17-5-2011
Challenge the future
DelftUniversity ofTechnology
Shear capacity of Reinforced Concrete Slabsexperimental study
Eva Lantsoght
2Shear in reinforced concrete slabs – wheel loads close to support
Overview
• Background• Project description
• Current practice
• Experiments• Results and discussion
• Loading history
• Distance to support
• Concrete compressive strength
• Preliminary conclusions
3Shear in reinforced concrete slabs – wheel loads close to support
Key message
Slabs under wheel loads behave
differently in shear than beams
4Shear in reinforced concrete slabs – wheel loads close to support
Background
Project description (1)
• Capacity of existing bridges in NL• TU Delft
• Concrete Structures
• Structural Mechanics
• TNO
• RWS
• 3715 relevant structures• 2020 built before 1976
• Study: bridge categories and specific details
Highways in the Netherlands
5Shear in reinforced concrete slabs – wheel loads close to support
Background
Project description (2)
• Concrete Structures• Long-term tensile strength
• Beam shear – sustained loads
• Continuous girders – shear
• Prestressed slabs – punching + CMA
• Slab bridges - shear/punching
Concrete bridges
6Shear in reinforced concrete slabs – wheel loads close to support
Background
Project description (3)
Shear failure of the de la Concorde bridge, Laval
The Netherlands: 60% of bridges built before 1975
Traffic volume and loads have increased
7Shear in reinforced concrete slabs – wheel loads close to support
Background
Project description (4)
• Wheel loads: tire contact area + loading
• Tandem loads for local verification
• Eurocode Tire contact area: 400mm x 400mm
• Larger than physical contact area
Tandem loads, EC2
8Shear in reinforced concrete slabs – wheel loads close to support
Background
Project description (5)
• Wheel loads: tire contact area + loading
• Values for load model 1
Load model 1
9Shear in reinforced concrete slabs – wheel loads close to support
Background
Project description (6)
Shear span to depth ratioLoad spreading towards the support
Influence of the support
10Shear in reinforced concrete slabs – wheel loads close to support
Background
Current practice
• Design: shear capacity of slabs• Flexural failure before shear failure
• Punching shear formulas
• Beam shear formulas over effective width
Beam shear, one-way shear Punching shear, two-way shear
11Shear in reinforced concrete slabs – wheel loads close to support
Goals
• Assess shear capacity of slabs
under concentrated loads
• Determine effective width in
shear
12Shear in reinforced concrete slabs – wheel loads close to support
Experiments
Test setup
Size: 5m x 2,5m x 0,3m
13Shear in reinforced concrete slabs – wheel loads close to support
Experiments
Test setup
Continuous support, Line supports
14Shear in reinforced concrete slabs – wheel loads close to support
Experiments
Test setup
Load: vary a/d and position along width
15Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Loading history
• Experiments:• Lower bound for cracked bridges
• Loading in vicinity of failure
• Influence of local failure
• Connecting existing cracks
• Opening existing cracks
• +/- 84% of peak load undamaged
specimenS2T1 cracks
Slabs with large cracks:
still 84% of uncracked load is carried!
16Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Distance to support (1)
beams: influence of archingslabs: decrease due to smaller effective width
17Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Distance to support (1)
beams: influence of archingslabs: decrease due to smaller effective width
Lower bound: 2d
Influence of the distance to the support on the shear capacity of slabs?
18Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Distance to support (2)
Influence of distance to support on measured peak load
19Shear in reinforced concrete slabs – wheel loads close to support
• Smaller increase than expected from EC2
• Reasons:• Cracking behavior
• Possible paths for strut
• Larger effective a/d ratio
Results and discussion
Distance to support (3)
Different behavior for slabs and beams!
20Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Concrete compressive strength (1)
( )1/ 3
, , 1 min 1100 ( )Rd c Rd c l ck cp w cp wV C k f k b d v k b dρ σ σ= + ≥ +
'1
6c c wV f b dλ=
• fc’ as parameter in code formulas
• Shear strength related to tensile strength
Test slabs with normal strength and high strength concrete
Eurocode
ACI code
21Shear in reinforced concrete slabs – wheel loads close to support
0
200
400
600
800
1000
1200
1400
1600
0 10 20 30 40 50 60 70 80 90
fc' (MPa)
Pu
(kN
)
S3
S4
S7
S8
S2
Results and discussion
Concrete compressive strength (2)
Influence of concrete compressive strength on measured ultimate load
22Shear in reinforced concrete slabs – wheel loads close to support
Preliminary conclusions
• Locally failed decks • 84% of peak load
• Redistribution capacity of slabs
• Distance to support• Smaller influence than for
beams
• Suggest different behavior
• Concrete compressive strength• No measured influence
S4T2 Dominant shear crack
23Shear in reinforced concrete slabs – wheel loads close to support
Key message
Slabs under wheel loads behave
differently in shear than beams
24Shear in reinforced concrete slabs – wheel loads close to support
Contact:
Eva Lantsoght
+31(0)152787449