Buildings are designed to resist gravity — the weight of the structure, its contents, and the people inside it. But earthquakes impose forces that act horizontally, shaking a building side to side and sometimes twisting it simultaneously. These lateral forces are fundamentally different from gravity loads, and a building that stands perfectly well under its own weight may fail catastrophically when subjected to strong ground shaking if it was not designed or evaluated for seismic loads.
Understanding how engineers analyze lateral loads and seismic forces provides valuable context for property owners, particularly those dealing with damage assessments, retrofit decisions, or insurance claims after an earthquake.
What Lateral Loads Are
A lateral load is any force that acts horizontally on a structure. Wind is one source of lateral loading. Earthquake ground motion is another — and in seismically active regions like Southern California, it is typically the governing lateral load case for building design.
During an earthquake, the ground moves beneath a building. Because the building has mass and inertia, it resists the ground's movement, which generates forces throughout the structure. These inertial forces are proportional to the building's mass and the intensity of the ground acceleration. Heavier buildings experience greater seismic forces. Buildings on sites with soft soils experience amplified ground motion compared to those on firm rock.
The seismic forces are not applied at a single point. They are distributed throughout the building, with each floor level experiencing forces related to its mass and height above the base. Upper floors generally experience greater forces and displacements than lower floors, which is why damage in multi-story buildings often concentrates at upper levels or at transitions in stiffness.
The Lateral Force-Resisting System
Every building that is designed to withstand earthquakes includes a lateral force-resisting system (LFRS) — the collection of structural elements that resist horizontal forces and transfer them down through the structure to the foundation and into the ground.
Common types of lateral force-resisting systems include:
Shear walls are vertical wall elements, typically made of reinforced concrete, reinforced masonry, or wood structural panels, that resist lateral forces through in-plane shear. Shear walls are common in both residential and commercial construction.
Braced frames use diagonal steel members to create triangulated configurations that resist lateral forces. Concentrically braced frames (with members meeting at a point) and eccentrically braced frames (with an intentional offset) perform differently under seismic loading.
Moment frames resist lateral forces through the bending strength and stiffness of beams and columns rigidly connected at their joints. Steel moment frames are common in mid-rise and high-rise buildings. After the Northridge earthquake revealed widespread fractures in welded steel moment frame connections, significant changes were made to design and construction standards for these systems.
Dual systems combine two types of lateral force-resisting systems, such as a moment frame acting together with shear walls, to provide redundancy and improved performance.
How Engineers Calculate Seismic Forces
The California Building Code (CBC), based on the International Building Code and ASCE 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures), provides the procedures engineers use to determine seismic design forces.
The equivalent lateral force procedure is the simplest approach and is used for regular, relatively low-rise buildings. It calculates a base shear — the total horizontal force at the building's base — using factors that account for the site's seismic hazard level, the soil conditions, the building's fundamental period of vibration, its weight, and the type and ductility of its lateral force-resisting system. The base shear is then distributed to each floor level based on its height and weight.
Modal response spectrum analysis is used for taller, more complex, or irregular buildings. This method considers the building's multiple modes of vibration and their associated periods and participation factors, combining the responses from each mode to estimate the total seismic demand on each element.
Nonlinear analysis methods, including pushover analysis and nonlinear response history analysis, are used for performance-based evaluations and for buildings where the standard code procedures do not adequately capture the expected behavior. These methods model the building's response as it moves into the inelastic range — where structural elements begin to yield and deform beyond their elastic limits.
Evaluating Existing Buildings
When engineers evaluate an existing building — whether after earthquake damage or as part of a seismic risk assessment — they compare the building's capacity (what it can resist) to the seismic demand (what forces the earthquake imposes). For more, see How Forensic Engineers Evaluate Earthquake Damage.
The evaluation often follows guidelines published by ASCE 41 (Seismic Evaluation and Retrofit of Existing Buildings), which provides standardized procedures for assessing existing structures against defined performance objectives. These performance levels range from Operational (building remains fully functional) to Collapse Prevention (building remains standing but may be severely damaged).
Older buildings designed before modern seismic codes were adopted frequently have deficiencies that become apparent during evaluation: inadequate connections, insufficient lateral bracing, soft or weak stories, and structural irregularities that concentrate damage.
Why This Matters for Property Owners
Property owners benefit from understanding lateral load concepts in several practical contexts. During a damage assessment, an engineer's findings about the building's lateral system performance explain why certain damage patterns occurred and what repairs are needed. When considering a retrofit, understanding the lateral system helps owners appreciate the scope and purpose of the proposed work. In insurance disputes, lateral load analysis can demonstrate that observed damage is consistent with the seismic forces the building experienced. For more, see Earthquake Damage Disputes.
Key Takeaways
Seismic forces impose lateral loads on buildings that are fundamentally different from the gravity loads structures are most obviously designed to resist. Engineers evaluate earthquake risk by analyzing the building's lateral force-resisting system and comparing its capacity to the demands imposed by expected ground motion. This analysis, grounded in building codes and engineering standards, forms the technical basis for damage assessments, retrofit designs, and expert opinions in earthquake-related disputes.