SEMINAR ON

BEHAVIOUR AND FAILURE MECHANISM OF INFILL WALLS

BYKAVYASHREE S.2nd Semester,

M.Tech in Structural Engineering,

BIT, Bangalore.

UNDER THE GUIDANCE OFP. M. RAVINDRA

Associate ProfessorDept. of Civil Engineering

BIT, Bangalore.

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 WallsHigher stiffness and lower displacementHigher strength.Lower ductility requirements.Frame design for small lateral loads.Reduce contribution of frame in lateral

resisting.

Design practice of Infill wallsInfills 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 WallsUnequal 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 WallsThe 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 StoreyOne 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 CrackingHorizontal Sliding Along the Mortar

Joints

Shear CrackingCracking Due to Diagonal Tension

Compression FailureCrushing of the Loaded Corner

Flexural CrackingIn 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.

ConclusionsThe 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…