6-learning from failures

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LEARNING FROM FAILURE Dr.Vinod Hosur, Professor, Civil Engg.Dept., Gogte Institute of Technology, Belgaum

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Understanding the force path in the structures. It includes the gravity and lateral load resisting systems.

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LEARNING FROM FAILURE

Dr.Vinod Hosur, Professor, Civil Engg.Dept., Gogte Institute of Technology, Belgaum

REQUIREMENTS OF GOOD DESIGN AND CONSTRUCTIONStructural Integrity (Redundancy) and DuctilityGood lateral load resistance systemUniform disposition and continuity in structural mass and stiffnessContinuous and direct load (force) path without stress concentrationSimple, symmetric plan with good aspect ratio & without re-entrant cornersSoil-Foundation-Structure Compatibility and integral actionWhat affects building performance & damage?Shape (configuration) of building: Square or rectangular usually perform better than L, T, U, H, +, O, or a combination of these. Construction material: steel, concrete, wood, brick. Concrete is the most widely used construction material in the world. Ductile materials perform better than brittle ones. Ductile materials include steel and aluminum. Brittle materials include brick, stone and unstrengthened concrete. Load resisting systemHeight of the building: (i.e. natural frequency)Previous earthquake damageIntended function of the building (e.g. hospital, fire station, office building)Proximity to other buildingsSoil beneath the buildingMagnitude and duration of the earthquakeDirection and frequency of shaking

FLOATING COLUMN ?

First, the apartment building was constructedThen the plan called for an underground garage to be dug out.The excavated soil was piled up on the other side of the building. Heavy rains resulted in water seeping into the ground..The building began to shift and the concrete pilings were snapped due to the uneven lateral pressures.The building began to tilt.They built 13 stories on grade, with no basement,and tied it all downto hollow pilings with no rebar.

Notice that the soft-storey is subject to severe deformation demands during seismic shaking.

DuctilityDuctility can be defined as the ability of material to undergo large deformations without rupture before failure.

The ductility of RC member is increased by,

Restricting tension steel to under reinforced section design. An increase in compression steel content. An increase in concrete compressive strength. An increase in ductility in steel rather than yield strength.. Provision of proper confining reinforcement.. Provision of adequate development and anchorage length. Provision of Redundancy in the structure.

NECESSITY OF DUCTILE DETAILING

Ductile detailing is provided in structures so as to give them adequate ductility to facilitate failure by yielding prior to brittle failure (shear, bond, anchorage failures) so that the structure resists severe earthquake shocks without collapse.

Ductile detailing is a must for the following structures. The structures located in seismic zone IV and V. The structures located in seismic zone III and has the important factor(I), greater than 1. such as schools, hospitals, bridges The industrial structures located in seismic zone III . The structures of height more than 5 stories and located in seismic zone III.

DUCTILE DETAILING OF FLEXURAL MEMBER

Shear- flexure failure of beams

Shear failure in beam column joint

Structural Behaviour of RCC Frames with Masonry infill

Masonry infill walls confined by reinforced concrete RC frames on all four sides play a vital role in resisting the lateral seismic loads on buildings.

MI walls have a very high initial lateral stiffness and low deformability.

Masonry infill (MI) walls are remarkable in increasing the initial stiffness of reinforced concrete RC frames, and being the stiffer component, attract most of the lateral seismic shear forces on buildings, thereby reducing the demand on the RC frame members

The introduction of MI in RC frames changes the lateral-load transfer mechanism of the structure from predominant frame action to predominant truss action , which is responsible for reduction in bending moments and increase in axial forces in the frame members Two Approaches in the Standards that consider or do not consider the role of MI walls while designing RC frames.

A very few codes specifically recommend isolating the MI from the RC frames such that the stiffness of MI does not play any role in the overall stiffness of the frame (NZS-3101 1995), The isolation helps to prevent the problems associated with the brittle behavior and asymmetric placement of MI.

Another group of national codes prefers to take advantage of certain characteristics of MI walls such as high initial lateral stiffness and these codes tend to maximize the role of MI as a first line of defense against seismic actions, and to minimize their potential detrimental effects through proper selection of their layout and quality control

Presence of non-isolated MI walls in buildings increases both the mass and stiffness of buildings; however, the contribution of latter is more significant. Consequently, the natural periods of an MI-RC frame are normally lower than that of the corresponding bare frame. Therefore, the seismic design forces for MI frames are generally higher than those for the bare frames.

FEMA-306 recommend modeling the MI as equivalent diagonal struts

RCC Framed Structure with Masonry infill

Failure of Pier of Delhi Metro

DUCTILITY ?

CONGESTION OF REINFORCEMENT

INVITATION FOR FAILURE ?

Improper bracing or shoring during construction,

REQUIREMENTS OF GOOD DESIGN AND CONSTRUCTIONStructural Integrity (Redundancy) and DuctilityGood lateral load resistance systemUniform disposition and continuity in structural mass and stiffnessContinuous and direct load (force) path without stress concentrationSimple, symmetric plan with good aspect ratio & without re-entrant cornersSoil-Foundation-Structure Compatibility and integral action

THANK YOU