A case study on how Collapse Load Sequence works
The collapse load sequence (LDSEQ line) applies load conditions on the structure sequentially using the load factors and a number of steps defined on the LDSEQ input line. During collapse analysis, model deflections, strains, stresses, and plasticity are automatically carried over from the current load step to the next load step.
To illustrate how the load sequence works, consider the following example, where there are four load conditions are defined:
- DEAD load for vertical load
- WAV1: first wave load
- WAV2: second wave load
- WAVF: Final wave load
In the collapse input file, we can apply the above load conditions sequentially, as defined in the following example:
First, the DEAD load condition is applied one step up to load factor 1.0. Then, WAVI uses
10 steps until it reaches load factor 1.0. After that, WAV2 similarly uses 10 steps. Finally,
the final WAVF is applied using 20 steps until it reaches load factor 2.0.
The following screenshot shows the loads, the model deformation, and the plasticity at different load steps:
As shown above, the Collapse program applies the load conditions sequentially to the model, and plastic strains and stresses are carried over from one step to another.
Notes:
1. The model file with the final deflection does NOT contain plastic strains or residual stresses. Its main purpose is to provide a model for performing linear analysis and code checks on the model after an extreme event to evaluate structures. For example, consider a ship's impact accident happens, and the structure (mainly members not affected by the ship impact) needs to be evaluated (code check) after the accident.
2. The model with final deflection only contains the model and not piles. As mentioned earlier, users can use SACS Collapse progressive sequential analysis to include pile plasticity in the analysis.
3. The model file with the final deflection shape is generated for every load sequence defined in the Collapse input file. They are named sequentially based on the number of load sequences defined in the collapse input file. For example, if the collapse input file has two load sequences, SACINP001 .DAT and SACSlNP002.DAT will be generated. Note that these files will be overwritten every time the Collapse program runs.
4. SACS currently doesn't support the restart option due to technical difficulties and the size of the data needed to be stored.