final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 58
Grillages
Our attention has been drawn, as a result of several
failures, to the over-loading of universal beams which
has resulted in tilting of the flanges and buckling of
the webs. The whole question of web stiffness is one
on which more information is required. We have
observed how much attention is given to the subject in
certain overseas countries including the USA where
welded metal fillets and timber packing pieces are
widely used in grillages. Where crossed beams are
used in highly stressed grillages the flanges should be
fixed together by means of beam clamps or equally
effective devices. Particular attention should be paid
to the effects of horizontal movement at all stages of
construction and loading. Longitudinal movement of
the ends of beams during loading can produce
horizontal forces or moments sufficient to fail the
webs or even to roll the grillage beams over if they
are not properly supported.
We recommend that particular attention to these
points is given in the proposed code of practice for
falsework.
The weakness of webs is highlighted in reports of
research at Aston University. Professor Holmes, Head
of the Civil Engineering Department, has measured
skin stresses at the junction of web and flange. ยท The
same loads applied to the new universal beams produce
stresses twice or even four times as great as in traditional rolled steel joists. These differences all point
to the need for a new tabulation of beam data and
we recommend that the research work on which such
tables could be based is put in hand.
We have found a lack of understanding among
designers of the way to use the web capacity tables for
universal beams. The calculation of the stiff bearing
lengths of one joist supporting another is difficult
and there are no tabulated values for each serial size.
When timber is used on or under steel beams there is
no stiffening of the section because the timber deforms.
The effect of this deformation on stresses at the web/
flange junctions is another point that Professor
Holmes found significant.
Proprietary equipment
Section 6 of the new Act states that it shall be the duty
of any person who designs, manufactures, imports
or supplies any article for use at work:
(a) to ensure, so far as is reasonably practicable,
that the article is so designed and constructed as
to be safe and without risks to health when properly
used;
(b) to carry out or arrange for the carrying out of
such testing and examination as may be necessary
for the performance of the duty imposed on him by
the preceding paragraph;
(c) to take such steps as are necessary to secure that
there will be available in connection with the use
of the article at work adequate information about
the use for which it is designed and has been tested,
and about any conditions necessary to ensure that,
when put to that use, it will be safe and without
risks to health.
The committee welcomes these requirements and the
following points should be read in that context.
Design by manufacturer
Proprietary units should be designed and constructed
such that the stresses set up from normal use and the
abuse to be expected on construction sites will not
cause damage detrimental to the strength and stability
of the unit. Even if units are designed with this consideration in mind they should be handled with care
at all times.
In order to reduce the risks of failure the units should
be designed in a way which eliminates as far as possible
all dangers of incorrect assembly. They must also be
designed for extreme conditions of use, for example,
manufacturers of forkheads should ensure that there
is always a residual length of shank in the load bearing
tube when the nut is set at the lower end.
It is important that items which are similar but which
may have different properties and therefore different
load-carrying capacities should be clearly identifiable
by the man on the site.
Design by falsework designer
It is essential that the falsework designer has a proper
understanding of the use of proprietary units. This can
only be achieved by training, experience and adequate
communication between manufacturer and user.
When designing for the use of different proprietary
systems within the same falsework structure, the compatibility of the systems must be considered, particularly at the interfaces.
The designer must pay attention to the bracing of
proprietary systems even if the relevant trade literature
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