final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 29
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Similarly special three-dimensional effects arise when
the supports are not themselves parallel - for example
the piers supporting a curved roadway.
Grillages
Universal beams are often used in falsework as packing
pieces in order to cater for differences in level between
falsework units. An example would be the making
up of height between the top of a unit construction
and the support beams. They may also be used to
spread the load from a number of beams to a different
number of supports, as at Loddon. Improvement in
the stability of grillages, particularly for foundations,
may be made by packing concrete round the crossed
joists to obviate crippling. When this is undesirable
the crossed members of the grillages may be prevented
from crippling by the use of sleeved bolts between the
webs, or in the case of timber beam grillages placing
them close together to enhance their stability. Such
precautions are not often taken in temporary works.
In temporary works the materials have to be re-used
on the job and frequently have to be preserved for
future works - perhaps of a different type. The
components have to be dismantled whether they are
to be used again or not, and frequently they have to
be dismantled under load in such a way that the
permanent structure supported by them during construction can deflect and carry the load itself.
Most grillages, therefore are constructed without
actually fixing one member to another and so suffer
the disadvantages discussed earlier with regard to
interconnection. A particular hazard arises where the
height to base ratio of the component beams is high,
because this increases the possibility of overturning
under lateral forces.
Another special difficulty arises when steel beams are
used to form grillages. It must be remembered that
the section of such beams is chosen to provide stiffness
against bending: the thick flanges and thin web
produce a suitable section to resist bending, but one
which is not ideal for supporting and distributing concentrated loads. Nor is the section well adapted to
taking eccentric loads which can easily lead to buckling of the web. The difficulties are exacerbated by
variations in the accuracy of rolling, the thinness of the
web, and sharpness of the fillet radii of modem sections. At least two of the collapses studied were
attributed to overstressing of the web of universal
beams used as load spreaders : and cases have also
been reported where the essential checks on the strength
of the grillages bad never been done.
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Heavy duty grillage beams provided with web stiffeners at load
transfer points with separate components securely interconnected
This does appear to be one of the areas where technical
knowledge is not entirely adequate. We would therefore recommend as an important subject for continued
research the behaviour of standard section rolled
steel beams under concentrated loads, applied
eccentrically to the web.
Tolerances and eccentricities
The Report No 27 produced by the Construction
Industry Research and Information Association
(CIRIA) gives the results of a survey, made on a
number of sites, of the accuracy of construction that
is currently accepted. It appears that an appreciable
number of props are inclined at 1 ½ or more to the
vertical and others are displaced by an inch or more
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