final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 27
(c) not realising that the results of ground investigations done for the permanent structure may not be
reliable guides to the nature of the ground on
which the falsework will rely for support
Founding falsework on an inclined surface such as the
side of an embankment raises particular problems. (d) not allowin/for the changing conditions caused
We were shown examples where the restraints against
by weather, diversion of surface water, or flooding
the base plates slipping down the slope were clearly
and effects of flood water, erosion, freezing and
inadequate. Even minor settlement can produce
thawing, or the effects of site traffic etc
disruption in a load bearing falsework.
(e) not providing adequate restraints for bases to
falsework founded on inclined surfaces such as
Timbers, often old railway sleepers, which are used
bridge embankments
to spread the loads from the bases of scaffold columns
(f) not anticipating effects of differential movement
or proprietary struts, may be short and worn. If the
and settlement
soleplates are resting on the edges and ends of the
sleepers, the load will not be properly spread. We were (g) using materials of unsatisfactory quality, e.g, in
the foundations, scaffold boards, etc
shown examples of standards which were not ade(h)
not placing sole plates horizontally or otherquately supported - in an extreme case one was
wise
bedding them properly
resting on the edge of a pile of bricks. An y base material containing silt or fine sand which may permit (i) placing structural supports at the edges of timber
sole plates, or in some instances not even providing
movement after wetting is potentially unreliable, and
sole plates
any badly compacted material will cause trouble.
U) not providing adequate base supports above
It is difficult to stabilise sleepers against some overground.
turning movement - grillages which have intermittent (k) not resisting the temptation to use at random
lower beams improperly dug into the ground may
the material on site, for example soft wood under
introduce bending stresses in the upper layer of cross
the falsework legs
sleepers. Close boarded grillage mats with sleepers
in both directions are costly. Crawler mats with close (1) making modifications on site to the falsework
foundations to cater for changed conditions without
sleepers in one direction must be stiffened by cross
prior consultation with the designer or supervisor
timbers and very evenly founded or prepared.
to ensure that the modifications are sound.
sion of surface water, freezing and thawing, or the
provision of an access road.
It is evident that the same care should be taken over
the base supports as over all other parts of the falsework, but that this is not always done.
In some cases timber foundations should not be used.
Concrete into which timber can be intimately bedded
results in a better spreading of the load because it can
be cast wider and thicker and be dug into the formation to make perfect contact with it.
Although very few of the collapses studied could be
attributed directly to a foundation failure, emergency
action was needed in some instances to prevent an
accident. The evidence received indicated a definite
need for greater thought to avoid a number of pitfalls,
including:
(a) not taking the trouble to find out the true nature
of the ground or the formation on which the falsework is to be constructed
(b) not informing the designer of pertinent data and
conditions, e.g, proximity of deep excavations, need
for site access lanes between the falsework supports
and nature of traffic to be expected thereon; ground
water table and possibility of surface water during
construction
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In contrast the trouble-free results achieved in other
jobs on which the falsework foundations were treated
throughout with exemplary care, highlight the wisdom
of foreseeing and avoiding such pitfalls.
We appreciate that it would be impracticable to expect
all these items to receive full treatment on every construction project. But items such as (g) to (k) should
always be considered even on the smallest of building
jobs to avoid unnecessary risk to life and limb. Our
impression is that the primary reason for the bad
cases is ignorance of the possible consequences of
failure and not deliberate skimping.
Support of beams
Many of the examples we studied suggested that the
behaviour of beams under load had not been properly
understood and allowed for.
One common fault in smaller jobs was the inadequate
bearing surface provided for the supports. In some
cases the beam was not long enough to extend over
the full length of the forkhead. In some the forkhead
