Flat Slabs Research

Jenny BurridgeHead of Structural Engineering

MPA The Concrete Centre

• Set up in 2003• “to enable designers to

realise the full potentialof concrete”

eCO2 Study

• Cost Model Study• Commercial

• Hospital

• School

• Twenty designs

• Cradle to Gate

• Full building except M&E

eCO2 Study

Study Method

Results

Flat Slab

Flat Slabs…• Thin structure• Flat soffits• No beams

• 80% of concrete framedbuildings are flat slabs

• 4 million m2 built eachyear in UK

Flat Slab

• Sized for punchingshear and deflection

• Additionalreinforcement addedto span steel toreduce deflections

• Typically 25% extraon a 7.5m x 7.5mgrid with 250mmdepth

• 5kg/m2 or 4.4kgeCO2 /m2

18 000 tonnes CO2 per year

Deflection

Knowledge of deflections in real slabs limited• Cardington

• St George’s Wharf

Typically design for deflection is by span to effectivedepth ratio

6.323.418

CantileverMultiple SpanSimply Supported

BS 8110

Deflection – EC2Basic span to depth ratios

12

14

16

18

20

22

24

26

28

30

32

34

36

0.30% 0.80% 1.30% 1.80%Percentage of tension reinforcement (As/bd)

Span

to d

epth

ratio

(l/d

)

fck = 20fck = 25fck = 28fck = 30fck = 32fck = 35fck = 40fck = 45fck = 50

Deflection Limits

Knowledge of deflectionlimits limited

Typical limit (both BS8110and EC2):

• Span/250 for alldeflections

• Span/500 for deflectionsoccurring afterconstruction

(from work done in 1950s.)

Research Questions

• How much do flat slabsdeflect?

• Membrane action ininternal slabs?

• What limit is acceptable?• Is additional

reinforcement required?• Where is additional

reinforcement required?

Punching Shear

• Can govern the depth of aflat slab

• Further research requiredinto the mechanism ofpunching shear

• Punching shearreinforcement in flat slabsincreased by introductionof EC2 (in comparisonwith BS8110)

• Furtherreinforcement/depthlikely in next iteration ofEC2