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

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

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


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


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


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

[email protected]

+31(0)152787449


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


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







Failure ModePu (kN)SS/CSName



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