Why the cable analysis results look weird?

The cable problem is in the paper “Analysis of Cable Structures” by A.H.Pevrot, A.M.Goulois.
image
Cables in the above imgae was modelled in both Sap2000 and Sofistik.

The cable has undeformed length of 100m. The section area of the cable is 1m^2, the elastic modulus
is 3E4 MPa and the coefficient of thermal expansion is 6.5E-6. The cable has vertical load of 1kN/m along undeformed length and has uniform temperature of 100℃.

The reaction result from Sap2000 show an acceptable comparison with the results in the paper.

The difference between the vertical reactions
= 2565177.3 - 2565077.3 = 100 kN
= vertical load (1kN/m) * undeformed length of 100m

Below is the teddy code

+PROG AQUA
HEAD
CTRL FACE NEG
UNIT 5 DIG 3 SET OUT
ECHO SECT EXTR
NORM GB 50010-2010
STEE NO TYPE ES ALFA
1 S 3e4 6.5e-6
SVAL NO MNO A
1 1 1
END
+PROG SofimshC
HEAD
SYST 3D GDIR NEGZ GDIV -1000
CTRL MESH 1
SPT NO X Y Z FIX
1 100 0 30 PP
2 0 0 90 PP
SLN NO NPA NPE SDIV STYP SNO GRP
1 1 2 -20 CE 1 1
END
PROG SOFILOAD
HEAD
LC NO
1
CABL FROM TO TYPE PA
‘GRP’ 1 PG 1
‘GRP’ 1 DT 100
END
PROG ASE
SYST PROB TH3
CTRL CABL 1
LEN0 L0 TYPE GRP
100 TOTA 1
LC 1
END

But the result are weird

  1. The sum of load is 120kN, why? It should be 1kN/m x 100m (unstressed length) =100kN

  2. This the reaction in X direction

  3. This the reaction in Z direction

Dear netsonic_Sofistik,
SAP 2000 gives weird results after nonlinear calcs. Results in Peyrot/Goulois paper also.
Positive temperature made compression in cable. A negative temperature difference of 100 units creates a tension of 19500 units for the given parameters, which cannot cause the shown reactions.
Please note also that for the chain system like in your code we need use CTRL CABL 0.

Thanks for the input.
We also tried Staad pro, which gave the very similar results as Sap2000.

+PROG AQUA
HEAD
CTRL FACE NEG
UNIT 5 DIG 3 SET OUT
ECHO SECT EXTR
NORM GB 50010-2010
STEE NO TYPE ES ALFA GAM
1 S 3e4 6.5e-6 1
SVAL NO MNO A
1 1 1
END
+PROG SofimshC
HEAD
SYST 3D GDIR NEGZ GDIV -1000
CTRL MESH 1
SPT NO X Y Z FIX
1 100 0 30 PP
2 80 0 30 PP
3 60 0 30 PP
4 40 0 30 PP
5 20 0 30 PP
6 0 0 30 PP
10 0 0 90 PP
SLN NO NPA NPE SDIV STYP SNO GRP
1 1 10 -20 CE 1 1
2 2 10 -20 CE 1 1
3 3 10 -20 CE 1 1
4 4 10 -20 CE 1 1
5 5 10 -20 CE 1 1
6 6 10 -20 CE 1 1
END
PROG SOFILOAD
HEAD
LC NO
1
CABL FROM TO TYPE PA
‘GRP’ 1 DT 100
END
PROG ASE
SYST PROB TH3
CTRL CABL 0
LEN0 L0 TYPE GRP
100 TOTA 1
LC 1 FACD 1
LCC 10
END

But the results are still weird
The reactions in X direction

The reactions in Z direction

Actually you did most things wrong which you could have done wrong in a nonlinear analysis using sofistik. But this is normal…

  1. len0 is made for elements. you have 20 elements so you element length of interest is 100m/5 = 20m

  2. You are doing a loadcombination in sofiload and analyse it in ase.
    The better way is to define loadcases in sofiload and combine them in ase

  3. Depending on the behavior of you cable in the global structure, you can decide between two methods of modeling the cable.

a) ctrl cabl 1 → inner cable sag is activated, you trust on the background calc regarding the axial cabl stiffness due consideration of the cable ag

b) ctrl cabl 0 → you are deviding the cable into elements and the effects of the cable sag are considered due the discretization of the cable and the upcomming degrees of freedom

To compensate my implicit insults of the first sentence, I prepared the example for you. Please note: To get the different cable geometries of the example (x position of the second anchor point) I defined a support displacement. You can factorize the lcc (load case copy) to get further results. Factor of -20 means an displacement of 20m in direction to the origin of coordiante system. This gives you the result for the second case of your example.

+PROG AQUA urs:1
HEAD
CTRL FACE NEG
UNIT 5 DIG 3 SET OUT
ECHO SECT EXTR
NORM GB 50010-2010
STEE NO TYPE ES ALFA
1 S 3e4 6.5e-6
SVAL NO MNO A
1 1 1
END

+PROG SofimshC urs:2
HEAD
SYST 3D GDIR NEGZ GDIV -1000
CTRL MESH 1

SPT NO X Y Z FIX
1 100 0 30 PP
2 0 0 90 PP

SLN NO NPA NPE SDIV STYP SNO GRP
1 1 2 -20 CE 1 1

END
PROG SOFILOAD urs:3
HEAD
LC NO
1
CABL FROM TO TYPE PA
‘GRP’ 1 PG 1
$ ‘GRP’ 1 DT 100
LC NO
2
CABL FROM TO TYPE PA
$ ‘GRP’ 1 PG 1
‘GRP’ 1 DT 100

END

PROG ASE urs:4
head
SYST PROB TH3
CTRL ITER 3 V2 1
CTRL CABL 1
LEN0 L0 TYPE GRP
100/20 TOTA 1

LC 10
LCC 1
$LCC 2

END

PROG ASE urs:5
head
SYST PROB TH3 PLC 10
CTRL ITER 3 V2 1
$CTRL CABL 1
$LEN0 L0 TYPE GRP
$ 100/20 TOTA 1

LC 11
LCC 1
LCC 2 PLC NEW

END

PROG SOFILOAD urs:6
HEAD
LC NO
3
NODE NO TYPE P1
1 WXX 1000

END

PROG ASE urs:7
head
SYST PROB TH3 PLC 11 iter 9999
CTRL ITER 3 V2 1
$CTRL CABL 1
$LEN0 L0 TYPE GRP
$ 100/20 TOTA 1

LC 12
LCC 1
$LCC 2 PLC NEW
LCC 3 fact -20

END

1 Like

Thank you for the example.

My complete teddy code is attached.
Cables.dat (6.2 KB)

  1. The reactions under dead and tempeture load agree well with those from Sap2000 or Staad
    Sofistik
    image

Sap2000
image

Staad
image

  1. The reactions under dead load only doesn’t agree well with them
    Sofistik
    image

Sap2000
image

Staad
image

  1. As for the cable support with 20m disp, your code as shown below gave the results that don’t agress well with the them from Sap2000 or Staad.
PROG ASE urs:7
head
SYST PROB TH3 PLC 11 iter 9999
CTRL ITER 3 V2 1
$CTRL CABL 1
$LEN0 L0 TYPE GRP
$ 100/20 TOTA 1
LC 12
LCC 1
$LCC 2 PLC NEW
LCC 3 fact -20 

Sofistik
image

Sap2000
image

Staad
image

So I add one more ASE to get better results

PROG	ASE			
SYST	PROB	TH3	PLC	1
CTRL	ITER	3	V2 1	
LC	21			
	LCC	10		
	LCC	30	FACT	20
END				
				
PROG	ASE			
SYST	PROB	TH3	PLC	21
CTRL	ITER	3	V2 1	
LC	2			
	LCC	10		
	LCC	30	FACT	20
END				

Sofistik
image

  1. As for the cable support with 40m disp, I use the code below, which is similar to the code for the previous case with 20m disp, but the result doesn’t agree with them from Sap2000 or Staad.
PROG	ASE			
SYST	PROB	TH3	PLC	1
CTRL	ITER	3	V2 1	
LC	31			
	LCC	10		
	LCC	30	FACT	40
END				
				
PROG	ASE			
SYST	PROB	TH3	PLC	31
CTRL	ITER	3	V2 1	
LC	3			
	LCC	10		
	LCC	30	FACT	40
END				

Sofistik
image

Sap2000
image

Staad
image

sean_cable_test.gra (2.8 KB)
sean_cable_test.dat (1.1 KB)
Now, you can see the same results in SOFiSTiK

sean_cable_test_001.gra (3.8 KB)
sean_cable_test_001.dat (1.9 KB)
Dear, netsonic.
Please see updated dat.
A such input (primary load case is previous displacement stage for the next displacement stage) gets the desire results. Values are almost the same as in SAP2000.

1 Like