Applies To
	Product(s):	STAAD.Pro
	Version(s):	All
	Environment:	N/A
	Area:	Modeling Solutions
	Subarea:	Supports
	Original Author:	Bentley Technical Support Group

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 (defined through REPEAT LOADs) since selfweight and gravity loads resist overturning/uplift.  

Since it is really only the load combinations (defined through REPEAT LOADs) 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-

SUPPORTS
1 TO 102 PLATE MAT DIRECT Y SUBGRADE 12 COMPRESSION
LOAD 1 LOADTYPE Dead TITLE DEAD LOAD
SELFWEIGHT Y -1
ELEMENT LOAD
1 TO 102 PR GY -0.025
LOAD 2 LOADTYPE None TITLE LIVE LOAD
JOINT LOAD
1 2 FY -217
8 9 FY -109
5 FY -308.7
6 FY -617.4
22 23 FY -410
29 30 FY -205
26 FY -542.7
27 FY -1085.4
43 44 50 51 71 72 78 79 FY -307.5
47 54 82 FY -264.2
48 55 76 83 FY -528.3
92 93 FY -205
99 100 FY -410
103 FY -487
104 FY -974
113 114 FY -109
120 121 FY -217
124 FY -273.3
125 FY -546.6
LOAD 3 LOADTYPE Wind TITLE WIND LOAD
JOINT LOAD
127 TO 130 FZ 25
LOAD 4 LOADTYPE None TITLE Load Combination
REPEAT LOAD
1 1.0 2 1.0 3 1.0
PERFORM ANALYSIS
FINISH

Modified-

SUPPORTS
1 TO 102 PLATE MAT DIRECT Y SUBGRADE 12
LOAD 1 LOADTYPE Dead TITLE DEAD LOAD
SELFWEIGHT Y -1
ELEMENT LOAD
1 TO 102 PR GY -0.025
LOAD 2 LOADTYPE None TITLE LIVE LOAD
JOINT LOAD
1 2 FY -217
8 9 FY -109
5 FY -308.7
6 FY -617.4
22 23 FY -410
29 30 FY -205
26 FY -542.7
27 FY -1085.4
43 44 50 51 71 72 78 79 FY -307.5
47 54 82 FY -264.2
48 55 76 83 FY -528.3
92 93 FY -205
99 100 FY -410
103 FY -487
104 FY -974
113 114 FY -109
120 121 FY -217
124 FY -273.3
125 FY -546.6
LOAD 3 LOADTYPE Wind TITLE WIND LOAD
JOINT LOAD
127 TO 130 FZ 25
PERFORM ANALYSIS
CHANGE
SUPPORTS
1 TO 102 PLATE MAT DIRECT Y SUBGRADE 12 COMPRESSION
LOAD 4 LOADTYPE None TITLE Load Combination
REPEAT LOAD
1 1.0 2 1.0 3 1.0
PERFORM ANALYSIS
FINISH

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 (defined through REPEAT LOADs), 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 (defined through REPEAT LOADs).  

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.