1. Structural Integrity in the Design of Concrete Structures
Jamie Farny
Lawrence Novak

2. Characteristics of Concrete
Basic Concept
Strong in Compression
Weak in Tension
Factor of 10!!

3. Ronan Point (1968) Gas Explosion on 18th Floor of 22 Floors

4. Ronan Point Load-Bearing Precast Panels

5. Explosion in Lower Story Localized Damage

6. Unpredictable and Unforeseen Events
Acts of Terrorism (Buildings are Symbols and hence Targets)
Earthquakes
Hurricane Winds
Vehicle Impact
Fire
Material Defects
Design Shortcoming / Analytical Modeling
Failure of Non-Structural Systems
Loads & Effects
System Response

7. Advantages of Redundancy
Improved Robustness
Distributed Structural System
Resistance to Unpredictable and Unforeseen Events

8. Disadvantages of Redundancy
Possible Less Optimal Use of Materials
More Interconnected Systems
Impact on Member Proportions and Other Disciplines
Can Impact Economic Viability

9. Redundancy vs. Robustness
Multiple Load Paths
Robustness
Resistance to Extreme Events
Similar Terms, But Not Identical

10. Historical Schools of Thought
Tie Everything Together
Ignore the Problem, Current Practice is Sufficient
Identify Most Important Member and Increase Factor of Safety
Historically very little Written by Practicing Engineers Regarding Philosophy of Many Projects

11. Functional Resilience
90 West St.
Built in 1907
Winecoff Hotel.
Built in 1913
Buildings that are built to only satisfy the minimum life safety provisions of building codes should not be considered green, sustainable, or high performance--regardless of the their energy conservation, water conservation, or efficient use of material resources, and indoor environmental quality. Truly high performance, green, or sustainable buildings must also be long-lasting, durable, and disaster resistant. While there are many examples, these two iconic structures demonstrate the resilience of robust construction…
Damaged by WTC collapse, uncontrolled fire for 5 days, and reopened as apartment building in 2005
Completely gutted by fire in 1946, hotel in 1951, housing for elderly, vacant for 20 years, and became the Ellis Hotel in 2007

14. Structural Integrity Concerns - 1980s
It was reported that a building under construction but structurally complete had collapsed. The cause was a violent shaking of a piece of mechanical equipment that had been improperly hooked up.

15. Code Implications
In the inquiry that followed the collapse, it was determined that the building met the ACI code, because the code had no requirements for connecting members together to resist such accidental loads.

16. Progressive Collapse vs. Structural Integrity
Design Against Progressive Collapse Requires Extensive Analysis and Design Assuming Loss of One Member at a Time
ACI 318 Structural Integrity Requirements Call for Minor Changes in Detailing of Reinforcement – No Analysis or Design Needed

17. What is Structural Integrity?
It is the Intent of the Structural Integrity Provisions to Improve the Redundancy and Ductility of Structures and thereby Reduce the Risk of Failure or Collapse of Parts or All of a Building Due to Damage Occurring to a Relatively Small Area of a Building.
The Overall Integrity of the Structure can be Substantially Enhanced by Minor Changes in the Detailing of Reinforcement.
The Intent is to Avoid “House of Cards” Type Collapses.

18. Structural Integrity Reinforcement
Member
318-11
318-14
General Provisions
7.13
4.10
Beams & Joists (’89)
9.7.7
Nonprestressed Two-Way Slabs (‘89)
Prestressed Two-Way Slabs (‘08)
&
Precast (’95)
16.5

19. Without Structural Integrity, the Impact of a Minor Problem will Result in a Disproportionate Damage

20. 12.11 — Development of Positive Moment Reinforcement
— At least one-third the positive moment reinforcement in simple members and one-fourth the positive moment reinforcement in continuous members shall extend along the same face of member into the support. In beams, such reinforcement shall extend into the support at least 6 in.

21. 12.12 — Development of Negative Moment Reinforcement
— At least one-third the total tension reinforcement provided for negative moment at a support shall have an embedment length beyond the point of inflection not less than d, 12db, or ln/16, whichever is greater.

22. Conventional Flexural Reinforcement ― 3-Span Beam
Bending Moment Diagram
Greater of: d
12db
Span/16
( )
Flexural Reinforcement
6 in.
( )

23. Conventional Flexural Reinforcement ― 3-Span Beam
Bending Moment Diagram
Greater of: d
12db
Span/16
( )
Flexural Reinforcement
6 in.
( )

24. 7.13.2.2(b) – Perimeter Beams Continuous - Class B Tension Splice
- Mechanical / Welded Splice

25. Why?

26. Why?

27. Why?

28. Why?

29. Why?

30. Why?

31. 7.13 – Requirements for Structural Integrity
Continuous Top and/or Bottom
Structural Integrity Reinforcement to Pass through Column Core
Class B Splices (pre ACI was Class A)

32. 7.13.2.2(a) – Perimeter Beams Continuous Class B Tension Splice
- Mechanical / Welded Splice

33. – Perimeter Beams
Min. 135o Hook or

34. R – Perimeter Beams
Crosstie (90 Deg. on Slab side - do not alternate) OR
Note orientation of crosstie relative to perimeter and slab.
Unacceptable OK

35. Integrity Reinforcement Concept – Perimeter Beams
Mobilize Catenary Action
Prevent Top Bars from Tearing Out
Resist torsion
What is a “Perimeter Beam”?

36. 7.13.2.1 – Joist Construction Standard Hook Minimum One Bottom Bar
- Continuous
- Class B Tension Splice
- Mechanical / Welded Splice

37. 7.13.2.4 – Other Beams Continuous Class B Tension Splice
- Mechanical / Welded Splice

38. 13.3.8.5 – Two-Way Slab Integrity Reinforcement
All bottom bars or wires within the column strip, in each direction, shall be continuous or spliced with Class B tension splices or with mechanical or welded splices satisfying Splices shall be located as shown in Fig At least two of the column strip bottom bars or wires in each direction shall pass within the column core and shall be anchored at exterior supports.

39. Slab Rebar
Courtesy of Hai Dinh, Univ. of Michigan

40. Thoughts
The Current rules “help” prevent collapse when subjected to various misfortunes
Inconsistent Level of Risk for different members
Now, we are faced with deliberate acts designed to be as destructive as possible
ACI Committee 377 – Performance Based Structural Integrity & Resilience of Concrete Structures
ASCE/SEI Committee on Disproportionate Collapse

41. Engineering Judgment Load Path: Completing the Triangles
Onterie Center, Chicago
Load Path: Completing the Triangles

42. Balancing Act

43. “Lack of” Engineering Judgment
Completing the Triangles

44. “Skyscrapers seem solid, immutable, as blank and indestructible as mountains,”

45. Sometimes, they cast off pieces of themselves like so much ballast”
“… but buildings sway in the wind, they settle, they crumble, they corrode.
Sometimes, they cast off pieces of themselves like so much ballast”

46. Parting Thought

47. Questions?