6694 (Figure 11). PD 6694 (Table 7) also defines the horizontal surcharge pressures to be applied to the structure.
Horizontal loads in terms of braking and acceleration forces are defined within the Eurocode, and these are applied in conjunction with the relevant vertical loads.
BS EN 1992-1-1:2002 – Eurocode 2: Design of concrete structures – Part 1-1: General rules and rules for buildings
Having established the appropriate applied loads, and from an analysis to determine the resultant bending moments and shear forces, the detailed concrete design is undertaken, in accordance with this document.
BS 8500-1:2002 – Concrete – Complementary British Standard to BS EN 206-1, Part 1: method of specifying and guidance for the specifier.
The performance of the culvert structure and its defined working life will be determined by defining a suitable concrete specification, and by adopting suitable cover to the reinforcement.
Section 4 of Eurocode 2 deals with durability and cover requirements. The Exposure Classes are as EN 206-1, but the UK adopts BS BS8500-1, to define a suitable concrete specification, taking account of corrosion due to carbonation (XC), chlorides (XD), sea water (XS), & freeze/thaw (XF). Generally, XD2 is adopted for structures buried below 1.0m and XD3 for shallower structures. The required concrete specification is described by its strength class (e.g. C20/25), and an associated minimum cement content and water cement ratio.
For particular cases of exposure (high sulphate conditions), it may be necessary to refer to the BRE Special Digest 1:2005 — Concrete in aggressive ground, which gives guidance on concrete specification and additional protective measures.
Intended working life is generally specified at 100years. However, the Highways Agency (IAN 95 07) accepts that adopting these parameters is deemed to provide a working life of 120 years.
Precast concrete generally adopts a higher concrete specification, when compared to in-situ concrete. This allows the adoption of a lesser cover to the reinforcement, and when considered in conjunction with a smaller Δc value, (allowance for rebar deviation), will result in a more economic design.
BS EN 13369:2001 — Common rules for precast concrete products
This standard is a generic document, which sets out requirements to the range of products which are produced in a factory environment. It is intended to act as a reference document which provides guidance on the various issues associated with precast concrete elements and their manufacturing, (e.g. durability, tolerances material requirements, testing). It is intended to provide a more consistent approach to standardization in the field of precast concrete products. General references to more specific Eurocodes are included.
BS EN 14844 — 2006 — Precast concrete products — Box culverts
In a similar manner to BS EN 13369, the standard provides guidance on materials, testing, and geometry etc., and production requirements, together with reference to design criteria, although this is generally in the form of cross references to the standards mentioned previously.
This is a ‘harmonized’ standard and fully encompasses the requirements of the EC. Consequently, this standard leads to the CE marking now required on box culverts.
As evident, several documents must be referenced to attain a suitable and compliant design for a box culvert. It is also evident that specifying the initial design parameters early on is crucial to guarantee the delivery of a product that aligns with the client’s needs.