Loading, Load Cases & Load Combinations
Learning Objectives
- Organize physical actions into clear primary and reusable reference-load definitions.
- Apply joint, member, element/surface, floor-distributed, selfweight, thermal, moving, wind and seismic actions using correct directions and units.
- Choose between member-distribution and FE surface-loading workflows based on the analytical slab idealization.
- Build and verify seismic/dynamic mass definitions separately from static force combinations.
- Distinguish linear result combinations from simultaneously applied repeat/concurrent load cases.
- Document code/edition and combination basis rather than copying unverified factors.
- Verify signs, generated-load totals, reactions and governing cases before design.
Load factors are versioned design data
A classroom combination is not automatically the correct combination for a project. Real models must declare the governing standard, edition, load categories, direction conventions and project-specific requirements. The interactive combinations below are intentionally limited teaching examples for sign/factor mechanics.
Primary Cases and Reference Loads
Primary load case
A loading/analysis case that STAAD solves as part of the structural analysis: for example a dead, live, wind-direction, temperature, seismic, moving-load, or explicitly combined concurrent case.
Reference load
A reusable load-definition block that can be referenced by other loading or mass workflows. Bentley documentation distinguishes reference loads from independently analyzed primary load cases. This makes them useful for organizing repeated load components without generating unnecessary standalone analysis results.
Reference-load QA
- The reference definition contains only the intended physical components.
- Reused components are not double-counted in primary cases or mass.
- The mass source and static-force cases remain independently auditable.
- Load categories/names describe physical meaning, not merely case numbers.
Selfweight, Gravity and Density
Selfweight
A generated load based on assigned material density/weight density and element geometry. Wrong density, units, omitted properties or an incorrect global direction can therefore corrupt both dead-load response and any mass definition derived from those quantities.
Direction check
Many STAAD models use global Y as vertical, but the project model convention is authoritative. Confirm the active vertical axis and inspect generated gravity arrows instead of relying on habit.
How Loads Enter the Analytical Model
Joint loads
Concentrated forces/moments applied at analytical joints. Use them when the physical action is concentrated or has already been reduced to a joint action.
Member loads
Loads along one-dimensional members can represent concentrated, uniform, varying, thermal and other supported actions. Confirm whether direction is global, local, projected, or otherwise defined by the selected command.
Element / pressure loads
When slabs, walls, tanks or mats are explicitly modeled using plate/surface elements, pressure should ordinarily be applied to those analytical elements as appropriate to the model. Surface-load sign depends on element orientation/local axes where local directions are used.
Floor-load member distribution
Bentley documents floor-loading workflows that convert an area pressure into supporting-member loads for framed floor idealizations. Use this when the design intent is to distribute floor pressure to members rather than to solve an explicit slab FE model. Always compare the sum of generated member loads with pressure × loaded area.
Legacy AREA LOAD is not the default modern choice
Bentley technical guidance describes AREA LOAD as a more limited legacy member-load distribution workflow and recommends FLOOR LOAD for the broader floor-distribution use case. Exact one-way/two-way options, enclosed-zone definitions and syntax vary by installed STAAD release; verify the current documentation.
Wind Uplift and Sign Conventions
Positive magnitude vs physical direction
In this course, is entered as a positive uplift magnitude. An uplift stability teaching expression therefore subtracts it from stabilizing downward gravity. This explicit convention prevents the common error of using a positive magnitude and also algebraically adding it in the downward direction.
Load Sign & Combination Laboratory
Positive D and L act downward. W↑ is entered as a positive uplift magnitude, so it must subtract from downward gravity action.
Load-sign QA
- Display each physical load case separately before combining it.
- Confirm gravity arrows point toward the intended downward global direction.
- Confirm pressure/suction/uplift acts on the intended face/direction.
- Check local member loads against member-local axes.
- Check imposed displacement and temperature signs.
- Hand-sum at least one representative floor/line/load region.
Wind and Seismic Generators
Automation still requires engineering inputs
Generated lateral loads depend on site/design parameters, exposure/terrain assumptions, effective areas, structural classification/system, directions, mass source and the selected standard/edition. A generated load is only as defensible as those inputs.
Wind-speed sensitivity
Many velocity-pressure formulations contain a V² dependence; the complete governing equation includes the coefficients defined by the applicable standard.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Velocity/dynamic pressure measure | - | |
| Wind speed | - |
Equivalent-lateral-force base-shear concept
A conceptual relationship used by many seismic procedures after the response coefficient has been determined from the governing standard.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Total design base shear | - | |
| Code-derived seismic response coefficient, including all applicable limits | - | |
| Effective seismic weight | - |
Do not teach one incomplete Cs equation as the whole seismic procedure
Current seismic standards impose system-, period-, spectrum- and limit-dependent requirements on . For course arithmetic, treat as an already validated input unless the lesson explicitly cites and implements a particular governing standard/edition in full.
Mass source is not a static load combination
Eigen/dynamic analysis requires mass. Confirm which dead, live, equipment and other components contribute under the governing project standard, and check for omission/double counting independently from static load cases.
Moving and Thermal Actions
Moving loads
Bridge, crane and vehicle problems require positions to move through the structure so response envelopes capture critical locations. Verify path, axle/wheel spacing, increments, direction, impact/dynamic assumptions where applicable, and envelope completeness.
Free thermal expansion
Unrestrained prismatic member under a uniform temperature change.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Coefficient of thermal expansion | - | |
| Original length | - | |
| Temperature change | - |
Thermal force requires restraint
Free uniform thermal expansion causes deformation without restraint force. Thermal force/stress develops when supports or connected members restrain that deformation, so boundary conditions are central to interpretation.
LOAD COMBINATION vs REPEAT / Concurrent Loading
Linear result combination
For a linear structural response, previously solved primary-case results can be algebraically superimposed using specified factors. This is efficient when the combined load does not need to change stiffness, geometry or element activity during the solution.
Repeat / concurrent load case
Bentley documents REPEAT LOAD as a primary loading state assembled from component cases and solved with those loads acting together. This distinction matters for analyses such as P-Delta and models with tension/compression-only or other load-dependent behavior, where post-analysis superposition is not equivalent to simultaneous loading.
Load Combination Envelope
Compare signed scalar teaching loads. A structural response quantity can be governed by a different case.
This does not mean the case governs moment, shear, reaction, drift, uplift stability, or member design. Those response quantities require structural analysis/enveloping.
Envelopes preserve purpose and provenance
Group cases/combinations according to their engineering purpose—such as serviceability versus strength/design—and retain the governing case/location when reporting maxima. “Largest scalar load” in the teaching widget is not automatically the case that governs a structural response quantity.
Before running design
- Governing standard and edition are recorded.
- Primary/reference load definitions are not double-counted.
- Floor/element loading matches the analytical slab idealization.
- Wind/seismic directions and signs are verified graphically.
- Generated-load totals are checked against independent area/weight calculations.
- Mass source is independently reviewed.
- Result-combination vs repeat/concurrent requirements match the analysis type.
- Service and strength/design envelopes contain only intended cases.
- Good STAAD loading starts with physical cases, clear directions, units, provenance and independent totals.
- Reference loads are reusable definitions rather than independently analyzed primary cases.
- Floor-load distribution and explicit plate/element loading represent different analytical slab idealizations.
- Wind uplift needs an explicit sign convention.
- Dynamic mass needs its own QA.
- Linear result combinations and simultaneous repeat/concurrent load cases are not interchangeable in nonlinear/second-order workflows.
- Code-generated factors remain versioned engineering data and must be verified against the governing standard/edition.