An earthquake is the shaking and vibration of the ground surface caused by movement along a fault plane or by volcanic activity. The motion comes from a sudden release of energy that sends seismic waves through the earth, and those waves are what a structure actually feels. The UBC 1997 seismic design process converts this ground motion into calculable lateral forces, and once the steps are laid out in sequence, the procedure is far simpler than its reputation suggests. The code is an older prescriptive framework, yet the thinking behind it still drives the current practice of seismic design of buildings, which has since expanded into detailing requirements and performance-based approaches.
This article walks through the workflow: the earthquake basics that define the loading, the two design concepts that shape the lateral system, the static and dynamic analysis options, and the load combinations that turn analysis into member forces. Each stage builds on the one before it.
Earthquake Basics and the Waves That Carry the Load
The first step in seismic design is understanding what arrives at the building site. Ground motion is not a single push but a series of waves that travel at different speeds and shake the structure differently. The architectural layout and the building envelope design process set how much mass the structure carries and how it is distributed, so envelope decisions made early in a project directly affect the forces the seismic system must resist. Heavy cladding, roof assemblies, and parapets all add to the seismic weight, and an irregular footprint concentrates stress at reentrant corners.
Three Types of Seismic Waves
Seismologists group the waves into three basic types, each interacting with a building differently. The classification comes from how ground particles move relative to the direction of travel.
| Wave type | Particle motion | Travel speed | Typical effect on buildings |
|---|---|---|---|
| Primary (P) waves | Compressional push and pull along the direction of travel | Fastest; arrive first | Sharp vertical and horizontal jolt |
| Secondary (S) waves | Shear motion perpendicular to the direction of travel | Slower than P waves | Strong horizontal shaking, the main driver of structural damage |
| Surface waves | Rolling motion along the ground surface | Slowest | Prolonged rocking, most damaging to tall and flexible structures |
Locating the Earthquake Source
Engineers locate the earthquake using the arrival times of the primary and secondary waves. Because P waves travel faster than S waves, the gap between the two arrivals grows with distance from the source, and readings from several stations triangulate the epicenter. The distance, depth, and magnitude then feed into site-specific ground motion estimates used for design.
Design Concepts for Earthquake Loads
Two concepts dominate how structural systems are organized for earthquake resistance, and nearly every lateral system in practice is a version of one of them:
- Strong beam and weak column: beams are proportioned to yield before the columns they frame into, so plastic hinges form in the beams and the frame keeps its stability.
- Ductile cantilever wall system: walls act as vertical cantilevers fixed at the foundation, concentrating damage in a controlled hinge zone at the base.
Strong Beam and Weak Column
In a strong beam and weak column frame, the designer deliberately makes columns stronger and stiffer than beams so that under strong shaking the beams form plastic hinges first. A hinged beam keeps carrying gravity load while it dissipates energy, whereas a hinged column can turn an entire floor into a mechanism. The rule is simple to state and hard to hold in practice, because it goes against the instinct to strengthen the largest member.
Ductile Cantilever Wall System
The ductile cantilever wall system works on the opposite principle: one stiff wall carries most of the lateral load, cantilevering from the foundation like a deep vertical beam. The design intent is to force the plastic hinge to form at the wall base, where it is accessible and repairable, while the wall above stays elastic. Wall geometry, boundary elements, and confinement reinforcement control where and how the hinge develops.
The two concepts are not limited to tall buildings. On residential projects the same load path thinking applies to the small details that fail first in an earthquake, and gable end walls are the classic example. A properly framed gable end wall ties the roof diaphragm into the wall below, and the gable end and eave design guide from Fine Homebuilding covers the connection details that keep these elements from pulling apart during wind and seismic events.
Types of Seismic Analysis
UBC 1997 recognizes two families of seismic analysis, and the choice depends on the regularity and height of the structure. Static analysis applies the earthquake as equivalent lateral forces, while dynamic analysis simulates the actual response of the building to ground motion.
Static Analysis
In static analysis the earthquake is converted into equivalent static loads, and the main factor is the calculation of the base shear. The base shear in UBC 1997 comes from the zone factor, the importance factor, the seismic coefficient for site soil conditions, and the response modification coefficient, applied to the structural weight, then distributed over the height of the building. The procedure follows a fixed sequence:
- Calculate the base shear from the code parameters for zone, importance, soil, and structural system.
- Model the mass of each floor as a lumped mass acting at the floor level.
- Distribute the base shear to each floor in proportion to its mass and height.
- Run the static analysis with the lateral forces applied, including material nonlinearity where the model allows it.
The Lumped Mass Method
The lumped mass method treats each floor as a point mass at the floor level, a reasonable approximation because floor slabs are stiff in their own plane. Consider a four-storey building: the mass of each floor is lumped at that floor’s level, and the base shear is distributed among those four points by relative mass and height. The same logic carries into dynamic methods, where the lumped masses become the nodes of the response model.
Dynamic Analysis
Dynamic analysis accounts for the time-dependent nature of the loading and captures both material and load nonlinearity. Two methods are used in practice:
- Response spectrum analysis: the structure is excited with a smoothed spectrum of ground motion peaks, and the maximum response of each mode is combined statistically.
- Time history analysis: a recorded or synthetic ground motion record is applied step by step, and the response is integrated over time.
Dynamic methods demand more from the structural model, so they are reserved for irregular, tall, or critical structures. Steel frames respond well to these methods because the material’s predictable stress-strain behavior makes the inelastic model reliable, and the structural steel design principles that govern framing layout, connection design, and modern construction applications are the same ones that keep a moment frame ductile under cyclic loading.
UBC 1997 Load Combinations
Once the seismic forces are known, they enter the load combinations that define the ultimate limit state. UBC 1997 prescribes the following combinations for strength design, where D is dead load, L is live load, Lr is roof live load including permanent reductions, S is snow load, W is wind load, and E is earthquake load:
| No. | Combination | Notes |
|---|---|---|
| 1 | 1.4 D | Dead load only, factored |
| 2 | 1.2 D + 1.6 L + 0.5 (Lr or S) | Gravity dominated |
| 3 | 1.2 D + 1.6 (Lr or S) + (f1 L or 0.8 W) | Roof or snow dominated |
| 4 | 1.2 D + 1.3 W + f1 L + 0.5 (Lr or S) | Wind dominated |
| 5 | 1.2 D + 1.0 E + (f1 L + f2 S) | Seismic dominated |
| 6 | 0.9 D ± (1.0 E or 1.3 W) | Uplift and overturning check |
Combination Factors for Live and Snow Loads
The factors f1 and f2 adjust the companion loads acting with the principal load. The live load factor f1 is 1.0 for floors in places of public assembly, live loads exceeding 4.9 kN/m2, and garage live loads, and 0.5 everywhere else:
- Floors in places of public assembly
- Live loads greater than 4.9 kN/m2
- Garage live loads
The snow factor f2 is 0.7 for roof configurations such as sawtooth roofs that do not shed snow off the structure, and 0.2 for other roof configurations.
The 1.1 Amplification for Concrete and Masonry
When a combination includes seismic forces, the factored load combinations are multiplied by 1.1 for concrete and masonry structures. The amplification covers the higher uncertainty in the strength of these materials under cyclic, strain-rate-sensitive loading. Material-specific adjustments are standard practice across civil engineering; pavement design principles apply the same idea when they treat flexible and rigid pavements differently, since one set of rules for both would be either wasteful or unsafe.
Serviceability Checks and the Lateral Load Path
Design does not stop at strength. UBC 1997 also defines serviceability load combinations that keep the structure usable under ordinary conditions:
- D
- D + L + (Lr or S)
- D + (W or E)
These combinations control deflections, drift, and cracking so the building stays functional after a design-level event.
Shear Walls and the Lateral Load Path
The lateral system only works if forces can travel from the roof and floors down to the foundation. Diaphragms collect the inertial forces, pass them to vertical elements, and those elements deliver them to the ground. Shear walls are the most common vertical element in low- and mid-rise construction, and the shear wall design and construction methods governing lateral force resisting systems for wind and seismic resistance determine whether the load path stays continuous or breaks at an intermediate floor.
Detailing rules support the analysis at every level. Boundary elements are confined where hinges form, wall openings are offset so piers do not become too slender, and collectors are tied into the diaphragm so the load actually reaches the wall. These details, more than the elastic analysis, decide whether a building survives the shaking it was designed for.
The same thinking extends into the spaces people occupy. After a strong earthquake, a building is only useful if occupants can move through it safely and keep using it while repairs are made, which is why layouts that follow universal design principles, such as accessible kitchen design and construction standards for independent living, matter for recovery as much as daily comfort. Seismic design in the UBC tradition is a chain from ground motion to detailing, and each link depends on the one before it.