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