Earthquake Resistant Building Design Basics

Introduction

Earthquakes are among the most destructive natural disasters, causing significant damage to buildings and infrastructure. Earthquake-resistant building design is the process of designing structures that can withstand seismic forces and minimize damage during an earthquake. Proper design helps protect human lives and reduces economic losses.

What is Earthquake Resistant Design?

Earthquake-resistant design refers to the techniques and engineering practices used to make buildings safer during earthquakes. These structures are designed to absorb and dissipate seismic energy without collapsing.

Earthquake-resistant building design is the process of designing a building so that it can resist earthquake forces, control excessive movement and reduce the risk of collapse and severe damage.

In the United States, seismic design is primarily governed through the locally adopted building code and referenced standards, including ASCE/SEI 7. ASCE identifies ASCE/SEI 7-22 as the current edition of its nationally adopted loading standard, covering seismic design, ground motions and load combinations.

Important: The examples below are educational. Actual seismic design requires site-specific hazard data, geotechnical information, structural analysis and review by a qualified structural engineer.

1. What Is Earthquake-Resistant Building Design?

Earthquake-resistant design aims to ensure that a building has an appropriate combination of:

Strength
Stiffness
Ductility
Stability
Redundancy
Proper load paths
Adequate connections

The goal is not necessarily to prevent every crack or every instance of damage. Rather, seismic design seeks to achieve the performance required by the applicable code and project objectives, including protection against collapse under specified earthquake demands.

2. Why Earthquake-Resistant Design Is Important

Earthquake forces are different from ordinary gravity loads because they produce significant horizontal and dynamic forces.

During an earthquake, the ground moves while the building's mass tends to resist that movement.

This creates:

Inertial forces
Lateral forces
Story drift
Torsion
Structural deformation
Connection forces

A properly designed seismic-force-resisting system provides a controlled path for these forces to travel from the building mass through the structural elements and finally into the foundation and ground.

3. Main Principles of Earthquake-Resistant Design

1. Simple and Regular Configuration

Regular buildings generally have a more predictable seismic response.

Important considerations include:

Uniform mass distribution
Uniform stiffness
Continuous structural elements
Balanced lateral resistance

FEMA notes that irregular distributions of mass, strength and stiffness can concentrate damage and may trigger additional seismic design requirements.

2. Adequate Strength

Structural members must have sufficient strength to resist the applicable seismic demands.

Components may include:

Columns
Beams
Shear walls
Braces
Diaphragms
Connections
Foundations

3. Ductility

Ductility is the ability of a structural system to undergo significant deformation while maintaining useful strength.

Ductile behavior is important because severe earthquake shaking can cause structural elements to enter the inelastic range.

Proper seismic detailing helps structures dissipate earthquake energy in a controlled manner.

4. Continuous Load Path

Earthquake forces need a continuous path:

Floor/Roof → Diaphragm → Shear Wall/Frame/Brace → Foundation → Soil

A weak connection anywhere along this path can compromise the overall system.

4. Seismic Design Systems

Common lateral-force-resisting systems include:

Moment Frames

Beams and columns resist lateral forces through bending and frame action.

Braced Frames

Diagonal braces provide lateral resistance.

Common systems include:

Concentrically braced frames
Eccentrically braced frames
Special braced frames

Shear Walls

Reinforced-concrete or reinforced-masonry walls resist significant lateral forces.

Dual Systems

A combination of:

Moment frames
Shear walls or braced frames

may be used.

ASCE seismic design guidance covers selection of seismic-force-resisting systems and analysis methods.

5. Seismic Design Factors

A seismic design generally considers factors such as:

Building location
Ground motion
Soil/site conditions
Risk category
Building occupancy
Structural system
Building height
Structural irregularities
Foundation conditions

ASCE 7 provides seismic hazard and ground-motion information through its requirements and Hazard Tool.

6. Soil and Site Conditions

The same building can have different seismic design requirements depending on the site.

Important information can include:

Soil profile
Shear-wave velocity
Groundwater
Bearing conditions
Liquefaction potential
Site class
Fault/geologic conditions

A geotechnical investigation may be required or appropriate depending on the project.

7. Building Irregularities

Irregularities are particularly important in earthquake design.

Horizontal/Plan Irregularities

Examples include:

Torsional irregularity
Re-entrant corners
Diaphragm discontinuities

Vertical Irregularities

Examples include:

Soft story
Weak story
Sudden stiffness changes
Discontinuous walls
Discontinuous vertical elements

FEMA specifically identifies soft/weak stories, torsional irregularity, plan irregularities and discontinuous structural systems as conditions that can produce undesirable concentrations of earthquake damage.

8. Soft Story Problem

A soft story occurs when one level has substantially less lateral stiffness than the levels above.

A common example is:

Open ground-floor parking + heavily partitioned upper floors

During an earthquake, deformation can become concentrated at the weak/soft level.

This can significantly increase seismic risk.

9. Torsional Irregularity

Torsion occurs when earthquake forces cause a building to twist as well as move laterally.

It can be caused by:

Uneven stiffness
Uneven mass
Poor placement of shear walls
Asymmetric structural layout

A balanced structural layout can help control torsional response.

10. Foundation Design for Earthquakes

The foundation must safely transfer seismic forces into the ground.

Depending on the building and site, foundations may include:

Spread footings
Combined footings
Mat foundations
Pile foundations
Drilled shafts

Foundation design may need to consider:

Vertical forces
Horizontal forces
Overturning
Sliding
Uplift
Soil capacity
Settlement
Liquefaction

11. Diaphragm Action

Floor and roof systems can act as diaphragms.

They transfer lateral earthquake forces to:

Shear walls
Braced frames
Moment frames

The diaphragm must have adequate strength, stiffness and connections for the applicable demands.

12. Story Drift

Story drift is the relative lateral displacement between adjacent floors.

For example:

Floor 2 moves 1.8 inches
Floor 1 moves 0.8 inches

Then:

$Δ = 1.8 - 0.8$ $Δ = 1.0 in c h$

The engineer compares the calculated drift with the applicable code limits and project requirements.

ASCE seismic design procedures specifically address story drift, drift limits and P-Delta effects.

13. P-Delta Effects

When a building is laterally displaced, gravity loads acting through that displacement can create additional moments.

These are known as P-Delta effects.

For taller or more flexible structures, P-Delta effects can become particularly important and must be evaluated as required by the applicable design procedure.

14. Earthquake Load Analysis

ASCE 7 provides several seismic analysis approaches.

Common methods include:

Equivalent Lateral Force (ELF)

Often used for buildings that satisfy the applicable conditions for this simplified analysis procedure.

Modal Response Spectrum Analysis

Useful for structures where dynamic characteristics need to be considered more explicitly.

Linear Response History Analysis

Uses earthquake ground-motion records to evaluate structural response.

Nonlinear Response History Analysis

Can provide more detailed information about nonlinear structural behavior.

ASCE identifies these four analysis approaches in its seismic-analysis guidance.

15. Basic Seismic Force Example

For educational purposes, consider a simplified building with:

Effective seismic weight, W = 2,000 kips
Hypothetical seismic coefficient, Cs = 0.10

A simplified base-shear relationship is:

$V = C_{s} W$

Therefore:

$V = 0.10 \times 2, 000$ $V = 200 ki p s$

Base Shear = 200 kips

This is only an illustrative calculation. The actual seismic coefficient must be determined using the applicable ASCE 7 provisions, site hazard parameters, structural system, response modification factors, risk category and other requirements.

16. Distribution of Seismic Forces

Once the design base shear is determined, the seismic force is distributed through the building according to the applicable code procedure.

Conceptually:

Roof/Floor Masses → Diaphragms → Vertical Lateral Systems → Foundation

Higher floors can attract significant lateral force because of their elevation and mass.

The exact force distribution depends on the applicable analysis procedure and building characteristics.

17. Reinforced Concrete Seismic Design

For reinforced-concrete buildings, seismic detailing may address:

Beam-column joints
Column confinement
Beam reinforcement
Shear reinforcement
Development length
Lap splices
Shear walls
Boundary elements
Foundation connections

The objective is to provide the required strength and ductile behavior.

18. Steel Building Seismic Design

Steel buildings may use:

Moment frames
Braced frames
Special braced frames
Buckling-restrained braces
Dual systems

Important elements include:

Beam-column connections
Brace connections
Column splices
Base plates
Anchor rods
Collector elements

Seismic detailing requirements depend on the selected structural system and applicable standards.

19. Nonstructural Components

Earthquake safety is not limited to the main structural frame.

Nonstructural components can include:

Ceilings
Mechanical equipment
Electrical equipment
Pipes
HVAC systems
Partitions
Exterior cladding
Storage racks
Suspended equipment

These components may need seismic restraint or anchorage according to the applicable requirements.

20. Earthquake-Resistant Building Example

Consider a 5-story reinforced-concrete office building.

Assume the conceptual design uses:

Reinforced-concrete moment frames
Reinforced-concrete shear walls
Regular floor plan
Symmetrical shear-wall arrangement
Reinforced-concrete foundation system

Design sequence

Step 1: Determine building location and seismic hazard.

Step 2: Obtain geotechnical information.

Step 3: Determine building risk category and applicable design criteria.

Step 4: Select the seismic-force-resisting system.

Step 5: Calculate seismic loads.

Step 6: Develop the structural model.

Step 7: Analyze lateral forces and drift.

Step 8: Design beams, columns and shear walls.

Step 9: Design diaphragms and connections.

Step 10: Design foundations.

Step 11: Detail reinforcement and connections for seismic performance.

Step 12: Review drawings and construction requirements.

21. Example of Good vs. Poor Building Configuration

Better Configuration

Regular plan
Balanced shear walls
Continuous columns
Continuous load path
Similar stiffness between stories
Properly detailed connections

Riskier Configuration

Large setbacks
Major openings
Discontinuous shear walls
Very flexible ground floor
Highly asymmetric layout
Sudden changes in stiffness

FEMA emphasizes that regular structures generally distribute earthquake effects more uniformly, while irregular structures can concentrate damage in particular locations.

22. Advantages of Earthquake-Resistant Design

1. Reduces Collapse Risk

Proper seismic design helps provide resistance to severe earthquake demands.

2. Protects Life Safety

Reducing the likelihood of structural collapse is a primary objective.

3. Controls Structural Damage

Ductile and properly detailed systems can provide controlled deformation.

4. Improves Building Resilience

Buildings can be designed with performance objectives that go beyond minimum life-safety considerations.

5. Protects Critical Facilities

Appropriate seismic design is especially important for facilities that need to remain operational after earthquakes.

6. Improves Long-Term Reliability

A properly engineered seismic system provides a predictable structural load path.

23. Disadvantages and Challenges

1. Higher Initial Cost

Seismic design can require additional:

Reinforcement
Structural steel
Connections
Shear walls
Bracing
Foundation capacity

2. More Complex Design

Seismic analysis and detailing can be more complicated than gravity-only design.

3. Construction Quality Is Critical

Poor detailing or incorrect installation can reduce the intended seismic performance.

4. Architectural Restrictions

Shear walls, braces and structural elements can affect:

Open floor plans
Windows
Doors
Interior layouts

5. Specialized Engineering

Complex buildings may require advanced seismic analysis and experienced structural engineers.

24. Common Earthquake-Resistant Design Mistakes

1. Ignoring Site Conditions

Seismic design should account for the actual site and applicable ground-motion parameters.

2. Creating a Soft Story

Large reductions in stiffness or strength at one level can create a dangerous concentration of deformation.

3. Poor Load Path

Earthquake forces must have a continuous path from the building mass to the foundation.

4. Ignoring Torsion

Asymmetric structural layouts can produce significant twisting.

5. Poor Connection Detailing

Connections must be designed for the demands associated with the selected seismic system.

6. Ignoring Nonstructural Components

Equipment and building contents can also become hazards during earthquakes.

7. Poor Construction Quality

Even a good design can perform poorly if reinforcement, connections, anchorage or other components are not constructed as specified.

25. Earthquake-Resistant Design Checklist

Site

☐ Seismic hazard determined
☐ Site conditions evaluated
☐ Geotechnical information obtained
☐ Liquefaction considered where applicable
☐ Foundation conditions evaluated

Structural System

☐ Appropriate seismic-force-resisting system selected
☐ Regularity evaluated
☐ Torsion evaluated
☐ Soft/weak stories checked
☐ Load path established
☐ Redundancy considered

Analysis

☐ Seismic weight determined
☐ Seismic forces calculated
☐ Base shear determined
☐ Story forces determined
☐ Story drift checked
☐ P-Delta effects evaluated where required
☐ Appropriate analysis method selected

Design

☐ Beams designed
☐ Columns designed
☐ Shear walls/braces designed
☐ Connections designed
☐ Diaphragms designed
☐ Foundations designed
☐ Nonstructural components considered

Construction

☐ Reinforcement detailing checked
☐ Connections installed correctly
☐ Anchorages checked
☐ Special inspections performed where required
☐ Approved drawings followed

FAQ

What is an earthquake-resistant building?

It is a building designed and constructed to resist earthquake-induced forces and deformations according to the applicable seismic design requirements.

What is the most important principle of earthquake-resistant design?

A reliable continuous load path, combined with adequate strength, stiffness, ductility and appropriate structural configuration, is fundamental.

Which buildings are most vulnerable to earthquakes?

Vulnerability depends on many factors, including structural system, age, detailing, site conditions, irregularities, construction quality and seismic hazard.

What is a soft story?

A soft story is a building level with substantially lower lateral stiffness than the levels above. It can cause earthquake deformation to concentrate at that level. FEMA identifies soft/weak stories as important seismic irregularities.

What is seismic base shear?

Base shear is the design-level lateral force at the base of the building used in applicable seismic analysis procedures.

Can a building be completely earthquake-proof?

No building can be guaranteed to be completely damage-proof for every possible earthquake. Seismic design is based on specified hazards, performance objectives and code requirements.

Is earthquake-resistant design required everywhere in the USA?

Seismic design requirements depend on the building location, adopted code, seismic hazard, building characteristics and other factors. ASCE 7 provides nationally used seismic-load criteria, but the locally adopted code controls the project.

What is ductility in earthquake design?

Ductility is the ability of a structural system or component to undergo significant deformation while retaining required strength and stability.

Why are regular buildings preferred?

Regular buildings generally have more predictable distributions of mass, stiffness and strength, while irregular configurations can concentrate earthquake damage.

What is the difference between seismic design and seismic retrofit?

Seismic design establishes earthquake resistance for a new building. Seismic retrofit modifies an existing building to improve its earthquake performance.

Does the foundation need earthquake design?

Yes. The foundation and soil interface must safely transfer applicable seismic forces and account for conditions such as overturning, sliding, uplift, settlement and, where relevant, liquefaction.

Who should design an earthquake-resistant building?

The structural design should be performed or reviewed by an appropriately qualified structural engineer, with geotechnical input where required.

Conclusion

Earthquake-resistant building design combines appropriate structural configuration, seismic-force-resisting systems, ductile detailing, adequate connections, foundation design and site-specific seismic information.