PROGRESSIVE COLLAPSE ANALYSIS OF MULTI STOREY STEEL ...

PROGRESSIVE COLLAPSE ANALYSIS OF MULTI STOREY STEEL STRUCTURE WITH

BRACING SYSTEMS

Rekha M N1, Dr. S Vijaya2, Dr. B SHIVAKUMARA SWAMY3 1PG student in Structural Engineering, Dr. AIT, Bengaluru, Karnataka, India

2,3Professor, Department of Civil Engineering, Dr. AIT, Bengaluru, Karnataka, India

Abstract:Progressive collapse refers to the phenomenon in which the local damage of a primary

structural element leads to total or partial structural system failure, without any proportionality

between the initial and final damage. Even if the probability of structural collapse is low, if it

occurs, it can cause significant losses. In the past few decades, many incidents of the total or

partial collapse of structures due to fire, explosions or impacts have occurred. In the present study,

Multi storey structure is considered. The modelling and analysis are carried out using E-tabs

software. The different bracing systems are used to analysis the structural behavior. The Structure

is later verified for progressive collapse analysis at 3 different locations such as corner, Centre

and middle of the structure. The progressive analysis is carried out and results are extracted and

discussed. The displacement control can be easily achieved by providing bracings. The

displacement of bare frame models can be reduced by 55%, 55% and 63% by adopting bracings

such as Diagonal bracing, V Bracing and X Bracing systems. The progressive collapse analysis

shows the failure of upper level beams. It is estimated that, the maximum increase in percentage of

DCR value for beams are around 38%, 52% and 59% for location A, B and C respectively. And

hence the beams are super designed for these additional capacities to avoid progressive collapse.

From the overall results, it is concluded that, the bracings are very much necessary for the

reductions of displacement and drift effect in a building. However, its Importance is not of much

use in case of progressive collapse state. The X Bracings are preferred for bracing system.

Keywords: Progressive collapse, equivalent static analysis, bracings, displacements, storey drift,

base shear, time period, acceleration, E-Tabs

1. Introduction

A structure could be subjected to more than one critical action during its entire service period. It

may be a manmade or natural disaster can lead to structural instability which influences the partial

or complete collapse of the building. Natural disasters include earthquake, wind blast or fire. And

these manmade disasters include terrorist attack or gas cylinder explosion or instant removal of the

primary structural element. progressive collapse is one outcome of these critical loads.

The progressive collapse can be defined as a situation where local failure of a primary structural

component leads to total collapse of the structure. Progressive collapse happens when relatively

local structural damage, causes a chain reaction of structure elements failures, disproportionate to

the initial damage, causing in partial or full collapse of the building. Local damage that initiates

progressive collapse of building is called initiating damage. In general, progressive collapse occurs

in a very short time in seconds. It is also possible that it can be characterized by the loss of load-

carrying capacity of a relatively small portion of a building due to a typical load which, in turn

initiates a fall of failures affecting a main portion of the structure.

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NOVYI MIR Research Journal

Volume 5, Issue 11, 2020

To minimize the progressive collapse of the structure bracings provides stability and resists

lateral loads. In case of steel structure to resist the lateral force and increase thestiffness of steel

frame, bracings play very vital role. Bracing will make structure indeterminate. But it stiffens the

structure and also helps to resist the sway of the structure. Bracings are straight member and carry

only axial forces. Generally, the use of bracings instead of Shear walls provides lower stiffness and

resistance for a structure but it should not beforgotten that such a system has lower weightand more

useful for architectural purposes. Use of braces for seismic rehabilitation of structures should not

cause any torsion disorder and designers should be aware of increasing the axial loads of columns

in bracing panels. The probable uplift in columns and foundations should be controlled too.

A. Bracings

Bracing systems are very simple to construct and also to understand the theory behind it. This

system is the economic way to construct a lateral load resisting system. These systems are in

expensive and works exactly like truss behaviour. It will not transfer the bending moment. It can

only transfer the axial loads. Bracing mainly take care of lateral loads whereas frames take care of

axial loads.

The primary function of bracings is to provide stability and resist lateral loads, either from diagonal

steel members or from concrete ‘core’. Due to bracing, displacement of the structure gets reduced

considerably it is up to 90% but in case of X-bracing material required will be more and hence V

or Inverted V bracing effectively resist the displacement as compared to all other types of bracings.

Using bracing systems axial reaction is reduced and hence the footing size also gets reduced. Also,

reduction in moment at base will definitely help to reduce the size of the footing and due to bracing

the torsional moment in base column increases.

In this study the different bracings such as Diagonal bracing, V Bracing and X Bracing systems are

used to analysis the structural behavior. The Structure is later verified for progressive collapse

analysis at 3 different locations such as corner, Centre and middle of the structure. The progressive

analysis is carried out and results are extracted and discussed activity. However, fully braced

systems are having highest rigidity compared to partially braced system. The biggest aim of the

engineer is to achieve the highest stability of the structure with nominal structural cost. The Shear

wall and Bracing systems are found to be suitable in achieving this. These systems are also reliable

and works effective under lateral force effect.

In this present study, 20 storey building is considered for the analysis. And attempts are made to

study the behavior of the structure in severe wind and earthquake zones of India and seven models

(bare frame, bare frame with V bracing, bare frame with inverted V bracing, bare frame with

diagonal bracing, bare frame with X bracing, bare frame with shear wall at centre and bare frame

with shear wall at ends) are analyzed using ETABS 2017 software by equivalent static method of

analysis, dynamic response spectrum method of analysis and wind analysis. The results of analysis

in terms of time period, frequency, displacements, storey drifts, base shear are obtained.

2. Objectives The following objectives are considered in the present studies

1. To Study the behaviour of Multi storey steel structure.

2. To understand the behaviour of structure when accidental collapse of columns for various

locations.

3. To study the various bracings system such as Diagonal, V and X bracings.

4. To understand the comparison of different models based on the parameters such as

displacement, Storey drift, base shear, time period, acceleration.

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3. Methodology Steel structure is modelled, designed as per IS-800-2007 the models are analysed for Static

Analysis and progressive collapse. The modelling is carried out using FEM based ETABS

software. Results are obtained from gravity loads and then lateral loads are applied to check the behaviour of

the models and Results are extracted for X direction only.

4. MODELLING AND ANALYSIS In the present study, 8 storey structure is considered. Totally sixteen number of models are created

and analysed. The model details are listed below. There are four major models, i.e., M1, M2, M3

and M4 represents are. M1 - Multi storey steel structure without bracings.

M2 - Multi storey steel structure with diagonal bracings.

M3 - Multi storey steel structure with V bracings.

M4 - Multi storey steel structure with X bracings.

However, the remaining models are the same models with removal of column location A, B and C

respectively and here it is representing model with removal of column at position A only and

showing its graphical representation.

The model is 8 storey height with regular structure.

Seismic Zone III

Seismic Zone Factor (Z) 0.16

Importance Factor (I) 1.5

Response Reduction Factor

4

Damping Ratio 0.05

Soil Type Hard Soil (Type I)

Height of the building 32m (8 Storey)

Story to story Height 4.0 m

Span Length 5m

Column used ISMB

Thickness of Slab 125 mm

Floor Finish 1.0 KN/m2

Live Load 3.0 KN/m2

Grade of Concrete(fck) M35

Grade of Structural Steel (fys)

Fe 350

Grade of Reinforcing Steel

Fe 500

Table 1: Material properties and design parameters

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Fig 1:M1- Multi storey steel structure with no bracings

Fig 2:M2- Multi storey steel structure with diagonal bracings

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Fig 3:M3- Multi storey steel structure with V bracings

Fig 4:M4- Multi storey steel structure with X bracings

5. Results and Discussion The models are first loaded with gravity loads and then lateral loads are applied to check the

behaviour of the models. Since, the models are symmetrical in both X and Y direction, the results

are extracted for X direction only.

The structural results of various analysis of M1, M2, M3 and M4 are discussed below.

A. Displacement Equivalent static analysis

The displacement of models in X direction is tabulated and presented below.

Fig 5 :Storey v/s Displacement in x direction_EQX

Figure 5 representing storey v/s displacement of equivalent static analysis. We can observe that as

the storey increases displacement also increases irrespective of the model.

The displacement of the model 1 is more compared to all other models with bracings. The

displacement is reducing drastically for models with bracings. However, the model with X bracing

exhibiting lowest displacement among other types of bracings because the lateral stiffness of the

system increases and the stability of the system improves.

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100

ST

OR

EY

in

No

's

DISPLACEMENT in mm

EQUIVALENT STATIC ANALYSIS

MODEL 1

MODEL 2

MODEL 3

MODEL 4

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B. Storey Drift Equivalent static analysis

Fig 6: Storey v/s Drift in x direction_EQX

Figure 6 representing storey v/s Drift of equivalent static analysis and drift is defined as ratio of

displacement of two consecutive floor to height of that floor.

The drift values are almost same in case of models with bracings i.e., M2, M3 and M4.

However, it is noticed that, the model without bracing are having largest drift values. This

proves, bracings can high influence on drift control.

C. Base shear Equivalent static analysis Base shear is the shear force at base or foundation level. The following table indicates the

base shear value for different configurations.

Fig 7: Base shear comparison of M1, M2, M3 and M4

Figure 7 showing the base shear values are same for all the models and the model 2 exhibiting

more shear compared to all other models shear forces in columns increases from unbraced to

braced system i.e., only minor variations. However, static analysis base shear will be only

dependent upon the weight of the structure and hence all the base shear values are similar.

0

2

4

6

8

10

0 5

10

15

20

ST

OR

EY

DRIFT

EQUIVALENT STATIC ANALYSIS

MODEL 1

MODEL 2

MODEL 3

MODEL 4

1060 1070 1080 1090 1100 1110

MODEL 1

MODEL 2

MODEL 3

MODEL 4

MODEL 1 MODEL 2 MODEL 3 MODEL 4

Series1 1082 1107 1086 1089

BASE SHEAR

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D. Time period Equivalent static analysis

Fig 8: Time period v/s Mode shapes

Figure 8 representing time period v/s mode shape the time period of the model M1 is more and

flexible comparedto all other models. The introduction of bracing reduces the flexibility. However,

the X bracing model is showing highest rate of reduction of time period and hence rigid compared

to other models.

E. Frequency Equivalent static analysis

Fig 9: Frequency v/s Mode shapes

From the figure 9, the model having highest rigidity factors are having more frequency. The model

M4 with X bracing possess highest frequency than other models.

F. Progressive Collapse Analysis

Progressive collapse analysis has been performed as per GSA specification. In case of regular

framed structure removal of one column at corner (Location A), middle (Location B) at exterior

frame and middle at immediate interior frame at middle (Location C) of the building is done as per

GSA specification. Since in this work, Structural systems has been considered, where distance

between columns are 6 m, column is assumed to be collapsed (supporting Storey 1) at locations as

shown in Figures below.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0 5 10 15

TIM

E P

ER

IOD

MODE SHAPE

MODEL 1

MODEL 2

MODEL 3

MODEL 4

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0 10 20

FR

EQ

UE

NC

Y

MODE SHAPE

MODEL 1

MODEL 2

MODEL 3

MODEL 4

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Fig 10: Assumed column removal location A

1. DCR ratios for beams

Fig 11: DCR Ratio of Beam 71 for Column Removal @ Location A

Figure 11 representing the beams at storey

However, the models with bracings are showing adverse effect with increase in DCR ratios for

Model M2, M3 and M4.

0.731

0

0.2

0.4

0.6

0.8

1

1.2

MODEL 1

DC

R R

AT

IO

C O L U M N R E M O V A L L O C A T I O N

Fig 10: Assumed column removal location A Plan view

DCR ratios for beams

Fig 11: DCR Ratio of Beam 71 for Column Removal @ Location A

Figure 11 representing the beams at storey 1 is failing due to removal of column location A.


0.731 0.738 0.736 0.733

1.0961.14 1.126 1.125


MODELS

C O L U M N R E M O V A L L O C A T I O N A

NO PC

PC. LOC A

1 is failing due to removal of column location A.


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2.DCR ratios for column

Fig 12: DCR Ratio of Column 25 for Column Removal @ Location A

Figure 12 showing the DCR ratio of Models with bracings are more compared to model M1. This

diagonal bracing model is worst among all other types of bracing system, since the model 2 is

having highest DCR Ratio.

5.CONCLUSIONS

Based on the results and discussions, the following conclusions are drawn,


reductions of displacement and drift effect in a

much use in case of progressive collapse state. The X Bracings are preferred for bracing

system.

The progressive collapse analysis shows the failure of upper level beams. It is estimated that,

the maximum increase in percentage of DCR value for beams are around 38%, 52% and 59%

for location A, B and C respectively. And hence the beams are super designed for these

additional capacities to avoid progressive collapse.

In the similar way, the Axial load increas

For Column removal location A, B and C respectively. And hence additional loads should be

predicted earlier to avoid Progressive collapse.

From the progressive collapse analysis, it is found that, the DCR ra

for removal adjacent.

6. REFERENCES

[1] Meng-Hao Tsai, Tsuei

Building with Exterior Non

[2] HadiFaghihmaleki (2017), “Assessment of Robustness Index and Progressive Collapse in

the RC Frame with Shear Wall Structure under Blast Loading”. Journal of applied

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[3] H.M. Salema , A.K. El

design of reinforced concrete structures against progressive collapse”. Engineering

structures 33(2011) 3341

0.825

0

0.2

0.4

0.6

0.8

1

1.2

1.4

MODEL 1

DC

R R

AT

IO

C O L U M N R E M O V A L L O C A T I O N

2.DCR ratios for column




Based on the results and discussions, the following conclusions are drawn,


reductions of displacement and drift effect in a building. However, its Importance is not of



increase in percentage of DCR value for beams are around 38%, 52% and 59%


additional capacities to avoid progressive collapse.

In the similar way, the Axial load increase in the column is found to be 25%, 25% and 20%.


predicted earlier to avoid Progressive collapse.

From the progressive collapse analysis, it is found that, the DCR ratio of column increase only

Hao Tsai, Tsuei-Chiang Huang (2014), “Progressive Collapse Analysis of an RC

Building with Exterior Non-Structural Walls”. Science Direct, 14, 377-384.

(2017), “Assessment of Robustness Index and Progressive Collapse in


Research on industrial Engineering, Volume 4, No 1, 59-66.

H.M. Salema , A.K. El-Fouly b, , H.S. Tagel-Dinb, (2011), “Toward an economic


structures 33(2011) 3341-3350.

0.8250.912

0.832 0.835

1.1221.238

1.14 1.145


MODELS

C O L U M N R E M O V A L L O C A T I O N A

NO PC

PC. LOC A





building. However, its Importance is not of



increase in percentage of DCR value for beams are around 38%, 52% and 59%


e in the column is found to be 25%, 25% and 20%.


tio of column increase only

Chiang Huang (2014), “Progressive Collapse Analysis of an RC

384.

(2017), “Assessment of Robustness Index and Progressive Collapse in


b, (2011), “Toward an economic


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[4] Mehrdad Sasani (2008), “Response of a reinforced concrete infilled-frame structure to

removal of two adjacent columns”. Engineering structures 30, 2478-2491.

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[6] Ifteqhar Ahmed Khan, B. Narender (2019), “Effect of Steel Bracing in Progressive

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[7] Ashna T E1 ,Nivya John (2015), “Progressive Collapse Analysis of a Regular

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[8] PanosPantidis, Ph.D, Liling Cao, Ph.D, and Simos Gerasimidis, Ph.D.3 (2020), “Partial

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