ANALISIS OF INFILLED FRAME STRUCTURES

Universidad Nacional de CuyoArgentina

Francisco Crisafulli

SEMINAR ONMASONRY AND EARTHEN STRUCTURES

Universidade do Minho

Analysis of infilled frames. Why?

• New buildings, in some countries.• Old buildings that need to be retrofitted.

Argentina VenezuelaPortugal

Infilled frames

NEESR-SG: Seismic Performance Assessment and Retrof it of Non-ductile RC Frames with Infill Walls. University of California San Diego, University of Colorado at Boulder and Stanford University. http://infill.ucsd.edu/

Infilled framesDynamic test of a 3-storey RC infilled frame in Italy. Project NEARB - OPCM 3274. EUCENTRE, Pavia.

Analysis of infilled frames

In order to develop adequate and rational models we need to understand de structural response of infilled frames.

• Masonry : composite material (bricks or blocks and • Masonry : composite material (bricks or blocks and mortar joints).

• Reinforced concrete (or steel) frame .

• Panel-frame interfaces .

Structural response

80

100

120

V (

kN)

Integral infilled frame

Non-integral infilled frame

0

20

40

60

0 2 4 6 8 10 12 14 16 18 20

Bas

e sh

ear,

V (

Lateral displacement, ∆∆∆∆ (mm)

Initial Slackness

Bare frame

Structural responseB

ase

sh

ea

r

Lateral displacement

Ba

se s

he

ar

Separation starts

Structural response

Lateral displacement

Ba

se s

he

ar

Craking of masonry

Separation starts

Structural response

After separation, the structure behaves as a truss in which the masonry wall can be approximately represented by a compressive strut.

Structural response

Internal forces in the reinforced concrete frame

(a) Bending moment (b) Shear force

(c) Axial force

Structural responseB

ase

sh

ea

r

Yielding of thereinforcement

Lateral displacement

Ba

se s

he

ar

Craking of masonry

Separation starts

Structural response

Ba

se s

hea

r

Yielding of thereinforcement

Lateral displacement

Ba

se s

hea

r

Craking of masonry

Separation starts

Degradation

Structural response

The structural response is very complex and usually 4

different stages can be distinguish:

• Initial stage. Monolithic wall

• Cracking masonry.

• Yielding of the reinforcement.

• Degradation.

Truss mechanism

The wall partially separates from the frame.

The frame restrain the shear deformation of the masonry wall.

Partial separation at the panel-frame interfaces

Types of failure

Damage or failure of the masonry panel:• Shear-friction failure• Diagonal tension failure• Compressive failure

Types of failure

Damage or failure of the masonry panel:• Shear-friction failure• Diagonal tension failure• Compressive failure

Types of failure

Damage or failure of the masonry panel:• Shear-friction failure• Diagonal tension failure• Compressive failure:

1. Failure of the diagonal strut2. Crushing of the corners,.2. Crushing of the corners,.

Types of failure

Failure modes of the RC frame:

Flexural plastic mechanism

Failure due to

Plastic hinges at span length

Yielding of the reinforcement

Plastic hinges at member ends

Shear failure of the columns

Failure due to axial loads

Beam-columnjoints failure

Bar anchorage failure

Yielding of the reinforcement

Sliding shear failure

Silakhor Earthquake, Iran. March 2006 (Moghadam, 2006).

Chile Earthquake. March 1985.

Sliding shear failure

α cos f A = V ystc

At the ultimate stage, limited experimentalevidence indicates that the dowel action ismainly caused by to the kinking mechanismin the longitudinal reinforcement.

EERI, Confined Masonry Design Group. http://www.confinedmasonry.org/

Analysis of infilled frames

Infilled frames are complex structures which exhibit a highly nonlinear inelastic behaviour, This fact complicates the analysis and explains why infill panels has been considered as "non-structural elements", despite their strong influence on the global response.

Modelling techniques:

• Refined or micro-models: based on the use of many elements (usually different types).

• Simplified or macro-models: diagonal strut model (with single or multiple struts.

Analysis of infilled framesRefined finite element models

P. Shing, 2007)

Analysis of infilled framesRefined finite element models

P. Shing, 2007)

Analysis of infilled frames

Finite element models with ABAQUSS

Elements:

• RC frame• RC frame• Masonry• Interfaces

Analysis of infilled framesFinite element models with ABAQUSS

Maximum load

Maximum displacement

Analysis of infilled framesNEESR-SG: Seismic Performance Assessment and Retrof it of Non-ductile RC Frames with Infill Walls.

Maximum load

Maximum displacement

Equivalent strut model

The equivalent strut model was suggested by Poliakov and implemented by Holmes and Stafford Smith in the 1960s.

Later, many researchers improved the model. Today the strut model is accepted as a simple and rational way to represent the effect of the masonry panel.

A ms

msA /2

msA /2

msA /4

A /4ms

A /2ms

Macro-model for inelastic analysis

Panel element based on rational considerations of infill behavior.

Advantages and limitations.

Implemented in RUAUMOKO and SeismoStruct

Struts

Shear spring

Macro-model for inelastic analysis

(3 dof)

(3 dof)1

3

v

External node

Internal node h z

43

2

4

uϕ

Truss mechanism

Dummy node(2 dof)

h z

1

2θθθθ1111

θθθθ2222

Macro-model for inelastic analysis

Hysteretic behavior of the strut under axial load.

Axi

al s

tres

s, f

m

' θmf

Axial strain, εεεεm

Macro-model for inelastic analysis

43

u

vϕ

Shear mechanism

1

2

Macro-model for inelastic analysis

Hysteretic behavior of the shear springττ ττ

τmax

τo Bond failure

Shear strain, γγγγ

She

ar s

tres

s,

ττ ττ

Gm Gm

−τmax

Macro-model for inelastic analysis

100

Late

ral F

orce

(kN

)

Displacement history. Pushver analysis

-100

0-45 0 45

Lateral displacement (mm)

Late

ral F

orce

(kN

)

Resultados experimentales

Resultados analíticos

Experimental resultsSimulation

fn

fp

ττττ

Evaluation of the masonry strength

1. Evaluate the strength of masonry under shear and compression, based on geometrical and mechanical properties of the materials.

2. Calculate the compressive strength of masonry in the direction of the diagonal strut.

msmc AfR ' = θ θθθθ

Evaluation of the strength

Masonry strength under shear and compression:

Modified Mann-Müller Theory

ττττm

Mohr-Coulomb criterion

fnShear-frictionfailure

Diagonal tensionfailure

Compressivefailure

f'm

τoµ∗

f'to

τ∗

o

1

Evaluation of the strength

Strut compressive strength (for different angles)

6

7

8

mθθ θθ

(MP

a)

0

1

2

3

4

5

20 30 40 50 60 70

Com

pres

sive

stre

ngth

, f'

m

Angle θ θ θ θ (degs)

Shear-friction failure

Diagonal tensionfailure

Proposed macro-model

Shear and axial springs

masonry struts

Shear and axial springs :

• Hysteretic behavior.

• Axial-shear interaction.

PhD thesis Mr. G. Torrisi.

Conclusions

� The strut model gives an adequate estimation of the stiffness of the infilled frame and the axial forces induced in the surrounding frame.

� Refined finite element models may represent adequately the structural response, provided that adequately the structural response, provided that the model is properly calibrated.

� Refined model are difficult to apply in the case of multi-storey buildings.

Conclusions

� Multi-strut macro models represents a compromise solution and they can be use for the analysis of large structures.

� The main uncertainties in these models are the area of the struts and their strength.area of the struts and their strength.

Simplicity that is based on rationality is the ultimate sophistication.S. Veletsos

Thank you for your attention

IMERIS

Universidad

Nacional de Cuyo

Influence of the loading system

(a) Pushing load (b) Pulling load

New reinforcement details proposed to improve the structural response of confined masonry (Crisafulli, 1997; Crisafulli, Carr and Park 2000).