Experimental investigation of the behavior of flat slabs

Brisid IsufiFaculdade de Ciências e Tecnologia, UNL

Supervisors: António M. P. Ramos

Válter J. G. Lúcio

Experimental investigation of the behavior of flat

slabs with studs as shear reinforcement

Department of Civil Engineering

Caparica, March 7, 2018

EDIFICE Conference 2018Department of Civil Engineering

Contents

• Introduction

• Experimental campaign

• Main results and conclusions


Introduction

In seismic regions:

Flat slabs: slabs directly supported on the columns, i.e., without beams.

advantages disadvantages

freedom in interior

architectural design

less labour intensive

installations: electrical,

HVAC, plumbing etc.

simpler formwork, simpler

reinforcement layout

reduced overall building

height and mass

risk of progressive collapse

uncertain ultimate drift

not entirely covered

by design codes

existing buildings


before after

Images collected by Galvis et al (2017) from Google Street View and media reports

Widespread

in Portugal

too.

2017 Puebla-Morelos earthquake, Mexico City


How to enhance punching shear strength?

+ high strength concrete, fibre reinforced concrete etc.

+ other proprietary

products.

In this research:

+ drop panels, thickening;


Studs as shear reinforcement

Headed studs are among the most efficient means of enhancing the

punching shear strength of flat slabs (supported by several monotonic and

cyclic loading tests).

easy to install

good anchorage

conditions

available tests indicate

that studs perform

better than stirrups

difficult to produce on site

considerable differences

between the specimens

found in literature

+ -

New tests proposed in

this research

Why studs?1) continuation of the research effort of the

last few years at FCT/UNL;

2) contribution to the available literature.


Experimental campaign

1.85m

thickness: 150mm- 1 reference specimen C-Ref

- 4 flat slab specimens with stud

shear reinforcement

- naming convention:

C-SSRn

Type of shear reinforcement:

SSR=Shear Studs Reinforcement

n = no. of perimeters of

shear reinforcement

Type of loading: C- horizontal

reversed cyclic loading

C-Ref

C-SSR3

C-SSR5a

C-SSR5b

C-SSR5c

No shear reinforcement

Studs shear reinforcement

varying gravity load:

a) in absolute value (in kN)

b) as a fraction of the

concentric punching

shear resistance (GSR)


Longitudinal reinforcement

- The same longitudinal

reinforcement for all

specimens.

- Top reinforcement

ratio ≈1%

- Instrumented bars as

shown in the figure

- Cover: 20mm

A

A


Shear reinforcement layout

C-SSR3 C-SSR5...

The same EC2 outer perimeter.


Production of studs

C-SSR3 C-SSR5...

70mm

Ø8mm



column

slab edge



Materials

Main mechanical properties

of concrete and steel are

determined by laboratory

tests.

Longitudinal reinforcement yield strength ≈ 540 MPa

Shear reinforcement yield strength ≈ 485 MPa

Specimen fc (MPa)

C-Ref 62.3

C-SSR3 41.2

C-SSR5a 27.0

C-SSR5b 57.6

C-SSR5c 69.9


Test setup

1.85m

thickness: 150mm

This setup ensures:

- equal vertical displacements at the opposite slab borders;

- equal magnitude shear forces, bending moments and rotations at the slab

borders;

- mobility of the line of inflection location along the longitudinal direction;

- application of high vertical load ratios.


Loading protocol

The loading sequence:

1) application of gravity load until a target GSR is

reached (resistance based on Eurocode 2):

55% for C-Ref, C-SSR3, C-SSR5a, C-SSR5b

65% for C-SSR5c

2) application of reversed horizontal

cyclic displacements until failure.

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

140

-7.0%

-6.0%

-5.0%

-4.0%

-3.0%

-2.0%

-1.0%

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

7.0%

dri

ftra

tio

[%]

ho

rizo

nta

ldis

pla

ce

me

nt

[mm

]

time

gravity load

Modified sequence for C-SSR5b:

1) Phase I: protocol followed until 3% drift

2) Phase II: specimen unloaded, protocol

restarted and followed until failure.

Purpose: to observe the strength and stiffness

of a flat slab – column assembly that has been

previously subjected to a major seismic event.


Instrumentation

Strains:

-Strain gauges in 15 studs of C-SSR3 and

in 25 studs of C-SSR5 specimens

-Four top bars at two locations;

-Two bottom bars at two locations;

Displacements and rotations:

- 18 displacement transducers

- 1 disp. transducer at the actuator

- 2 inclinometers at borders

Forces:

- 4 load cells for gravity loading;

- 2 load cells at test setup struts;

- 1 actuator load cell


Time-lapse video of the cyclic loading test of specimen C-SSR5c


Results: C-Ref

-120 -80 -40 0 40 80 120

-60

-40

-20

0

20

40

60

-6 -4 -2 0 2 4 6

-120

-80

-40

0

40

80

120

Drift (%)

Un

ba

lan

ce

dM

om

en

t(k

Nm

)

Ho

rizon

talF

orc

e(k

N)

Horizontal Displacement (mm)

-Failure mode: punching shear failure

-Maximum attained drift ratio: 1.0%.


Results: C-SSR3

-Failure mode: punching outside the

shear reinforced zone

-Maximum attained drift ratio: 4.0%.

-140-120-100 -80 -60 -40 -20 0 20 40 60 80 100 120 140-80

-60

-40

-20

0

20

40

60

80-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

-160

-120

-80

-40

0

40

80

120

160

Drift (%)

Un

ba

lan

ced

mo

me

nt

(kN

m)

Ho

rizo

nta

lfo

rce

(kN

)

Horizontal displacement (mm)


Results: C-SSR5a

-Failure mode: gradual loss of strength.

-Maximum drift: 6.0%, no punching

failure.

-140-120-100 -80 -60 -40 -20 0 20 40 60 80 100 120 140-80

-60

-40

-20

0

20

40

60

80-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

-160

-120

-80

-40

0

40

80

120

160

Drift (%)

Un

ba

lan

ced

mo

me

nt

(kN

m)

Ho

rizo

nta

lfo

rce

(kN

)

Horizontal displacement (mm)

Bottom face:

This is the slab with the smallest

Vg (although the same GSR)


Results: C-SSR5b

-Failure mode: punching failure outside the shear reinforced zone

-Maximum drift prior to punching failure: 5.5%.

-120 -80 -40 0 40 80 120

-60

-40

-20

0

20

40

60

-6 -4 -2 0 2 4 6

-120

-80

-40

0

40

80

120

Phase I

Phase II

Drift (%)

Un

ba

lan

ce

d M

om

en

t (k

Nm

)

Ho

rizo

nta

l F

orc

e (

kN

)



Results: C-SSR5c

-Failure mode: punching failure outside the shear reinforced zone

-Maximum drift prior to punching failure: 4.0%.

-120 -80 -40 0 40 80 120

-60

-40

-20

0

20

40

60

-6 -4 -2 0 2 4 6

-120

-80

-40

0

40

80

120

Drift (%)

Un

ba

lan

ce

dM

om

en

t(k

Nm

)

Ho

rizon

talF

orc

e(k

N)



Comparison

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

700 1 2 3 4 5 6 7

0

20

40

60

80

100

120

140

C-Ref

C-SSR3

C-SSR5a

C-SSR5b (envelope)

C-SSR5c

Drift (%)

Un

ba

lan

ce

d M

om

en

t (k

Nm

)

Ho

rizo

nta

l F

orc

e (

kN

)



Conclusions

• Significant drift capacity enhancement was achieved by specimens with studs;

• Increase of gravity loads led to decrease of the ultimate drift;

• Extent of shear reinforcement influenced drift capacity (specimen C-SSR3 vs. C-SSR5b);

• Post-earthquake behaviour: specimen C-SSR5bsustained high drifts (5.5%);


Thank you!

Documents

Experimental investigation of the behavior of flat slabs