final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 53
The connections of the braces to the foot ties must be
adequate to accept the accumulated tensions or
thrusts going down the braces and must not apply
excessive lateral force on to the weak portions of these
foot ties or to the uprights.
Use of guys
In general the use of guys is not advocated since they
have a number of disadvantages.
The primary one is that the guy has a high extension
under load and therefore permits increased movement.
This effect is made worse when the guy is long so that
its inherent elasticity is increased by the sag. Other
disadvantages are summarised in the following
paragraphs.
The guy tension produces extra vertical loads in the
structure at one point.
Since the guys act only in tension two cannot supplement each other when situated on opposite sides of a
falsework. Furthermore since the guys act in opposition they must be tightened simultaneously.
The increased flexibility can allow oscillations to
build up. Indeed the wind forces on the guys may
themselves introduce oscillation, particularly under
icing conditions.
It is difficult to provide a firm anchorage which can
withstand tension, particularly in weak ground.
Although appearing firm the fixing may give way a
little under load and allow unexpected movement.
Where two or more levels of guys are used tensions
may be introduced in the falsework and these must be
anticipated.
The lateral forces carried by intermediate laces cannot
be transferred easily or effectively to the guys and
must be resisted by internal bracing in any event.
In general therefore the use of guys is not advocated.
However in the case of very high falsework structures
in open countryside - for example bridge soffits over
valleys - it may be impracticable to use extensive
bracing to the falsework towers which are tall and
slender. We have seen guys in use, particularly in
California, for stabilising such structures. We recognise also the need for guys as temporary stabilisers of
falsework under construction.
The problems already listed must be studied very
carefully before a guyed system of stabilisation is
selected. Wire attachments, however, to parts of a
permanent structure at levels approximately the same
as the lacing levels can be used successfully provided
that a multiplicity of such connections can be made
so that individually the forces are small.
Badly placed, irregular, non-vertical rows of props do not lend themselves to fixing of the
necessary Iacmg tubes, which have been omitted at this site
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