Is a higher class the answer?

Barry Turner explains that changing access covers from BS EN124 class D400 to class E600 for road applications could be a costly exercise that may not deliver the expected improvements.

Often when a D400 access cover has failed prematurely, a common reaction is to replace it with one of the subsequent load class (E600), in the hope this will prevent failure in the future.

While the change from D400 to E600 itself is not normally detrimental, it incurs additional cost and is unlikely to solve the problem.

It’s important to remember that the key difference between the requirements for a D400 cover and an E600 is simply the ability to take a higher static load in the centre of the cover/s. What this means in practice is that by thickening the central beam, a D400 cover can be classified as E600.

Of course, an E600 cover assembly is entirely appropriate in group 5 locations such as forklift loading areas, plant driveways or docks, where the cover itself is exposed to high point-loads, particularly vehicles with non-pneumatic tyres.

Considering the construction of a manhole cover assembly for a moment helps put this into context; a cover consists of the top plate and supporting beam structure and is held within a perimeter frame which retains the road, has a flange for embedding in and spreading load on the underlying construction and seats for taking the load of the cover.

The supporting structure normally comprises a main beam for taking the test load and secondary structure of smaller beams to reinforce the remainder of the top plate.

Structural failure of the main beam is unheard of. The D400 design load of 400kN represents a safety factor which is many times in excess of a UK maximum axle load and of course, at most, covers will only ever see half of that. The only way a main beam might fail would be associated other factors such as manufacturing defects, excessive corrosion or inappropriate installation.

Changing the specification of cover and frame from D400 to E600 unfortunately, therefore, doesn’t deliver product features which counteract common modes of failure or correctly considered installation materials and methods.

This hypothesis is supported by the findings in WRc research report CP373, which indicates that the true reason for premature failures is most often related to the foundation support beneath the access-cover’s frame.

“Selecting access cover frame types which spread traffic loads to supporting foundations evenly should be a higher priority than simply increasing a product’s load class.” 

Following this logic, selecting access cover frame types which spread traffic loads to supporting foundations evenly should be a higher priority than simply increasing a product’s load class.

Likewise, the foundation structure should also be a top priority during construction-material selection.

Wrekin Products’ UniteTM D400 is particularly notable here. Challenging manufacturing conventions, the first design was created more than 15 years ago with performance longevity and delivering the lowest possible whole life cost in mind. Now in its third evolution, the design delivers unrivalled load dissipation to ensure bedding mortar is not compromised, the flanges themselves are also bulbous and rounded to further remove stress concentrations.

A raft of other features include substantial top plate support, bedding interlock features, safe lifting provisions and a super-strong structural main beam to reduce movement and provide against more-aggressive corrosion environments.

Trial installations on the M25 confirmed the theory that a well-designed D400 is better than many E600 products when installed in an intensely trafficked Group 4 location.

Service life

The project was designed to identify the most cost-effective, safe and reliable way to use the hard shoulder of the M25 during peak usage hours. This posed several engineering problems, a major one of which was to consider the service life of the access covers that would now be located in the wheel-track stretch of one of Europe’s busiest motorways.

For the trial, carried out by Mouchel for Highways England, Wrekin was requested to supply a D400 product design for installation between junctions 8 & 9 of the northbound carriageway (a domain which would normally see the selection of E600 products).

Numerous BS EN124 class E600 units were offered for the trials from a wide selection of UK manufacturers and suppliers, whilst Wrekin was the only manufacturer to supply a D400 product, namely Unite.

All products were installed using similar techniques and construction materials.

Notably, all of the E600 units installed for the trial failed after three months of service, with fractures occurring on both the cover and frame elements. Evidence gained from the failed installations showed that the intense traffic loads were being transmitted through the covers to the substructure unevenly.  This load-dissipation resulted in a chain of degradation, beginning with foundation distress and culminating in fracture failure of the ironwork assemblies.

The Wrekin D400 product did not exhibit any evidence of distress either during or after this three-month period.

The conclusions from these illustrative trials clearly indicated that a well-designed D400 product can outperform poorly-designed (but EN124 conforming) E600 products in any given Group 4 application.

Barry Turner, Technical Manager, Wrekin Products