1. Advantages and Disadvantages of Statically Indeterminate Structures
Lesson 2
Advantages and Disadvantages of Statically Indeterminate Structures

2. Advantages/Disadvantages

3. Geometric changes cause indirect stresses

4. Geometric changes cause indirect stresses
Statically Determinate
Statically Indeterminate

5. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Fabrication Error
If this is accidentally
Fabricated too long…
Statically Determinate
Statically Indeterminate

6. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Fabrication Error
If this is accidentally
Fabricated too long…
All joints just move to
New positions
Statically Determinate
Statically Indeterminate

7. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Fabrication Error
If this is accidentally
Fabricated too long…
Statically Determinate
Statically Indeterminate

8. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Fabrication Error
If this is accidentally
Fabricated too long…
Then it must be
Compressed into place
Statically Determinate
Statically Indeterminate

9. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Fabrication Error
Which puts these in tension
If this is accidentally
Fabricated too long…
Then it must be
Compressed into place
Statically Determinate
Statically Indeterminate

10. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Fabrication Error
Which puts forces on all members
If this is accidentally
Fabricated too long…
Then it must be
Compressed into place
Statically Determinate
Statically Indeterminate

11. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Fabrication Error
Which puts forces on all members
No Stresses. Members go together.
Members must be forced to fit.
Statically Determinate
Statically Indeterminate

12. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Statically Determinate
Statically Indeterminate

13. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 2. Temperature Changes
SUN
Heated
Statically Determinate
Statically Indeterminate

14. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 2. Temperature Changes
SUN
Heated
No Stresses. Positions just change
Statically Determinate
Statically Indeterminate

15. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 2. Temperature Changes
Joints restricted by truss-action
SUN
SUN
Heated
Heated
No Stresses. Positions just change
Statically Determinate
Statically Indeterminate

16. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 2. Temperature Changes
SUN
SUN
Heated
Heated
No Stresses. Positions just change
Thermal causes stresses
Statically Determinate
Statically Indeterminate

17. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 2. Temperature Changes
SUN
SUN
If Expansion Joint does not
Allow expansion, this duct
Is statically indeterminate
Heated
Heated
No Stresses. Positions just change
Thermal causes stresses
Statically Determinate
Statically Indeterminate

18. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 2. Temperature Changes
SUN
SUN
If Expansion Joint does not
Allow expansion, this duct
Is statically indeterminate
Heated
Heated
No Stresses. Positions just change
800F high-temperature gas causes it to
Crumple like a beer can
Thermal causes stresses
Statically Determinate
Statically Indeterminate

19. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses
Simple Beam
Simple Beam
Pier
Pier
Pier
Statically Determinate
Statically Indeterminate

20. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 3. Foundation Settlement
Simple Beam
Simple Beam
Pier
Pier
Pier
Any Settlement:
Statically Determinate
Statically Indeterminate

21. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 3. Foundation Settlement
Simple Beam
Simple Beam
Pier
Pier
Pier
Any Settlement: No Curvature. No moment
Statically Determinate
Statically Indeterminate

22. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 3. Foundation Settlement
Simple Beam
Simple Beam
Pier
Pier
Pier
Any Settlement: No Curvature. No moment
Statically Determinate
Statically Indeterminate

23. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 3. Foundation Settlement
Simple Beam
Simple Beam
Continuous Beam
Pier
Pier
Pier
Pier
Pier
Pier
Any Settlement: No Curvature. No moment
Statically Determinate
Statically Indeterminate

24. Statically Indeterminate Disadvantages
Geometric changes cause indirect stresses 3. Foundation Settlement
Simple Beam
Simple Beam
Continuous Beam
Pier
Pier
Pier
Pier
Pier
Pier
Any Settlement: No Curvature. No moment
Any Settlement: Curvature! Moment!
Statically Determinate
Statically Indeterminate

25. Pier
Pier
Pier
Pier
Pier
Pier
Statically Determinate
Statically Indeterminate

26. Statically Indeterminate Advantages
Pier
Pier
Pier
Pier
Pier
Pier M M
Statically Determinate
Statically Indeterminate

27. Statically Indeterminate Advantages
Pier
Pier
Pier
Pier
Pier
Pier
wL2/8 M M
Statically Determinate
Statically Indeterminate

28. Statically Indeterminate Advantages
1. Lower Stresses
Pier
Pier
Pier
Pier
Pier
Pier
wL2/8
wL2/21.4 M M M M
Statically Determinate
Statically Indeterminate

29. Statically Indeterminate Advantages
2. Greater Stiffness
Pier
Pier
Pier
Pier
Pier
Pier
D =
Statically Determinate
Statically Indeterminate

30. Statically Indeterminate Advantages
2. Greater Stiffness
Pier
Pier
Pier
Pier
Pier
Pier
D = 5wL4/384EI
Statically Determinate
Statically Indeterminate

31. Statically Indeterminate Advantages
2. Greater Stiffness
Pier
Pier
Pier
Pier
Pier
Pier
D = 5wL4/384EI
Statically Determinate
Statically Indeterminate

32. Statically Indeterminate Advantages
2. Greater Stiffness
Pier
Pier
Pier
Pier
Pier
Pier
D = 2.1wL4/384EI
D = 5wL4/384EI
Statically Determinate
Statically Indeterminate

33. Statically Indeterminate Advantages
2. Greater Stiffness
Pier
Pier
Pier
Pier
Pier
Pier
Statically Determinate
DOIs = ??
Statically Indeterminate

34. Statically Indeterminate Advantages
2. Greater Stiffness
Pier
Pier
Pier
Pier
Pier
Pier
Statically Determinate
DOIs = 0
Statically Indeterminate
DOIs = 1

35. Statically Indeterminate Advantages
Pier
Pier
Pier
Pier
A catastrophic event eliminates the middle pier
Statically Determinate
DOIs = 0
Statically Indeterminate
DOIs = 1

36. Statically Indeterminate Advantages
3. Greater Robustness – May survive redundant loss of massive overload
Pier
Pier
Pier
Pier
Will NOT Survive
(zero redundancy)
MAY Survive
(if the beam can take a
Substantial increase in moment)
A catastrophic event eliminates the middle pier
Statically Determinate
DOIs = 0
Statically Indeterminate
DOIs = 1