Safety Levels are a Choice
Thinking of road safety barriers, design engineers are required to understand environmental conditions and then provide protection for all road users. Their role, as professionals, is to objectively evaluate the alternatives and based on their knowledge and experience, to specify appropriate containment levels. Essentially, “What performance do I need to build into this road safety barrier to ensure that in the event of an accident the road users suffer the least level of harm?” and “What design standards and regulations guide my decision?”
As technology progresses, corresponding standards are re-evaluated. We are more aware than ever that static force cannot be directly compared with impact energy. Actual crash testing has shown that it is insufficient to compare calculations based on static forces with results which have been proven in crash testing situations. Is it possible to compare static force with impact force?
Crash test provide indisputable evidence of performance. TMH7 is still only a theoretical calculation. SA in most cases appears to be insistent upon the use of TMH7 for median barriers and this is in spite of an SANS standard in respect of crash tested standards for Road (Vehicle) Restraint Systems
Deltabloc is in the fortunate position that their products comply fully with TMH7 to an excess of 50kN and has crash tested systems to the new requirement of 200kN.
Further, and more alarmingly SA still insists on the installation of guardrail on wooden posts, which is effectively an American standard dating back to 1956 and which has not change to take account of the increase in traffic volumes and speeds.
When a vehicle impacts a barrier, it impacts at angles of up to 45˚. According to studies by the CSIR, the average angle of vehicular impacts in South Africa is 13˚. Crash tests are normally carried out at an angle of impact of 20˚ and the speed will vary depending upon the size of the vehicle.
Crash tests show what happens when a vehicle impacts a barrier. The top of the barrier at the impact point is tilted by the impact of the vehicle. When an articulated truck trailer combination hits a barrier it has a triple impact. The outcome is a “harder” and higher impact than that from a rigid vehicle.
The bottom of the barrier does not take / absorb the same amount of impact energy as the top of the barrier. In practice, most of the impact on the lower half of the barrier is absorbed by the tyres.
How do we know that the above analysis is factually correct?
Deltabloc have carried out dozens of crash tests going back to the 1994 with the most current crash test being carried out January 2015. The organisation’s scientifically controlled crash tests provide known containment levels and working width results. This means clients can design with certainty, to their required performance levels. Their decisions are based on reliable and factual information.
|VARIOUS DELTABLOC DB80 SYSTEMS|
|SYSTEM nomenclature||Containment level||Working width||Displacement|
|Semi-rigid DB80 – free standing||H 1||W 4||1280mm|
|Semi-rigid DB80 – plate mount “bridge”||H2||W 4|
|Near rigid DB80E – embedded in asphalt||H2||W 1||0|
|“Rigid” DB80AS-A – absorption link||H2||W 1||120mm|
|Rigid DB80 EPB – cast-in-situ||H2||W 2|
The crash tests also illustrate the minimal level of damage to the system following a severe impact from a heavy vehicle.