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T.P.D.M. V.2.3
(Al/2001)
3.9.3.18 The standard thriebeam can reduce the incidence of rolling over in large vehicle collisions
more effectively. This is accomplished by increasing the beam mounting height to 875 mm
and using a stronger blockout. It is therefore preferable to the untensioned corrugated beam
when there is high frequency of traffic accidents involving vehicles of high centre of gravity.
3.9.3.19 In the event that the standard thriebeam is deemed not sufficient because frequent impact by
heavy vehicles is expected, the standard blockout for thriebeam shall be replaced by a 350
mm deep modified blockout. Such blockout has a notch at the bottom.which allows the
lower portion of the beam and the flange of the spacer block to bend in during collision by
vehicles of high centre of gravity, keeping the rail face nearly vertical and thereby retaining
the vehicle from rolling over.
3.9.3.20 The use of standard/modified thriebeam can be considered where a higher rigidity of barrier
fence than corrugated beam is required but concrete profile barrier is not warranted due to
the associated cost, accident risk and space availability, etc. This type of barrier can more
effectively reduce the incidence of rolling over by high vehicles when impacted. The
thriebeam barriers should be installed at the following locations:
(i)
(ii)
On roads with a speed limit of 70 km/h or above, on the outside curve of the bend of
radius R4 or below and on top of the downhill slope;
On roads with a speed limit of 50 km/h, on R3 or below road bends adjacent to
downhill slope.
Concrete Profile Barrier
3.9.3.21 The theory of the concrete profile barrier is that when a vehicle strikes at an angle of 15° or
less the impact energy is absorbed in compressing the suspension of the vehicle. The front
wheels of the vehicle climb up the 55° slope and on contact with the upper slope the wheels
are turned parallel to the barrier's longitudinal axis and the vehicle is redirected.
3.9.3.22 The concrete barrier will contain most vehicles within the carriageway, though at high
angle, serious damage can result.
3.9.3.23 Concrete profile barriers require little maintenance, therefore they are ideal for locations
where maintenance would cause considerable traffic disruption. However, because of the
damage that can be caused to vehicles and their occupants by high angle impacts, the
potential risk of this type of accident occurring, particularly on roads having a design speed
or speed limit of 80 km/h or more, should also be taken into account when considering the
use of this type of barrier fence.
3.9.3.24 Although the concrete profile barrier requires minimum space, it can have an inhibiting
effect on motorists and therefore a parallel hard strip should always be provided between the
barrier and the edge of the carriageway. Where the hard strip is 1500 mm or more the hard
strip should be delineated by 200 mm wide continuous white line, where it is less than 1500
mm wide the hard strip should be 100 mm wide continuous white line. The hard strip
should never be less than 300 mm wide, and preferably should be wider.
3.9.3.25 While planting is aesthetically pleasing on central reservations, care should be taken,
particularly when used in conjunction with concrete profile barriers that sight lines are not
obscured either at the time of planting or by subsequent growth.