Presentation on theme: "In Collaboration with NUST 15-16 May 2007 In Collaboration with NUST 15-16 May 2007 PAKISTAN ENGINEERING COUNCIL Two Days Short Course on EARTHQUAKE ENGINEERING."— Presentation transcript:
In Collaboration with NUST 15-16 May 2007 In Collaboration with NUST 15-16 May 2007 PAKISTAN ENGINEERING COUNCIL Two Days Short Course on EARTHQUAKE ENGINEERING
Earthquake protection - present Shear walls are made of reinforced concrete and add strength and stiffness to the building. Shear walls can well handle short duration earthquakes, but if the duration of an earthquake is longer, the stresses developed in the shear walls become so high that they may fail and cause building collapse. Shear walls in a structure can raise the structural cost by 7-10 %
Earthquake protection - present Braced frames and moment-resistant frames maintain structural integrity by flexibility
Earthquake protection - concept Approach - force vs energy –Use of seismic forces –Based on acceleration –Review resulting lateral drift/movement Conflict - for stiff soil site, larger the acceleration –Larger the seismic force –Stronger the structure –Stiffer the structure –Higher the seismic acceleration/forces
Earthquake protection - concept Strong stiff structure is good conceptually and lateral drift is also minimum Contradiction comes from energy dissipation which comes from larger displacements Dissipation of seismic forces require large displacements, drift control requires smaller displacements Useful seismic systems should have predictable stable and non degradable cyclic behavior Base isolation, Moment frames with dampers and Dual-resistance systems with built-in redundancy
Earthquake protection - trends A building can itself dissipate or dampen the energy from earthquakes. However, the capacity of buildings to dissipate energy before they begin to deform and damage is quite limited. By equipping a building with devices which have high damping capacity, we can greatly decrease the seismic energy entering the building, and thus decrease building damage. Damping devices are usually installed as part of bracing systems. This arrangement provides the column with additional support. Most earthquake ground motion is in a horizontal direction; so, it is a building's columns which normally undergo the most displacement relative to the motion of the ground.
Earthquake protection - trends The most common advanced technique is base isolation. The structure is supported by a series of bearing pads which are placed between the building and its foundation. The bearing pads are made from layers of rubber sandwiched together with layers of steel. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.
Fluid dampers The fluid damper consists of a stainless steel piston with bronze orifice head. It is filled with silicone oil. The piston head utilizes specially shaped passages which alter the flow of the damper fluid and thus alter the resistance characteristics of the damper.
Linear friction dampers Linear friction dampers consist of sliding steel plates and work on the principal that when two metal surfaces slide, friction heat is produced and energy gets dissipated. These types of dampers are susceptible to corrosion and cold welding which has a direct effect on the yielding threshold.
Rotational friction dampers The rotational friction damper dissipates energy by friction of two plates rotating around a friction pad. It is the same principle as when a car is braked and its kinetic energy is dissipated through the rotational movement in the disc brake around a friction pad. The harder the brake is clamped, the more energy is dissipated.
Energy dissipation various methods The hysteresis (force-deflection curve) shows the efficiency of a damper during an earthquake. The (blue) area inside the curve is the total dissipated energy. The curve shows that the rotational friction damper dissipates more energy than any other damper today. FrictionYielding metal ViscoelasticFluid
Strong bending moment Anti-EQ design to make substructure (piers) of bridges safe for this bending moment. Fixed support Movable supports The most suitable approach to seismic protection by replacing conventional bearings by HDRB To protect superstructure To protect substructure
ELASTOMERIC - BASED SYSTEMS Base Isolation Separate Type Standard Laminated Rubber Bearing + Steel-bar Damper + Lead Plug Damper + Hydraulic Damper Lead rubber bearing High damping rubber bearing Built-in type
MECHANICAL CHARACTERISTICS OF ELASTOMETRIC BEARINGS Horizontal stiffness K H =GA/t r G- Shear modulus of elastomer, A is a full cross section area, t r -total thickness of rubber Vertical stiffness K v =E c A/t r E c -instantaneous compression modulus of rubber steel composite and is controlled by shape factor S, for circular pad of radius R and thickness t, S=R/2t, for square pad of site a and thickness t, S=a/4t For single circular pad E c =6GS 2 For square pad E c = 6.73GS 2
Natural rubber/ high damping bearings At 100% shear strain damping is increased between 10%-20%. Damping is neither viscous nor hysteretic (between). Many steel shims (vertical load capacity & stiffness and prevent lateral bulging of rubber. Thick steel endplate Composite material rubber+steel shims Rubber sheets are vulcanized and bonded onto thin steel Plates under pressure and heat Neoprene becomes extremely stiff at -40 C and natural rubber at -55 C
LEAD RUBBER BEARING (the most frequently used) Energy dissipation core (reduced EQ forces & displacement by energy dissipation) Internal rubber leria+ steel reinforces plate Cover rubber Protecting steel plates from corrosion Thick steel endplate Steel plates in bearing force lead plug to deformed in shear. Lead must fit tightly in element bearing, lead plug is slightly larger than hole (1%) and forcing it in.
Friction pendulum (spherically shaped sliding bearing)-the biggest in the world Spherical stainless steel surface Slider Lateral force that slide the structure depend of curvature and vertical load Energy dissipation is generated by friction between slider and spherical surface Coefficient of friction is not constant (varies with time and temperature)
Laminated Rubber Bearing (low-damping natural) + Steel Rod Damper Material in shear quite linear up to Shear strain of 100% damping 2-3% Purpose to have linear viscous dynamic model (rubber fitted the model damper was not exactly linear viscous element) Advantage Simple to manufacture Disadvantage require elaborate connections and metallic dampers are prone to low-cycle fatigue
OLD NEW Seismic Retrofit of Dangsan Railway Bridge (1999) –Replacement of the old bridge due to cracks in superstructure –New superstructure –Increasing seismic capacity using seismic isolator –Reuse of foundations –Retrofit of concrete piers