Progressive Collapse of Concrete Buildings

PROGRESSIVE COLLAPSE RESISTANCEOF CONCRETE BUILDINGS

Ying Tian

Department of Civil and Environmental EngineeringUniversity of Nevada Las VegasOUTLINE

Historical events of progressive collapse

Design standards and available approaches

Gaps in existing knowledge and research needs

Experimental study of progressive collapse resistance ofRC 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 initiallocal failure from element to element resulting, eventually, in the collapse of an entire structure or a disproportionately large part of it.

--- ASCE 07-10HISTORICAL 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)

(King and Delatte, 2004)

Commonwealth Avenue apartment, 1971, Boston

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 belowthe transfer girder and may have destroyed or severely damaged columns G24. 168 people died.

Sampoong Department Store, Seoul, South Korea

RC flat-plate structurePunching shear failure initiated from an interior slab-column connection at the top story. Contributing factors for the failure included reduced slab effective depth and a35% 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, iscurrently 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 LoadResistance and the Alternate Path approaches.

Indirect design DOD procedure

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

Indirect Design - emphasizes providing minimumlevels 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 ofSupporting Column

(undamped SDOF system)mg

P

3.5

P3

P = 0.9Pu

u 2.5

2

P = 0.7Pu

1.5

Displacement / Static DisplacementP 1

P = mg

0.5 m 5% damping ratio

0t 0 0.5 1 1.5 2 2.5Time (s)Three analysis procedures permitted:

Linear Static (consider M-factor)

Nonlinear Static (consider Nonlinear Dynamic Increase factor)

Nonlinear Dynamic

Forcedriven nonlinear static analysisLoad applied considers DIF for tributary area surrounding the lost elementDynamic Increase Factor (DIF) for concrete structures

(Marchand et al. 2009) Protection Engineering ConsultantsGAP 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 PolytechnicUniversity (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

Ptu

Tensile arch(catenary) action

Vertical LoadPcu Compressive arch action

Pyu

Capacity based on yield-line theory

cu Deflection tu

(Bao, 2008)

Compressive arch action and catenary action

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 (1/2scale), 3 under different loadingspeed (1/3scale)Test variables: (1) reinforcement ratio, (2) spantodepth ratio, and (3) loading speed

Following concretecrushing

Prior to final failure

A3: 2.7 m x 0.3 m x 0.15 m, Pcu = 249 kN, PACI = 147 kNB1: 4.2 m x 0.3 m x 0.15 m, Pcu = 125 kN, PACI = 77 kNB2: 5.7 m x 0.3 m x 0.15 m, Pcu = 83 kN, PACI = 55 kNAll 3 14 at top and bottom, = 1.13%

A4

A1

(a)

A5

A2

A6

A3

with symmetrical reinforcement with asymmetrical reinforcement

3 3

2.5 2.5

Strength Enhancement Factor 2 2

1.5 1.5 B1

10 0.3 0.6 0.9 1.2 1.5

Flexural Reinforcement Ratio (%) 1

(b)

A6

A

3

B3

B2with symmetrical reinforcement with asymmetrical reinforcement

0 2 4 6 8 10

Span / Depth (ln /h )Strength Enhancement Factor

Effect of Reinforcement Ratio Effect of Spantodepth ratio200 45

15030

100

1550

0 00 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

-50

-15

-100

-150

-200

Specimen C1Specimen C2Specimen C3Peak Load Pcu

Center Deflection / Beam Depth (/h )

-30

-45(kip)Horizontal Reaction N(kip)Vertical Load P(kN)Vertical Load P(kN)Horizontal Reaction N

C1: 2.7 m x 0.2 m x 0.1 m, loading rate 0.2 mm/s, Pcu = 91.6 kNC2: 2.7 m x 0.2 m x 0.1 m, loading rate 2 mm/s, Pcu = 96.4 kNC3: 2.7 m x 0.2 m x 0.1 m, loading rate 20 mm/s, Pcu = 108 kNAll 2 12 at top and bottom, = 1.3%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 ratioFour specimens were tested: D1 to D4, 5700 mm x 300 mm x 150 mm (1/2scale)D1: no axial restraint was appliedD1 and D2: = 1.2 %, D3: 1.8 %, D4: 2.4%Each specimen was tested multiple times with different weight of mass blocksLoad release time less than 10% of natural period

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

Higher weight of mass blocks: detect the dynamic loadcarrying capacityDynamic response under lower level of load

(a)

Midspan deflction

Quarterspan deflection

15

Deflection (mm)10

5

0

(b)

45

30

15

Horizontal Force (kN)0Restraining Moment (kN-m)45

30

15

0

(c)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Time (s)

P = 53.5

kN

D4

P = 44.0 kN

P = 28.9 kN

Dynamic response under higher level of load

Flexura

Concre

l yielding

te crushing

D1

P = 23.9 kN

P = 18.8 kN

D2

P = 44.0 k

PN

= 38.9 kN

90

Center Deflection (mm)60

30

0

90

Center Deflection (mm)60

30

0

P

D3

= 54.6 kN

0 0.5 1 1.5 0 0.5 1 1.5

Time (s) Time (s)Dynamic response of axial restraining force and restraining moment

150 150 150

100 100 100

50 50 50

(b)

0 0 0

-50 -50 -50

-100 -100

(a)

At peak deflection At peak deflection -100

-150 -150 -150

-200

0 0.05 0.1 0.15 0.2 0.25 0.3 -200

0 0.05 0.1 0.15 0.2 0.25 0.3 -200

AtConcrete Crushing

(c)

0 0.05 0.1 0.15 0.2 0.25 0.3Restraining Moment(kN-m)Axial Force(kN)Time (s) Time (s) Time (s)

Specimen D2 Specimen D3 Specimen D4

Concrete Spalling

Diagonal Crack

Edge Column Center Column

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

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

Damage PatternObservations 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 RCFRAME 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- (or M-p) 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 stress- strain 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 adoptedConcrete 1 was used to define material property for concreteConfined 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 andSritharan (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)

100

50 Pu (ACI)

00 50

100

150

200

250 300

-50

Load and Axial Force (kN)-100

-150

-200

Vertical Displacement at Center Column (mm)

Symmetrically reinforced beam ( = 1.5%)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

Twoway sh

ar strength (ACI 3

1808)

G1.0

G0.5

Fir

st Yielding

e4

3

24

1Inclined Crack

00 0.5 1 1.5

Center Deflection (in.)

(Tested at University of Texas at Austin) 40Test results of slab-column connections by (Elstner and Hognestad, 1956)

(=0.99%)

(=0.50%)

(=0.50%)

For flatplates with lowtomoderate reinforcement ratios, punching shearfailure is actually controlled by flexure rather than shear.Calibration of modeling parameter

60 50 1Specimen A-13, = 0.55% Specimen 6AH, = 0.6% Specimen T-2Vertical Shear (kips)50 40 0.8

403030Applied Load 2020

10 10

P2>P1P1 0.6

0.4

Torque (tonf-m)0.2

Column

Slab

Lateral Load

00 0.2 0.4 0.6 0.8Deflection (in) 00 0.5 1 1.5 2 2.5Deflection (in) 00 0.003 0.006 0.009 0.012Twist Angle (rad)Vertical Shear (kips)Test Result FE Simulation Result

Simulation results for a one story flat-plate building

Peak Dynamic Rotation Demand (rad.)STRUCTURAL ENGINEERING LABORATORY AT UNLV

Renovated from a gymnasiumStrong floor

Strong floor: 32 ft long, 28 ft wide, and4 ft thick reinforced concrete slab witha matrix of embedded anchors

Anchor unitCONCLUSIONS

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?