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

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

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

Ronan Point Load-Bearing Precast Panels

Explosion in Lower Story Localized Damage

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

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

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

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

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

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

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.

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.

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

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.

Structural Integrity Reinforcement Member 318-11 318-14 General Provisions 7.13 4.10 Beams & Joists (’89) 7.13.2-.5 9.7.7 Nonprestressed Two-Way Slabs (‘89) 13.3.8.5 8.7.4.2 Prestressed Two-Way Slabs (‘08) 18.12.6 & 18.12.7 8.7.5.6 Precast (’95) 16.5 16.2.1.8

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

12.11 — Development of Positive Moment Reinforcement 12.11.1 — 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.

12.12 — Development of Negative Moment Reinforcement 12.12.3 — 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.

Conventional Flexural Reinforcement ― 3-Span Beam Bending Moment Diagram Greater of: d 12db Span/16 (12.12.3) Flexural Reinforcement 6 in. (12.11.1)

Conventional Flexural Reinforcement ― 3-Span Beam Bending Moment Diagram Greater of: d 12db Span/16 (12.12.3) Flexural Reinforcement 6 in. (12.11.1)

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

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7.13 – Requirements for Structural Integrity Continuous Top and/or Bottom Structural Integrity Reinforcement to Pass through Column Core Class B Splices (pre ACI 318-08 was Class A)

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

7.13.2.3 – Perimeter Beams Min. 135o Hook or

R7.13.2 – Perimeter Beams Crosstie (90 Deg. on Slab side - do not alternate) OR Note orientation of crosstie relative to perimeter and slab. Unacceptable OK

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

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

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

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 12.14.3. Splices shall be located as shown in Fig. 13.3.8. 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.

Slab Rebar Courtesy of Hai Dinh, Univ. of Michigan

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

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

Balancing Act

“Lack of” Engineering Judgment Completing the Triangles

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

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”

Parting Thought

Questions?