Crisafulli F.J. –PPT-Analysis of infill frame structures

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ANALISIS OF INFILLED FRAME STRUCTURES

Francisco Crisafulli Universidad Nacional de Cuyo Argentina

SEMINAR ON MASONRY AND EARTHEN STRUCTURES Universidade do Minho

Analysis of infilled frames. Why? • New buildings, in some countries. • Old buildings that need to be retrofitted.

Argentina

Portugal

Venezuela

Infilled frames NEESR-SG: Seismic Performance Assessment and Retrofit of Nonductile RC Frames with Infill Walls. University of California San Diego, University of Colorado at Boulder and Stanford University. http://infill.ucsd.edu/

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

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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 mortar joints). • Reinforced concrete (or steel) frame. • Panel-frame interfaces.

Structural response 120

Integral infilled frame 100

Base shear, V ((kN)

80

Non-integral infilled frame 60

Initial Slackness 40

Bare frame 20

0 0

2

4

6

8

10

12

14

Lateral displacement, ∆ (mm)

16

18

20

Base shear

Structural response

Separation starts

Lateral displacement

Base shear

Structural response

Craking of masonry

Separation starts

Lateral displacement

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 response

Base shear

Yielding of the reinforcement Craking of masonry

Separation starts

Lateral displacement

Structural response

Base shear

Yielding of the reinforcement Craking of masonry

Separation starts

Lateral displacement

Degradation

Structural response The structural response is very complex and usually 4 different stages can be distinguish:

• Initial stage.

Monolithic wall

Partial separation at the panel-frame interfaces

Truss mechanism

• Cracking masonry. • Yielding of the reinforcement. • Degradation.

The wall partially separates from the frame. The frame restrain the shear deformation of the masonry wall.

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 strut 2. Crushing of the corners,.

Types of failure Failure modes of the RC frame: Flexural plastic mechanism

Failure due to axial loads

Plastic hinges at member ends Plastic hinges at span length Yielding of the reinforcement Bar anchorage failure

Shear failure of the columns Beam-column joints failure

Sliding shear failure

Chile Earthquake. March 1985.

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

Sliding shear failure

At the ultimate stage, limited experimental evidence indicates that the dowel action is mainly caused by to the kinking mechanism in the longitudinal reinforcement.

V c = A st f y cos α 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 frames Refined finite element models

P. Shing, 2007)

Analysis of infilled frames Refined finite element models

P. Shing, 2007)

Analysis of infilled frames Finite element models with ABAQUSS

Elements: • RC frame • Masonry • Interfaces

Analysis of infilled frames Finite element models with ABAQUSS Maximum load

Maximum displacement

Analysis of infilled frames NEESR-SG: Seismic Performance Assessment and Retrofit of Nonductile 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 /2

A ms A ms /2

A ms /4 A ms /2 A ms /4

Macro-model for inelastic analysis Panel element based on rational considerations of infill behavior. Advantages and limitations. Implemented in RUAUMOKO and SeismoStruct

Shear spring

Struts

Macro-model for inelastic analysis v 3 ϕ u

hz

4 4

3

2

1

External node (3 dof) Internal node (3 dof)

hz

Dummy node (2 dof) θ1

θ2

2 1 Truss mechanism

Macro-model for inelastic analysis

Axial stress, fm

Hysteretic behavior of the strut under axial load.

f 'mθ

Axial strain, ε m

Macro-model for inelastic analysis v 3 ϕ u

4

2 1 Shear mechanism

Macro-model for inelastic analysis Hysteretic behavior of the shear spring

τmax τo Shear stress, τ

Bond failure

Gm

Gm

−τmax Shear strain, γ

Displacement history. Pushver analysis

Lateral Force (kN)

Macro-model for inelastic analysis 100

0 -45

0

45

Lateral displacement (mm)

Experimental results Resultados experimentales Simulation analíticos Resultados -100

Evaluation of the masonry strength fn

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

τ fp

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

θ

Rc = f 'mθ Ams

Evaluation of the strength Masonry strength under shear and compression: Modified Mann-Müller Theory τm Mohr-Coulomb criterion

µ∗

τo τ∗

1

o

f'm f'to

Shear-friction failure

Diagonal tension failure

Compressive failure

fn

Evaluation of the strength Strut compressive strength (for different angles)

Compressive strength, f 'm θ (MPa) mθ

8 7 6 5 4

Diagonal tension failure

3 2

Shear-friction failure

1 0 20

30

40

50

60

70

Angle θ (degs)

Proposed macro-model

Shear and axial springs: springs • Hysteretic behavior. • Axial-shear interaction.

masonry struts

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 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.

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).