Earthquake resistance design

Columns can sustain two types of Damage, namely axial- flexural (or combined compression bending) Failure and shear failure. Shear damage is brittle and must be avoided in columns as by providing transverse ties at close spacing which carry the horizontal shear forces and hold concrete and vertical bars together. Horizontal Bands and its Role: Horizontal bands are the most important Earthquake-resistant feature in masonry buildings, since it holds a masonry building as a Single unit by tying all the walls together. There are four types of bands in a typical Masonry building, namely gable band, roof band, lintel band and plinth band. Lintel band is the most Important of all, since it ties the walls together and also breaks the monotonous continuity of wall.The gable band is employed only in Buildings with pitched or sloped roofs. In buildings with flat R. C or reinforced brick roofs, the Roof band is not required. In buildings with pitched r sloped roof, the Roof band is very importan t. Plinth bands are primarily used where uneven settlement of foundation in soil undergoes bending and pulling actions. It will be better to use ARC bands Shear Wall: Reinforced concrete buildings often have vertical plate-like ARC walls called Shear Walls. These walls generally start at foundation level and are contain souse throughout the building height. Their thickness varies from mm to mm. Shear walls are usually provided along both length and width of buildings.Shear Core Shear walls are like vertically-oriented wide beams that carry earthquake odds downwards to the foundation. Just like reinforced concrete (ARC) beams and columns, ARC 6 shear walls also perform much better if designed to be ductile.. Shear walls, if provide around the elevator core or stair well is known as shear core. Boundary Elements: Under the large overturning effects caused by horizontal earthquake forces, edges of shear walls experience high compressive and tensile stresses. To ensure that shear walls behave in a ductile way, concrete in the wall end regions must be reinforced in a special manner to sustain these load reversals without loosing strength.End regions of a wall with increased infotainment are called boundary elements which have high bending strength. Boundary Elements (Design, Location) Short and Long Columns: During past earthquakes, reinforced concrete (ARC) frame buildings that have columns of different heights within one storey, suffered more damage in the shorter columns as compared to taller columns in the same storey. Short Column Behavior: Poor behavior of short columns is due to the fact that in an earthquake, a tall column and a short column of same cross-section move horizontally by same amount. However, the short column is stiffer as compared to the tall column, ND it attracts larger earthquake force. Therefore it cause X-shaped cracks.Short Column (Failure, Location with Mezzanine floor) Stiffness of a column means resistance to deformation -? the larger is the stiffness, larger is the force required to deform it. This behavior is called Short Column Effect. Design Phenomenon: 7 If it is not possible to avoid short columns, this effect must be addressed in structural design. As per Indian Standard the reinforcement must extend beyond the short column into the columns vertically above. In case of stone or brick masonry the width has to be increased accordingly for short column. BEAM COLUMN JOINT: The points where the beams and columns intersect is a beam column joint. Since they too made of same material we can't expect to have more strength. So have to take care on these unavoidable joints.During earthquake the upper bars and lower bars act in a different direction causing elongation or damage of joint. Design Strategy: In design practice large column size, having large closed loops are placed inside. These should follow some design specification. Normally we will go for the anchoring of the bars at the ends. Micro concreting can b e gone in the congested junction. Beam Column Joint (Location, Failure without proper anchorage) Hidden Beams: These are also called as concealed beams which have their depth equal to that of the slab. These can be provided either on longer or on the shorter span. When provided along longer span it is found that the load carrying capacity increase to 135% with an economical increase of just 0. 4 -? 0. 5%.These beams are designed for negative bending moment which is caused due to load reversal expected during earthquake. 8 Hidden Beam Plastic Hinge: As moment increases, the linear stress distribution form persists and the extreme fiber stress reaches the yield stress value. Further increase in the bending moment cannot produce any increased fiber-stress but causes yield to spread into the inner fibers. As the bending moment Increases more and more fibers reach the yield stress until the final state, the whole of the section will yield. The complete yielding across the section of a be am is termed as plastic hinge. The section now carries the maximum bending moment without strain hardening taking place. The beam can carry no further load.Any further load will only result in increased deflection. The beam will behave as if t is hinged at the plastic section and a condition of collapse has been reached. Reduced Beam Section: This is a section of beam which is provided along the length of steel beams. These will have their area of cross section lesser than the proceeding section to an extent that it will just act as a plastic hinge. In case of steel section also a circular arc will be cut in the required flange portion of span. Reduced Beam Section Pre tensioning Technique: In case Of domes and shell Structures, the lateral thrust experienced will be more. This fault is answered well by pre tensioned concrete.In case of huge trustees like nuclear 9 rectors, large spanning domes we will be having a thin walled cylindrical tube of diameter about 10 to 15 CM and steel rods will be packed tightly. Pre tension elements (After & Before Concreting) Then stressing will be done as per design and then the micro concrete is injected in pressure into the tube. This setup is then done with normal concreting. It will resist the lateral thrust in an effective manner. Techniques to Adopt on Sky Scrappers: While speaking about large multistoried buildings we can't simply go in for normal strengthening of beams, columns, and other structural elements. There we had an alternative to speak about some elements such as Bearing, Bracing, Friction pendulum and Dampers which are primarily meant to take Vibration produced by lateral force.Rubber Bearings: Rubber bearings are made from layers Of rubber with thin steel plates between them, and a thick steel plate on the top and bottom. The bearings are placed between the bottom of a building and its foundation . The bearings are designed to be very stiff and strong for vertical load to carry the weight of the building an d designed to be much weaker for horizontal loads, so that they can move sideways due to lateral thrust. Rubber Bearing Viscous Dampers Viscous Dampers: Viscous fluid dampers are meant as shock absorbers. They consist of a closed cylinder containing a viscous fluid and a piston having small holes in its head. As the 10 piston move in and out of the cylinder oil is forced in and out causing friction.The damper is usually installed as part of a building's bracing system using single diagonals. As the building sways to and fro, the piston is forced In and out of the cylinder. Friction Dampers: Friction dampers are designed to have moving parts that will slide over each other. The damper is made up from a set Of steel plates, with slotted holes in hem, and they are bolted together. At high enough forces, the plates can slide over each other creating friction causing energy dissipation. The plates are specially treated to increase the friction between them. Friction Dampers Cross Bearing s (In foundation) Cross Bracing: These are very common in case of vertical load distribution.But we can also adopt this technique to foundation, in which the entire building will be laid in a cross horizontal bracing rather than placing it directly on foundation. It will distribute the load to joints and through foundation finally. Friction bearing (Location, Appearance) 11 Friction Pendulum: Considering about the large multistory buildings, we can always expect some appreciable movement in it base due to the vibration. Instead to resist against it completely we can allow the structure to deform at its foundation level by provision of friction pendulum without damaging the structural integrity.