PUNCHING SHEAR RETROFIT USING GFRP SHEAR · PDF fileINTRODUCTION |Shear bolt reinforcement yRetrofit method for existing, previously built, reinforced concrete flat slabs. yBolts will

PUNCHING SHEAR RETROFIT USING GFRP SHEAR BOLTSNicholas Lawler, B.A.Sc

Masters of Applied Science Seminar

University of Waterloo, Waterloo, Ontario

July 15th, 2008

PRESENTATION OUTLINE

IntroductionResearch Program

Shear Bolt Reinforcement

Design of ReinforcementTesting ProgramResultsConslusionsFuture ConsiderationsQuestions?

INTRODUCTION

Shear bolt reinforcementRetrofit method for existing, previously built, reinforced concrete flat slabs.

Bolts will prevent a brittle, punching shear (two-way shear) failure.

Provide ductility at failure

Can be applied at any time in the service life of a structure.

Bolts are anchored at each face of the slab surface.

“headed shear reinforcement” details are outlined in Clause 13.3.8 of the 2005 CAN/CSA concrete design manual.

PRINCIPAL OF A SHEAR BOLT

Installation requires drilling of small holes through the slab, in a predetermined pattern.

Minimal prestressing is required, but bolts must be tight against the slab.

Bolts are not bonded to the slab.

Bolts intercept punching cone, and restrict it’s development as tension elements. Plan view

Stem

Washer and Nut

Head

UW TESTING PROGRAM HISTORY

Researcher Year Support Condition

Bolt Type Loading

E.F. El-Salakawy 1998 Edge Steel Static

B. Adetifa 2003 Interior Steel Static

W. Bu 2007 Interior Steel Quasi-Static

N.D. Lawler 2008 Interior GFRP Quasi-static / static

Punching shear retrofit with headed shear studs (bolts) has been developed at UW.2008 research will focus on determining if a GFRP based bolt is feasible as punching shear reinforcement.

DESIGN OF GFRP REINFORCEMENT

Tensile forces in shear bolts are caused by restraining expansion of the slab, due to punching cone formation.

By restraining this expansion, the connection can be strengthened.

Thus, development of the tensile stresses in the bolts through anchorage is critical.

DESIGN OF GFRP REINFORCEMENTCRIMPING TECHNIQUE

In the hydroelectric industry high tension insulators with GFRP rods are ‘crimped’ to metal fittings.

Residual stresses transfer through the metal fittings, forming a high strength bond between two materials.

Using this principal, a technique and system was developed.

Pictures c/o K-Line Insulators

Smooth GFRP rod core, with crimped ends (K-Line Type)Core rod from the hydro industry

REINFORCEMENT TYPES TESTED

“SchöckComBAR” core rod, with crimped ends.RC composite reinforcement from Germany, just approved for use in Canada

Also make a headed stud, research is being done into adapting it for use in punching shear applications.

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REINFORCEMENT TYPES TESTED

REINFORCEMENT TYPES TESTED

“Strongwell Fibrebolt” an off the shelf product. Comprised of a complete composite nut and bolt system

TEST RESULTS, QUASI-STATIC LOADING, INTERIOR CONNECTION 1500 x 1500 x 120 Slab (mm)200 x 200 x 700 Column (mm)Predermined cyclic loading pattern developed. Drift ratio ranges from 1% to 7%.One slab was tested with two openings as shown.Both reinforced with “K-Line” type bolts.

H

H

HY

ST

ER

ES

IS P

LO

T –

CO

NT

RO

L S

LA

B V

S. S

N5

•Vert. Load = 160kN

•Same loading path

Control:

•Max. Load – 63kN

•Max. Disp. – 20mm

SN5:

•5 Rows of GFRP shear bolts

•Max. Load – 50kN

•Max. Disp. – 28mm

•No pinching, more energy is dissipated.

-80

-60

-40

-20

0

20

40

60

80

-40 -30 -20 -10 0 10 20 30 40

Displacement (mm)

Load

(kN

)

-80

-60

-40

-20

0

20

40

60

80

-40 -30 -20 -10 0 10 20 30 40

Displacement (mm)

Hor

izon

tal L

oad

(kN

)20kN @ 30mm

48kN @ 33mm

HY

ST

ER

SIS

PL

OT

–C

ON

TR

OL

SL

AB

VS

. SN

6

•Vert. Load = 160kN

•Same loading path

•2-100mm openings, in the moment direction

Control:

•Max. Load – 45kN

•Max. Disp. – 10mm

SN5:

•6 Rows of GFRP shear bolts

•Max. Load – 35kN

•Max. Disp. – 18mm

•No pinching.

20kN @ 23mm

20kN @ 35mm

TEST RESULTS, INTERIOR CONNECTION – STATIC LOADING

1500 x 1500 x 120 Slab (mm)150 x 150 x 150 Column (mm)Yield line theory predicts flexural strength capacity of 360kN.

CSA A23.3 predicts a punching shear capacity of control specimen of 220kN.

Simply supported edge

VOpening

FO

RC

E V

S. D

ISP

LA

CE

ME

NT

PL

OT

S –

ST

AT

IC L

OA

DIN

G

SB1 (Control):

•Max. Load – 253kN

•Max. Disp. – 12mm

SB4:

•4 Rows of steel 3/8”shear bolts

•Max. Load – 350kN

•Max. Disp. – 28mm

SN1:

•4 Rows of Fibrebolts

•Max. Load – 199kN

•Max. Disp. – 23.6mm

SN2:

•4 Rows of K-Line Type

•Max. Load – 278kN

•Max. Disp. – 36mm

SN3 (Radial):

•4 Rows of K-Line Type

•Max. Load – 310kN

•Max. Disp. – 42mm

SN4:

•4 Rows of Schöck Type

•Max. Load – 332kN

•Max. Disp. – 36mm

SN1: FAILURE OF FIBREBOLTS TO PROVIDE PUNCHING SHEAR (TENSILE) RESISTANCE

SN2 & SN3: K-LINE TYPE BOLTS PROVIDE PUNCHING SHEAR STRENGTHENING

SN4: RESULTS FROM SCHÖCK TYPE BOLTS

CONCLUSIONSA GFRP tensile resisting element can be used to strengthen a slab-column connection for punching shear.

Anchorage and tightening of this element to the slab face is critical to it’s performance.

By allowing movement between cracks, more energy was dissipated under cyclic loading.

The field crimping process was found to be a feasible way to provide anchorage, provided the required tensile loads are low.

GFRP shear bolts provide connection ductility in both static and quasi-static loadings.

RECOMMENDATIONS AND FUTURE WORKAfter more detailed development, slab-column connections could be retrofitted for punching shear strength with GFRP non-corrosive shear bolts.

A method for controlling the amount of energy dissipation under cyclic loading should be investigated.

Further study should be made into the crimping technique, making it more efficient.

The size of the aluminum head on one side needs to be reduced, to be almost flush with the slab face.

Study should be made into using the shear bolt retrofit technique for reinforcement after overloading events (earthquakes, structural failure) or abnormal loadings.

QUESTIONS?Thank You!