Requires: 2D Design Level

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 automatic 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': Applies to only live load cases.
  • 'W or S only': Applies to load cases that only contain wind or snow loads.
  • 'D + L': Applies to combinations of dead and live loads, but nothing else.
  • 'Other': Applies to any 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' definitions in Load Case Manager. If you later change the material specification 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
Steel (AISC)	Either, select the specification in the design parameters
Steel (CSA)	Strength (LRFD) only.
Cold-Formed Steel (NAS)	Either, select the specification in the design parameters
Concrete (ACI, CSA)	Strength (LRFD) only.
Wood (NDS)	Either, select the specification in the design parameters
Aluminum (ADM)	Either, select the specification in the design parameters
Connections (VAConnect)	Strength (LRFD) only

Live Load Reduction

It is possible to check design groups for specific live load reduction levels. The process is two-fold. First use the load case manager to define the live load reduction levels that you wish to use. Note that additional load combinations are generated for each level, which has a significant performance impact. The second step is to specify that a design group should be checked for one of these live load reduction levels. That design group will then ignore the full live load combinations and load combinations for other reduction levels.

Checking Results Superposition

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.

Results	Checked?
+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!