Product(s):	SACS
Version(s):	All
Area:	Collapse

Problem

How can I consider the nonlinear spring load displacement relation for energy absorption of shock cell for boat landing support and rubber strip for frontal member in boat impact analysis? I have set of data of Reaction Force vs Deflection and Energy Absorption vs Deflection from vendor. How to use SACS Collapse to simulate the scenario?

Solution

You can refer NLSPJJ command in SACS collapse analysis. Since you have the vendor data for Reaction Force vs Deflection and Energy Absorption vs Deflection of the shock cell or fenders, you can simulate the same as spring stiffness using the above said line. The first and second joint names are the members starting and end joints, which will have the shock cell. In collapse analysis model, you need to delete the member after assigning the spring data. 

This line will ask for deflection at 6 degrees of global direction (3 linear and 3 rotational). Based on the degree of freedom provided in Column 18-19, corresponding force or moment shall be given in SACS. In one line you can provide 4 sets of data, but you can add as many lines as required by inserting the same command.

Make sure to check SACS default unit system before applying vendor data, as you may need to convert the original data to SACS unit.

Mostly, vendor provides deflection vs force data for a set, and you need to input the data in both positive and negative magnitude, in a symmetrical form. The same philosophy could be applied for boat landing member rubber strips as well.

User can verify the results by simulating cases with and without springs.

Let us try to calculate a scenario with the following data:

Assume,

1. Total energy absorbed by the structure = 1 MJ (This can be seen at the end of collapse listing file)
2. Initial applied load = 10 kN, which shall be increased in sequence

 

You will see for ‘without springs’ case the load factor is A and ‘with springs’ the load factor is B. “A” will always be lesser than “B” when results will be compared. You can see the load factors in collapse log file.

If we do the reverse engineering, we see without spring the structure is capable of absorbing 1 MJ energy by experiencing a force of F1 = AX10 kN. But with spring case, same energy absorption can be done with an impact force of F2 = BX10 kN.  Since, A<B, F1 will always be lesser than F2. This confirms structure is capable of absorbing higher force with spring than that without spring. That is how the springs are working in the model.

 

 

A random data and its input in SACS collapse input file is provided below as an example. Here member A100-B100 will have the shock cell.

Vendor data as

Deflection (cm)	40	35	32	30	28	26	25	22	20	18	16	15	12.5	10	7.5	5	2.5
Force (kN)	350	315	275	245	220	190	165	145	125	110	95	85	75	65	50	40	25

The corresponding SACS input shall be as below:

NLSPJJ A100B100  DX       -350. -40.00  -315. -35.00  -275.   -32.  -245.   -30.                                       

NLSPJJ           DX       -220.   -28.  -190.   -26.  -165.   -25.  -145.   -22.                                       

NLSPJJ           DX       -125.   -20.  -110.   -18.   -95.   -16.   -85. -15.00                                       

NLSPJJ           DX        -75. -12.50   -65. -10.00   -50. -7.500   -40. -5.000                                       

NLSPJJ           DX        -25. -2.500    0.0  0.0      25.  2.500    40.  5.000                                       

NLSPJJ           DX         50.  7.500    65.  10.00    75.  12.50    85.  15.00                                       

NLSPJJ           DX         95.    16.   110.    18.   125.    20.   145.    22.                                       

NLSPJJ           DX        165.    25.   190.    26.   220.    28.   245.    30.                                       

NLSPJJ           DX        275.    32.   315.    35.   350.    40.        

 

*********************************

NLSPJJ A100B100  DY       -350. -40.00  -315. -35.00  -275.   -32.  -245.   -30.                                       

NLSPJJ           DY       -220.   -28.  -190.   -26.  -165.   -25.  -145.   -22.                                       

NLSPJJ           DY       -125.   -20.  -110.   -18.   -95.   -16.   -85. -15.00                                       

NLSPJJ           DY        -75. -12.50   -65. -10.00   -50. -7.500   -40. -5.000                                        

NLSPJJ           DY        -25. -2.500    0.0  0.0      25.  2.500    40.  5.000                                       

NLSPJJ           DY         50.  7.500    65.  10.00    75.  12.50    85.  15.00                                        

NLSPJJ           DY         95.    16.   110.    18.   125.    20.   145.    22.                                       

NLSPJJ           DY        165.    25.   190.    26.   220.    28.   245.    30.                                       

NLSPJJ           DY        275.    32.   315.    35.   350.    40.            

*********************************

NLSPJJ A100B100  DZ       -350. -40.00  -315. -35.00  -275.   -32.  -245.   -30.                                       

NLSPJJ           DZ       -220.   -28.  -190.   -26.  -165.   -25.  -145.   -22.                                       

NLSPJJ           DZ       -125.   -20.  -110.   -18.   -95.   -16.   -85. -15.00                                       

NLSPJJ           DZ        -75. -12.50   -65. -10.00   -50. -7.500   -40. -5.000                                       

NLSPJJ           DZ        -25. -2.500    0.0  0.0      25.  2.500    40.  5.000                                       

NLSPJJ           DZ         50.  7.500    65.  10.00    75.  12.50    85.  15.00                                       

NLSPJJ           DZ         95.    16.   110.    18.   125.    20.   145.    22.                                       

NLSPJJ           DZ        165.    25.   190.    26.   220.    28.   245.    30.                                       

NLSPJJ           DZ        275.    32.   315.    35.   350.    40.