Progressive Collapse of Concrete Buildings

PROGRESSIVE COLLAPSE RESISTANCE OF CONCRETE BUILDINGS

Ying Tian Department of Civil and Environmental Engineering University of Nevada Las Vegas

OUTLINE • • • •

Historical events of progressive collapse Design standards and available approaches Gaps in existing knowledge and research needs Experimental study of progressive collapse resistance of RC beams • Numerical simulation of axially restrained RC frame beams • Numerical simulation of RC flat-plate buildings at the risk of progressive collapse • Structural laboratory at UNLV

“Progressive collapse is defined as the spread of an initial local failure from element to element resulting, eventually, in the collapse of an entire structure or a disproportionately large part of it.” --- ASCE 07-10

HISTORICAL EVENTS OF PROGRESSIVE COLLAPSE Ronan Point apartment, 1968, UK • •

•

Precast concrete wall and floor system. Explosion caused by a gas leak blew out one of the precast wall panels on the 18th floor, triggering the partial collapse of the building. Attention to progressive collapse was initiated.

(Nair, 2004)

Commonwealth Avenue apartment, 1971, Boston • •

• •

(King and Delatte, 2004)

RC flat-plate structure Likely construction over-load, poor material properties in cold weather, and inadequate positioning slab top bars caused punching shear failure at roof level. Punching shear failure propagated to the ground level. Attention to progressive collapse was initiated.

Alfred P. Murrah Building, 1995, Oklahoma City, Oklahoma • • •

•

RC frame structure with transfer girders designed in accordance with ACI 318-71. Discontinuity of reinforcement in both the positive and negative moment reinforcement. The blast from the bomb destroyed column G20 below the transfer girder and may have destroyed or severely damaged columns G24. 168 people died.

Sampoong Department Store, Seoul, South Korea • • • •

RC flat-plate structure Punching shear failure initiated from an interior slab-column connection at the top story. Contributing factors for the failure included reduced slab effective depth and a 35% increase in dead loads due to the change of use at the 5th floor (Gardner et al. 2002). Killed 501 people.

DESIGN STANDARDS

Both consider progressive collapse as dynamic and nonlinear event.

ASCE/SEI Committee, Disproportionate Collapse Standards and Guidance, is  currently develop new standard modified from DOD ‐2009. 

Design Approaches • Indirect Design - emphasizes providing minimum levels of strength, continuity, and ductility to ensure structural integrity. • Direct Design - includes the Specific Load Resistance and the Alternate Path approaches.

Indirect design – DOD procedure

Relies on an integrated system of tie forces for developing tensile membrane or catenary action. Horizontal ties and vertical ties.

• Indirect Design - emphasizes providing minimum levels of strength, continuity, and ductility to ensure structural integrity. • Building must bridge across a removed element.

Location of column removal considered in DOD 2009

Moment before column removal

Moment after column removal

Dynamic Loading Effects Due To Sudden Removal of Supporting Column (undamped SDOF system)

mg P P u

P

Displacement / Static Displacement

3.5

P = 0.9Pu

3 2.5 2

P = 0.7Pu

1.5 1

P = mg

0.5

m

5% damping ratio

0

t

0

0.5

1

1.5

Time (s)

2

2.5

Three analysis procedures permitted: • Linear Static (consider M-factor) • Nonlinear Static (consider Nonlinear Dynamic Increase factor) • Nonlinear Dynamic

Force‐driven nonlinear static analysis Load applied considers DIF for tributary area surrounding the lost element

Dynamic Increase Factor (DIF) for concrete structures

(Marchand et al. 2009) –Protection Engineering Consultants

GAP IN EXISTING KNOWLEDGE AND RESEARCH NEEDS • Actual strength of critical element such as beams and beam-column joints • Actual deformation capacity of critical element such as beams under large deformation • Participation of slabs in resisting progressive collapse • Risk of progressive collapse of flat-plate structures • Appropriate retrofit techniques for progressive collapse prevention

EXPERIMENTAL RESEARCH • In collaboration with Dr. Youpo Su at Hebei Polytechnic University (China) • Investigated flexural capacity of RC frame beams where axial restrains exist • Both static and dynamic loading tests were conducted.

Typical Behavior of RC Frame Beams

Vertical Load

Ptu Tensile arch (catenary) action

Pcu

Compressive arch action

Pyu Capacity based on yield-line theory

δcu

Deflection

(Bao, 2008)

Compressive arch action and catenary action

δtu

Prototype Structure and Test Specimen

Prototype structure and typical geometry of test specimen

Monotonic Loading Test Setup

12 specimens were tested: 9 under static loading (1/2‐scale), 3 under different loading  speed (1/3‐scale) Test variables: (1) reinforcement ratio, (2) span‐to‐depth ratio, and (3) loading speed

Following concrete  crushing

Prior to final failure

A3: 2.7 m x 0.3 m x 0.15 m, Pcu = 249 kN, PACI = 147 kN B1: 4.2 m x 0.3 m x 0.15 m, Pcu = 125 kN, PACI = 77 kN B2: 5.7 m x 0.3 m x 0.15 m, Pcu = 83 kN, PACI = 55 kN All 3φ 14 at top and bottom, ρ = 1.13%

3

3 A4 A1

2.5

A5 A2 A6

2

A3

1.5

with symmetrical reinforcement with asymmetrical reinforcement

(b) Strength Enhancement Factor α

Strength Enhancement Factor α

(a) 2.5 A6

2 B3

A3 B1

1.5

B2

with symmetrical reinforcement with asymmetrical reinforcement 1

1 0

0.3

0.6

0.9

1.2

Flexural Reinforcement Ratio (%)

Effect of Reinforcement Ratio

1.5

0

2

4

6

8

Span / Depth (ln /h )

Effect of Span‐to‐depth ratio

10

150 30 100 15 50

Horizontal Reaction N (kN)

0

0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-50 -15 -100 -150

Specimen C1 Specimen C2 Specimen C3 Peak Load Pcu

-200

-30

-45

Center Deflection / Beam Depth (δ/h )

C1: 2.7 m x 0.2 m x 0.1 m, loading rate 0.2 mm/s, Pcu = 91.6 kN C2: 2.7 m x 0.2 m x 0.1 m, loading rate 2 mm/s, Pcu = 96.4 kN C3: 2.7 m x 0.2 m x 0.1 m, loading rate 20 mm/s, Pcu = 108 kN All 2φ 12 at top and bottom, ρ = 1.3%

Vertical Load P (kip)

45

Horizontal Reaction N (kip)

Vertical Load P (kN)

200

Observations from monotonic loading tests • Compressive arch action resulting from axial restraint contributed at least 50% extra loading capacity beyond the capacity estimated without considering axial restraining forces and strain harderning. • Load resistance under catenary action may not provide higher capacity than under compressive arch action. • High loading speed slightly increases beam flexural stiffness and load resistance.

Dynamic Loading Tests

Test variables: Load level, reinforcement ratio Four specimens were tested: D1 to D4, 5700 mm x 300 mm x 150 mm (1/2‐scale) D1: no axial restraint was applied D1 and D2: ρ = 1.2 %, D3: 1.8 %, D4: 2.4% Each specimen was tested multiple times with different weight of mass blocks Load release time less than 10% of natural period

Lower weight of mass blocks: study the dynamic response of a specimen  within its elastic range  

Higher weight of mass blocks: detect the dynamic load‐carrying capacity

Dynamic response under lower level of load 15

Deflection (mm)

(a) 10

Midspan deflection

5

Quarterspan deflection

Restraining Moment (kN-m)

Horizontal Force (kN)

0 45

(b) 30

15

0 45

(c) 30

15

0 0

0.1

0.2

0.3

0.4

Time (s)

0.5

0.6

0.7

0.8

Dynamic response under higher level of load 90

D1

Center Deflection (mm)

Flexural yielding

D2

Concrete crushing 60

P = 44.0 kN

P = 38.9 kN

30

P = 23.9 kN P = 18.8 kN 0 90

Center Deflection (mm)

D3

D4

P = 53.5 kN

P = 54.6 kN

60

P = 44.0 kN

30

P = 28.9 kN

0 0

0.5

1

Time (s)

1.5 0

0.5

1

Time (s)

1.5

Axial Force (kN)

Restraining Moment (kN-m)

Dynamic response of axial restraining force and restraining moment 150

150

150

100

100

100

50

50

50

0

0

0

-50

-50

-50

-100

-100

At peak deflection

-150

-100

At peak deflection

-150

-150

(a)

(b)

-200

(c)

-200 0

At Concrete Crushing

0.05

0.1

0.15

0.2

Time (s)

Specimen D2

0.25

0.3

-200 0

0.05

0.1

0.15

0.2

Time (s)

Specimen D3

0.25

0.3

0

0.05

0.1

0.15

0.2

Time (s)

Specimen D4

0.25

0.3

Concrete Spalling

Diagonal Crack Edge Column

Center Column

(a) Damage pattern of Specimen D3 (P = 54.6 kN)

Damage pattern of Specimen D3 (P = 53.5 kN, collapsed)

Damage Pattern

Damage pattern of Specimen D3 (P = 54.6 kN, approximately the  load capacity)

Observations from dynamic loading tests • Typically assumed 5% damping ratio for cracked concrete structures was verified. • Compressive arch action still exists under dynamic loading scenario considered by DOD and can significantly increase the dynamic loading capacity. • Dynamic increase factor of 2 could be too conservative for force controlled actions. • Another series of tests is being conducted to further identify dynamic loading effects (mainly evaluate DIF proposed by DOD and dynamic deformation capacity).

NUMERICAL SIMULATION OF AXIALLY RESTRAINED RC FRAME BEAMS (ONGOING) •

• •

•

Current DOD progressive collapse design guideline considers dynamic loading condition. The response of structure from an analysis (deformation and force demand) can be highly sensitive to the definition of beam flexural capacity. To reduce uncertainty in an analysis, appropriate nonlinear model is need for frame beams surrounding the lost column. Using traditional ACI code approach to define M-ϕ (or M-θp) in a nonlinear analysis cannot effectively capture the dynamic response under both compressive arch action and catenary action. Numerical analysis needs to consider the geometry nonlinearity when solving system equations.

Using fiber section to define flexural property • • • • • • •

Cross section is divided into several layers (regions) to have fibers along the beam or column. Material property is defined at stressstrain level. Confinement effects due to transverse reinforcement can be explicitly considered. Can be used for irregular cross sections. Current fiber section can only define flexural and axial loading behavior. Involves higher computational cost. Available in SAP newer editions.

Simulation of axially restrained beams tested

• • •

• • Concrete property  (Concrete 1 model)

• •

OPENSEES was adopted Concrete 1 was used to define material property for concrete Confined concrete model for peak stress and ultimate compress strain proposed by Scott et al. (1982) was use for cover concrete and core concrete. Steel 2 was used to define material property for reinforcing bars. Model (Bond_SP01) proposed by Zhao and Sritharan (2007) was considered. Zero-length section was used to define bond-slip property. Ultimate goal: nonlinear static and dynamic analysis of multi-story RC frame building designed w/ seismic loading (assisted by Ken Zhang) and w/o seismic loading (assisted by Sang-in Choi).

Simulation results 200

150

Load (measured)

Average Axial Force (measured)

Load (calculated)

Axial Force (calculated)

Load and Axial Force (kN)

100

Pu (ACI)

50

0 0

50

100

150

200

250

-50

-100

-150

-200

Vertical Displacement at Center Column (mm)

Symmetrically reinforced beam (ρ = 1.5%)

300

NUMERICAL SIMULATION OF RC FLAT-PLATES (ONGOING) • Flat-plate buildings, especially those designed prior to 1980s, could be vulnerable to a progressive collapse. • ABAQUS using shell elements is used to conduct nonlinear analysis. • Concrete damaged plasticity model was used to simulate the property of concrete under tri-axial state of stress. • Rebar layer was used to simulate tension and compression mats of slab flexural reinforcement. • Preliminary analyses have been conducted. • Research assisted by Jinrong Liu.

Behavior of two slab-column connections under simulated gravity loading

5

Two‐way shear strength (ACI 318‐08)

4

G1.0 3

G0.5 2

4” 1

Inclined Crack First Yielding

0 0

0.5

1

1.5

Center Deflection (in.)

(Tested at University of Texas at Austin)

40

Test results of slab-column connections by (Elstner and Hognestad, 1956)

(ρ=0.99%) (ρ=0.50%) (ρ=0.50%)

For flat‐plates with low‐to‐moderate reinforcement ratios, punching shear  failure is actually controlled by flexure rather than shear.

Calibration of modeling parameter 1

50

Specimen A-13, ρ = 0.55%

Specimen 6AH, ρ = 0.6%

40 30

Applied Load

20 10 0

Specimen T-2 0.8

40

Torque (tonf-m)

50

Vertical Shear (kips)

Vertical Shear (kips)

60

30

P2>P1

P1

20

0.2

0.4

0.6

0.8

Slab 0.4

Column 0.2

10

Lateral Load

0

0 0

0.6

0

0.5

Deflection (in)

1

1.5

2

2.5

0

0.003

Deflection (in)

Test Result

FE Simulation Result

Simulation results for a one story flat-plate building

Peak Dynamic Rotation Demand (rad.)

0.006

Twist Angle (rad)

0.009

0.012

STRUCTURAL ENGINEERING LABORATORY AT UNLV

Renovated from a gymnasium

Strong floor

Strong floor: 32 ft long, 28 ft wide, and  4 ft thick reinforced concrete slab with  a matrix of embedded anchors Anchor unit

CONCLUSIONS • Lateral restraining effect existing in an actual moment frame may significantly increase beam flexural capacity. • Even though such effect is generally neglected in a normal design, it can be considered for progressive collapse resistance under extreme loading conditions. • Fiber section can best describe the strength and stiffness properties of RC frame beams. • Flat-plate buildings, especially older flat-plates, could be at high risk of progressive collapse. • Input for industry is needed to better improve current design practice for progressive collapse.

Thank You QUESTIONS?