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!!
2
Ronan Point
(1968)
Gas
Explosion on
18th Floor
of 22 Floors
3
Ronan Point
Load-Bearing
Precast Panels
4
Explosion in
Lower Story
Localized
Damage
5
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

Redundancy


Robustness


Multiple Load Paths
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
Damaged by WTC collapse, Completely gutted by fire in
uncontrolled fire for 5 days, 1946, hotel in 1951, housing for
elderly, vacant for 20 years, and
and reopened as apartment
became the Ellis Hotel in 2007
building in 2005
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.
14
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.
15
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
16
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.
17
Structural Integrity
Reinforcement
Member
318-11
318-14
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
18.12.6 &
18.12.7
8.7.5.6
16.5
16.2.1.8
General Provisions
Prestressed Two-Way Slabs (‘08)
Precast (’95)
18
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.
20
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.
21
Conventional Flexural Reinforcement ― 3-Span Beam
Greater of:
-d
-12db
-Span/16
Bending Moment Diagram
(12.12.3)
(12.11.1)
Flexural Reinforcement
6 in.
Conventional Flexural Reinforcement ― 3-Span Beam
Greater of:
-d
-12db
-Span/16
Bending Moment Diagram
(12.12.3)
(12.11.1)
Flexural Reinforcement
6 in.
7.13.2.2(b) – Perimeter Beams

s1
A

s2
A

s1
A /4
L arg est of As1 / 4 or As2 / 4 ( Min. 2 Bars )
- Continuous
- Class B Tension Splice
- Mechanical / Welded Splice
24
Why?
25
Why?
26
Why?
27
Why?
28
Why?
29
Why?
30
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)
31
7.13.2.2(a) – Perimeter Beams

s1
A

s2
A
L arg est of As1 / 6 or As2 / 6 ( Min. 2 Bars )
- Continuous
- Class B Tension Splice
- Mechanical / Welded Splice
32
7.13.2.3 – Perimeter Beams
Min. 135o Hook
or
33
R7.13.2 – Perimeter Beams
Crosstie (90 Deg. on Slab side
- do not alternate)
OR
Unacceptable
OK
34
Integrity Reinforcement
Concept – Perimeter Beams
• Mobilize Catenary Action
• Prevent Top Bars from Tearing Out
• Resist torsion
What is a “Perimeter Beam”?
35
7.13.2.1 – Joist Construction
Standard Hook
Minimum One Bottom Bar
- Continuous
- Class B Tension Splice
- Mechanical / Welded Splice
36
7.13.2.4 – Other Beams

s1
A

s2
A
As1 / 4
L arg est of As1 / 4 or As2 / 4 ( Min. 2 Bars )
- Continuous
- Class B Tension Splice
- Mechanical / Welded Splice
37
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.
38
Slab Rebar
Courtesy of Hai Dinh, Univ. of Michigan
39
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
40
Onterie Center, Chicago
Engineering Judgment
Load Path: Completing the Triangles
41
Balancing Act
42
“Lack of” Engineering Judgment
Completing the Triangles
43
“Skyscrapers seem solid,
immutable, as blank and
indestructible as mountains,”
“… 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
46
Questions?
47