Loading for Design
Requires: Design Level
- Setting Up Load Combinations
- Matching Load Combinations to Design Groups
- Live Load Reduction?
- Checking Result Superpositions
Setting Up Load Combinations
The design software checks only the load combinations you define in Load Case Manager, no other types of load cases are used by the design software. You may define any number of load combinations to check, using atomatic building code combinations or your own custom load combinations. Each load combination is assigned to one of the following design categories:
- No Design: The load combination is ignored in all design checks.
- Allowable (ASD): Used for force or stress checks at service level.
- Strength (LRFD): Used for force or stress checks at ultimate strength level.
- Deflection: Used to check deflections. Note that deflection
combinations fall into one of four IBC-defined categories based on the types of
loads they contain:
- 'L Only': A combination for just live loads
- 'L + D': A combination of dead and live loads, but nothing else.
- 'W or S': Any combination including wind, snow or both types of loads (and any other types).
- 'Other': Anything load combination that does not match one of the above.
- Allowable & Deflection: Used for force checks and deflection checks.
Matching Load Combinations to Design Groups
The type of load combinations you use depends on the type of materials or design groups you are using. The following table shows how the type of group maps to load combination checks. When design groups are initially created the type of load combination is chosen based on 'current' defnitions in Load Case Manager. If you later change the material specifiction for design from ASD to LRFD you may also need to go back to Load Case Manager and add or change appropriate load combinations.
|Design Group Type||Strength or Allowable Load Combination Type|
|AISC, CISC Steel||Either, select the specification in the design parameters|
|NAS Cold-formed Steel||Either, select the specification in the design parameters|
|ACI Concrete||Strength (LRFD) only.|
|NDS Wood||Either, select the specification in the design parameters|
|ADM Aluminum||Either, select the specification in the design parameters|
|Connections (VAConnect)||Strength (LRFD) only|
Live Load Reduction?
Automated live load reduction is not supported in VisualAnalysis. The definition in the building-code makes it extremely difficult to implement in a finite element solution where "load path" is not part of the bookkeeping system during analysis! This is a very 'thorny' issue to implement efficiently and correctly. If this feature is important to you, or you have suggestions for implementing it in a practical way, please let us know.
Checking Results Superpositions
VisualAnalysis has the ability to run design checks on superposition result combinations. These are advanced features for combining moving load or dynamic results with more traditional static analysis results. The superposition consists of two sets of results culled from many results instances or times: upper and lower extremes. We currently design-check each of these independently, and while this may provide more information to the design algorithms and catch situations that might control the design, it may very well miss certain combinations of forces that would control.
|+Fx, +Mz, +My||Yes|
|-Fx, +Mz, +My||No|
|+Fx, -Mz, -My||No|
|-Fx, -Mz, -My||Yes|
For example, we will check the maximum axial force (e.g. tension) in a member with the maximum moment. We will check the minimum axial force (e.g. compression) in a member with the minimum moment. But 'cross-over' checks are not currently made, and depending on the shape, material used, bracing, magnitudes of forces, etc., other combinations may actually control the design! So VisualAnalysis could be quite unconservative in this regard!