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Experimental study of shear in reinforced concrete one way slabs subjected to concentrated loads
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14-6-2011
Challenge the future
DelftUniversity ofTechnology
Shear in One-Way SlabsSubjected to Concentrated Loads
Eva Lantsoght, Dr. Cor van der Veen, Prof. Joost Walraven
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2Shear in reinforced concrete slabs – wheel loads close to support
Overview
• Background• Design practice
• Previous research
• Experiments• Results and discussion
• Shear span to depth ratio
• Comparison to EN 1992-1-1 and Regan’s method
• Conclusions
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3Shear in reinforced concrete slabs – wheel loads close to support
Background
Design practice (1)
• 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
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4Shear in reinforced concrete slabs – wheel loads close to support
Background
Design practice (2)
• One-way shear: beam shear formulas
• Code formulas: empirical
• Most experiments:• Beams
• Heavily reinforced
• Slender (a/d ≥ 2,5)
• Small size
• Concentrated loadamount of shear experiments done
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5Shear in reinforced concrete slabs – wheel loads close to support
Background
Design practice (3)
45° load spreading 45° load spreading – French practice
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6Shear in reinforced concrete slabs – wheel loads close to support
Background
Design practice (4)
• Effective width• Assume uniform stress
• Maximum stress over effective
width
• Load spreading 45° for design
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7Shear in reinforced concrete slabs – wheel loads close to support
Background
Design practice (5)
• Lower bound: 2d
• Loads closer to support:• Smaller beff
• Smaller Vult
• In beams: direct load transfer• Larger Vult
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8Shear in reinforced concrete slabs – wheel loads close to support
Background
Previous research
• Limited amount of experimental data available• Regan (1982):
• Tests at simple and continuous support
• Small slabs (1,6m x 1,2m x 0,1m): size effect?
• Conclusion:
• Increase in shear capacity with decrease in shear span
• More shear capacity at continuous support
• Small slabs: tests on larger scale needed
• Formula of Regan
• Subdivision of perimeter
• Different contributions to ultimate load
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9Shear in reinforced concrete slabs – wheel loads close to support
Goals
• Assess shear capacity of slabs
under concentrated loads
• Determine effective width in
shear
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10Shear in reinforced concrete slabs – wheel loads close to support
Experiments
Test setup
Size: 5m x 2,5m x 0,3m
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11Shear in reinforced concrete slabs – wheel loads close to support
Experiments
Test setup
Continuous support, Line supports
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12Shear in reinforced concrete slabs – wheel loads close to support
Experiments
Test setup
Load: vary a/d and position along width
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13Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Shear span to depth ratio (1)
• Decrease: smaller effective width
• Increase due to direct load transfer
• EC2: β = av/2d for 0,5d ≤ av ≤ 2d
• S3/S4 to S5/S6: capacity 2x
• β in MC 2010
Influence of the distance to the support
on the shear capacity of slabs?
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14Shear 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
•Smaller increase than expected from EC2
•Different behavior for beams and slabs
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15Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Comparison to EC2 and Regan (1)
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16Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Comparison to EC2 and Regan (2)
• Eurocode: underestimates capacity slabs
• French national annex:• Higher strength for slabs under point load (redistribution)
• Better estimate
• Systematically low predictions for high strength concrete
• Regan’s formula:• Developed for shear in slabs under point load
• Based on punching shear perimeter
• Best prediction
Shear in slabs is not a fully one-way shear failure, a certain amount of two-way shear distribution is possible
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17Shear in reinforced concrete slabs – wheel loads close to support
Conclusions (1)
• Shear span to depth ratio• Clear influence on capacity
• Smaller influence than for
beams
• Suggest different behavior
• French National Annex• Better results
• Unsafe for high strength
concrete
• Regan: Best design tool S4T2 Dominant shear crack
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18Shear in reinforced concrete slabs – wheel loads close to support
Conclusions (2)
• Recommendations:• Moment distribution at support
• Higher minimum shear stress
for slabs
• Concentrated loads
• Transverse redistribution
• Direct load transfer
• Different behavior slabs vs.
beams
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19Shear in reinforced concrete slabs – wheel loads close to support
Key message
Slabs under concentrated
loads behave differently in
shear than beams
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20Shear in reinforced concrete slabs – wheel loads close to support
Contact:
Eva Lantsoght
+31(0)152787449
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21Shear in reinforced concrete slabs – wheel loads close to support
Experiments
Specimens
Slab fc’ (MPa)
fct (MPa)
ρl (%)
ρt (%)
a/d M/S Aload (mm x mm)
S1 35,8 3,1 0,996 0,132 2,26 M 200 x 200
S2 34,5 2,9 0,996 0,132 2,26 M 300 x 300
S3 51,6 4,1 0,996 0,258 2,26 M 300 x 300
S4 51,7 4,2 0,996 0,182 2,26 S 300 x 300
S5 48,2 3,8 0,996 0,258 1,51 M 300 x 300
S6 50,6 3,9 0,996 0,258 1,51 S 300 x 300
S7 82,1 6,2 0,996 0,258 2,26 S 300 x 300
S8 77,0 6,0 0,996 0,258 2,26 M 300 x 300
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22Shear in reinforced concrete slabs – wheel loads close to support
Experiments
Test results
Wide beam shear1356CSS8T2
Wide beam shear1481SSS8T1
Wide beam shear + Punching shear1063SSS7T5
Wide beam shear + Punching shear1136CSS7T3
Wide beam shear + Punching shear1172CSS7T2
Wide beam shear + Punching shear1121SSS7T1
Wide beam shear1347SSS6T5
Wide beam shear1366SSS6T4
Wide beam shear1423CSS6T2
Wide beam shear1446CSS6T1
Wide beam shear1755SSS5T4
Wide beam shear1804CSS5T1
Failure ModePu (kN)SS/CSName
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23Shear in reinforced concrete slabs – wheel loads close to support
Results and discussion
Comparison to EC2 and Regan
0,0980,1051,0670,1570,1611,0240,1230,2401,952S1 – S8, CS
0,0980,1151,1780,1290,1311,0140,1000,1931,933S1 – S8, SS
0,1080,1221,1280,1380,1411,0180,1080,2101,941S1 – S8
COVSTDAVGCOVSTDAVGCOVSTDAVG
Pu/PReganPu/VRdcminsPu/VRdcTest data