The load patterning algorithm takes a given load combination and creates a set of sub load combinations for it to check. For example, a load combination containing wind would not apply both pressure and suction at the same time, but instead apply all the loads with both pressure followed by all the loads with suction.
Mandatory Patterns
Wind, Seismic, and Earth all have mandatory load patterning that the user cannot disable.
Load Type | Description |
|---|---|
Wind | Splits the load combination into one with pressure, and one with suction. |
Seismic | Due to not knowing the direction of the acceleration from the seismic event, the seismic loads are all reversed in direction and run again. |
Earth | Typically fill soil can be removed and can potentially be a resisting force. Because of this a second combination is run replacing all earth loads with a minimum earth load factor instead of the load combinations factor, typically 0. |
Live Load Patterning
Live loads are patterned first by grouping them based on their span and by which side of the member is load is on. Specifically, for point loads we look at the direction of the applied load and for point moments we look at the sigh of the applied moment. From here, we alternate the loads based on their applied moments from one span to the next, so for a 3 span system the two cases are +, -, + and -, +, - where the sign denotes the sign of the applied moments.
Note that this does not guarantee the controlling case is found. For a typical simply supported structure under uniform smooth loading with a continuous moment diagram this would be the typical way to pattern the loading. Because point moments cause discontinuities in the moment curve and reverse the sign of the diagram instantaneously, they are difficult to categorize in this way. For cases where this is a concern the user should manually pattern loads.