WIRE ROPE SAFETY FENCE
TYPICAL PROCEDURES FOR CONCRETE FOUNDATIONS
1. The Standard Drawings show the
required positions of the anchorage foundations. The positioning of the anchorage is critical to avoid any problems
when the ropes are installed.
2. The steel anchor frames need
to be located at the correct height as indicated by the Standard Drawing LB/126 or LB/127. The drawing also shows
the pocket made in the top of the concrete block to provide access for the wire ropes to engage in the slots the
steel anchor frame. The concrete should also be kept clear of the two smaller holes in the anchor frame adjacent
to the main slots. These two smaller holes are where the check ropes are connected. Vibratory compaction of the
concrete should be carried out.
3. It is extremely important that
the anchor frame is angled back at 5o from the vertical. A levelling plate is welded to the side of the anchor
for this purpose. A spirit level can be used to position the levelling plate dead level, which then ensures the
correct angle for the anchor frame. Any installation outside an angle of +/- 2o from the 5o specified should be
referred back to LB Wire Ropes for further instructions.
4. Plastic sockets are supplied
which are cast in-situ in the post foundations to provide a socket into which the Brifen posts are an easy sliding
fit. Care should be taken to avoid ramming a post into the socket to push it into position in the concrete, since
this could push the bottom out of the plastic socket.
5. With a Brifen post inserted
into the plastic socket a string line should be used to ensure a good alignment of the post foundations. The string
line also helps to ensure that the depth of each socket remains constant. This is obviously important so that when
the posts are inserted in the sockets, the tops of the posts are in alignment and follow the contour of the road.
A Brifen post should remain in the plastic socket until the concrete has cured. This ensures that the socket does
not get distorted and cause the post to be a tight fit. Also, the weight of the post prevents the socket from being
lifted up by the hydrostatic pressure of the wet concrete.
6. The spacing of the post foundations
is specified for each individual fence length. Each concrete foundations should be 350mm diameter and to a depth
of 950mm. If the ground conditions are extremely hard it is permissible to reduce the depth of the concrete foundation,
provided that the completed foundation can resist a bending moment of 6,000Nm. This can easily be tested by placing
a post into a concrete foundation and using a lever hoist and load cell anchored to a truck parked alongside. A
force of 10,000 N (approx. 1 tonne force) should be applied at a height of 600mm above ground level. If the foundation
can withstand this bending moment with minimal movement, then it is proven to be acceptable. If the concrete foundations
are to the full depth of 950mm it is usually unnecessary to perform such a test unless the ground conditions are
unusually soft.
7. When the post foundations are
being poured, one of the steel reinforcing rings should be inserted 60mm below ground level. When the concrete
has been poured and the plastic socket is in position, a 20mm crown should be formed on the top surface. A simple
tool can be made up to provide a good surface finish.
8. Also attached is a copy of
Appendix A from British Department of Transport specification TD 32/89. This gives further details of excavation
and concrete requirements.
TD 32/89 APPENDIX A - A.4 INSTALLATION
Excavations
1. Excavations for concrete post
foundations and anchor blocks shall have vertical sides and shall be formed to the line and level described in
the Contract.
Post Foundations and Anchor Blocks
2. Precast concrete post foundations
shall be inserted on a firm level base. Any lateral overbreak of the excavation shall be filled with grade C7.5P
concrete.
3. In-situ concrete post foundations
and anchor blocks shall be constructed using concrete to grade C25P to BS 5328. Temporary formwork shall be used
where the sides of excavations cannot be kept vertical for in-situ concrete post foundations and anchor blocks.
The formwork shall be installed immediately after excavation. Any lateral overbreak of the excavation shall be
filled with grade C7.5P concrete.
4. All anchor frames, check posts,
sockets and reinforcing rings shall be positioned to the line and levels described in the Contract and they shall
be secured against displacement during placing of the concrete.
5. Concrete shall be compacted
using internal vibration an shall be thoroughly worked around any embedded metal and into corners of formwork or
excavations to produce a dense mass substantially free from voids and in such manner as to prevent segregation.
Immediately on completion of compaction exposed surfaces shall be covered for a minimum period of 7 days (unless
the Engineer agrees a shorter period) by covering with an impermeable sheeting set to prohibit excess of moisture.
The sheeting shall be removed at the end of the curing period. Formwork shall be struck without damage to the concrete
not less than 7 days after placing the concrete. Formed and unformed surfaces shall be free from voids, honeycombing
or other defects. Small blemishes caused by entrapped air or water are acceptable.
|
|
||
|
|
|
