Earthquake Resistant Structure: Essential Design Principles for Seismic Safety

Past earthquakes around the world have demonstrated that many common buildings and typical construction methods lack basic resistance to seismic forces. In most cases, however, this resistance can be achieved by following simple, inexpensive principles of good building construction practices. Experience shows that adherence to these rules will not prevent all damage in moderate or large earthquakes, but life-threatening collapses can be prevented and damage limited to repairable proportions. Understanding the fundamentals of Earthquake Resistant Design 3 is essential for engineers, builders, and students seeking to create safer structures in seismically active regions.

Understanding the Fundamentals of Earthquake Resistant Design

Earthquake resistant design is built upon a set of straightforward principles derived from studying how buildings perform during seismic events. These principles fall into several broad categories that guide engineers from the initial planning stage through to detailed structural design.

General Principles from Past Earthquakes

From extensive study of earthquake damage patterns, several general principles have emerged that form the backbone of seismic design:

1. Structures should not be brittle and should not collapse suddenly. Instead, buildings should be tough and able to deflect or deform a considerable amount before failure.
2. Resisting elements such as bracing or shear walls must be provided evenly throughout the building in both directions side to side as well as top to bottom.
3. All elements such as walls and the roof should be tied together to act as an integrated unit during earthquake shaking, transferring forces across connections and preventing separation.
4. The building must be connected to a good foundation. Wet soil should be avoided, and the foundation must be well tied together as well as tied to the wall. Where soft soils exist, strengthening must be provided.
5. All materials used must be of good quality and protected from sun, rain, insects and other weakening actions so that their strength lasts throughout the building’s lifetime.
6. Unreinforced earth and masonry have no reliable strength in tension and are brittle in compression. They must be suitably reinforced by steel or wood.

The Role of Ductility, Deformability and Damageability

Three interconnected concepts are central to understanding how buildings withstand seismic forces: ductility, deformability and damageability. These qualities determine whether a structure survives an earthquake with repairable damage or suffers catastrophic failure.

Ductility formally refers to the ratio of displacement just prior to ultimate collapse to the displacement at first damage or yield. Some materials are inherently ductile, such as steel, wrought iron and wood, while others such as cast iron are not. A ductile structure can absorb significant energy through deformation before failing.

Deformability is the ability of a structure to displace or deform substantial amounts without collapsing. Besides relying on the ductility of materials and components, deformability requires that structures be well proportioned, regular and well tied together so that excessive stress concentrations are avoided and forces can be transmitted from one component to another even through large deformations.

Damageability refers to the ability of a structure to undergo substantial damage without partial or total collapse. This is a desirable quality because it allows buildings to be repaired after an earthquake rather than demolished. A key to good damageability is redundancy, which means providing several supports for key structural members such as ridge beams. Central columns or walls supporting excessively large portions of a building should be avoided.

Categorizing Buildings for Seismic Resistance

For categorizing buildings with the purpose of achieving seismic resistance at economical cost, three parameters are significant. Their combination determines the extent of seismic strengthening required. For a deeper look, refer to Earthquake Resistant Design which covers building categorization in more detail.

1. The seismic intensity zone where the building is located
2. How important the building is for post-disaster functionality
3. How stiff the foundation soil is

Seismic Zones and Their Significance

In most countries, macro-level seismic zones are defined on the basis of seismic intensity scales. The zones indicate the level of risk a region faces:

• Zone A: Risk of widespread collapse and destruction. This is the highest risk category requiring the most stringent design measures.
• Zone B: Risk of collapse and heavy damage. Buildings in this zone require significant seismic strengthening.
• Zone C: Risk of minor damage. While the risk is lower, basic seismic design principles still apply.

Building Importance Classification

The importance of a building determines the level of seismic strengthening it requires. Buildings are classified into two broad categories:

• Important buildings: Hospitals, clinics, communication buildings, fire and police stations, water supply facilities, cinemas, meeting halls, schools, and cultural treasures such as museums, monuments and temples. These facilities must remain functional after an earthquake.
• Ordinary buildings: Housing, hostels, offices, warehouses, factories and similar structures. While these also need seismic protection, the design criteria may be less stringent than for essential facilities.

Foundation Soil Types and Bearing Capacity

The type of soil on which a building rests significantly affects its seismic performance. Three soil types are considered in seismic design:

Buildings can be constructed on firm and soft soils with appropriate design measures, but it is dangerous to build on weak soils. Appropriate soil investigations should be carried out to establish the allowable bearing capacity before construction.

Site Selection and Foundation Considerations

The choice of site for a building from a seismic point of view is mainly concerned with the stability of the ground. Two primary site hazards must be evaluated before construction, and understanding Types of Earthquake Resistant Masonry Walls Construction can help guide site-specific design choices.

Choice of Site for Seismic Stability

Stability of Slopes

Hillside slopes liable to slide during an earthquake should be avoided. Only stable slopes should be chosen to locate a building. Where building on slopes is unavoidable, it is preferable to have several blocks on terraces rather than one large block with footings at very different elevations. Sites subject to rock falls must be avoided entirely.

Very Loose Sand and Sensitive Clays

These two types of soil are liable to be destroyed by earthquake shaking to such an extent that they lose their original structure and undergo compaction. This results in large unequal settlement that can severely damage the building. If loose cohesionless soils are saturated with water, they are apt to lose their shear resistance altogether during shaking and become liquefied, a phenomenon known as soil liquefaction.

Foundation Types for Different Soil Conditions

For making a building truly earthquake resistant, it is necessary to choose an appropriate foundation type. Since loads from typical low-height buildings are light, providing the required bearing area is not usually a problem. The depth of footing should go below the zone of deep freezing in cold countries and below the level of shrinkage cracks in clayey soils.

Firm Soil Foundations

In firm soil conditions, any type of footing can be used. The footing should have a firm base of lime or cement concrete with the requisite width. In Zone A, it is desirable to connect individual reinforced concrete column footings by means of RC beams just below plinth level, intersecting at right angles. This interconnection helps distribute seismic forces more evenly throughout the foundation system.

Soft Soil Foundations

In soft soil, it is desirable to use a plinth band in all walls and to connect individual column footings by means of plinth beams. Continuous reinforced concrete footings are considered most effective from earthquake considerations as well as for avoiding differential settlements under normal vertical loads. These should ordinarily be provided continuously under all walls with the following specifications:

• Continuous footings should be reinforced both in the top and bottom faces
• Width of the footing should be wide enough to make contact pressures uniform
• Depth of footing should be below the lowest level of weathering

Structural Safety Requirements and Isolation Concepts

Beyond site selection and foundations, the structural system itself must meet specific safety requirements to resist earthquake forces effectively. For additional information on advanced seismic design approaches, see Earthquake Resistant Design 2.

Key Structural Safety Requirements

The following requirements must be satisfied to ensure structural safety during an earthquake:

1. A freestanding wall is difficult to achieve in unreinforced masonry in Zone A. Therefore, all partitions inside buildings must be held on the sides as well as the top. Parapets must be reinforced and held to the main structural slabs or frames.
2. Horizontal reinforcement in walls is required for transferring their own out-of-plane inertia load horizontally to shear walls.
3. Walls must be effectively tied together to avoid separation at vertical joints due to ground shaking.
4. Shear walls must be present along both axes of the building to resist lateral forces in every direction.
5. A shear wall must be capable of resisting all horizontal forces due to its own mass and those transmitted to it from other structural elements.
6. Roof or floor elements must be tied together and be capable of exhibiting diaphragm action, distributing lateral loads to vertical resisting elements.
7. Trusses must be anchored to the supporting walls and have an arrangement for transferring their inertia force to end walls.

The Concept of Base Isolation

Base isolation is a technique aimed at reducing the transmission of seismic forces from the ground to the structure. One suggested technique is to place two layers of good quality plastic between the structure and its foundation so that the plastic layers may slide over each other during seismic shaking. This sliding action decouples the building from ground motion, significantly reducing the forces that reach the superstructure. Modern base isolation systems have evolved far beyond this simple concept, using sophisticated bearings made of rubber and steel, but the underlying principle remains the same.

Requirements for Walls, Roofs and Trusses

Each structural element in a building has specific seismic requirements:

Earthquake resistant design is not about making buildings earthquake-proof, which is economically impractical. Rather, it is about ensuring that buildings can withstand seismic forces without collapsing, protecting lives and allowing for repair after moderate events. By following the principles of proper site selection, appropriate foundation design, structural regularity, ductility, and good construction quality, engineers can create buildings that perform well during earthquakes at minimal additional cost.