DUCTILITY AND PREVENTION

OF STRUCTURAL FAILURE

TOPICS• Types of Loading• Structural Distress under Various Loading Conditions• Ductility Provisions and Structural Repair/Retrofit• Relevant Research at UAP• Conclusions

Types of Loading

Structural Distress under Various Loading

Conditions• Quasi-Static Loads

• Machine Vibration

• Impact Loads

• Blast Loading

• Cyclonic Storm Loading

Vertical Loads

• Overload from service requirement and careless use

• Poor construction practices and material quality

Quasi-Static Loads

Cracks in Beams and Columns

Ultimate Collapse of Structure

Support Settlement

• Overloaded super-structure and sub-structure

• Filling up lands, ponds, with soft infill

• No/inaccurate soil test and no soil improvement

(a) Building before support settlement, (b) Uniform settlement, (c) Differential settlement

Cracks indicating Differential Support Settlement

Extreme Temperature (Fire)

0 100 200 300 400 500 600 700 800 900Temperature C

10

15

20

25

30

35

40

45

50

Co

mp

ress

ive

stre

ng

th (

MP

a)

The age (days)

30

60

90

o

Fig. 7(a): The effect of fire flame on the compressive strength at 1-hour of exposure

Effect of temperature on (a) Steel yield strength, (b) Concrete

compressive strength

• Steel melts as in September 11, 2001

• Dehydration of paste in the concrete matrix

Impact LoadsProgressive Failure of Slabs

Progressive Failure of slabs in (a) USA, (b) Bangladesh

• Sudden drop of top slab causes a large impact load

• Creates a series of slab failures heaped like a pack of cards (called a ‘pancake’ failure)

Vehicular Impact on Bridge Railings

Railing crash involving (a) smaller vehicle, (b) larger vehicle

Vehicular Impact on Bridge Railings

Arrangements for vehicular-impact test of RC railings

Machine Vibration• Machines and Power Generators

• Careless Placement and Design

• May cause Resonance and Fatigue

Fig. 11: Dynamic amplification of machine vibration

Dynamic Amplification of Machine Vibration

• One blast can change history

• Extremist views and access to explosives

• Very sudden and very high pressure

Blast Loading

September 11, 2001

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

0 10 20 30 40 50

Distance R (m)

Dyn

amic

Bla

st P

ress

ure

(psi

)

1 kg

10 kg 100 kg

10000 kg

1000 kg500 kg

0 10 20 30 40 50

Distance R (m)

Distance R (m)

Fig. 14: Variation of blast pressure with distance, for explosives of different weightsVariation of Blast Pressure with Distance

Nature of Blast Loading

Controlled Demolition

Controlled Demolition

• Ever-changing urban infrastructure in this country

• Predicament in the demolition of a single building

Cyclones in Bangladesh

Hydraulic Loading

Date Year

Max. Wind Speed(Kmph))

Storm Surge Ht. (m)

Deaths09 Oct

1960

162 3 3,00030 Oct

1960

210 4.5~6 5,14909

May196

1146 2.5~3 11,466

28 May

1963

203 4~5 11,52011

May196

5162 4 19,279

12 Nov

1970

223 6~105,00,0

0025 May

1985

154 3~5 11,06929

April199

1225 6~8

1,38,00015

Nov200

7240 5~6 3,406

25 May

2009

120 2~3 330

Loads due to Surge (BNBC, 1993)

Coastal RegionSurge Height at Sea Coast,

hT (m)T = 50-

yearT = 100-

yearTeknaf to Cox's Bazar 4.5 5.8

Chakaria to Anwara, Maheshkhali-Kutubdia Islands

7.1 8.6

Chittagong to Noakhali 7.9 9.6

Sandwip, Hatiya and all islands in this region

7.9 9.6

Bhola to Barguna 6.2 7.7

Sarankhola to Shyamnagar 5.3 6.4

Ductility Provisions and Structural

Repair/Retrofit• Ductility Provisions in

Structural Design

• Methods of Structural Retrofitting

Ductility Provisions in Structural Design

Provisions for Quasi-Static Load• Steel yielding preferred to

Concrete crushing• Balanced Steel Ratio (b),

Maximum (max) and Minimum Steel Ratio (min)

• Column Ties and Spirals, latter is more ductile

Behavior of tied and spirally reinforced columns (Nilson)

Provisions for Impact Load

Arrangements of free fall tests on concrete slabs

Provisions for Machine Vibration

Fig. 19: Machines supported on shock-absorbing springs

Provisions for Cyclone Load

Coastal forest and vegetation (a) diminished tsunami wave height, (b) prevented destruction of

houses at West Java

Blast Resistant Design

Pair of Links

Pair of Links

(a) Beam-Column connection details (b) CFRP wrapped Column

Blast Resistant Planning

Methods of Structural Retrofitting

Jacketing and Confinement

Steel jacketed columns (a) circular, (b) rectangular with

elliptical jacket

FRP jacketed (a) Circular Columns,

(b) Square Columns

Jacketing and Confinement with

transverse ties

Global Strategies- Adding shear wall, infill wall, wing wall- Adding bracing- Wall thickening- Mass reduction (using lighter materials)- Supplemental damping (TMD, TLD)- Base Isolation (shock absorber)

Local Strategies- Jacketing of Beams, Columns, Joints- Strengthening of individual footings

Seismic Retrofitting

Makes stiffer

Makes strong

er

Jacketing of Columns

Retrofitting Beam-Column Frames

Relevant Research at UAP

• Numerical Study on Design of Blast Resistant Buildings

• Dynamic Response of Coastal Structures to Ocean Wave Loading

• Dynamic Response of RC Railing to Vehicular Impact

• Transverse/Compression Reinforcement in RC Beams

Numerical Study on Design of Blast Resistant Buildings

Response to Blast Load for Ru/Fm = 0.10~2.0 and Damping Ratio (a)

0%, (b) 5% (a) Damped SDOF system with elastic fully-plastic k, (b) Blast

Loading

k c

m y(t), F(t)

y

R

k

ye ym

t

F(t)

Fm

td

0

1

10

100

0 1 10

0

1

10

100

0 1 10

ym/y

e

td/Tn td/Tn

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 0.9

1.0 1.2

1.5

2.0

0.1

0.2

0.3

0.4

0.5

0.6 0.7

0.8 0.9

1.0 1.2

1.5 2.0

Column

k (k/ft) ye (ft)yu (ft)

Ru (k) m (k-s2/ft)

Tn (s) yu/ye

6-00N1.44E+0

31.06E-

020.43 15.2 29.35 0.90 40.3

6-00M1.27E+0

39.45E-

033.83 12.0 29.35 0.96 406

6-1001.33E+0

31.30E-

026.14 17.3 29.35 0.93 472

6-10001.11E+0

31.69E-

026.14 18.7 29.35 1.02 364

W (kg) td/Tn

6-StoriedR = 3m R = 10m R = 30m

1000.0125 356 0.68 0.0160.0250 847 1.55 0.0330.0500 1859 4.57 0.069

10000.0125 5242 51 0.1940.0250 11423 142 0.4160.0500 23818 347 0.857

100000.0125 55190 1246 6.910.0250 118559 2802 22.970.0500 245327 5943 65.90

Ductility Demand (ym/ye) for Different Loading Conditions

Ductility Ratio (yu/ye) for 6-Storied Building

Dynamic Response of Coastal Structures to Ocean Wave

Loading

(a) Moment-Curvature Relationship, (b) Curvature vs. Time for GF column of 6-

Storied Building for 50-Year Storm

-1500

-1000

-500

0

500

1000

1500

-0.006 -0.003 0.000 0.003 0.006

Curvature (rad/ft)

Mom

ent (

k-ft

)

-0.06

-0.04

-0.02

0.00

0.02

0 40 80 120 160

Time (sec)

GF

Col

umn

Cur

vatu

re (

rad/

ft)

W

WC

WCW

Dynamic Response of RC Railing to Vehicular Impact

2-19mm

290mm

200mm

3-19mm

150mm

190mm

2-19mm 2-19mm

-100

-50

0

50

100

150

-1.0 -0.5 0.0 0.5 1.0

Curvature (rad/m)

Mom

ent (k

N-m

)

Static SR = 100/s

-60

-40

-20

0

20

40

60

-1.0 -0.5 0.0 0.5 1.0

Curvature (rad/m)

Mo

men

t (k

N-m

)

Static SR = 100/s

Moment-curvature relationship of Railing and Rail Post for

different strain rates

Cross-sections of Railing and Rail Post

0.0

0.2

0.4

0.6

0.00 0.02 0.04 0.06 0.08 0.10

Time (sec)

Def

lect

ion

(m)

100, 30 100, 90 50, 30

0.0

0.2

0.4

0.6

0.00 0.02 0.04 0.06 0.08 0.10

Time (sec)

Def

lect

ion

(m)

W = 2 t W = 4 t W = 1 t

ult Ref of various Posts Damping Ratio

Weight (ton)

Velocity (kmph), Angle()

Top Middle

Side 4% 2% 4 1 100, 90 50, 30

250 330 168 187 377 390 413 244 517 193

Maximum Deflections (mm) from Parametric Studies

Dynamic Response showing effect of (a) Vehicular Weight, (b) Velocity and

Angle

Experimental Work on Column Retrofit

• Careful assessment of structural loads, and better construction practice necessary – Member jacketing and confinement

• Proper assessment of soil properties necessary from accurate soil testing – Soil strengthening measures

• Member detailing measures and shock-absorbing devices can be used to improve structural performance to Impact loads

Conclusions

• Machine Vibrations should either be transferred to rigid sub-structure or supported on flexible spring/damper

• Large stand-off distance, shock absorbers and member ductility necessary for Blast Resistant Design

• Measures to resist cyclonic storms (combination of wave, current and wind forces) include protective vegetation and member ductility

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