Module 1: Introduction to Structural Analysis and Loads
Learning Objectives
- Classify common structural systems by their dominant load-carrying behavior.
- Distinguish service-level and strength-level design philosophies without mixing their load combinations or resistance checks.
- Identify major permanent, occupancy, environmental, and imposed actions relevant to building analysis.
- Trace gravity and lateral load paths from their point of application to the foundation.
- Determine tributary widths and areas for preliminary load distribution.
- Apply the governing project code and load combinations consistently to structural analysis.
Load Path
A load path is the continuous sequence of structural components and connections through which an applied action is transferred to the supporting ground.
Tributary Area
Tributary area is the portion of an area load assigned to a supporting member or point, commonly bounded by lines located halfway to adjacent supports when the load-distribution assumptions justify that idealization.
Common Structural Systems
- Beams and girders carry transverse loads primarily through shear and flexure.
- Trusses idealize straight members connected at joints and carry joint-applied loads primarily through axial tension or compression.
- Rigid frames transfer axial force, shear, and bending moment through moment-resisting joints.
- Cables carry tensile force and adopt a geometry that depends on loading and support conditions.
- Arches develop significant compression and horizontal thrust while carrying load through curved geometry.
- Diaphragms and vertical lateral systems collect and transfer lateral forces to frames, braced systems, or walls and then to the foundation.
Service-Level and Strength-Level Checks
Allowable-stress or service-level procedures and strength or limit-state procedures use different combinations of load effects and different resistance formats. Never take a load combination from one design philosophy and compare it against a resistance calibrated for another. For Philippine projects, use the edition and provisions explicitly adopted by the governing project documents and authority.
Philippine Code Basis
The course uses the National Structural Code of the Philippines (NSCP) as the Philippine building-load context. The NSCP Volume I, 7th Edition (2015), published by ASEP, includes minimum design-load provisions. Always verify the exact edition and project-specific governing requirements before professional use; the educational simulator is not a substitute for the code text.
Major Load Categories
- Dead load: permanent weight of structural and nonstructural construction and fixed equipment.
- Live load: occupancy-related movable or transient loading.
- Wind action: pressure and suction governed by wind climate, exposure, topography, height, geometry, and pressure coefficients.
- Earthquake action: inertial effects associated with ground motion, structural mass, dynamic characteristics, and code-defined seismic parameters.
- Rain and ponding effects: roof actions that can increase when drainage is inadequate or deformation changes water depth.
- Soil, fluid, and other environmental actions: lateral or vertical actions from retained materials, hydrostatic pressure, temperature, shrinkage, or other imposed effects.
- Support movement: settlement or imposed displacement that can create internal forces in constrained or indeterminate systems.
Loads versus Load Effects
A load such as a floor pressure in is an action. The resulting reaction, member axial force, shear, bending moment, torsion, or displacement is a load effect. Keep the action, analysis model, and resulting effect distinct when applying combinations and resistance checks.
Load Combinations
Load combinations account for the fact that different actions are unlikely to reach their maximum values simultaneously and that safety formats require calibrated factors. Use only the combinations from the governing code and design method. Do not invent factors or combine factored and unfactored effects within the same check.
Interactive Exploration
Use the load-combination simulator to observe how changing individual action magnitudes changes the governing combined demand. Treat the displayed combinations as an educational implementation tied to its stated code basis, and confirm professional calculations directly against the governing code.
LRFD Load Combinations Simulator (NSCP 2015)
Input Loads (kN)
Factored Combinations
Governing Load Combination:
Combo 5: 1.2D + 1.0E + 1.0L = 250.0 kN
Gravity Load Path
A typical floor gravity load may travel from slab or deck to secondary beams, then to girders, columns or walls, foundations, and finally soil or rock. Every transfer requires a structural component or connection capable of transmitting the corresponding force.
Lateral Load Path
Wind or earthquake action is commonly collected by the building envelope and floor or roof diaphragms, transferred through collectors and connections into vertical lateral-force-resisting elements, and delivered through the foundation into the ground. A discontinuity, weak connection, or unintended flexible link can invalidate the assumed path.
Load Path and Tributary Distribution Workflow
Use the workflow to avoid the common error of calculating a member load before identifying how the area load reaches that member. The tributary idealization is selected only after the supporting geometry and load path are understood.
Applied area, line, point, or environmental action → Identify the supporting structural system; Identify the supporting structural system → Trace continuous load path to the foundation; Trace continuous load path to the foundation → Is tributary distribution an appropriate idealization?; Is tributary distribution an appropriate idealization? — Yes → Determine tributary width or area; Is tributary distribution an appropriate idealization? — No → Use direct structural distribution or refined analysis; Determine tributary width or area → Convert action to member or joint loads; Use direct structural distribution or refined analysis → Convert action to member or joint loads; Convert action to member or joint loads → Apply governing load combinations consistently; Apply governing load combinations consistently → Analyze reactions and internal load effects; Analyze reactions and internal load effects → Verify equilibrium and continuity of load path
- Applied area, line, point, or environmental action: terminator
- Identify the supporting structural system: process
- Trace continuous load path to the foundation: process
- Is tributary distribution an appropriate idealization?: decision
- Determine tributary width or area: process
- Use direct structural distribution or refined analysis: process
- Convert action to member or joint loads: process
- Apply governing load combinations consistently: process
- Analyze reactions and internal load effects: process
- Verify equilibrium and continuity of load path: terminator
Area Load to Line Load
A uniform area load may be converted to an equivalent line load when a tributary width is appropriate.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Equivalent line load, commonly in kN/m | - | |
| Uniform area load, commonly in kN/m^2 | - | |
| Tributary width assigned to the member | - |
Area Load to Concentrated Joint or Column Load
A uniform area load may be converted to a concentrated load using a tributary area when the distribution model is appropriate.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Equivalent concentrated load | - | |
| Uniform area load | - | |
| Tributary area | - |
Interactive Exploration
Use the tributary-area simulator to change bay dimensions and support location. Confirm that interior, edge, and corner tributary regions change with the geometry rather than relying on memorized fractions without a plan view.
The highlighted rectangle is the floor area assigned to the center column. Increasing either bay spacing increases tributary area; increasing floor load changes the force without changing geometry.
Tributary Area Is an Idealization
Half-distance boundaries are most defensible for regular systems with comparable support stiffness and a clear one-way or simplified distribution mechanism. Two-way slabs, irregular framing, transfer systems, discontinuities, large stiffness differences, or diaphragms may require a more refined load-distribution model.
- Structural analysis begins by identifying the system and tracing a continuous load path to the ground.
- Loads are actions; reactions, internal forces, and deformations are load effects.
- Service-level and strength-level methods must use internally consistent load combinations and resistance formats.
- The NSCP provides the Philippine building-load context, but the exact governing edition and project requirements must be verified.
- Tributary widths and areas convert distributed actions into member or joint loads only when the idealization fits the structural system.
- Existing interactive load-combination and tributary-area simulations should be used to test physical trends, not as substitutes for the governing code.