I have a model containing compression only soil springs in which the wind loads are producing very large displacements. How do I resolve this?
It is not uncommon for structures supported by compression only soil springs to encounter stability issues when analyzing primary load cases, particularly lateral loads such as wind or seismic. This is because these load cases are not "realistic" in the sense that they'd never actually occur in reality (for example, you'd never have wind load acting on a structure by itself without the dead load acting simultaneously.) When the lateral load is applied without the stabilizing effects of gravity loads, many of the soil springs are pulled into tension and therefore turned off. As a result, the effects of overturning/uplift dominate and displacements become excessive. Typically this behavior doesn't affect the load combinations since selfweight and gravity loads resist overturning/uplift.
Since it is really only the load combinations that need to be considered for design, the large displacements can be gotten rid of by setting up the model so the compression only behavior is not considered for the primary load cases (but is active for the combined loads). This can be done my going into the STAAD Editor and specifying the SUPPORT command twice: once for the primary load cases (without the COMPRESSION command included, and a second time for the load combinations with the COMPRESSION command included).
Consider the example below showing how this procedure is done. The original command syntax is provided, followed by the modified commands edited as described above:
Original-
SUPPORTS1 TO 102 PLATE MAT DIRECT Y SUBGRADE 12 COMPRESSIONLOAD 1 LOADTYPE Dead TITLE DEAD LOADSELFWEIGHT Y -1 ELEMENT LOAD1 TO 102 PR GY -0.025LOAD 2 LOADTYPE None TITLE LIVE LOADJOINT LOAD1 2 FY -2178 9 FY -1095 FY -308.76 FY -617.422 23 FY -41029 30 FY -20526 FY -542.727 FY -1085.443 44 50 51 71 72 78 79 FY -307.547 54 82 FY -264.248 55 76 83 FY -528.392 93 FY -20599 100 FY -410103 FY -487104 FY -974113 114 FY -109120 121 FY -217124 FY -273.3125 FY -546.6LOAD 3 LOADTYPE Wind TITLE WIND LOADJOINT LOAD127 TO 130 FZ 25LOAD 4 LOADTYPE None TITLE Load CombinationREPEAT LOAD1 1.0 2 1.0 3 1.0 PERFORM ANALYSISFINISH
Modified-
SUPPORTS1 TO 102 PLATE MAT DIRECT Y SUBGRADE 12 LOAD 1 LOADTYPE Dead TITLE DEAD LOADSELFWEIGHT Y -1 ELEMENT LOAD1 TO 102 PR GY -0.025LOAD 2 LOADTYPE None TITLE LIVE LOADJOINT LOAD1 2 FY -2178 9 FY -1095 FY -308.76 FY -617.422 23 FY -41029 30 FY -20526 FY -542.727 FY -1085.443 44 50 51 71 72 78 79 FY -307.547 54 82 FY -264.248 55 76 83 FY -528.392 93 FY -20599 100 FY -410103 FY -487104 FY -974113 114 FY -109120 121 FY -217124 FY -273.3125 FY -546.6LOAD 3 LOADTYPE Wind TITLE WIND LOADJOINT LOAD127 TO 130 FZ 25PERFORM ANALYSISCHANGESUPPORTS1 TO 102 PLATE MAT DIRECT Y SUBGRADE 12 COMPRESSIONLOAD 4 LOADTYPE None TITLE Load CombinationREPEAT LOAD1 1.0 2 1.0 3 1.0 PERFORM ANALYSISFINISH
As you can see, in the original there is only one SUPPORT command located before the first primary load case. In the modified scenario this support definition has been changed to eliminate the COMPRESSION specification. As a result, the primary load cases underneath not subject to compression-only soil spring behavior (i.e. the support springs can take tension or compression.) Following the last primary load case, and just prior to the load combination, a PERFORM ANALYSIS and CHANGE command have been added. These commands allow for a second SUPPORT definition to be specified, which you'll find directly underneath. In it the COMPRESSION specification is present, meaning the compression only soil spring behavior will be considered for all subsequent load combinations.
As a result, in the modified scenario when the analysis is run the large displacements no longer occur due to wind load while the correct compression only soil spring behavior is still captured for the load combinations.