Construct Earthquake Resistant Buildings by Simple Means

The engineering science is continuing to advance in response to seismic threats. There have been significant breakthroughs in the field. However, most of them are very complex and require exceptional machinery. Not to mention, expensive as well. However, there are some simple ways to build a structure that will be resistant to earthquake damages up to a certain level.

In areas where seismic activity is not too harsh, we can utilize these techniques to same money and complexity but make the building resistant to seismic activities.

Structure Stiffness: The most traditional way to fight quakes is to use stronger materials to construct the building. Stiffer or heavier members can be used to fight the lateral forces generated during seismic activities. For special quake-proof structures, ACI codes prescribe at least 10” thick members.

Geometrical Absorption: The building can be planned in such a regular and special geometrical shape that it disperses the seismic forces evenly so that no particular member experiences excessive force. This naturally fares much better than a poorly-planned unsymmetrical building.

For existing buildings that are structurally asymmetrical, you can use seismic joints and expansion points in places where the forces are dispersed unevenly. Providing extra columns, shear walls, and framing can make the weaker section withstand the extra forces to a good level. Parking levels should have extra reinforced columns in order to negate the soft story effect.

Lateral Force Resistance: Using three types of lateral force resisting systems, we can try to negate much of the seismic forces. These are:

1. Moment Resisting Frame System: it is designed to resist all types of earthquake generated forces acting on the structure. They can be customized to fit the seismic activity scale of the region.

2. Building Frame System: these are designed to resist gravitational loads only, but they function excellently in that. A shear wall is added to resist the lateral forces acting on structure.

3. Dual Frame System: this is a combination of the above two systems. Shear walls along with moment resisting frames work excellently to fight off the vibrations and displacements from an earthquake. But, of course, they are more complex and costlier to build.

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Published By
Rajib Dey
www.constructioncost.co
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What are Shear Keys and How to Use Them

A building and structure has to bear not only vertical loads, but also lateral loads. This can occur due to many design or natural reasons. To counteract this lateral load, Shear Keys are used. Let us see today what exactly are shear keys and how can you use them in your construction.

A building can face a lot of lateral load due to many reasons. Some of these include earthquake loads, sliding forces, water pressure, wind pressure etc. This often occurs with bridges, retaining walls, basements, extremely tall buildings, precast buildings and culverts, masonry walls where seismic activity is stronger, and steel columns and piers.

Shear keys can be constructed by concrete in precast buildings, and of steel in steel structures. Sometimes, steel reinforcements are made to play the part of shear keys as well. They improve the lateral stability of a building.

Placement of Shear Keys: Where the shear keys will be placed differs a lot depending upon the structure they need to support. Let us discuss them below.

Bridges: In small to medium bridge structures, shear keys are placed in the abutments of the bridge. This provides lateral transverse support to the structure above during sidewise movement.

During an earthquake or similar strong lateral force application, the shear keys act as a sacrifice. Therefore, the strong seismic forces are prevented from entering the abutment piles.

They act better when they are placed on the outer side of the bridge. However, this makes them difficult to inspect or repair.

Retaining Walls: You should place shear keys at the base of retaining walls, just beneath the stem. This prevents sliding of the base during strong lateral force application.

The shear keys placed beneath the wall must be nearly twice in width than their depth for the best performance. Generally, they are 508mm by 381 mm, respectively.

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Published By
Rajib Dey
www.constructioncost.co
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Seismic Zones in India

In the field of seismology, seismic zones are, areas divided based on the frequency and intensity of expected earthquake. Indian subcontinent comprises of four seismic zones those are- II, III, IV and V. These zones are categorized based on scientific research related to seismicity and earthquake occurrence in the past years.

Earlier India was divided into five zones, but then, The Bureau of Indian Standards [IS 1893 (Part I):2002] decided to group the country into four seismic zones, where, the first and second zone being unified.

The Bureau of Indian Standards is responsible for publishing seismic activities in terms of seismic hazard maps and codes. They brought out a total of three versions of seismic zones; a six zone map in 1962, a seven zone map in 1966 and a five zone map in 1970/1984.

Seismic Active Zone:

Seismic Zone II: This is the area that suffers least damage of the other three zones. Intensity of earthquake lies between intensities V to VI of MM scale (MM – Modified Mercalli Intensity scale).

Zone II covers those areas which are not covered by the other zones listed below.

Seismic Zone III: Zone III receives moderate damage. This damage corresponds to intensity VII of MM scale.

States that lie under this zone are- Tamil Nadu, Orissa, Andhra Pradesh, Maharashtra, Chhattisgarh, Bihar, Jharkhand, Bihar, Madhya Pradesh, Kerala, Gujarat, Goa, Lakshadweep islands, West Bengal, Karnataka, parts of Punjab and some remaining parts of Uttar Pradesh.

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Published By
Rajib Dey
www.constructioncost.co
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Impacts of earthquake on structures

Earthquake produces severe damages to the structures. For this purpose, thorough knowledge about the seismic effects on a structure is required. The designers and contractors should be capable of analyzing the effect of seismic forces on buildings to adopt protective measures against failures and collapses.

When earthquake strikes on structures, it develops damaging inertia forces which lead to deformations as well as horizontal and vertical shaking.

Given below, detail explanation about these effects :-

Impacts of Earthquake on Structures

1. Inertia Forces in Structures: The formation of inertia forces in a structure refers to one of the seismic influences that adversely damage the structure. When ground shaking occurs due to earthquake, the base of the building proceeds but the roof remains motionless. As the walls and columns are connected with it, the roof is pulled by the base of the building.

The susceptibility of the roof structure to stand at its original position is known as inertia. The inertia forces lead to shearing of the structure that can consolidate stresses on the fragile walls or joints in the structure causing failure or perhaps total collapse. Lastly, more mass signifies greater inertia force and due to this lighter buildings can resist the earthquake shaking efficiently.

2. Impact of Deformations in Structures: When a building undergoes earthquake along with ground shaking, the base of the building proceeds with the ground shaking. But, the roof movement varies from that of the base of the structure. This variation in the movement produces internal forces in columns and as a result the column goes back to its original position.

These internal forces are known as stiffness forces. The stiffness forces become greater when the sizes of columns are raised. The stiffness force in a column belongs to the column stiffness times the relative displacement among its ends.

3. Horizontal and Vertical Shaking: Earthquake contributes to shaking of the ground in all the three directions X, Y and Z, and the ground shakes indiscriminately from side to side along each of these axis directions. Normally, the purpose of designing the structures is to resist the vertical loads in order that the vertical shaking resulting from earthquakes (either adds or subtracts vertical loads) is controlled through safety factors provided in the design to sustain vertical loads.

However, horizontal shaking along X and Y directions is dangerous for the operation of the structure as it develops inertia forces and lateral displacement and consequently sufficient load transfer path should be arranged to resist its detrimental influences on the structure.

Exact inertia force transfer path is formed through adequate design of floor slab, walls or columns, and connections among these structural components. It should be noted that the walls and columns are vital structural components in transmitting the inertial forces. The masonry walls and thin reinforce concrete columns create weak points in the inertia force transfer path.

4. Other Effects: Due to earthquake various other effects may occur which range from liquefaction, tsunami, and landslides. These belong to the indirect effects of strong earthquakes that can lead to significant devastation.

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Published By
Rajib Dey
www.constructioncost.co
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How to build up an earthquake-resistant structure

Earthquake means quick shaking of the ground due to the shift of rock and tectonic plates underground. The ground appears as solid, but the topmost crust of earth is deep and long periods of time produce pressure to develop among plates and fissures.

When the pressure is applied, seismic vibrations and fierce shaking reverberate to the surface which instantly impact miles of land. Once the initial quake hits, aftershocks happen to create further damage.

The buildings should withstand radical movement and foundation shifts so as to reduce damage and safeguard the people inside and around them. Earthquake-resistant building designs should be created based on the following characteristics which impact their structural integrity: stiffness and strength, regularity, redundancy, foundations, and load paths.

Stiffness and Strength: While creating design for earthquake-resistant buildings, safety professionals suggest sufficient vertical and lateral stiffness and strength – specifically lateral. Structures are likely to deal with the vertical movement resulting from quakes superior to the lateral, or horizontal, movement.

Devoid of taking earthquakes into consideration, professionals still concentrate on a building’s vertical stiffness and strength since it has to support itself. However, earthquakes present new directional forces and owing to these, buildings will shift left and right, and, if not constructed perfectly, will rapidly destabilize.

Regularity: This characteristic pertains to the movement of the building if pushed in lateral directions. According to safety professionals and building designers, the building should move uniformly in order to disperse the energy devoid of placing extra force on one side or another. When a building is uneven, then flaws will be detectable while the building shakes. The flaws will be settled and the structure will experience concentrated damage – which settles the structure completely.

Redundancy: It is a vital safety characteristic while designing for safety. Redundancy assures that there exist several strategies in place whether one fails. These can possibly be add to the building cost, but redundancies become vital if/when a natural disaster like an earthquake happens. Safety professionals suggest to uniformly allocate mass and strength during the structure so strength isn’t entirely dependent on one factor.

Foundations: A steady foundation is a vital characteristic of constructing a large structure irrespective of natural disaster risks. It is important for a building’s long-term existence, and a robust foundation is required to withstand an earthquakes powerful forces. Different areas have unique foundational characteristics that define how a structure’s base needs to be reinforced. Professionals should stringently monitor reactions of the ground and movement prior to starting of construction. The buildings developed to withstand fierce earthquakes contain deep foundations and driven piles. To settle these radical measures, the foundations should be joined properly to facilitate moving as a unit.

Continuous Load Path: Binding into the stable foundation characteristic, structural and nonstructural elements of a building should be interlinked in order that inertial forces are dispersed. Numerous points of strengths and redundancies distribute the force rather than the quake partitioning the foundation apart. It refers to the continuous load path characteristic that safety professionals, architects, and engineers should be aware of at the time of creating the design. When the structure is not entirely fastened jointly, elements fail to shift separately and collapsing can occur. The incessant load path is the journey of earthquake through the building – laterally and vertically. The path should be intact so that it can disperse an earthquake’s powerful shudders.



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Published By
Rajib Dey
www.constructioncost.co
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