COMPRESSION SPRING ONLY FOR UPLIFT WATER PRESSURE FOR STRUCTURE UNDERGROUND

Hi,

I am modelling a box section built underground, it has to be checked for upward buoyancy load. To find the moments and shear forces I had applied compression only spring for upward load, can anyone check the correctness of the approach adopted.

STAAD PLANE BOX ANALYSIS OF SIZE 0.6 X 0.6
START JOB INFORMATION
ENGINEER DATE 03-Mar-18
END JOB INFORMATION
UNIT METER MTON
JOINT COORDINATES
1 0 0 0; 2 0.121 0 0; 3 0.242 0 0; 4 0.363 0 0; 5 0.483 0 0;
6 0.604 0 0; 7 0.725 0 0; 8 0 0.75 0; 9 0.725 0.75 0; 10 0.0625 0 0;
11 0.6625 0 0;
MEMBER INCIDENCES
1 1 10; 2 2 3; 3 3 4; 4 4 5; 5 5 6; 6 11 7; 7 1 8; 8 7 9; 9 8 9;
10 10 2; 11 6 11;
MEMBER PROPERTY AMERICAN
1 TO 6 10 11 PRIS YD 0.15 ZD 1
7 8 PRIS YD 0.125 ZD 1
9 PRIS YD 0.15 ZD 1
DEFINE MATERIAL START
ISOTROPIC MATERIAL
E 3e+006
POISSON 0.15
DENSITY 2.5
ALPHA 1e-005
DAMP 0.05
END DEFINE MATERIAL
CONSTANTS
MATERIAL MATERIAL ALL
SUPPORTS
1 FIXED BUT FZ MX MY MZ KFY 46.875
7 FIXED BUT FX FZ MX MY MZ KFY 46.875
3 TO 5 FIXED BUT FX FZ MX MY MZ KFY 181.25
10 11 FIXED BUT FX FZ MX MY MZ KFY 90.625
2 6 FIXED BUT FX FZ MX MY MZ KFY 134.375
LOAD 1 DEAD LOADS
SELFWEIGHT Y -1
MEMBER LOAD
9 UNI GY -10
LOAD 2 PLL PEDESTRIAN LIVE LOAD
MEMBER LOAD
9 UNI GY -0.4
LOAD 3 EARTH PRESSURE LEFT
MEMBER LOAD
7 TRAP GX 5.825 5.075 0 0.75
LOAD 4 EARTH PRESSURE RIGHT
MEMBER LOAD
8 TRAP GX -5.825 -5.075 0 0.75
LOAD 5 LL SURCHARGE LEFT
MEMBER LOAD
7 TRAP GX 1.2 1.2 0 0.75
LOAD 6 LL SURCHARGE RIGHT
MEMBER LOAD
8 TRAP GX -1.2 -1.2 0 0.75
LOAD 7 EARTH PRESSURE SUBMERGED
MEMBER LOAD
7 TRAP GX 7.238 6.113 0 0.75
8 TRAP GX -7.238 -6.113 0 0.75

PERFORM ANALYSIS
CHANGE
SUPPORTS
1 FIXED BUT FZ MX MY MZ KFY 46.875
7 FIXED BUT FX FZ MX MY MZ KFY 46.875
3 TO 5 FIXED BUT FX FZ MX MY MZ KFY 181.25
10 11 FIXED BUT FX FZ MX MY MZ KFY 90.625
2 6 FIXED BUT FX FZ MX MY MZ KFY 134.375
SPRING COMPRESSION
2 to 6 10 11 KFY
LOAD 8 UPWARD WATER PRESSURE
MEMBER LOAD
1 TO 6 10 11 UNI GY 0.75
LOAD 9 COMBINATION LOAD CASE 9 (FACTORED)
REPEAT LOAD
1 1.5 2 1.5 3 1.5 4 1.5 5 1.5 6 1.5
LOAD 10 COMBINATION LOAD CASE 10 (FACTORED SUBMERGED EARTH PRESSURE)
REPEAT LOAD
1 1.5 2 1.5 7 1.5 5 1.5 6 1.5 8 1.5
LOAD 11 COMBINATION LOAD CASE 11 (UNFACTORED)
REPEAT LOAD
1 1.0 2 1.0 3 1.0 4 1.0 5 1.0 6 1.0
LOAD 12 COMBINATION LOAD CASE 12 (UNFACTORED SUBMERGED EARTH PRESSURE)
REPEAT LOAD
1 1.0 2 1.0 7 1.0 5 1.0 6 1.0 8 1.0
PERFORM ANALYSIS PRINT STATIC CHECK
FINISH