TYPES & EFFECT OF IRREGULARITIES IN THE STRUCTURE INTRODUCTION: Earthquake resistant design of reinforced concrete buildings is a continuing area of research since the earthquake engineering has started not only in India but in other developed countries also. The buildings still damage due to some one or the other reason during earthquakes. The building configuration has been described as regular or irregular in term of size and shape of the building, arrangement of structural elements and mass. Regular building configuration are almost symmetrical (in plan and elevation) about the axis and have uniform distribution of lateral forceresisting structure such that, it provides a continuous load path for both gravity and lateral loads. A building that lacks of symmetry and has discontinuity in geometry, mass, or load resisting element is called irregular. These irregularities may cause interruption of force flow and stress concentrations. Asymmetrical arrangements of mass and stiffness of elements may cause a large torsional force where the center of mass does not coincide with the center of rigidity. The section 7 of IS 1893 (Part 1): 2002 [1] enlists the irregularity in building configuration system. These irregularities are categorized in two types; (i)
(ii)
Vertical irregularities referring to sudden change of strength, stiffness, geometry and mass results in irregular distribution of forces and/or deformation over the height of building. Horizontal irregularities which refer to asymmetrical plan shapes (e.g. L-, T-, U-, F-) or discontinuities in the horizontal resisting elements (diaphragms) such as cut-outs, large openings, re-entrant corner and other abrupt changes resulting in torsion, diaphragm deformation and stress concentration.
There are numerous examples enlisted in damage report of past earthquake in which the causes of failure of multistoried reinforced concrete building in irregularities in configuration. This paper describes the types of irregularities and possible causes of damage with some recommendation. VERTICAL IRREGULARITIES Vertical Discontinuities in Load Path: One of the major contributors to structural damage in structure during strong earthquake is that discontinuities in the load path or load transfer. The structure should contain a continuous load path for transfer of the seismic force, which develops due to accelerations of individual elements, to the ground. Failure to provide adequate strength and toughness of individual elements in the system, or failure to tie individual elements together can result in distress or complete collapse of the system. Therefore, all the structural and non-structural elements must be adequately tied to structural system. The load path must be complete and sufficiently strong. The general load path is as follows: earthquake forces, which originate in all the elements of the building are delivered through structural connections to horizontal diaphragms. The diaphragms distribute these forces to vertical resisting components such as columns, shear walls, frames and
other vertical elements in the structural system which transfer the forces into the foundation. The diaphragms must have adequate stiffness to transmitting these forces. The failure due to discontinuity of vertical elements of lateral load resisting system has been among the most notable and spectacular. One common example of this type of discontinuity occurs in Bhuj earthquake in which, infill walls that are present in upper floors are discontinued in the lower floor (floating column concept). Another example of discontinuous shear wall is the Olive View Hospital, which nearly collapsed due to excessive deformation in the first two stories during the 1972 San Fernando earthquake. Irregularity in Strength and Stiffness: A “weak” storey is defined as one in which the storey’s lateral strength is less than 80 percent of that in the storey above. The storey’s lateral strength is total strength of all seismic resisting element sharing the storey shear for the direction under consideration i.e. the shear capacity of the column. The deficiency that usually makes a storey weak is inadequate strength of the frame columns. A “soft storey is one in which the lateral stiffness is less than 70% of that in the storey immediately above, or less than 80% of combined stiffness of the three stories above” [1]. The essential characteristic of a “weak” or “soft” storey consists of a discontinuity of strength or stiffness, which occurs at the second storey connections. Figure 1 shows that this discontinuity is caused by lesser strength, or increased flexibility, the structure results in extreme deflections in the first storey of the structure, which in turn results in concentration of forces at the second storey connections. The failures of reinforced concrete buildings due to soft stories have remained the main reason in past earthquake. In the Bhuj earthquake of 2001, researchers determined that soft first stories were a major contribution of serious failure. Figure 1 showing failure of building with soft story during the Bhuj earthquake in 2001
Mass Irregularity: Source: www.google.co.in Mass irregularities are considered to exist where the effective mass of any storey is more than 200% of the effective mass of an adjacent storey [1]. The effective mass is the real mass consisting of the dead weight of the floor plus the actual weight of partition and equipment. Excess mass can lead to increase in lateral inertial force, reduced ductility of vertical load resisting elements. Irregularity of mass distribution in vertical and horizontal planes can result in irregular response and complex dynamics as shown in figure 2. The characteristics swaying mode of a building during an earthquake implies that masses placed in the upper stories of building produce considerably more unfavorable effects than masses placed lower down. The center of gravity of lateral forces is shifted above the base in the case of heavy masses in upper floors resulting in large bending moments [2]. Where mass irregularities exist, check the lateral-force resisting elements using a dynamic analysis for a more realistic lateral load distribution of the base shear. It is believed that the Mansi Complex, a
multistoried building has failed in Bhuj earthquake due to massive swimming pool at the upper floor. Figure 2 showing failure of RC building having mass irregularity during earthquake.
Vertical Geometric Irregularity: A vertical setback is a geometric irregularity in a vertical plane. It is considered, when the horizontal dimension of lateral force resisting system in any storey is more than 150% of that in an adjacent storey [1]. The setback can also be visualized as a vertical re-entrant corner. The general solution of a setback problem is the total seismic separation in plan through separation section, so that portions of the building are free to vibrate independently.
Source: www.google.co.in
Figure 3 showing failure of setback building during Mexico earthquake in 1985.
Proximity of Adjacent Building: Source: www.google.co.in Pounding damage is caused by hitting of two buildings constructed in close proximity with each other. Pounding may result in irregular response of adjacent buildings of different heights due to different dynamic characteristics. This problem arises when buildings are built without separation right up to property lines in order to make maximum use of the space. When floor of these buildings are constructed of the same height, damage due to pounding usually is not serious. If this is not the case, there are two problems. When the floors of adjacent buildings are at different elevations, the floor of each structure can act like rams, battering columns of the other building. When one of the building is higher than the other, the lower building can act as a base for the upper part of the higher building; the lower building receives an unexpected large lateral load while the higher building suffers from a major stiffness discontinuity at the level of the lower building. Damage due to pounding can be minimized by drift control, building separation, and aligning Figure 4 shows pounding between a six story building and floors in adjacent buildings. a two storey building in Kocaeli, Turkey earthquake 1999. PLAN IRREGULARITIES Source: www.google.co.in
Torsion irregularities: Torsion irregularity shall be considered when floor diaphragms are rigid in their own plan in relation to the vertical structural elements that resist the lateral forces. The lateral force resisting
elements should be a well-balanced system that is not subjected to significant torsion. Significant torsion will be taken as the condition were the distance between the storey's Centre of rigidity and storey's center of mass is greater than 20% of the width of the structure in the either major plan dimension. Torsion or excessive lateral deflection is generated in asymmetrical buildings that may result in permanent set or even partial collapse. Re-entrant corners Source: www.google.co.in The re-entrant, lack of continuity or "inside" corner is the common characteristic of overall building Figure 5 showing unbalanced location of configurations that, in plan, assume the shape of perimeter an L,T,H,+, or tocombination wall leading torsional forcedof andthese shapes occurs due to lack of tensile capacity and forces concentration. Theearthquake, re-entrant1964 corners of partial collapse in Alaska the building are subjected to two types of problems. The first is that they tend to produce variation in rigidity, and hence differential motions between different parts of the building, resulting in a local stress concentration at the notch of the re-entrant corner. The second problem is torsion. In figure, an L-shaped building is subjected to a ground motion of Alaska earthquake, attempt to move differently at their notch, pulling and pushing each other. So the stress concentration are high at notch. To avoid this type of damage, either provide a separation joint between two wings of the building or tie the building together strongly in the system of stress concentration and locate resistance elements to increase the tensile capacity at re-entrant corner. Non-parallel system The vertical load resisting elements are not parallel or symmetrical about the major orthogonal axis of the lateral force resisting system. This condition results in a high probability of torsional forces under a ground motion, because the center of mass Figure 6 showing damage concentration at the and resistance does not coincide. This problem intersection of two wings of an L-shaped school, Alaska Source: www.google.co.in is often exaggerated in the triangular or wedge shaped buildings resulting from street inter-sections at an acute angle. The narrower portion of the building will tend to be more flexible than the wider ones, which will increase the tendency of torsion. To design these types of buildings, special care must be exercised to reduce the effect of torsion or to increase torsional resistance of the narrow parts of the building. Figure 7 showing building collapse at intersection of glouster and mancester streets, Lorca eartquake, 2011
Source: www.google.co.in
Diaphragm Discontinuity The diaphragm is a horizontal resistance element that transfer between vertical resistance elements. The diaphragm discontinuity may occur with abrupt variations in stiffness, including
those having cut-out or open areas greater than 50% of the gross enclosed diaphragm area, or change in effective diaphragm stiffness of more than 50% from one storey to the next. The diaphragm acts as a horizontal beam, and its edge act as flanges. It is obvious that opening cut in tension flanges of a beam will seriously weaken its load carrying capacity. In a number of buildings there has been evidence of roof diaphragms, which is caused by tearing of the diaphragm.
REFERENCES
Figure 8 showing failure resulting from diaphragm flexibility in Loma Prieta, 1989. Source: www.google.co.in
IS 1893, “criteria for earthquake resistant design of structure (part 1) General Provision and buildings”, Bureau of Indian Standard, 2002.
Pankaj Agarwal and Manish shrikhande, “Earthquake Resistant Design of Structure”, July 2014. S.k. Duggal, “Earthquake Resistant Design of Structure”, 2013. www.google.com www.wikipedia.com
