Collapse Analysis of Steel Structure Using E-Tabs

 
Journal of Civil Engineering Technology and Research
Volume 2, Number 1 (2014), pp.159-168
© Delton Books
http://www.deltonbooks.com

        Collapse Analysis of Steel Structure Using E-Tabs
                           Namita Shedbal1 & Radhakrishna*2
    1
     PG Student, Department of Civil Engineering, R. V. College of Engineering, R. V.
                    Vidyaniketan Post, Mysore Road, Bangalore 560059.
                                           India,
2
  Associate Professor, Department of Civil Engineering, R. V. College of Engineering, R. V.
                    Vidyaniketan Post, Mysore Road, Bangalore 560059.
                                           India,
                              Email: radhakrishna@rvce.edu.in

                                           Abstract:
     The collapse analysis was carried out using linear analysis and the non-linear
     static analysis. The linear analysis procedure was performed using the
     combination of service loads, such as dead and live load applied on the building.
     Response was evaluated by the ratio of static to dynamic shear to be 1 and also
     the demand to capacity ratio (DCR) which shall not exceed the value of 1
     according to the GSA guidelines. The non-linear static analysis also called as the
     Pushover analysis is a procedure under permanent vertical loads and gradually
     increasing lateral loads in accordance with a certain predefined pattern. With the
     increase in the magnitude of the loading, weak links and failure modes of the
     structure were found.
             The analysis was carried out using software, ETABS according to Indian
     Standard codes. ETABS is an engineering software product that caters to multi-
     story building analysis and design. Analysis and design was carried out to get the
     final output of design details.
             After the linear analysis, member forces were known. It was found out
     that the ratio of static to dynamic shear was equivalent to 1 and also DCR values
     were less than 1. From this it can be concluded that the structure does satisfy the
     GSA progressive collapse criteria. After the pushover analysis, the plastic hinges
     were formed in the building. The joints, at which the plastic hinges were formed,
     were strengthened by increasing the size of the sections and then re-running the
     analysis. From this it can be concluded that the structure is safe by GSA
     guidelines if the performance level of plastic hinges formed, is CP(collapse
     prevention)for beams and LS (life safety) for columns. Hence the collapse can be
     prevented.

     Keywords: linear analysis, pushover analysis, ETABS, collapse.
160                                                          Namita Shedbal & Radhakrishna

Introduction:
        In a structure, when major load carrying members are removed or failed, due to
unforeseen reason suddenly, the remaining structural elements will not be able to support the
weight of the building and hence fail. When this occurs, the local initial failure starts
spreading from element to element which leads in the collapse of entire structure or a large
part of it.
        Although progressive collapse is not a new concern to structural engineers, recent
widely publicized collapses have brought the issue to the fore. Following the bombing and
partial collapse of Alfred Murrah Federal Building in 1995, an executive order was issued by
the Federal government to establish construction standards for federal buildings vulnerable to
terrorist attacks. In response to this order, General Services Administration and the
Interagency Security Committee have issued criteria documents ISC 2004. These documents
require progressive collapse resistance to be incorporated into the design of new federal
building construction, but are silent with regard to the methodology. Detailed information
regarding methodologies to resist progressive collapse can be found in documents issued by
the General Services Administration GSA 2003 and the Department of Defense DoD 2005.

Review of Literature:
Research on progressive collapse has been the focus during the past few years because of the
increasing rate of victims resulting from natural disasters like earthquake, human-made
disasters, e.g., bomb blasts, fires and vehicular impacts etc (Jinkoo Kim , Jun-Hee Park,
2010). The use of connection details such as Side Plate TM, developed for earthquakes, the
use of cables imbedded in reinforced concrete beams to activate catenary action, and the use
of mega-trusses in high-rise buildings to resist progressive collapse (Crawford, 2002). The
use of hat-bracing at the top of structures may increase the resistance to progressive collapse
(Suzuki et al, 2003). The relationship between seismic designs and the blast or progressive
collapse-resisting capacity states that the seismic design details developed for special moment
frames in high seismic zones would provide better resistance to external explosion or impact
load than the less-rigorous design details of ordinary moment frames (Hayes et al, 2005). The
mechanism of the progressive collapse can also be prevented by using seismically designed
braced steel frames (Khandelwal.et al, 2009).

Both linear and nonlinear analysis methods can be used to simulate progressive collapse. The
linear analysis method can be readily adopted (GSA, 2003) where the demand-capacity ratio
of the structure is evaluated repeatedly. However, it’s recommended that the nonlinear
analysis method should be used for progressive collapse because the result of the linear
analysis can be too conservative and is sensitive to input parameters (Powell, 2005). In this
paper, the collapse analysis is carried out using linear analysis and the non-linear static
analysis. And also base shears are calculated to know the susceptibility of building to
collapse during the linear analysis.Base shear is an estimate of the maximum expected lateral
force that will occur due to seismic ground motion at the base of a structure. (JagMohan,
Mohamed, 2005). Calculation of base shear depends on soil conditions at the site, proximity
to potential sources of seismic activity, probability of significant seismic ground motion, the
level of ductility and over strength associated with various structural configurations and the
total weight of the structure, the fundamental (natural) period of vibration of the structure
when subjected to dynamic loading.
Collapse Analysis of Steel Structure Using E-Tabs                                          161

        The non-linear static analysis also called as the Pushover analysis is a procedure
under permanent vertical loads and gradually increasing lateral loads in accordance with a
certain predefined pattern. The advantage of this procedure is its ability to account for
nonlinear effects (Shalva, Elizabeth, 2006).After the analysis, hinges formed will be
monitored and a static pushover curve will be obtained to know the behavior of the hinges
(Clough, Penzien, 1993). The standard pushover curve is as shown in the Fig.1.

                       Fig.1. Static pushover curve (Force v/s deformation)
Where,

   ·     Point A corresponds to unloaded condition.
   ·     Point B represents yielding of the element.
   ·     The ordinate at C corresponds to nominal strength and abscissa at C corresponds to
         the deformation at which significant strength degradation begins.
   ·     The drop from C to D represents the initial failure of the element and resistance to
         lateral loads beyond point C is usually unreliable.
   ·     The residual resistance from D to E allows the frame elements to sustain gravity
         loads. Beyond point E, the maximum deformation capacity, gravity load can no
         longer be sustained (Srinivasu, Panduranga Rao, 2013).

Analysis:
The building considered for the analysis was an industrial live steel building with a height of
28.06 m. The ground floor is made of RCC and the rest is made of steel. This building was
analyzed for the collapse analysis. Above the ground floor, there is a floor called
Storage1.Above this level, there are 5 more levels called 1 st, 2nd, Crane, Crane-2, Crane-1. All
the supports were modeled as fixed supports .The analysis will be carried out using software,
ETABS according to Indian Standard codes. The 3D model building and its elevation are
shown in Figures 2 and 3. The material properties of the sections used for the building are
shown in Table 1. The details of dead loads considered for the building is given in the Table
2.
162                                                         Namita Shedbal & Radhakrishna

Fig.2. Three dimensional model of building              Fig.3. Elevation of the building
                                   Table.1. Material properties
        Name           Type            E            Unit        Design Strengths,
                                     MPa          Weight              MPa
                                                   KN/m³
      A615Gr60         Rebar       199947.98      76.9729          Fy=413.69
                                                                   Fu=620.53
        CONC          Concrete         24821.13    23.5616           Fc=27.58

        Fe345           Steel          210000.00   76.9729        Fy=345 , Fu=450

            M20       Concrete         22400.00    2.5000              Fc=20

            M30       Concrete         27386.13    30.000             Fc=300

        STEEL           Steel          199947.98   76.8195          Fy=344.74 ,
                                                                    Fu=448.16

                            Table.2.Dead load of the building

      STORY                     LOAD PATTERN          DIRECTION          FORCE
      CRANE-1                   Dead                  Gravity            80kN

      CRANE-2                   Dead                  Gravity            80kN

      CRANE                     Dead                  Gravity            80kN

      2ND                       Dead                  Gravity            80kN

      1ST                       Dead                  Gravity            150kN
Collapse Analysis of Steel Structure Using E-Tabs                                        163

      1ST                      Dead                     Gravity            125 kN

      1ST                      Dead(Distributed)        Gravity            30kN/m

      GROUND FLOOR             Dead (Distributed)       Gravity            36kN/m

      GROUND                   Dead (Distributed)       Gravity            27.6kN/m

        The building was analyzed using both linear and Non-linear static analysis. The
linear analysis alone cannot be used to know the failure behavior of the building as they
cannot represent failure states because they cannot track plastic deformations which absorb
much energy while a failure occurs. Since progressive collapse analyses have to handle
extreme responses of a structure, analysis methods must be able to handle material and
geometric nonlinearities. These nonlinearities are very important, because all member failure
phenomena involve material yielding. Hence Non-linear static analysis was also performed
on the building.

Linear analysis of the building:
        The linear analysis was carried out using the combination of service loads, such as
dead and live load applied on the building. However, it is limited to relatively simple
structures where both nonlinear effects and dynamic response effects can be easily and
intuitively predicted. The load patterns provided to the Etabs software which are used for the
linear analysis is as shown in Table 3.
                                     Table.3. Load patterns
               Load       Type      of Self-Weight              Code referred
               ID         load          Multiplier
               DEAD Dead                1                       -
               LIVE       Live          0                       -
               EX         Seismic       0                       IS1893 2002
               EY         Seismic       0                       IS1893 2002
               WXS        Wind          0                       Indian
                                                                IS875:1987
               EQUIP Dead               0                       -
               WXP        Wind          0                       Indian
                                                                IS875:1987
               WYP        Wind          0                       Indian
                                                                IS875:1987
               WYS        Wind          0                       Indian
                                                                IS875:1987

Loads estimation by Etabs:
Wind load is calculated for load patterns WXP, WXS, WYP, and WYS.
  Design Wind Speed, V z [IS 5.3]          V =V k k k             V = 43.12 kN.

Earthquake loading is calculated for the load patterns EX and EY. And the respective Base
shear is also calculated as shown in Table 4.
164                                                          Namita Shedbal & Radhakrishna

                               Table.4.Calculated base shear
                Direction   Period Used        W             Vb
                             (sec)              (kN)         (kN)
                X           0.43               30574.9166 2038.3278
                Y           0.41                30574.9166     2038.3278

The dynamic loads are also calculated for the loads patterns SPECX and SPECY defined in
the functions as response spectrum function which are included in the analysis by the
software.

Non-linear static analysis :
   Loads applied:
      The loads which are applied on the building are
  · Of the nonlinear static type
  · Displacement controlled up to the standards according to the Indian 1983:2002. Hinge
      interaction is also applied according to the “Steel Fema 356” (FEMA 356, 2000).The
      loads applied in terms of displacement are named as shown in the Table.5
                               Table.5. Nonlinear static loads
                            Load ID Type of Load
                            DEAD        Nonlinear Static
                            PUSH1       Nonlinear Static
                            PUSH2       Nonlinear Static
                            PUSH3       Nonlinear Static
                            PUSH4       Nonlinear Static
                            PUSH5       Nonlinear Static
                            PUSH6       Nonlinear Static

Results and discussion:
After the loads were applied, the analysis was run by the software ETABS. Results of the
linear analysis are shown in the Figures 4, 5 and 6. Fig 4 represents the Bending Moment of
the building for the dead load applied. Fig 5 represents the 1 st floor which has the maximum
Bending Moment due to crane loads applied.
Collapse Analysis of Steel Structure Using E
                                           E-Tabs                                165

Fig.4. BM distribution in the building              Fig.5. Floor with maximum Bending
                                                                               ending
Moment
166                                                             Namita Shedbal & Radhakrishna

                       Fig.6. Linear analysis of the building

Fig. 6 represents the linear analysis of the building under the applied loads. The failed    fail
members are normally indicted red in color. It is shown that none of the members have failed
for the assigned sections. During the analysis, it was found that all members were         re safe.
According to GSA guidelines, the ratios of the static to dynamic base shear, both in X and Y
directions should be equal to 1 for the building to be able to resist the lateral forces and hence
prevent the collapse. It was found that for the present analsysis, the ratio was less than 1 for
both the directions. The DCR values estimated by the software were less than 1 which
satisfies the GSA guidelines. Hence thee structure is not suseptible to progressive collapse.
 The different base
                ase shears are shown as follows:
    · Static base shear in X-direction,
                               direction, EX=
                                          E     2038 kN
    · Static base shear in Y-direction,
                               direction, EY= 2038kN
                                                2038k
    · Dynamic base shear in X-direction,
                                   direction, SPECX= 2107 kN
    · Dynamic base shear in Y-direction,
                                   direction, SPECY =20
                                                      =2097 kN
    · Ratio of EX/SPECX in X-direction
                                   direction ==0.967
    · Ratio of EY/SPECY in Y-direction
                                   direction ==0.971

     After the nonlinear analysis was run, the hinges wewere
                                                           re formed in the structure as shown in
the Fig.7. The status of the hinges was
                                      wa obtained from the color of the hinge as well as the
letters assigned to the color. The assigned letters are explained as:

   ·   B– Operational level,
   ·   IO – Immediate occupancy,
   ·   LS – Life safety,
   ·   CP – Collapse prevention,
   ·   C – Ultimate capacity for pushover ana
                                          analysis,
   ·   D – Residual strength for pushover analysis
Collapse Analysis of Steel Structure Using E
                                           E-Tabs                                         167

            Fig.7 Hinges formed in the building after nonlinear analysis
According to the GSA guidelines, the performance level of hinges formed in the structure
should be CP for beams and LS for columns (Tavakoli et al ,2012).In this building, most of
the hinges are in the operational level. There are 2 hinges in the beams which are in the
ultimate capacity level which can be brought to the CP level by strengthening the sections of
the respective beams. However no hinge is in the residual strength which implies that the
structure is not susceptible to progressive collapse.

Conclusions:
The following conclusions are drawn from the analysis of the building:

   ·   The industrial building has shown variety of failures like beam
                                                                   beam-column
                                                                          column joint failure,
       flexure failure. Flexure failures have been observed in beams.
   ·   The failures were eliminated by strengthening the sections using weak beam-strong
                                                                                  beam strong
       column concept after which all the membe
                                             membersrs passed the analysis which rendered the
       structure safe.
   ·   In linear analysis, the building was
                                         wa safe, as there was no failure and also the ratio of
       static to dynamic base shear is equivalent to 1.
   ·   In the pushover analysis, it has been observed that one subsequent
                                                                 sub sequent push to building,
       hinges started forming in beams first. Initially hinges were in A    A-B stage (below
                                                                                         below
       operational level) and subsequently proceeding to B-IO    B      (operational
                                                                         operational level to
       immediate occupancy level) stage. Overall performance of building is said to be B  B-IO
       stage.
References:
GSA. (2003) “Progressive
              Progressive collapse analysis and design guidelines for new federal office
      buildings and
                  nd major modernization projects”, Washington (DC, USA): The US
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Crawford JE. (2002), “Retrofit
                      Retrofit methods to mitigate progressive collapse”,
                                                                    collapse the multi-hazard
                                                                                       hazard
      mitigation council of the national institute of building science.
168                                                         Namita Shedbal & Radhakrishna

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