Collapse Analysis of Steel Structure Using E-Tabs
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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: firstname.lastname@example.org 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 General Services Administration. 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 Suzuki I, Wada A, Ohi K, Sakumoto Y, Fusimi M, Kamura H.(2003),” Study on high rise steel building structure that excels in redundancy”, part II evaluation of redundancy considering heat induced by fire and loss of vertical load resistant members. In: Proc. CIB-CTBUH international conference on tail building. p. 251–9. Hayes Jr, Woodson SC, Pekelnicky RG, Poland CD, Corley WG, Sozen M.(2005),” Can strengthening for earthquake improve blast and progressive collapse resistance?”, ASCE J StructEng;131(8):1157–77. Khandelwal K, El-Tawil S, Sadek F. (2009),” Progressive collapse analysis of seismically designed steel braced frames”, J Constr Steel Res; 65:699–708. Powell G. (2005),” Progressive collapses: case study using nonlinear analysis”, In: Proceedings of the 2005 structures congress and the 2005 forensic engineering symposium. H.R. Tavakoli, A. Rashidi Alashti & G.R. Abdollahzadeh, (2012),” 3-D Nonlinear Static Progressive Collapse Analysis of Multi-story Steel Braced Buildings”, Department of Civil Engineering, Babol University of Technology (BUT) Jinkoo Kim, Jun-Hee Park, (2010),”Sensitivity analysis of steel buildings subjected to column loss”, Korea Atomic Energy Research Institute, Daejeon, Republic of Korea. Shalva Marjanishvili, Elizabeth Agnew, (2006),”Comparison of Various Procedures for Progressive Collapse Analysis”, Journal of Performance of Constructed Facilities, Vol. 20. Clough, R. W, Penzien, J. (1993), “Dynamics of structures”, 2nd Ed., McGraw-Hill, New York. FEMA 356, (2000). Pre standard and commentary for the seismic rehabilitation of buildings, Washington. Srinivasu, Dr. Panduranga Rao, (2013),”Non-Linear Static Analysis of Multi-Storied Building”, International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 10. JagMohan Humar and, Mohamed A. Mahgoub, (2005),” Determination of seismic design forces by equivalent static load method”, Special Issue on the Proposed Earthquake Design Requirements of the National Building Code of Canada.
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