EN 1317
Inadequacies
For the past 18 months Varley and Gulliver has been giving presentations to engineers, local authorities and contractors in order to demonstrate the life threatening possibilities that exist as a result of the inadequacies in the European standard EN 1317. The deficiencies are due to the introduction of working widths, with no differentiation between bridge parapets and other safety barriers. With a safety barrier, an impacting vehicle will be supported by the ground whatever the working width, not so with a bridge parapet. Typically an aluminium bridge parapet with a W4 working width erected on a 450mm plinth will have a gap in excess of 500mm between the front face of the system and the back edge of the bridge structure. Parapets with W3 and W4 (1m and 1.3m) working widths are being placed on plinths narrower than what they were tested on. This was not an issue under the British Standard BS 6779, because the standard specified a maximum wheel penetration of 500mm, to ensure that when the parapet was impacted, there would not be a gap between the front face of the parapet and the back face of the bridge, nor would the wheels overhang the bridge structure. The only way to ensure a parapets performance is to do as the British Standard and limit the penetration of the vehicle or TEST IT on the minimum size of structure it will be placed. You cannot accurately calculate the effects of a dynamic impact into any vehicle restraint system.
Some sources have stated that misinformation has been generated regarding working widths and testing on 450mm plinths and are trying to allay any fears of placing parapets on structures where the plinth width is less than what they were tested on. A famous test of the Brifen wire rope safety fence system is being quoted to add some weight to this argument. The Brifen test was carried out at MIRA with a four rope system on a plinth adjacent to a water filled ditch. The vehicle impacted the fence which deflected approximately 1.5m (W5) allowing the vehicles nearside to traverse fresh air. The system then pulled the vehicle back on track. The reason for the vehicle not falling over the edge was not due to the velocity of the vehicle, as stated, but because each of the four ropes are tensioned to 25kN. This enables the system to act as an elastic band which grips and supports the vehicle and pulls it back. Bridge parapets are not tensioned and rely on the strength of their structure to resist the penetration of a wayward vehicle. If the vehicle wheels leave the structure the vehicle will roll and begin to drop as the parapet will not be able to offer the support that a tensioned rope system can.
The new draft of EN 1317 and The Highways Agency TD 19/06 document (see below) address this issue which reinforces what Varley and Gulliver have been saying for nearly 2 years to be fact not fiction.
During certain guardrail tests, such as the bridge parapet tests and while using a simulated bridge deck, the test vehicle and barrier must not in any way touch or take advantage of structures which will not be present on the final bridge installation, i.e. if the vehicle drops down behind the bridge installation it must not touch soil or supporting devices.
The dimensions of the edge detail shall be sufficient to demonstrate the actual performance of the vehicle and the tested system on the edge of a bridge, or structure.
The test shall demonstrate the minimum width of structure beyond the traffic face of the bridge parapet that is required to safely contain and redirect the vehicle
4.15 Ideally the parapet should be located so that, if it is impacted, there will be no gap arising between the edge of the bridge deck and the front face of the parapet that will affect the performance of the parapet. This must be demonstrated by information from the parapet manufacturer.
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For a Presentation on
BS EN 1317
contact
sales@v-and-g.co.uk
Varley and Gulliver Ltd
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