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Explanation By Example

Using USA LRFD combinations as an example
Base CombinationExpanded Combinations
1. 1.4D1.4D1
2. 1.2D + 1.6L + 0.5Lr1.2D + 1.6L + 0.5Lr
1.2D + 1.6L2 + 0.5Lr		2
3
3. 1.2D + 1.6L + 0.5S1.2D + 1.6L + 0.5S
1.2D + 1.6L2 + 0.5S		4
5
4. 1.2D + 1.6L + 0.5R1.2D + 1.6L + 0.5R
1.2D + 1.6L2 + 0.5R		6
7
5. 1.2D + 1.6Lr + f1L1.2D + 1.6Lr + f1L
1.2D + 1.6Lr + f1L2		8
9
6. 1.2D + 1.6Lr + 0.5W,dn1.2D + 1.6Lr + 0.5W,dn
1.2D + 1.6Lr + 0.5W,dn2		10
11
7. 1.2D + 1.6S + f1L1.2D + 1.6S + f1L
1.2D + 1.6S + f1L2		12
13
8. 1.2D + 1.6S + 0.5W,dn1.2D + 1.6S + 0.5W,dn
1.2D + 1.6S + 0.5W,dn2		14
15
9. 1.2D + 1.6R + f1L1.2D + 1.6R + f1L
1.2D + 1.6R + f1L2		16
17
10. 1.2D + 1.6R + 0.5W,dn1.2D + 1.6R + 0.5W,dn
1.2D + 1.6R + 0.5W,dn2		18
19
11. 1.2D + 1.0W,dn + f1L + 0.5Lr1.2D + 1.0W,dn + f1L + 0.5Lr
1.2D + 1.0W,dn + f1L2 + 0.5Lr
1.2D + 1.0W,dn2 + f1L + 0.5Lr
1.2D + 1.0W,dn2 + f1L2 + 0.5Lr		20
21
22
23
12. 1.2D + 1.0W,dn + f1L + 0.5S1.2D + 1.0W,dn + f1L + 0.5S
1.2D + 1.0W,dn + f1L2 + 0.5S
1.2D + 1.0W,dn2 + f1L + 0.5S
1.2D + 1.0W,dn2 + f1L2 + 0.5S		24
25
26
27
13. 1.2D + 1.0W,dn + f1L + 0.5R1.2D + 1.0W,dn + f1L + 0.5R
1.2D + 1.0W,dn + f1L2 + 0.5R
1.2D + 1.0W,dn2 + f1L + 0.5R
1.2D + 1.0W,dn2 + f1L2 + 0.5R		28
29
30
31
14. 1.2D + 1.0Ev + 1.0Eh + f1L + 0.2S1.2D + 1.0Ev + 1.0Eh + f1L + 0.2S
1.2D + 1.0Ev + 1.0Eh + f1L2 + 0.2S
1.2D + 1.0Ev + 1.0Eh2 + f1L + 0.2S
1.2D + 1.0Ev + 1.0Eh2 + f1L2 + 0.2S		32
33
34
35
15. 0.9D + 1.0W,up0.9D + 1.0W,up
0.9D + 1.0W,up2		36
37
16. 0.9D - 1.0Ev + 1.0Eh0.9D - 1.0Ev + 1.0Eh
0.9D - 1.0Ev + 1.0Eh2		38
39

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Key Points

  • There are many more “expanded” load combinations than “base” load combinations
  • You only INPUT the “base” load combinations into your “remote” widget. The solver automatically expands them out, based upon any alternate load types that you tell the solver exists.
  • The solver internally calculates everything using the expanded combinations.
  • Some solver OUTPUTS are base combinations, and some are expanded combinations. Be careful which is which!
Pay close attention to whether solver outputs are using base or expanded combinations, as this affects interpretation of results.

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The Weird Stuff

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Recombining Expanded into Base Combinations

Let’s take this combination as an example:
BaseExpanded
0.9D + 1.0W,up0.9D + 1.0W,up
0.9D + 1.0W,up2
Usually, wind uplift will be acting in the opposite direction of dead load. So let’s say the user sets:
  • W,up = “-0.9D”
  • W,up2 = “0”
This means:
  • remote.LCTable.maxR (maximum reaction) will be equal to 0.9D (= 0.9D + 0)
  • remote.LCTable.minR will be equal to 0 (= 0.9D - 0.9D)
  • The signed absolutely maximum remote.LCTable.R will be equal to 0.9D
Be very careful as to what result you’re actually looking for if you use these in your calculations!

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Sorting of Expanded Load Combinations

In all of the outputs that include expanded load combinations, they are returned in the order that the solver has generated them in. That means that all the alternate load types are included in later indices. The first 16 load combinations look like the base combinations, but are actually the expanded combinations - NOT recombined!

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Visual Example

The diagrams show the envelope considering all different types of Q (Q1, Q2, Q3). Observe that Q1, Q2 and Q3 are not considered in the same load combination. The images below show that the bending demand is the same across different load types.
Bending moment diagram Q1
Bending moment diagram Q2
Force diagram Q1
Force diagram Q2
Note that if you add another load Q3 in another location, this will only be shown in the diagram if the bending moment along the beam caused by Q3 is higher than the bending moment caused by the other loads:
Bending moment with Q3
Force diagram with Q3
Combined bending diagram
Combined force diagram
The envelope diagrams show the maximum effects from all non-concurrent load combinations, demonstrating how the solver handles multiple load types without combining them inappropriately.