STRUCTURE
UNSTABLE
TRACTION
STABLE
BRIDGE
SHELL
O
I
I
I
H
X
D
O
A
F
G
G
H
E
P
U
I
N
O
J
X
N
E
D
E
B
R
I
D
G
E
O
E
L
U
I
R
V
O
O
V
I
D
Q
B
J
C
G
U
R
D
T
B
R
J
A
E
N
E
O
T
C
Z
T
V
U
T
U
Q
A
H
A
C
V
U
Z
U
S
X
U
X
R
K
R
U
A
X
L
N
L
E
T
N
E
B
E
R
U
M
U
W
A
I
I
V
Q
Z
O
T
Y
A
I
U
K
E
L
B
A
T
S
S
