Behaviour and failure mechanism of infill walls

SEMINAR

ON

BEHAVIOUR AND FAILURE

MECHANISM OF INFILL WALLS

Introduction

Masonry infilled walls are provided within the reinforced

concrete structures without being analyzed as a combination of

concrete and brick elements, though in reality they act as a

single unit during earthquakes. The performance of such

structures during earthquakes has proved to be superior in

comparison to the bare frames in terms of stiffness, strength and

energy dissipation. There are plenty of researches done so far for

infilled frames, however partially infilled frames are still the topic

of interest. Though it has been understood that the infills play

significant role in enhancing the lateral stiffness of complete

structure, the past experience in various earthquakes have

proved that the partially infilled framed structures somehow are

affected adversely.

Types Of Infill Provisions

Influence Of Infill Walls

Infill walls provide durable and economical

partitions.

The presence of infill wall changes the behavior

of frame action into truss action.

The structural response is quite complex as it

involves an interaction of infill behavior,

reinforced concrete frames behavior and length

of contact between infill and frame.

Properties of infill walls

If the infills are very light and flexible, or

completely isolated from the RC frame,

presence of infills does not affect the

structural response of the system.

Infills are expected to remain in the

elastic range.

Infills are expected to suffer significant

damage during the seismic event.

Advantages of Infill Walls

Higher stiffness and lower displacement

Higher strength.

Lower ductility requirements.

Frame design for small lateral loads.

Reduce contribution of frame in lateral

resisting.

Design practice of Infill walls

Infills are adequately separated from the RC

frame such that they do not interfere with

the frame under lateral deformations.

Infills are built integral with the RC frame,

but considered as non-structural elements.

Infills are built integral with the RC frame,

and considered as structural elements.

Difficulties in Consideration Of Infill Walls In Structural Design

Computational complexity.

Structural uncertainties.

The non-linear behaviour of infilled

frames.

Various cracking patterns and

concentration of forces in structural

components.

Effects Of Infill Walls

Unequal distribution of lateral forces.

Vertical irregularities in strength and

stiffness.

Horizontal irregularities.

Inducing the effect of short column or

captive column in infilled frame.

Cause of casualities.

Behaviour Of Infill Walls

The structural load transfer mechanism is

changed from frame action to predominant

truss action.

Infills interfere with the lateral deformations

of the RC frame; separation of frame and

infill takes place along one diagonal and a

compression strut forms along the other.

Change In Lateral Load Transfer Mechanism Owing To Inclusion Of

Masonry Infill Walls

(a) Frame action in bare frame

(b) Predominant truss action in infilled frame

Behaviour Of Infill Walls

The state of stress in the infill gives rise

to a principal compressive stress along

the diagonal and a principal tensile stress

in the perpendicular direction.

When infills are strong, strength

contributed by the infills may be

comparable to the strength of the bare

frame itself.

Separation of frame and infill takes place along one diagonal and a compression strut forms along the other

In-plane Behavior

The in-plane capacity of the wall depend

on the relative strength of the masonry

and the mortar.

The level of the axial load significantly

controls the type of failure.

The crack propagation either follows the

mortar joints or passes through the

masonry units, or both.

Out-of-plane Behaviour

Crushing along the edges for low height

to thickness ratio.

Snap-through (small effect of arching)

for high height to thickness ratio i.e.

approximately between 20 and 30.

Behaviour of Partially Infilled Frames

In majority of hospitals, academic institutions

and commercial complexes, partial infills are

provided to attain light within the rooms.

It is observed that such walls on one hand

contribute in enhancing the lateral stiffness of

the structure while on the other hand they

play ironic role with an adverse effect called

"short column effect".

Behaviour of Partially Infilled Frames

Lateral deformation in bare frame Lateral deformation in partially infilled frame

Behaviour Of Infill Frames With Openings

In most cases, door or

window openings are provided in

masonry infill panels because of the

functional and ventilation requirements

of buildings.

Introducing openings in an infill wall

alters its behavior and adds complexity

in behavior.

Behaviour Of Infill Frames With Openings

Right loading Left loading

Soft Storey

One of the main reasons of failure of

structures due to earthquakes is

discontinuity of lateral force resisting

elements like bracing, shear wall or

infill in the first story.

Failure Mechanism Of Infilled Walls

Shear CrackingStepped Cracking Along the Mortar

Joints

Shear Cracking

Horizontal Sliding Along the Mortar Joints

Shear CrackingCracking Due to Diagonal Tension

Compression Failure

Crushing of the Loaded Corner

Flexural Cracking

In those cases where flexure effects are

predominating, such as multistory

infilled frames, and the columns of the

frame are very weak, flexure cracks can

open in the tensile side of the panel due

to the low tensile strength of the

masonry .

Conclusions Many structural engineers ignore such infills when

assessing the seismic vulnerability of these frames.

Consequently, more research is needed to evaluate

the strength and stiffness of masonry-infilled frames

with openings.

Masonry infill wall panels increase strength,

stiffness, overall ductility and energy dissipation of

the building. More importantly, they help in

drastically reducing the deformation and ductility

demand on RC frame members.

Conclusions

The better collapse performance of fully-

infilled frames is associated with the larger

strength and energy dissipation of the

system, associated with the added walls.

The presence of infills leads, in general, to

decreased shear forces on the frame

columns. However, in the case of infilled

frame with a soft ground story, the shear

forces acting on columns are considerably

higher than those obtained from the analysis

of the bare frame.

References I.S. 1893(Part I)-2002, “Criteria for Earthquake Resistant

Design of Structure, General Provisions and Buildings”,

Bureau of Indian Standards, New Delhi.

Asteris, P. G., Kakaletsis, D. J., Chrysostomou, C. Z. &

Smyrou, E. E. (2011).” Failure Modes of In-filled Frames”

Electronic Journal of Structural Engineering 11(1) , 11-20.

Das, D., Murty, C. V. R. (2004). “Brick masonry infills in

seismic design of RC framed buildings: Part 1 –Cost

implications” The Indian Concrete Journal, vol78 No7: 39-

43.

References• Wijanto L. S., (2007). “Seismic Assessment of

Unreinforced Masonry Walls”, A thesis, University of

Canterbury Christchurch, New Zealand.

• Zhang B., (2006). “Parametric Study On The Influence Of

Infills On The Displacement Capacity Of Rc Frames For

Earthquake Loss Estimation” A Dissertation, European

School For Advanced Studies In Reduction Of Seismic Risk.

• Pradhan, P. M., Pradhan, L.P., & Maskey, R.K. (2012). “ A

Review On Partial Infilled Frames Under Lateral Loads”,

Kathmandu University Journal Of Science, Engineering And

Technology, VOL. 8, No. I, 142-152.

Thank you…