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Site Observations
During a recent roof inspection it was
pointed out that an anchor post securing a running line
system was no longer vertical, as shown in photo 1. On
closer inspection it appeared that the baseplate securing
the post to the concrete deck had come loose. The anchorage
system was unsafe.
On a second roof seen a couple of days
earlier the galvanised steel anchor post had been fixed
to a metal deck, in advance of the insulation and membrane
being laid. As seen in photo 2, four rivets were missing
from one side of the baseplate, presumably because the
plate was too small to sit onto three crowns of the decking
profile. As a result, if the top of the anchor was pulled
towards the right then the safe anchorage would be totally
reliant on just two rivets.
From the subsequent desk study it was
confirmed that the pull-out strength of the two rivets
fixed into the 0.7mm thick steel deck was inadequate.
Since the posts were not fixed directly above the steel
support beams, a twisting type deflection could be expected
in the lightweight deck. Also, the shot fired studs holding
the deck down to the steel frame had inadequate pull-over
resistance to overcome the local over-turning moment.
The example is a particularly alarming
case as once the insulation and membrane had been laid
the means of attachment would have been hidden whilst
the anchor point would appear to be safe. It is critical
that for fixing into the building structure, the design
and installation of the whole system should be verified
by a qualified engineer to be capable of sustaining the
test force.
Testing
The relevant British Standard is BS EN
795: 1997 which defines the requirements and testing for
anchor devices. For example, where there is a horizontal
rigid anchor line there is a recommendation for a static
test with a force of 10 kN applied in the direction in
which the force can be applied in service. The force should
be maintained for three minutes during which time the
anchor device should hold the force. If more than one
person may use the device at the same time, then the test
load is increased.
10 kN or 1 tonne is a significant sideways
force. It is of the same order of magnitude as the force
in a rope pulled by a 5 man tug-o-war team! The engineer
needs to ensure that this lateral load and over-turning
moment can be transferred down into the structural framework
of the building.
Conclusion
Thankfully there are several firms of
well established specialists who design, install and test
safety related equipment. The examples of bad practice
are hopefully isolated, but they are a timely warning
to our industry. Potentially a man's life is at risk if
the anchorage is not secure.
If we cannot be confident that the anchor
point can take the applied load, then we would be better
off not having the anchor point at all.
References
1. HSG 33, 'Health and Safety in Roof
Work', 1998 (to order: telephone 01787 881165)
2. BS EN 795: 1997 'Protection Against Falls from a
Height - Anchor Devices - Requirements and Testing'.
© Keith Roberts, 2000
First published in Roofing Cladding
Insulation, April 2000
RCI
Technical Notes
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