Gravity Load Collapse and Vulnerability of Existing Buildings

H. Sezen1 1Dept. of Civil, Environmental, and Geodetic Engineering, Ohio State Univ., 470 Hitchcock Hall, 2070 Neil Ave., Columbus, OH 43210. E-mail: sezen.1@osu.edu Abstract Vertical collapse performance of existing multi-story buildings has been investigated through field experiments and computational simulations. Four reinforced concrete (RC), two masonry, and three steel frame buildings have been tested in recent years. During the experiments, single or multiple first-story columns were physically removed from the buildings, which were demolished immediately after the experiments. Due to lack of full-scale building test data, the data produced in this research has been a valuable addition to the state of knowledge on gravity collapse of buildings. The main goal of field experiments was to simulate the structural dynamic and static response of buildings that may experience collapse after sudden loss of column(s) due to, say column shear failure in a reinforced concrete building. Another objective was to investigate how the internal forces are redistributed within the building after each column is removed. In this study, two- and three-dimensional models of the buildings were used to simulate the building performance and collapse potential. Computational models and simulations were examined and compared to the experimental data from the field tests. It is noted that the current nonlinear modeling and progressive collapse assessment guidelines (GSA 2013) use nonlinear analysis parameters found in the seismic assessment of existing buildings (ASCE/SEI 41, 2013), e.g., performance limit states and m-factors to evaluate nonlinear performance. This study showed robustness of existing buildings and contribution of different structural components to collapse resistance. INTRODUCTION The main research goal is to understand vertical collapse of buildings after one or more axial load carrying elements are lost. Although the loss of columns or load bearing walls is load-independent in this research, research results can be used to predict building response after loss of vertical load carrying elements due to most extreme loads including earthquakes. The emphasis of this study is not on the sidesway collapse that can be due to low lateral strength or stiffness, P-Delta or other effects. This study focuses on global and vertical collapse triggered by a local failure. The initial local damage may be caused by, for example, steel connection fractures or shear and axial failure of reinforced concrete columns or masonry load bearing walls under seismic loading. The research aims to better understand the subsequent load

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redistribution within the building, collapse mechanism and to mitigate the risk by preventing collapse. The ongoing research project at The Ohio State University (OSU) involves testing of actual buildings to monitor their collapse behavior while one or more columns and load bearing walls are physically removed from the first story of the buildings. Structural response, including displacements and strains, was recorded during the experiments. The main research goal is to investigate gravity load redistribution and collapse response of these existing buildings thorough experimental testing and numerical simulations. The computational phase involves: 1) development of detailed and simplified building models for static and dynamic analysis during and after column/wall removals, 2) implementation of current guidelines for assessing collapse potential of the test buildings, and 3) comparison of simulated and measured building response data. American Society of Civil Engineers (ASCE 7, 2010), General Services Administration (GSA, 2003 and 2013), Department of Defense (Unified Facilities Criteria, UFC, 2009), and American Concrete Institute (ACI 318, 2011) have developed criteria and guidelines to evaluate, design and improve structural integrity and vertical collapse resistance of existing and new buildings. Vertical or progressive collapse is usually defined as total collapse of a building or a disproportionately large part of it, which may be a result of small or local structural failure leading to propagation of failure and collapse of the adjoining members in the building. Vertical collapse of building structures is typically initiated by loss of one or more vertical load carrying members such as columns or structural load carrying walls. After one or more columns or walls fail, an alternative load path is needed to transfer the load to other structural components. If the neighboring components are not designed to resist and redistribute the additional loads, failure will happen with further load redistribution until equilibrium is reached, sometimes after a sizeable part of the structure collapses. Two approaches, namely indirect and direct methods, have been generally used for providing resistance against vertical collapse. Indirect methods involve prescriptive requirements for minimum strength, ductility and element continuity to increase the overall robustness and integrity of the structure, as it is done in ACI 318-11. The direct design approach considers resistance to vertical collapse explicitly during the design process through the alternate path method or specific local resistance method. The alternate path method requires redistribution of the loads within the structure after the loss of a primary structural element. Generally, design code requirements prescribe simplified analysis procedures requiring instantaneous removal of certain vertical load carrying members. Collapse potential can be investigated using the demand-to-capacity ratio (DCR) calculated for each structural element. DCR is defined as the ratio of the force (moment, shear, or axial force) calculated after the instantaneous loss of a column and the corresponding capacity of the member. In this study, each test building are being analyzed and the

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corresponding DCRs are being calculated. The acceptance criteria for different performance levels, e.g., m-factors in ASCE 41-13, are then be used to assess the potential for vertical collapse, and immediate occupancy or life safety performance levels. BUILDING EXPERIMENTS Structural response of nine existing buildings was recorded during and after the removal of up to four perimeter columns or walls. The goal was to determine whether actual building performance could be predicted using the available analysis techniques. Basic modeling and analysis of the first few buildings have been completed (Table 1). All but two buildings had four stories. Seven of the test buildings were located on The Ohio State University campus in Columbus, Ohio.

Table 1 Properties of test buildings Building name (number of

stories) Location Construction and test year Frame No of columns,

walls removed

1. Ohio Student Union (4) Columbus, Ohio 1950 - 2007 Steel 4

2. BLC Company (2) Napersville, IL 1968 - 2008 Steel 4

3. Swallen’s Dept Store (4) Dayton, Ohio 1978 - 2010 RC 3

4. Boyd Laboratory (3) Columbus, Ohio 1933 - 2011 RC 1

5. Aviation Building (4) Columbus, Ohio 1952 - 2011 RC 1

6. Johnston Hall (4) Columbus, Ohio 1942 - 2011 RC 1

7. Haskett Hall (4) 8. Blackburn House (4)

Columbus, Ohio

Columbus, Ohio

1925 - 2012

1962 - 2014

Steel Masonry

1 4

9. Nosker House (4) Columbus, Ohio 1962 - 2015 Masonry 4

Ohio Student Union Building Four of the nine exterior columns were removed from The Ohio Student Union building, as shown in Figure 1. First story steel columns were first torched near their top and bottom. A small portion of the flange was left in place when the cross sections were cut. The middle column segment between the torched sections was then pulled out using a steel cable (Figure 1.c). The columns were removed within a short period of time representing an instantaneous column removal as recommended in the design guidelines. Columns and beams neighboring the removed columns were instrumented to monitor the redistribution of loads during the process of column removals. Detailed discussion of experiments and numerical simulations are provided in Song and Sezen (2013).

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(a) (b) (c)

Figure 1. (a) Ohio Union building, (b) four columns removed, and (c) middle of column removed

Static and dynamic analyses were performed by suddenly removing columns from the two- and three-dimensional frame models in SAP2000 (2013). Displacements and internal force demands calculated from linear static analysis showed that the building became most susceptible to vertical collapse after the last column was removed. Figure 2 shows the demand-to-capacity ratios (DCR) for all beams and columns after all four columns were removed. DCR is defined as the ratio of moment demand to plastic moment capacity of the member. Maximum moment demand is calculated from linear static analysis under factored dead loads because no live loads existed at the time of testing. Frame member numbers up to 45 are columns, and beams are numbered from 46 to 85 in Figure 2.b. After removal of the fourth column, DCR values remarkably increased, especially in columns. For all frame members, except for five columns, the building satisfied the GSA (2003) collapse criteria, which requires DCR ≤ 2.0 for columns and DCR ≤ 3.0 for beams. This suggests that collapse is expected after all columns were removed. However, it should be noted that GSA requirements are for the removal of one column only. The beams were less impacted than columns with all DCR values smaller than 2.0. The maximum DCR value, 2.83 was calculated for Column 10 in the top story.

Figure 2. Moment diagram and DCR values after removal of all four columns (left), and DCR values from linear static analysis after four columns are removed (right)

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Linear static analysis is commonly used to investigate collapse potential of a building. However, sudden column loss and subsequent load redistribution are dynamic phenomenon involving vibration of structural elements and internal dynamic forces influenced by damping, inertia and other factors. This is also inherently a nonlinear event in which structural elements may be stressed beyond their elastic limit to failure. In the current research, in nonlinear dynamic analysis, columns are removed dynamically by replacing the removed column by an equivalent axial load calculated from static analysis (Figure 3). To simulate dynamic response following instantaneous removal of the column, the equivalent load is removed suddenly using a combination of two time history inputs resulting in a step function (Figure 3). Nonlinear dynamic analysis is more realistic and accurate than linear static analysis, which is usually more conservative. In the Ohio Union building, internal forces and deformations calculated from linear static analysis were much larger than those from nonlinear dynamic analysis.

Figure 3. Column removal procedure for dynamic analysis, and time history function used to model sudden column loss in dynamic analysis

Bankers Life and Casualty Company Building The steel frame building, Bankers Life and Casualty Company (BLLC) building was located in a suburb Chicago, Illinois (Figure 4). The building was modeled and analyzed using linear static and nonlinear dynamic analysis procedures. The calculated DCR values, after all four columns were removed, are shown in Figure 4. DCR values for many structural members were larger than the limits specified by GSA (2003). Experimental details and analysis results are provided in Sezen et al. (2014).

Figure 4. Bankers Life and Casualty Company building before the experiment (left), and deflected 3-D SAP2000 model with corresponding DCR values after loss of four

columns (right)

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Strains calculated from static analysis of the 2-D and 3-D models are compared with the average strain measured by a selected strain gauge attached on a column during the removal of each of the four columns. Strains were calculated by considering the combination of axial load and bending moments from the 2-D and 3-D SAP2000 analyses. Details of strain calculations from SAP2000 models are provided in Sezen et al. (2014). Figure 5 compares the calculated and measured strains. 2-D model overestimates the measured response. The results from the 3-D model are in closer agreement with the experimental results than the 2-D model.

Figure 5. Comparison of measured and calculated strain values

Swallen’s Department Store Building Three first-story columns, including one corner column, were removed from the Swallen’s building (Figure 6). The RC flat slab structure with drop panels had no beams in the lower floors and included a grid of 130 circular columns with a uniform spacing of 20 ft. The demolition contractor used a processor to crush and remove the columns from the building. Redistribution of loads from the removed columns to the

Figure 6. Swallen’s Department Store building and its detailed 3-D computer model

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(f)

neighbouring columns was successfully monitored using 14 strain gauges attached on the longitudinal steel bars of selected columns around the removed columns. A series of models, ranging from very detailed to very simple, are developed and validated for various modeling simplifications (Morone and Sezen, 2014). Ultimately, a procedure is developed for creating simplified models and a spring model consisting of few structural elements (Figure 7).

-60

-50

-40

-30

-20

-10

0

10

0 5 10 15 20 25 30 35 40

Mic

ro-s

trai

n

Time (seconds)

Test Data

Calculated Strains

Figure 7. a) Single floor model, b) intermediate model, c) stick model, d) final simplified model, e) spring model, and f) comparison of measured strains and strains

calculated from detailed model in Figure 6 Blackburn House Four wall piers were removed from the first story of the four-story Blackburn House in. Only masonry block walls were used as the main vertical load carrying system in this building. Construction equipment was used to remove the exterior walls as shown in Figure 8. An unmanned aerial vehicle was used to capture and construct 3-D

Figure 8. Blackburn House (photo taken by drone - left), and four wall sections are removed (right)

(a) (b) (c)

(d) (e)

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images of the building. Similarly, several high resolution cameras were used to monitor the deformation of the walls above the removed wall piers using the 3-D markers shown on the right in Figure 8. Displacement sensors (LVDT’s) were also used to measure the vertical displacements in the tested region. Currently the finite element and simplified models of the building are being developed and analyzed, and laboratory experiments are being conducted on masonry materials retrieved from the building. Other Test Buildings The other five buildings (Boyd Laboratory, Johnston Hall, Aviation Building, Haskett Hall, and Nosker House) were demolished to make room for a new engineering building and new dormitories on The Ohio State University campus. Experiments and design details of Nosker House were very similar to those of Blackburn House (Figure 8). Boyd Laboratory was a three story reinforced concrete frame building. One of the first-story perimeter columns was removed using a 2-ton wrecking ball attached to a track hoe. Although the wrecking ball was used successfully to remove the column, it was not as effective as the processor or crusher, which were used to remove columns from the Swallen’s building (Figure 6) and Johnston Hall (Figure 9). Johnston Hall was a four story reinforced concrete frame structure with an approximately 130 ft by 45 ft rectangular floor plan including twelve bays in the longitudinal direction and three bays in the transverse direction. Typical story height of the building was approximately 13 ft. Vertical displacement of the joint above the removed column and strains at 18 locations on the beams and columns neighboring the removed column were recorded during the experiment. The column shown in Figure 9 was cut at mid-height with the help of concrete crushing/pinching demolition equipment in a short period of time. Vertical displacements, and changes in axial loads and moments in the neighboring beams and columns were recorded using LVDT’s and strain gauges attached on the steel bars of the columns and beams around the removed column. Linear static (LS) and nonlinear dynamic (NLD) analyses of 3-D model of the building were performed by removing the column from the model as described in Figure 3. Figure 9 compares the calculated strains from static and dynamic analyses with strains measured by three different strain gauges (S.G. 10, 11 and 12) attached on the column next to the removed column in Johnston Hall. Nonlinear dynamic analysis is proven to be more realistic and accurate than linear static analysis, which is simpler but usually more conservative. In almost all test buildings, internal forces and deformations calculated from linear static analysis were typically larger than those from nonlinear dynamic analysis. In Figure 9, the average measured axial strain is close to the calculated dynamic strains.

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Figure 9. Column being removed (left), and column removed (middle) from Johnston Hall; and measured and calculated strains in the column on the right side of the

removed column (right) The Aviation Building had a rectangular floor plan and typical story height of 12 ft. The corner column was removed using the same demolition equipment used for Johnston Hall (Figure 9). Computational simulations show that the ribbed floor slabs contributed significantly to distribute the gravity loads within the structure. As a result, 2-D frame model simulations were less reliable, and therefore a 3-D model had to be used to better understand how the loads were redistributed within the frame after the column was removed. Haskett Hall was a steel framed structure built in 1925 and was remodeled in 1970s in which additional secondary steel columns and beams were added. One column along the longitudinal side of the building near the corner was removed using the same demolition equipment used earlier for Johnston Hall experiment (Figure 9). A residual vertical displacement of 0.7 in. was recorded at the joint above the removed column. SUMMARY AND CONCLUSIONS The ongoing research project involves investigation of load redistribution and vertical collapse response of the existing buildings thorough experimental testing and numerical simulations. The experimental research involved testing of nine steel, reinforced concrete and masonry buildings by physically removing first story columns or load bearing masonry walls and monitoring the structural response during member removals. The computational phase involves development of detailed and simplified building models and analysis, and assessment vertical collapse potential of the selected buildings, and comparison of simulation results with the measured test data. The current experimental and computational research investigate the building response under load-independent loading by sudden member removals, which may occur during earthquakes or other extreme events. The test buildings are being modeled using the software SAP2000 considering effects of loading conditions, material properties and other modeling and analysis parameters. Analysis results suggest that test buildings would be within immediate occupancy and life safety performance levels, while some buildings exceeded collapse performance limits after multiple members are removed. None of the test buildings collapsed after the removal of columns or walls, however clearly some buildings performed close to

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collapse prevention or life safety limits. It can be concluded that, overall, existing steel, masonry and reinforced concrete buildings are quite robust against collapse. The contribution of floors slabs and infill walls are typically ignored in computer models and analysis while they seem to substantially increase robustness of existing buildings. This research has also shown that, in frame buildings, a large portion (approximately 70-80%) of the gravity loads carried by a column in a perimeter frame are carried by the neighboring columns in the same perimeter frame when that column is suddenly lost. The remaining loads (approximately 20-30%) on the removed column are carried by the interior columns. ACKNOWLEDGMENTS This research was partially funded by the National Science Foundation (CMMI 0745140, 1130397 and 1435446), and American Institute of Steel Construction; this is gratefully acknowledged. The author would like to thank undergraduate and graduate students involved in this research since 2007 including Brian Song, Kevin Giriunas, Gregory Ullom, Justin Morone, Shadab Lodhi, Curtis Wood, Ebiji Akah, John Wade, Hongsen Shi, Kai Li, Taha Koroglu, and Michael Savage. The author also would like to thank Loewendick Demolishing Contractors, Steve R. Rauch, Inc., Messer Construction Co., and The Ohio State University for providing access to the test buildings and help with the experiments. REFERENCES ACI-318. (2011). “Building code requirements for structural concrete”, American

Concrete Institute, Farmington Hills, MI. ASCE 7. (2010). “Minimum design loads for buildings and other structures”,

American Society of Civil Engineers, Reston, VA. ASCE/SEI 41. (2013). “Seismic evaluation and retrofit of existing buildings”,

American Society of Civil Engineers, Reston, VA. GSA. (2003 and 2013). “Progressive collapse analysis and design guidelines for new

federal office buildings and major modernization projects”, General Services Administration. Washington, D.C.

Morone, D.J. and Sezen, H. 2014. “Simplified collapse analysis using data from reinforced concrete building experiment”, ACI Structural Journal, 111(4), 925-934

SAP2000. (2013). Version 14, Analysis Reference Manual. Computers and Structures, Berkeley, CA

Sezen, H., Song, B.I. and Giriunas, K.A. 2014. “Progressive collapse testing and analysis of a steel frame building.” Journal Constructional Steel Research, 94, 76-83

Song, B.I. and Sezen, H. 2013. “Experimental and analytical progressive collapse assessment of a steel frame building”, Engineering Structures, 56, 664-672

UFC, Unified Facilities Criteria. (2009). “Design of buildings to resist progressive collapse”, UFC 4-023-03. U.S. Department of Defense.

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