final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 56
The increased reactions under the centre supports of
continuous beams can sometimes create a hazard in
struts with very rigid foundations particularly if both
the primary and secondary beams of a strutting
system are continuous on the same support.
Defects in joints due to previous use cannot be
tolerated. Cracked welds in components must be
ground out and re-welded only under expert guidance
and preferably not on site. The movement in bolted
or pinned joints should not exceed the design
specification.
Steel components
Tolerances in straightness should be limited to an
amount which depends upon the total unsupported
length of the member and its cross-sectional dimensions,
The commonest falsework components in the United
Kingdom are steel tubes and fittings, telescopic props,
proprietary framed supports and various steel
members.
The components actually used inevitably vary from
piece to piece. Not every prop is produced with
identical dimensions and the British Standards for
new material allow certain tolerances. In addition the
designer must allow for deterioration due to corrosion,
damage in handling, re-straightening and repair. These
may have different effects on materials of the same
nominal capacity. For example, scaffold tubes of
3ยท2 mm wall thickness are much more liable to damage
from mishandling, and the deterioration due to a
given depth of corrosion may be greater than that
with 4 mm wall tubes. This underlines the cardinal
point that the material specified by the designer or an
alternative acceptable to him must always be used.
The allowable stress for a falsework member is not
always easily determined. First, the original specification of the metal may not be available; secondly, its
characteristics may have been significantly modified
by age, repeated use or straightening; thirdly corrosion
may have a significant effect on failure loads and
hence on the aHowable stress.
When corrosion has reduced the cross section. area of
a member, the section properties of the reduced area
are not readily available to the designer. He usually
prefers to modify the allowable stress and use the
nominal and tabulated section properties rather than
to re-calculate the properties of the corroded section
and retain the normal allowable stress. Unless the reused steel is in 'as-new' condition, it is preferred for
simplicity and the avoidance of errors to reduce the
allowable stress by 15 %- This reduction is from the
original allowable stress or, when this is not known,
the old BS 15 steel stresses. This 15 % will generally
be adequate to cover corrosion, distortion and restraightening. In making such a recommendation, it
is assumed that any element which would clearly be
inadequate will be discarded. If there is more than a
10 % loss of section anywhere the item should be
rejected.
Components which have defects of thickness and
straightness should be given the most careful examination if they must be re-used.
The emphasis in the USA is towards fewer stouter
members rather than a multiplicity of smaller members.
Rolled beams are frequently used in falsework, but it
is not always realised that the smaller root radii of
the universal beams may result in a lower web capacity
than in the case of the older joists. Furthermore there
is evidence that the 'as rolled' dimensions are sometimes different from the nominal.
Careful attention must be given to these points in
design and in the use of published data and on-site
inspection.
Jacks to extensions
In a typical scaffold falsework it is usual to have
jacks at the bottom and top positions in the form of
adjustable base plates and adjustable forkheads
respectively. Where extendable jacks are used it must
be remembered that the point between the extension
and the prop is relatively weak in bending.
Instances occur where frequent re-use leads to slackness in the threads which makes the connection even
weaker. The jack designer should ensure that his
design does not permit excessive screw extension.
Furthermore, if pin-jointed forkheads and end plates designed to accommodate sloping soffits and foundations - are used there can be no moment applied at the
jack ends tending to keep them axially aligned. It is
therefore essential that particular attention be paid
to the lacing and bracing of jacks in such circumstances. The next lift may also be affected by the
reduced restraint at the junction points and require
stabilisation.
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