Progressive Collapse Analysis of Atypical Reinforced Concrete Framed Structure

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Progressive Collapse Analysis of Atypical Reinforced Concrete Framed Structure

Gagan B M

M.Tech Student Department of Civil Engineering

PES College of Engineering Mandya, Karnataka, India

Shivaraj G Nayak

Assistant Professor Department of Civil Engineering

PES College of Engineering Mandya, Karnataka, India

Abstract Nowadays man-made and natural hazards like terrorist attack, gas cylinder burst, fire, gas explosion, etc. leads to serious effect on the buildings. Progressive collapse is the result of these manmade and natural hazards. Progressive collapse takes place when one or more columns fails, this results in failure of adjacent members and finally leads to partial or whole structure failure. In the present study, L shape 12 story reinforced concrete framed structure is considered to assess the demand capacity ratio (DCR). A linear static analysis is carried out using ETABS 2016 Software Version. The analysis is carried out by removing columns at different locations one at a time as per General Service Administration (GSA) guidelines, and then DCR values for beam and column are evaluated. The results concluded that the column removed at center is more vulnerable for progressive collapse.

Keywords Progressive Collapse, DCR, Linear Static Analysis, GSA Guidelines, Column Removal, ETABS.

  1. INTRODUCTION

    The buildings are first designed and then planned for ultimate forces or stresses resistance. But if the load acting on the whole structure or a structural element exceeds the limiting value of this operational load or stress, the structure fails or any failure of structural element takes place. When load exceeds the operational loads, building or any element like beams and column fails its effects results in the failure of adjacent elements or higher storey members results in failure of whole structure. This pattern of continuous failure of structural members causes failure of whole structure. This phenomenon is called Progressive Collapse or progressive failure.

    When an abnormal load acts on the structure, then any one of the structural elements i.e. column, beam and slab are foremost to damage. The failure of vertical structural member or element i.e. column is more susceptible than damage of horizontal member i.e. beam. When any vertical member i.e. column got damaged due to impulsive effect of load, it leads to the distribution of loads to other adjoining or neighboring components of element. If the adjacent members of damaged member are capable of carrying extra load, then it will resist the load but if not, it fails to resist. When any of the adjacent structural members fails again, then its adjacent elements were supposed to have enough capacity to bear or else failure goes on getting higher causing a series of action of failure causing structural damage.

    A. Objectives

    • To understand the process (course of action) of progressive collapse of atypical structure.

    • To understand the sequence of failure of members under progressive collapse of atypical structure.

    • The performance and reaction of a structure under progressive collapse will be figured out.

    • To examine the sample buildings of different shapes using linear static procedure.

  2. GENERAL SERVICE ADMINISTRATION GUIDELINES

    According to this guideline, it is to ensure that when failure of members occurs at the commencement, this failure is referred as local failure and this local failure can be restricted at some point so that global failure i.e. failure of whole building can be prevented.

    GSA specifies locations for column removal as,

    • Exterior column removal in buildings longer direction.

    • Exterior column removal in buildings shorter direction.

    • Corner column removal.

    • Interior column removal.

    1. Linear Static Methods

      The loading is taken as per G.S.A guidelines that is [DL

      + 0.25LL] for before removal case and 2[DL + 0.25 LL] for after removal case. The design has been done as per IS: 456 code using ETABS software.

      Where,

      DL = Self weight and LL = Live load

    2. Demand Capacity Ratio Value

      According to G.S.A structural member are said to be safe or unsafe based on DCR value. The members are safe if the DCR value is within the specified limit or else it is unsafe.

      It is the ratio of force demanding by the structural member to ultimate capacity of the structural member.

      DCR = Demand by the member / Capacity of the member

      = W acting / W capacity

      W acting = Force taken by the element. The forces like BM, SF and AL are considered.

      W capacity = Ultimate force or capacity of the member in terms of BM, SF, AL or any combined force.

      According to G.S.A, the permissible value of DCR value is limited to 2 for regular structure and 1.5 for irregular structures.

  3. METHODOLOGY

    For the analysis, the structure is in L shape and consists of 12 storeys with bay size as 5 meters in X direction and Y direction. Height of each storey is taken as 3 meters.

    The details of the building are as follows

      1. Material information

        Concrete used is of M30 grade, fck- 30 N/mm2 Reinforcement steel, fy 500 N/mm2

      2. Slab thickness 150 mm

      3. Wall thickness 300 mm

      4. Beam Size: 300 X 450 mm

      5. Column dimension:

        300mm X 750mm 1st to 4th storey 300mm X 600mm 5th to 8th storey 300mm X 450mm 9th to 12th storey

      6. Load Consideration:

        Live load = 3 kN/m2

        Floor finish = 1.5 kN/m2

        Wall load = 13.8 kN/m

        Parapet load = 3.75 kN/m

      7. Earthquake loads:

    Zone factors = 0.10, 0.16, 0.24 and 0.36 Soil type III, response reduction factor = 3 Importance factor = 1

    Linear static analysis is carried for different seismic zones the following cases,

    Case 1- Middle base column removal on shorter side. Case 2- Corner base column removal.

    Case 3- Interior base column removal. Case 4- Center base column removal.

    Fig. 1. 3D Model of a12 storey L Shaped Building

    Fig. 2. Plan of L Shaped RC Structure

    Fig. 3. Locations of Column Removal

  4. RESULTS AND DISCUSSIONS

    To understand the concept of progressive failure, columns are removed at different locations for all the seismic zones and variation of Bending-moment observed from floor to floor, then DCR value is calculated. The graphs are plotted for all the cases as DCR v/s Storeys.

    1. Case 1: Middle Base Column [C23] Removal n Shorter Side.

      Fig. 4. D-C Ratio v/s Storey for all Zones of B20

      In this case, the DCR value exceeding the limiting permissible value 1.5 by all the beams from 11th storey to 1st storey except the beams in 12th storey. As the load in the 12th storey is less when compared to other storeys.

    2. Case 2: Corner Base Column [C18] Removal.

    In this case, the beams in all storeys except 12th storey exceeding and reaches the DCR value of 3.24. Hence there will be occurance of progressive collapse.

    D. Case 4: Center base column [C34] removal

    Fig. 7. D-C Ratio v/s Storey for all Zones of B12

    In this case, the beams in the 12th, 11th, 10th, 9th and 8th storeys are safe and within the limit but the beams in remaining storeys fails by exceeding the limiting acceptable value.

    Fig. 5. D-C Ratio v/s Storey for all Zones of B19

    In this case, the DCR value of beams are exceeding allowable limit of 1.5 and reaches 2.25 from 7th storey to1st storey. The beams in the remaining storeys are within the limit for all the zones and are safe from progressive collapse

    1. Case 3: Interior Base Column [C25] Removal

      Fig. 6. D-C Ratio v/s Storey for all Zones of B24

  5. CONCLUSIONS

In the present study, the columns are removed at various locations for L shaped building considering all the 5 zones. Linear static analysis was done as per guidelines of GSA. The following are the conclusion that we got from the study,

    • The DCR value of the columns adjacent to the removal column fails for all the 4 cases in all 5 zones

    • The structure is more susceptible to progressive collapse in case of center base column removal and less susceptible in case of corner base column removal.

    • The DCR values of beams for zone 5 are more than zone 2 and zone 5 is more susceptible to progressive collapse.

    • The DCR values are linearly varying from top to bottom storeys.

    • If the beam fails by exceeding the DCR value i.e. greater than 1.5, then beam needs to be redesign to resist progressive collapse. It can be made safe either by increasing dimension or by increasing concrete grade.

    • Increasing the size of the beam is more effective than increasing size of the column in avoiding progressive collapse or in delaying failure of beam.

    • Finally it can be concluded that the atypical structures are more critical than typical structures because in typical structure is of uniform geometry and can take load uniformly in both the direction.

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