Experimental study of shear in reinforced concrete one way slabs subjected to concentrated loads

Shear in One-Way Slabs
Subjected to Concentrated Loads
Eva Lantsoght, Dr. Cor van der Veen, Prof. Joost Walraven
14-6-2011

Delft
University of
Technology

Challenge the future

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

Shear in reinforced concrete slabs – wheel loads close to support 2

Background
Design practice (1)

Beam shear, one-way shear Punching shear, two-way shear

• Design: shear capacity of slabs
• Flexural failure before shear failure
• Punching shear formulas
• Beam shear formulas over effective width


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 load
amount of shear experiments done


Background
Design practice (3)

45° load spreading 45° load spreading – French practice


Background
Design practice (4)

• Effective width
• Assume uniform stress
• Maximum stress over effective
width

• Load spreading 45° for design


Background
Design practice (5)

• Lower bound: 2d

• Loads closer to support:
• Smaller beff
• Smaller Vult

• In beams: direct load transfer
• Larger Vult


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


Goals
• Assess shear capacity of slabs
under concentrated loads
• Determine effective width in
shear


Experiments
Test setup

Size: 5m x 2,5m x 0,3m


Experiments
Test setup

Continuous support, Line supports


Experiments
Test setup

Load: vary a/d and position along width

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?


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

Comparison to EC2 and Regan (1)


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

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


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


Key message

Slabs under concentrated
loads behave differently in
shear than beams


Contact:
Eva Lantsoght
E.O.L.Lantsoght@tudelft.nl
+31(0)152787449


Experiments
Specimens
Slab fc’ fct ρl ρt a/d M/S Aload
(MPa) (MPa) (%) (%) (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


Experiments
Test results
Name SS/CS Pu (kN) Failure Mode
S5T1 CS 1804 Wide beam shear
S5T4 SS 1755 Wide beam shear
S7T1 SS 1121 Wide beam shear + Punching shear
S7T2 CS 1172 Wide beam shear + Punching shear
S7T3 CS 1136 Wide beam shear + Punching shear
S7T5 SS 1063 Wide beam shear + Punching shear


Comparison to EC2 and Regan
Test data Pu/VRdc Pu/VRdcmins Pu/PRegan

AVG STD COV AVG STD COV AVG STD COV

S1 – S8 1,941 0,210 0,108 1,018 0,141 0,138 1,128 0,122 0,108

S1 – S8, SS 1,933 0,193 0,100 1,014 0,131 0,129 1,178 0,115 0,098

S1 – S8, CS 1,952 0,240 0,123 1,024 0,161 0,157 1,067 0,105 0,098


Experimental study of shear in reinforced concrete one way slabs subjected to concentrated loads

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