potentially plastic zones configurations in … 2011/presentations/day1... · 1 potentially plastic...

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POTENTIALLY PLASTIC ZONES

CONFIGURATIONS IN BOTTOM

COLUMNS OF ECCENTRICALLY

BRACED FRAMES

Helmuth Köber & Bogdan Ştefănescu & Şerban Dima

Steel Structures Department

Technical University of Civil Engineering Bucharest

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ANALYZED FRAMES: The present paper is intended to illustrate the advantages and

disadvantages of different structural details for the potentially

plastic zones located near the bottom end of the columns.

Several structural details were analyzed considering: reduced

flanges cross-sections and/or transversal and longitudinal

stiffeners for the bottom zone of the columns.

Two eccentrically braced frames located in

Bucharest were considered for the analyses.

For both frames the story height was 3.5m,

the span was 6.6m and the length of the

dissipative members was 1.2m.

Built-up I-shaped cross-sections were used

for all types of structural members.

Frame K Frame V

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ANALYZED FRAMES: Dynamic nonlinear analyses were performed for each structural

configuration. The N-S component of the Vrancea 1977

earthquake acceleration record was used. The acceleration

record was calibrated to a peak ground acceleration value of

approximately 0.24 times the acceleration of gravity.

The maximum values of the bending

moments, the axial forces and the plastic

deformations in the potential plastic zones

at the bottom end of the columns were

compared.

Four different constructive details were

considered for the bottom end of the first-

story columns.

Frame K Frame V

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Configuration 1:

Configuration 1 is the

reference analysis detail.

The column has the same

cross-section on the entire

height of the first story

column.

Transversal web stiffeners

(P3) were used to avoid early

local buckling in the

potentially plastic zone.

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Configuration 2: In the second considered detail an

additional longitudinal stiffener (P4)

was placed on the web. This pair of

stiffeners was used to reduce the

axial loading level in the flanges, to

make room for stresses generated by

bending moment.

Plates P5 were placed to facilitate the

axial load transfer from the column

flanges (P1) to the longitudinal web

stiffeners (P4) reducing at the same

time load concentrating effects.

Transversal web stiffeners (P3) are

kept in all configurations to reduce

the risk of local buckling in the

potentially plastic zone .

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Configuration 3:

The third configuration has a

reduced flange cross-section in the

potentially plastic zone

(resembling to dog-bone detail).

In configuration 3 the reduced

width of the flanges in the

potentially plastic zone is about

25% smaller than the flanges width

in the rest of the first-story column.

The longitudinal web stiffeners

were kept to assure about the

same axial capacity all along the

first-story column height.

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Configuration 4: In the fourth considered detail the

first configuration column cross-

section was kept for the potentially

plastic zone, whereas the rest of

the column has larger flanges in

order to increase the buckling

capacity of the first-story column.

The reduced flange width in the

potentially plastic zone is also

about 25% smaller than the flange

width in the rest of the first-story

column.

All connections between structural

members, as well as the

connections to the infrastructure

were considered as fixed.

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Forces at the bottom of the columns

The maximum bending moments recorded during

the dynamic nonlinear analyses at the bottom of the

columns in the other considered configurations are

nearly the same. Compared to these values, the

bending moments registered for configuration 3 are

about 20 ÷ 26% smaller.

The maximum axial forces recorded in the first-story

columns during dynamic nonlinear analyses are

quite the same for all considered configurations. It

seems that the considered constructive details do

not affect significant the values of the maximum

axial forces noticed in the first-story column.

Configuration 3 (with reduced column flanges in the potentially

plastic zones) leads to the smallest bending moment values at

the bottom end of the first-story columns.

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Maximum bending moments at the bottom of the first-story columns

Frame K Bending moments marginal columns

0

2000

4000

6000

8000

(kNm)

(kNm) 7437.82 7658.72 6212.11 7483.31

Config. 1 Config. 2 Config. 3 Config. 4

Frame K Bending moments central columns

0

250

500

750

1000

(kNm)

(kNm) 902.31 901.49 715.06 898.17


Frame V

Bending moments marginal columns

0

2000

4000

6000

8000

(kNm)

(kNm) 6742.01 7067.41 5700.23 6783.2


Frame V

Bending moments marginal columns

0

450

900

1350

1800

(kNm)

(kNm) 1669.28 1671.91 1395.34 1685.3


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Buckling resistance of first-story columns

The buckling resistance of first-story columns was evaluated

using relations (6.61, 6.62) and annex B from EN 1993-1-1:2005 .

It can be observed from the graphics in the figure below, that

configuration 4 provides the greatest buckling resistance for the

situations when inelastic deformations appear in the potentially

plastic zones at the bottom of the columns.

0.00

0.20

0.40

0.60

0.80

1.00

(Loading

State/

Capacity)

Frame V Marginal Columns

General Stability Criterion

Loading State/ Capacity 0.984 1.060 0.877 0.828

Config. 1 Config. 2 Config. 3 Config. 40.00

0.20

0.40

0.60

0.80

1.00

(Loading

State/

Capacity)

Frame K Central Columns

General Stability Criterion

Loading State/ Capacity 1.018 0.995 0.897 0.821


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Maximum plastic hinge rotations

The greatest plastic hinge rotations in the

potentially plastic zones of the first-story

columns were recorded during dynamic

nonlinear analyses for configuration 3.

The values of the maximum plastic hinge

rotations at the bottom of the columns for

the other considered configurations were in

the same range. Compared to these values,

the plastic rotations for configuration 3

were about 37 ÷ 45% greater.

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0

0.003

0.006

0.009

0.012

(rad)

Frame K - Marginal Columns


(radians) 0.009208 0.008734 0.013296 0.009415


0.003

0.006

0.009

0.012

(rad)

Frame K - Central Columns


(radians) 0.00982 0.009289 0.01393 0.010456


0

0.003

0.006

0.009

0.012

(rad)

Frame V - Marginal Columns


(radians) 0.008336 0.007993 0.011648 0.008597


0.003

0.006

0.009

0.012

(rad)

Frame V - Central Columns


(radians) 0.008782 0.008402 0.012022 0.00904


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Conclusions Compared to the other

considered constructive

details, configuration 3

leads to smaller bending

moments and greater

plastic hinges rotations at

the bottom of the columns.

The smaller bending

moment values conduct to

smaller anchor bolts for

the columns.

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Conclusions

Configuration 4 appears to

be the safest from the

point of view of assuring

the general stability of the

first-story column in the

situation when plastic

deformations occur in the

potentially plastic zone at

the bottom of the column.

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Conclusions

In configuration 3 and 4 the distribution of

plastic deformations along the first-story

columns is better controlled. The inelastic

deformations along the first-story column are

concentrated mainly in the column segments

with reduced flanges width.

Config. 3 Config. 4

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General Conclusion

In eccentrically braced frames subjected

to severe seismic actions configuration

3 or 4 bottom end details for first-story

columns should be used.

Config. 3 Config. 4

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Thank you very much for your

attention

potentially plastic zones configurations in … 2011/presentations/day1... · 1 potentially plastic...

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