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C A S E S T U D I E S O N A P P LY I N G B E S T P R A C T IC E T O I N - S I T U C O N C R ET E F R A M E D B U I L D I N G S

Early age construction loading

Introduction

www.bca.org.uk www.stephenson-ssc.co.uk www.wyg.com www.bre.com www.dti.gov.uk www.construct.org.uk www.stgeorgeplc.com www.concretecentre.com

Figure 1: Backpropping installed at St George Wharf

Early age construction loading can be the most significantload 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 Projectat Cardington was a joint initiative aimedat improving the performance of theconcrete frame industry. It led to thepreparation of a series of Best PracticeGuides, giving recommendations forimproving the process of constructing in-situ concrete frame buildings.

As part of a programme to disseminateand apply what has been learnt fromCardington, BRE has subsequentlyworked 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), theirengineers, White Young Green,and specialist concrete contractors,Stephenson, to develop and implementprocess improvements tailored to theSt. George Wharf site.

This work has led to a series ofinnovations being trialled, the results ofwhich are summarised in this series ofBest Practice Case Studies.

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Criteria for early age loadingWork carried out on the in-situ concrete

uilding at Cardington highlighted the

otential benefits that could be achieved

y adopting revised criteria for striking

nd the design of backpropping based on

erviceability. This led to the preparation

f a Best Practice guide, Early striking and

mproved backpropping for efficient flat

lab construction.

hese 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= estimated concrete strength at time

of application of construction load

confirmed by measurement

(e.g. Lok test)

u = specified characteristic cube

strength at 28 days

he existing approaches being used by

he contractor were based on BS 5975.

hese were compared with the revised

pproaches suggested by the work

om Cardington.

indings in relation to strikinghe expectation was that adopting the

ew criteria would allow striking at

ower concretestrengths than currently

ermitted. However this was found to very

much depend on the assumptions made.

s fairly optimistic assumptions were

sed for the existing criteria, it was not

onsidered prudent to revise the existing

trengths at striking on this project.

ased on a characteristic cube strengtht 28 days of 40 N/mm 2 the minimum

trengthsrequired to be achieved for

triking were 22 N/mm2 for slab pours

without balconies and 25 N/mm2 with

alconies.

he minimum age at which striking took

lace was three days. The results of air-

ured cubes indicated that these minimum

trengths were exceeded when the slabs

were struck.

he use of Lok tests has also been invest-

gated as an alternative to air-cured cubes

or determining the striking strength.

his is the subject of a companion case

tudy. The influence of the age of striking

n deflection is covered in a further

ase study.

Findings in relation tobackproppingWhen designing backpropping the criticalissue is the assumed distribution of loadbetween the levels of supporting slabs.

The conventional approach to the designof backpropping is to assume a uniformdistribution of load between supportingslabs. The number of supporting slabsrequired is then determined by the sparecapacity1 of each of the slabs to supportthe additional weight of the next slab tobe cast.

Work from Cardington suggested that, inpractice, without the input of significantlevels of preload into the backprops, andassuming the slabs to remain essentiallyelastic, there was very little benefit inhaving more than one level of back-propping. Further, the uppermost slab ofthe supporting slabs carries approximately70% of the load during the construction ofthe slab above. This can be showntheoretically by considering the stiffnessof the different slabs in relation to theprops and the arrangements of the false-work and backprops. It was found to holdtrue over a range of different arrangementsof backprops and backprop types (steel oraluminium).

As with many other residential develop-ments,the spare capacity of the slabs atSt George Wharf (3.1 kN/m2 unfactored),is very low because of the low level ofimposed load specified (1.5 kN/m2).This creates a dilemma since the workfrom Cardington would suggest that theslab immediately beneath that being castcould theoretically be overloaded unlesssteps are taken to prevent this.

Assuming an even distribution of loadbetween the slabs in accordance withconventional approaches, the weight ofa freshly cast slab (250 mm thick giving6 kN/m2) in addition to allowance forconstruction loads meant that for mostslabs two levels of backpropping wererequired and this was the backproppingarrangement adopted. This ignored anyreduction in the load factor appropriateto the loading.

In the case of the 15th floor transfer slab,which was 600 mm thick with a self-weight of 14.4 kN/m2, three levels ofbackpropping were employed. Such atransfer slab is often required if thecolumn grid layout changes above thelevel in question.

Although the contractor was not givenany instruction to preload the backpropsand did not present any calculationsmaking any assumption about this,it was found in practice that the levelsof preload measured in individualbackprops were high [3] and were suchthat, with one level of backpropping,the uppermost slab was not necessarilypredicted to be the most critically loaded.

In the context of the 15th floor transferslab it would not have been possible tojustify the construction of this slab usingthe new criteria unless the beneficialeffects of preload are taken into account.This again creates a problem since thepermanent works designer would befaced with specifying a level of preloadin the backprops that the contractor

would not be able to control or verify.In practice such problems can beovercome only by both the designer andcontractor taking a pragmatic approach[1].

Quality control on site concerning thetype, positioning, sequencing ofplacement and removal and tighteningof backprops is important and didnot always appear to be exercised atSt George Wharf in accordance withthe calculations presented. Because ofthe possibility of significant preloadbeing introduced into the backpropping,it is advisable to make allowance for thisin any assumptions made about theloads carried by the props themselves.

Interpretation of existingBest Practice guidanceconcerning backpropping1. The assumptions made in Table 1,

which comes from the Best PracticeGuide, were shown not to representtypical site practice, in particular

arranging for the backprops to befinger-tight and not relying on anypre-load in them. Where it proves

difficult to justify increasingthe load bearing capacity of thesupporting slab by following therecommendations given in the BestPractice Guide, consideration could begiven to applying appropriate levels ofpre-load in the backprops. This wouldjustify the assumption of a moreeven distribution of load betweensupporting slabs, as in conventionalapproaches. However, if this weremore than a nominal amount it wouldinvolve specification of a defined levelof preload in individual backpropsthat would be very difficult to controlin practice.

2. The Guide to flat slab formwork andfalsework [1] includes a CD with an

interactive Excel spreadsheet,illustrated in Figure 2 with sampledata. This allows the influence ofcracking of the slabs and the effectsof pre-load to be taken account of incalculations for up to two levels ofbackpropping. There is evidence tosuggest that there may be merit inextending the scope of the spread-sheet to allow additional levels ofbackpropping. However, as statedabove, the level of pre-load mightprove very difficult to control inpractice, especially for multiplefloors of backpropping.

3. The issue of the design of the back-

propping will be most acute forsituations where low imposed loadsare specified, such as in car parks and

residential developments, because ofthe limited spare capacity of the slabs.Exceeding the design service load of

the slabs by a small margin will not bea safety issue, but could have some

impact on serviceability performance.

The Permanent Works Designer shouldtherefore be involved in any decisions

to theoretically overload slabs andshould consider possible implicationsfor 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.

1Spare capacity is defined as the available

service load capacity less the self-weight

Figure 2: Backpropping spreadsheet

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Notes1. Assumes lower and supporting floors have been struck, have taken up their

deflected shape and are carrying their self-weight2. Floor l oading from imposed loads and self-weight is not considered

3. The strength of particular slabs to carry applied loads will have to be consideredseparately

4. All floors are suspended floors

NO ONE LEVELOF TWO LEVELS OFLOCATION LOAD BACK-

BACKPROPS BACKPROPSPROPS

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

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C A S E S T U D I E S O N A P P LY I N G B E S T P R A C T IC E T O I N - S I T U C O N C RE T E F R A M E D B U I L D I N G S

Use of the spreadsheetIn the spreadsheet the user may set avalue for the level of preload in individualbackprops. The default value is 6kN perbackprop [1], which is believed to becommonly achieved in practice.

Measurements of the preload in individualbackprops at St George Wharf variedconsiderably, but averaged 13 kN per prop.

From the point of view of the uppermostsupporting slab, it should be recognisedthat the relieving preload (as measuredin kN/m2) is dependent not only on thepreload in each backprop but also onthe number of backprops.

As an illustration of this, the level ofpreload chosen in each backprop fortrial use of the spreadsheet forSt George Wharf was 6kN per prop.

This gave a preload in kN/m2 similarto that actually measured.

The Best Practice Guide recommends theinstallation of backprops at the earliestopportunity to assist in the distributionof load between the supporting slabs.In many cases, as with St George Wharf,flying form systems are used which,in practice, usually means that theuppermost slab carries all of the weightof the falsework. The spreadsheetallows this loading to be specified(usually 0.5 kN/m2) and automatically

takes this into account when calculatingthe overall distribution of the loadbetween the slabs.

An average backprop stiffness of23 kN/mm was used, based on measuredaverage values for different types of props.Parameters were chosen to allow theinfluence of cracking on the slab tobe taken into account. These were basedon past experience. These parametersresulted in an equivalent reduced modulusof elasticity for a given concrete strength,but these calculated values could havebeen overridden if desired.

The number and location of the falseworksupports and backprops was specified onthe basis of the calculations presentedfor the project. As can be seen from theresults for this example, which is for anedge 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 closeto the equal thirds split suggested byconventional approaches.

In this example the results indicate that

the slab immediately beneath that beingcast is subject to a construction load verymarginally in excess of the designservice load.

The spreadsheet allows some interpolationbetween the two criteria set out inEquations 1 and 2 by virtue of an

Feff factor. This factor has been introducedin recognition that Equation 2 is notrelevant if the slab is uncracked. Equation2 was introduced to limit excessive strainsin the reinforcement. This is explainedfurther in Reference 1.

Conclusions1. The distribution of loads for the

supporting slabs at St George Wharfwas found to be close to that predictedby conventional approaches assumingan even distribution of load betweenslabs once the effect of preload wastaken into account. However preloadingof the props was not achieved in acontrolled manner and in practicewould be very difficult to do. This isemphasised by the variationsin prop loads measured.

2. If heavily tightened, loads measuredin individual backprops can besignificant, although variable, andaveraged 13 kN for the propsinstrumented at St George Wharf.This is believed to be higher than thelevels of preload generally measuredat Cardington. However the overalllevel of preloading achieved at StGeorge Wharf (estimated as 1 kN/m2)is not believed to be exceptional.

3. Although slabs may be predicted to be

overloaded, they may very well not beso in practice because of the marginson the actual construction loadallowed for.

4. Additional margins may be requiredfor the design of the backpropsthemselves to allow for unintentionalpreload induced in them duringinstallation and as a result ofsubsequent temperature changes.

5 . To achieve a controlled approach toearly age loading the most reliablemethod would be to follow the

existing guidance given in the BestPractice Guide, but the penalty of thisapproach is that the slabs will needto be designed for a higher loadingduring construction.

The work undertaken and the conclusionsreached in relation to the innovationsdescribed above should be viewed in thecontext of the particular project on whichthe innovations have been trialled.

This Case Study is underpinned by fullreports [2, 3] giving the background and

further information on the innovations.

References1. Guide to flat slab formwork and

falsework, by Eur Eng P. F. Pallett.Published by The Concrete Society onbehalf of Construct. Ref. CS 140, 2003.

2. Best practice in concrete frameconstruction: practical application atSt George Wharf, by R. Moss. REReport BR462, 2003.

3. Backprop forces and deflections in flatslabs: construction at St George Wharf,

by R. Vollum. BRE Report BR463, 2004.

AcknowledgementsThe support of the DTI for this projectcarried out under its Partners inInnovation scheme is gratefullyacknowledged.

The Best Practice Guide, Early strikingand improved backpropping for efficientflat slab construction, summarises workcarried out on these topics during the

construction of the in-situ concretebuilding 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 acompanion guide Early age strengthassessment of concrete on site.

Case Studies in this series of applyingbest practice:

St George Wharf project overview

Early age concrete strengthassessment

Early age construction loading

Reinforcement rationalisation andsupply

Slab deflections

Special concretes

Ref TCC/03/3First published 2004Price group AISBN 1-904818-04-8

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