02-01-2013, 02:19 PM
Improving Performance Of Building during Earthquake
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Abstract
Engineer does not attempt to make earthquake proof building. Instead, there intention is to make building earthquake resist. Earthquake resistant building, particularly their main elements, needs to be build with ductility in them. Now a day, reinforced concrete building is mostly use. Earthquake shaking generate inertia force in the building, which is proportional to the building mass. Design of beam in RC building for good seismic performance is very essential. Beam is a horizontal member in RC building, which fails due to flexural failure and Shear failure. Design of column in RC building for good seismic performance Indian standard IS13920-1993 prescribes design. Design of beam-column joint, and shear wall performance should be taken into consideration in RC building for good seismic performance. Vertical plate like RC wall in RC building called shear wall in addition to slab, beams and column. Shear walls are like vertically oriented wide beams that carry earthquake load downwards to foundation. We cannot afford to build concrete building meant to resist severe earthquake without shear walls. Reinforcement bar in RC walls, isolation technique and seismic damper are the most important for earthquake proof building. Earthquake resistant structure therefore depends on capacity of structure to resist the earthquake inertia force.
Improving Performance of Building
During Earthquake
1) Introduction
Earthquake is perceptible movement of earth surface. Primary cause of earthquake is the rapture of fault in earth crust and associated rapid slip on the faults. Large strain energy released during an earthquake travels as seismic wave in all direction through earth’s layers. There are basically two types of seismic wave as Body wave and Surface wave. Body wave consist of Primary wave (P-wave) and Secondary wave (S-wave) where as surface wave consist of Love wave and Raleigh wave.
S-wave in association with effect of Love wave causes maximum damage to structure by raking motion on the surface in horizontal and vertical direction. Seismic wave then passes through structural component such as foundation, beam, column, column-beam joint, slab that generate inertia force at top of structure due to which structure may collapse, this lead to loss of human being. So, to avoid this, performance of building during earthquake has to improve. As now days reinforced concrete buildings are mostly use, some design for improving performance of RCC building during earthquake are described.
2) Seismic design philosophy of building
In earthquake, minor shaking occurs frequently, moderate shaking occasionally and strong shaking rarely. Engineer does not attempt to make earthquake proof building. Instead, there intention is to make building earthquake resistant. Such building resists the effect of ground shaking, although they may get damaged severely but would not collapse during strong earthquake. There by safety of people and contents is assured in this type building, therefore earthquake design philosophy may be as fallows
a) Under minor but frequent shaking, the main members of building that carry vertical and horizontal forces should not be damaged, however building parts that do not carry load may sustain repairable damage.
b) Under moderate but occasional shaking the main members may sustain repairable damage, while other part of the building may be damaged such that they have to be replaced after earthquake.
c) Under strong but rare shaking the main members may sustain severe damage, but building should not collapse.
Earthquake resistant design should be concerned about ensuring that damage in building during earthquake are of acceptable variety and also that they occur at right place and in right amount. Therefore to save building from collapsing we need to allow some pre-determined part to undergo the acceptable type and level of damage.
Earthquake resistant building, particularly their main elements, needs to be build with ductility in them. Such building have the ability to sway back and forth during an earthquake and to withstand earthquake effect with some damage, but without collapse thus for earthquake resistant design we have to ensure ductile behavior of the building at location where damage may take place.
3) Behavior of reinforced concrete building during earthquake
Now a day, reinforced concrete building is commonly used in town and cites. It consists of frame of beam and column called RC frame. Earthquake shaking generate inertia force in the building, which is proportional to the building mass. These forces travel downward through slab and beam to column and wall then to the foundation from where they are dispersed to the ground. Therefore, lower storey experiences higher earthquake induced forces and therefore designed to be stronger than those in storey above as shown in figure.
Vertical space between column and floor are usually filled in with masonry wall called infill wall, are not connected to surrounding column and beam. When column receive horizontal force at the floor level, they try to move in horizontal direction but masonry tend to resist this movement. Due to their height and thickness these wall attract rather large horizontal force, but masonry is a brittle material. These walls develop cracks once their ability to carry horizontal load is exceeded. Thus infill wall develop cracks under severe ground shaking but help share the load of beam and column until cracking.
Therefore, earthquake performance of infill wall is improved by mortar of good strength, making proper masonry course sand proper packing of gap between RC frame and masonry infill walls.
4) Design of beam in RC building for good seismic performance
Beam is a horizontal member in RC building. Beam can sustain basically two type of failure namely flexural or bending failure and shear failure.
1. Flexural failure
As beam sag under increased loading, it can fail in two possible ways if more steel is present on tension face concrete crushes in compression this is brittle failure. If relatively less steel is present on tension face, steel yields first and redistribution occurs in the beam until eventually the concrete crushes in compression this is ductile failure. The ductile failure is characterized with many vertical cracks starting from beam face going toward its mid depth
as shown in figure.
2. Shear failure
Beam may fail due to shearing action. A shear cracks is inclined at 45° to the horizontal, it develops at mid depth near the support and grows toward the top and bottom face as shown in figure. Closed loops stirrups are provided to avoid such shearing action. Therefore, shear failure must be avoided.
Design strategy
Designing a beam involves selection of its materials properties, amount and displacement of steel to be provided in the beam must be determining by performing design calculation as per Indian standard IS: 13920-1993.
1. Longitudinal bar
Longitudinal bar are provided to resist flexural cracking on the side of the beam that stretches, since both top and bottom faces stretch during strong earthquake shaking. Longitudinal steel bars are required on both face at the end and on the bottom face at mid length. Indian standard code IS: 13920-1993 prescribes that
a) At least two bars go through the full length of the beam at the top as well as bottom of the beam.
b) At the ends of beams, the amount of steel provided at the bottom is at least half that at top.
2. Stirrups
Stirrups in RC beam help in three ways,
a) They carry the vertical shear force and thereby resist diagonal shear cracks.
b) They protect the concrete from bulging outward, due to flexure.
c) They prevent the buckling of compressed longitudinal bars due to flexure.
Indian standard IS: 13920-1993 prescribes that
1. The diameter of stirrups must be at least 6mm in beam more than 5m long.
2.Both ends of vertical stirrups should be bend into 135° hook and extended sufficiently beyond this hook to ensure that stirrups does not open out during an earthquake.
3. The spacing of vertical stirrups in any portion of the beam should be determined from calculation.
4. The maximum spacing of stirrups is less than half the depth of beam.
5. For a length of twice the depth of the beam from the face of the column an even more stringent spacing of stirrups is specified, namely half the spacing mentioned above.