CASE STUDI ES ON APPLYI NG BEST PRACTI CE TO I N- SI TU CONCRETE FRAMED BUI LDI NGS

Early age construction loading

Introduction
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Figure 1: Backpropping installed at St George Wharf
Early age construction loading can be the most significant
load experienced by multi-storey structures.
Key points
This Case Study looks at the experiences of applying new criteria for
the striking of slabs and the design of backpropping.
In residential developments the spare load bearing capacity of slabs used
in determining backpropping requirements is very low because of the low
level of imposed load specified. This is important because such spare
capacity needs to be available to support freshly cast concrete.
It was found in practice that the levels of preload measured in individual
backprops were high.
When the effect of preload was taken into account, the distribution of
loads for the supporting slabs was found to be close to that predicted
by conventional approaches assuming an even distribution of load
between slabs.
Quality control concerning the type, positioning, sequencing of placement
and removal and tightening of backprops is important and does not
always appear to be exercised.
The temporary works designer and permanent works designer should
work together to assess whether a higher design load should be used to
cater for the construction load conditions.
St George Wharf Case Study
The European Concrete Building Project
at Cardington was a joint initiative aimed
at improving the performance of the
concrete frame industry. It led to the
preparation of a series of Best Practice
Guides, giving recommendations for
improving the process of constructing in-
situ concrete frame buildings.
As part of a programme to disseminate
and apply what has been learnt from
Cardington, BRE has subsequently
worked directly with those involved in St
George Wharf, a high-profile, mixed-use,
phased project on the River Thames.
BRE worked jointly with the developers,
St. George (South London), their
engineers, White Young Green,
and specialist concrete contractors,
Stephenson, to develop and implement
process improvements tailored to the
St. George Wharf site.
This work has led to a series of
innovations being trialled, the results of
which are summarised in this series of
Best Practice Case Studies.
1
Criteria for early age loading
Work carried out on the in-situ concrete
building at Cardington highlighted the
potential benefits that could be achieved
by adopting revised criteria for striking
and the design of backpropping based on
serviceability. This led to the preparation
of a Best Practice guide, Early striking and
improved backpropping for efficient flat
slab construction.
These new criteria can be summarised as:
- (w/wser)(fcu/fc)
0.6
1.0 Equation 1
- (w/wser) 1.0 Equation 2
Where
w = construction load
wser = design service load
fc = estimated concrete strength at time
of application of construction load
confirmed by measurement
(e.g. Lok test)
fcu = specified characteristic cube
strength at 28 days
The existing approaches being used by
the contractor were based on BS 5975.
These were compared with the revised
approaches suggested by the work
from Cardington.
Findings in relation to striking
The expectation was that adopting the
new criteria would allow striking at
lower concrete strengths than currently
permitted. However this was found to very
much depend on the assumptions made.
As fairly optimistic assumptions were
used for the existing criteria, it was not
considered prudent to revise the existing
strengths at striking on this project.
Based on a characteristic cube strength
at 28 days of 40 N/mm
2
the minimum
strengths required to be achieved for
striking were 22 N/mm
2
for slab pours
without balconies and 25 N/mm
2
with
balconies.
The minimum age at which striking took
place was three days. The results of air-
cured cubes indicated that these minimum
strengths were exceeded when the slabs
were struck.
The use of Lok tests has also been invest-
igated as an alternative to air-cured cubes
for determining the striking strength.
This is the subject of a companion case
study. The influence of the age of striking
on deflection is covered in a further
case study.
Findings in relation to
backpropping
When designing backpropping the critical
issue is the assumed distribution of load
between the levels of supporting slabs.
The conventional approach to the design
of backpropping is to assume a uniform
distribution of load between supporting
slabs. The number of supporting slabs
required is then determined by the spare
capacity
1
of each of the slabs to support
the additional weight of the next slab to
be cast.
Work from Cardington suggested that, in
practice, without the input of significant
levels of preload into the backprops, and
assuming the slabs to remain essentially
elastic, there was very little benefit in
having more than one level of back-
propping. Further, the uppermost slab of
the supporting slabs carries approximately
70% of the load during the construction of
the slab above. This can be shown
theoretically by considering the stiffness
of the different slabs in relation to the
props and the arrangements of the false-
work and backprops. It was found to hold
true over a range of different arrangements
of backprops and backprop types (steel or
aluminium).
As with many other residential develop-
ments, the spare capacity of the slabs at
St George Wharf (3.1 kN/m
2
unfactored),
is very low because of the low level of
imposed load specified (1.5 kN/m
2
).
This creates a dilemma since the work
from Cardington would suggest that the
slab immediately beneath that being cast
could theoretically be overloaded unless
steps are taken to prevent this.
Assuming an even distribution of load
between the slabs in accordance with
conventional approaches, the weight of
a freshly cast slab (250 mm thick giving
6 kN/m
2
) in addition to allowance for
construction loads meant that for most
slabs two levels of backpropping were
required and this was the backpropping
arrangement adopted. This ignored any
reduction in the load factor appropriate
to the loading.
In the case of the 15th floor transfer slab,
which was 600 mm thick with a self-
weight of 14.4 kN/m
2
, three levels of
backpropping were employed. Such a
transfer slab is often required if the
column grid layout changes above the
level in question.
Although the contractor was not given
any instruction to preload the backprops
and did not present any calculations
making any assumption about this,
it was found in practice that the levels
of preload measured in individual
backprops were high [3] and were such
that, with one level of backpropping,
the uppermost slab was not necessarily
predicted to be the most critically loaded.
In the context of the 15th floor transfer
slab it would not have been possible to
justify the construction of this slab using
the new criteria unless the beneficial
effects of preload are taken into account.
This again creates a problem since the
permanent works designer would be
faced with specifying a level of preload
in the backprops that the contractor
would not be able to control or verify.
In practice such problems can be
overcome only by both the designer and
contractor taking a pragmatic approach[1].
Quality control on site concerning the
type, positioning, sequencing of
placement and removal and tightening
of backprops is important and did
not always appear to be exercised at
St George Wharf in accordance with
the calculations presented. Because of
the possibility of significant preload
being introduced into the backpropping,
it is advisable to make allowance for this
in any assumptions made about the
loads carried by the props themselves.
Interpretation of existing
Best Practice guidance
concerning backpropping
1. The assumptions made in Table 1,
which comes from the Best Practice
Guide, were shown not to represent
typical site practice, in particular
arranging for the backprops to be
finger-tight and not relying on any
pre-load in them. Where it proves
difficult to justify increasing
the load bearing capacity of the
supporting slab by following the
recommendations given in the Best
Practice Guide, consideration could be
given to applying appropriate levels of
pre-load in the backprops. This would
justify the assumption of a more
even distribution of load between
supporting slabs, as in conventional
approaches. However, if this were
more than a nominal amount it would
involve specification of a defined level
of preload in individual backprops
that would be very difficult to control
in practice.
2. The Guide to flat slab formwork and
falsework [1] includes a CD with an
interactive Excel spreadsheet,
illustrated in Figure 2 with sample
data. This allows the influence of
cracking of the slabs and the effects
of pre-load to be taken account of in
calculations for up to two levels of
backpropping. There is evidence to
suggest that there may be merit in
extending the scope of the spread-
sheet to allow additional levels of
backpropping. However, as stated
above, the level of pre-load might
prove very difficult to control in
practice, especially for multiple
floors of backpropping.
3. The issue of the design of the back-
propping will be most acute for
situations where low imposed loads
are specified, such as in car parks and
residential developments, because of
the limited spare capacity of the slabs.
Exceeding the design service load of
the slabs by a small margin will not be
a safety issue, but could have some
impact on serviceability performance.
The Permanent Works Designer should
therefore be involved in any decisions
to theoretically overload slabs and
should consider possible implications
for serviceability.
4. If the developer is
closely involved in
the design and
construction process,
as is the case with
St George, they can
perhaps take a more
informed decision as
to the relative merits
of accepting a higher
design load to cater
for the construction
load conditions.
1
Spare capacity is defined as the available
service load capacity less the self-weight
Figure 2: Backpropping spreadsheet
2 3
Notes
1. Assumes lower and supporting floors have been struck, have taken up their
deflected shape and are carrying their self-weight
2. Floor loading from imposed loads and self-weight is not considered
3. The strength of particular slabs to carry applied loads will have to be considered
separately
4. All floors are suspended floors
NO
ONE LEVEL OF TWO LEVELS OF
LOCATION LOAD BACK-
BACKPROPS BACKPROPS
PROPS
On slab On slab In props On slab In props
New slab being cast total 100% 100% 100%
Falsework/formwork wp 100% 100% 100%
On supporting slab(1) 100%wp 70%wp 65%wp
In backprops wb1 30%wp 35%wp
On lower slab (2) 30%wp 23%wp
In backprops wb2 12%wp
On lower slab (3) 12%wp
Table 1: Load distribution by backpropping
CASE STUDI ES ON APPLYI NG BEST PRACTI CE TO I N- SI TU CONCRETE FRAMED BUI LDI NGS
Use of the spreadsheet
In the spreadsheet the user may set a
value for the level of preload in individual
backprops. The default value is 6kN per
backprop [1], which is believed to be
commonly achieved in practice.
Measurements of the preload in individual
backprops at St George Wharf varied
considerably, but averaged 13 kN per prop.
From the point of view of the uppermost
supporting slab, it should be recognised
that the relieving preload (as measured
in kN/m
2
) is dependent not only on the
preload in each backprop but also on
the number of backprops.
As an illustration of this, the level of
preload chosen in each backprop for
trial use of the spreadsheet for
St George Wharf was 6kN per prop.
This gave a preload in kN/m
2
similar
to that actually measured.
The Best Practice Guide recommends the
installation of backprops at the earliest
opportunity to assist in the distribution
of load between the supporting slabs.
In many cases, as with St George Wharf,
flying form systems are used which,
in practice, usually means that the
uppermost slab carries all of the weight
of the falsework. The spreadsheet
allows this loading to be specified
(usually 0.5 kN/m
2
) and automatically
takes this into account when calculating
the overall distribution of the load
between the slabs.
An average backprop stiffness of
23 kN/mm was used, based on measured
average values for different types of props.
Parameters were chosen to allow the
influence of cracking on the slab to
be taken into account. These were based
on past experience. These parameters
resulted in an equivalent reduced modulus
of elasticity for a given concrete strength,
but these calculated values could have
been overridden if desired.
The number and location of the falsework
supports and backprops was specified on
the basis of the calculations presented
for the project. As can be seen from the
results for this example, which is for an
edge panel with two levels of backpropping,
the distribution of load between the slabs,
taking account of the preload in the back-
props, was predicted to be fairly close
to the equal thirds split suggested by
conventional approaches.
In this example the results indicate that
the slab immediately beneath that being
cast is subject to a construction load very
marginally in excess of the design
service load.
The spreadsheet allows some interpolation
between the two criteria set out in
Equations 1 and 2 by virtue of an
F
eff
factor. This factor has been introduced
in recognition that Equation 2 is not
relevant if the slab is uncracked. Equation
2 was introduced to limit excessive strains
in the reinforcement. This is explained
further in Reference 1.
Conclusions
1. The distribution of loads for the
supporting slabs at St George Wharf
was found to be close to that predicted
by conventional approaches assuming
an even distribution of load between
slabs once the effect of preload was
taken into account. However preloading
of the props was not achieved in a
controlled manner and in practice
would be very difficult to do. This is
emphasised by the variations
in prop loads measured.
2. If heavily tightened, loads measured
in individual backprops can be
significant, although variable, and
averaged 13 kN for the props
instrumented at St George Wharf.
This is believed to be higher than the
levels of preload generally measured
at Cardington. However the overall
level of preloading achieved at St
George Wharf (estimated as 1 kN/m
2
)
is not believed to be exceptional.
3. Although slabs may be predicted to be
overloaded, they may very well not be
so in practice because of the margins
on the actual construction load
allowed for.
4. Additional margins may be required
for the design of the backprops
themselves to allow for unintentional
preload induced in them during
installation and as a result of
subsequent temperature changes.
5 . To achieve a controlled approach to
early age loading the most reliable
method would be to follow the
existing guidance given in the Best
Practice Guide, but the penalty of this
approach is that the slabs will need
to be designed for a higher loading
during construction.
The work undertaken and the conclusions
reached in relation to the innovations
described above should be viewed in the
context of the particular project on which
the innovations have been trialled.
This Case Study is underpinned by full
reports [2, 3] giving the background and
further information on the innovations.
References
1. Guide to flat slab formwork and
falsework, by Eur Eng P. F. Pallett.
Published by The Concrete Society on
behalf of Construct. Ref. CS 140, 2003.
2. Best practice in concrete frame
construction: practical application at
St George Wharf, by R. Moss. RE
Report BR462, 2003.
3. Backprop forces and deflections in flat
slabs: construction at St George Wharf,
by R. Vollum. BRE Report BR463, 2004.
Acknowledgements
The support of the DTI for this project
carried out under its Partners in
Innovation scheme is gratefully
acknowledged.
The Best Practice Guide, Early striking
and improved backpropping for efficient
flat slab construction, summarises work
carried out on these topics during the
construction of the in-situ concrete
building at Cardington.
This can be downloaded free at www.rcc-
info.org.uk/pdf/Early_Striking_4pp_WEB.
PDF and at http://projects.bre.co.uk/
ConDiv/concrete%20frame/default.htm.
It should be read in conjunction with a
companion guide Early age strength
assessment of concrete on site.
Case Studies in this series of applying
best practice:
St George Wharf project overview
Early age concrete strength
assessment
Early age construction loading
Reinforcement rationalisation and
supply
Slab deflections
Special concretes
Ref TCC/03/3
First published 2004
Price group A
ISBN 1-904818-04-8
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4