final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 47
related to its whole weight the impact in an in situ
concrete construction is only related to the size of the
skip and the construction plant. It may well be argued
that there is then greater danger with a falsework
designed for a rel_atively light duty than for one
designed for a heavy load.
(iii) Access openings We have drawn attention elsewhere to the need for planning in advance the openings
in falsework where access for vehicles will be required.
We have mentioned the need for additional strengthening to accommodate loads resulting from impact by
a vehicle or its load. In addition to this, where an
access opening is to be used frequently or for public
traffic, substantial fenders, bollards, crash barriers,
etc, should be provided'. These must be of sufficient
strength and sufficiently embedded to prevent the
effects of accidental impact being transmitted to the
falsework proper. We have noted some examples of
excellent protection in the United Kingdom where
falsework · has been constructed on either side and
over an existing public highway. These recommendations are invariably adopted in the United States and
form part of the codes of practice of several bodies
concerned with falsework construction.
The 3 % horizontal load rule
In paragraph 31 of our Interim Report we recommended that "all falsework structures should be
designed to accommodate all identifiable horizontal
forces plus an additional allowance of 1 % of the
vertical load in any horizontal direction to allow for
the unknowns. But in no case should the allowance
for the horizontal load in any direction be less than
3 % of the vertical".
In writing .the recommendations we were thinking of
the structure as a whole. Certain horizontal forces
are identifiable and can be calculated. There may be
other forces which were not foreseen and to allow
for these, and for any underestimation of identified
forces, we added an amount equal to 1 % of the total
vertical load on the falsework.
If the summation or' the estimated horizontal forces
plus 1 % of the vertical was greater than 3 % of the
vertical loads then the design was to be based on the
summation. If the summation was less than 3 % then
the design was to be based on 3 %.
Our object was to ensure that each falsework structure
was properly braced, guyed or tied back so that it
was stable against lateral and longitudinal forces. We
were aware that previously the empirical value of
2½ % was commonly used. As lateral instability has so
48
frequently been shown to be the cause of collapse we
considered it imperative to increase the figure by 20 %
resulting in the value of 3 % we recommended in our
Interim Report. The estimated value of the horizontal
load applied to any particular point of the falsework,
however, is not necessarily a fixed proportion of the
vertical load. Wind forces, for example, will be greater
on some members than on others.
Furthermore a vertical strut which is effectively pin
jointed at its ends but which is not correctly aligned
can introduce local horizontal reactions which are
quite distinct from the external loads. For example if
the strut is 1 ½ 0 out of plumb it will produce a horizontal component of force at its ends equal to 2½ % of the
vertical load which it carries.
Thus when a falsework is loaded vertically at a
multiplicity of points, each point must be interconnected with adjacent points by continuous formwork, lacing or other means so that the known and
unknown horizontal forces can be collected together.
The spacing of the collecting points should not be
greater than about four times the average spacing of
the points of vertical load application.
In any structure which consists of a large number of
elements the internal reactions caused by misalignment will tend to cancel out. The net horizontal forces
must be transferred from the collection points to the
ground or other stable point. If struts are used for
this purpose it may be necessary to make intermediate
connections to control their effective length.· Such
connections may be made to other falsework members
and so increase their resistance to buckling.
The relevance of the present BS 449 in this context is
that it requires that at any point which is to be taken
as a node when calculating the effective buckling
length of a strut, the connections restraining sideways
movement must be capable of carrying 2½ % of the
load of the strut.
It is for the British Standards Institution Committee
to codify these practices in detail. However we would
confirm our recommendation that falsework structures
must be capable of withstanding a net horizontal load
in any direction of at least 3 % of the design vertical
load, or 1 % of the vertical load plus the estimated
horizontal loads if that sum is greater than 3 %.
Lateral stability
The basic problem of ensuring the lateral stability of
falsework can be shown as a development from elementary. propositions.
