Element not rotating right when using springs at end of elements

[Jeroen124]

Please take a look at the following script, it appears that the second bar is not rotating correctly (e.g. the bar becomes longer as it rotates)

# =============================================================================
# Simple code example 
# =============================================================================
from anastruct import SystemElements
import math

gnl = True
w = 1              # m
F1 = 2
k1 = 1             # kNm/rad
k2 = 1             # kNm/rad


# Properties  (mm)
A = 1e3
I = 1e6

# from MPa to kN/m2
E_yougs = 210000 * 1000

# from mm4 to m4
mm4_to_m4 = 1 / 1e12

# from mm2 to m2
mm2_to_m2 = 1 / 1e6

# conversion
EI = E_yougs * I * mm4_to_m4
EA = E_yougs * A * mm2_to_m2

# Add beams to the system.
ss = SystemElements()


ss.add_element(location=[[0, 0], [0, 0.5*w]], EI=EI, EA=EA)
ss.add_element(location=[[0, 0.5*w], [0, w]], EI=EI, EA=EA, spring={1: k1, 2: k2})

# add supports
ss.add_support_fixed(node_id=1)

# add loads
ss.point_load(Fx=-F1, Fy=-0, node_id=3)
ss.show_structure()
ss.solve(geometrical_non_linear=gnl,discretize_kwargs=dict(n=20) , max_iter=500)
ss.show_displacement(factor=1)


# =============================================================================
# Checking rotation
# =============================================================================

n2x =           ss.get_node_displacements(node_id=2)['ux']
n2y = 0.5*w+    ss.get_node_displacements(node_id=2)['uy']
n3x =           ss.get_node_displacements(node_id=3)['ux']
n3y = w+        ss.get_node_displacements(node_id=3)['uy']

length_upper_bar = ((n2x-n3x)**2+(n2y-n3y)**2)**0.5
print('Length upper bar is:',length_upper_bar,' which in reailty is:',0.5*w)


Rotation_anastruct = math.sin((n2x-n3x)/(0.5*w))

print('Rotation is:',Rotation_anastruct,' rad according to anastruct')

# Analytical calculation:
Rotation_hand_calc = (0.5*w*F1)/k1
print('Rotation is:',Rotation_hand_calc,' rad according to simple hand calc')