MID-RISE BUILDING PROGRESSIVE COLLAPSE FINITE ELEMENT MODELING WITH CONSIDERATION OF OCCUPANT EGRESSOF OCCUPANT EGRESS

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MID-RISE BUILDING PROGRESSIVE COLLAPSE FINITE ELEMENT MODELING WITH CONSIDERATION OF OCCUPANT EGRESS

ABSTRACT

 

A progressive collapse is characterized by initial local damage to a structural element leading to collapse of a large portion of the structure.  Recently, investigation of collapse behavior and designing for progressive collapse has been greatly influenced by an increase in terrorist attacks to civilian type structures around the world.  With the increased computing capabilities, numerical models have evolved into the primary method for evaluating structural response to extreme loading events.  While, numerical models are a valuable investigative tool, modeling parameters must be carefully considered in order to appropriately represent the nonlinear material and geometric

behavior inherent to a collapse event.

This study considers the effects of three modeling parameters: element composition, element discretization, and load application on the performance of building model at a number of failure locations.  Several preliminary investigations are considered to evaluate these parameters in beam and building bay models.  Insight from these models were applied to full building models to investigate the collapse performance of the building subject to two load cases: the DOD (2009) collapse design load combination and a critical pedestrian load case representative of emergency occupant egress on the

damaged structure.

Results indicate that building model behavior is sensitive to all of the aforementioned modeling parameters as well as computational analysis procedures.  None-the-less, with respect to load case, building behavior in response to the critical pedestrian egress loads were comparable to the UFC (2009) collapse design load and should certainly be considered for structures subject to extreme loading events.

 

 

TABLE OF CONTENTS

List of Figures………………………………………………………………………………………………….. vi

List of Tables……………………………………………………………………………………………………. x

Acknowledgements…………………………………………………………………………………………… xi

 

Chapter 1 Introduction ……………………………………………………………………………………. 1

1.1 Background ………………………………………………………………………………………. 1

1.2 Problem Statement …………………………………………………………………………….. 31.3 Scope and Limitations of Research ………………………………………………………. 5

1.4 Objectives …………………………………………………………………………………………. 61.5 Task List …………………………………………………………………………………………… 7

1.6 Organization of Thesis ……………………………………………………………………….. 8

Chapter 2 Literature Review …………………………………………………………………………… 9

2.1 Introduction ………………………………………………………………………………………. 9

2.2 Background ………………………………………………………………………………………. 10

2.3 Designing for Progressive Collapse ……………………………………………………… 13

2.3.1 Organizations and Guidelines Pertaining to Progressive Collapse …… 13

2.3.2 Introduction to Design Approaches …………………………………………….. 152.3.3 Occupancy Categories and Design Method Specification ………………. 21

2.3.4 Basis for Structural Evaluation …………………………………………………… 22

2.4 Numerical Modeling Considerations …………………………………………………….. 26

2.4.1 Analysis Software Selection ………………………………………………………. 262.4.2 Element Selection ……………………………………………………………………… 272.4.3 Plasticity Definition ………………………………………………………………….. 312.4.4 Surface Contact and Multi-Point Constraints ……………………………….. 35

2.4.5 Analysis Procedure Selection ……………………………………………………… 39

2.5 Occupant Egress Considerations ………………………………………………………….. 41

Chapter 3 Preliminary Investigations ……………………………………………………………….. 46

3.1 Introduction ………………………………………………………………………………………. 46

3.2 Cantilever Beam ………………………………………………………………………………… 463.3 Fixed-Fixed Beam ……………………………………………………………………………… 52

3.4 The Bay Models ………………………………………………………………………………… 58

3.5 Preliminary Model Results Summary and Conclusions …………………………… 74

Chapter 4 Description of Studied Building and Numerical Modeling …………………… 81

4.1 Building Geometry and Design ……………………………………………………………. 81

4.1.1 Building Geometry Details ………………………………………………………… 82

4.1.2 Design Loads and Load Combinations ………………………………………… 87

4.1.3 Design Procedure and Member Selection …………………………………….. 89

4.2 Abaqus Numerical Models ………………………………………………………………….. 90

4.2.1 Building Model Assembly …………………………………………………………. 90

4.2.2 Building Model Variations …………………………………………………………. 91

Chapter 5 Building Model Investigations ………………………………………………………….. 93

5.1 Gravity Models ………………………………………………………………………………….. 93

5.2 UFC Design Load Models …………………………………………………………………… 103

5.3 Pedestrian Load and UFC Design Load Comparison ……………………………… 113

5.3.1 Introduction ……………………………………………………………………………… 1135.3.2 Application of Pedestrian Load …………………………………………………… 114

5.3.3 Investigation Results …………………………………………………………………. 118

Chapter 6 Summary, Conclusions, and Recommendations ………………………………….. 126

6.1 Summary and Conclusions ………………………………………………………………….. 1266.2. Recommendations …………………………………………………………………………….. 129

6.3 Future Research …………………………………………………………………………………. 130

6.3. 1 Numerical Modeling Analysis Computation ……………………………….. 130

6.3.2 Concrete properties and composite behavior ………………………………… 1306.3.3 Pedestrian load applications ……………………………………………………….. 131

6.3.4 Full-scale testing. ……………………………………………………………………… 131

REFERENCES ……………………………………………………………………………………………… 132

Chapter 1  

 

Introduction

1.1 Background

Prevention of progressive collapse has been an increased focus of research in the United States due to the increase of terrorist attacks targeting civil infrastructure, such as the attacks on the Alfred P. Murrah Federal Building and the World Trade Center.

Progressive collapse is defined in the American Society of Civil Engineers/ Structural

Engineering Institute Standard 7-10: Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) as “the spread of an initial local failure from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it.”

Government and military facilities have long been subject to terrorist attacks but recently civilian structures: office buildings, hospitals, transportation centers, entertainment venues, etc. have been the focus of these attacks.  Collapse in a civilian structure is particular devastating; along with physical destruction, collapse events can have worldwide economic impacts, as well as tremendous injury and death.  Modern research focuses on mitigate structural damage during a collapse event, and ultimately, how to save lives.

The two inherent methods for limiting structure risk to terrorist attack and/or collapse are: a) reducing its susceptibly to extreme or abnormal loading events and/ or

 

reducing the magnitude of these loads, and b) increasing its force-resistance system capabilities to withstand localized member failure(s).  Although quantifying the risk of a particular structure is difficult, its force-resisting system, the number and demographic of its occupants, and its exposure to extreme or abnormal loading events are given careful consideration when investigating preventative techniques.

While decreasing structure exposure to terrorist attack or other extreme loading is very important, this research focuses on the latter of the two preventive techniques introduced above by investigating structural response to a local failure.  During a collapse event, extreme loads will exceed the intended demand capacity of the structure and nonlinear material and geometric behavior will govern structural response.  Depending on the nature and location of the failures, a variety of possible collapse mechanisms may form.

To prevent the formation of collapse mechanisms, designers must understand how local failures affect global response of the structure.  This holistic approach will lead to collapse resistant solutions non-specific to the local failure locations.  Unfortunately, cost and space prohibit researchers from evaluating full-scale models subject to collapse.  Because of these limitations, researchers use numerical finite element models to investigate structural performance during collapse.  The numerical models are significantly cheaper than considering laboratory testing and data collection, although modeling parameters must be carefully considered to accurately depict the nonlinear behavior inherent during structural collapse.

 

 

Researchers implementing numerical models generally focus on the immediate response of the structure upon failure of one or more local component(s).  Additional research is needed to consider load events on the damaged structure after the initial local failure(s).  Life safety considerations should not be overlooked during extreme events, although, occupant egress during a collapse events have been appropriately investigated.  Therefore, this investigation will consider the impact of occupant evacuation on the damaged structure.  The response of the damaged building subject to occupant egress loads will be compared with that of the state-of-the-art collapse design loads.

The building model evaluated is a 10-story, 152′-0″ × 192′-0″ (46.3 m × 58.5 m) steel moment – framed building with 8 above grade office stories and 2 underground parking levels.  This building features unrestricted public access on the lobby and parking levels and has a large number of occupants; therefore, it can be considered as having a high risk to extreme or abnormal loading.  These building characteristics and features are common in many structures throughout the United States; therefore, this building serves as a relevant subject for collapse investigation.

1.2 Problem Statement

Finite element applications are being called upon more and more by academia and industry alike.  Although promising, the application of finite element models for progressive collapse is still needs much development.  Potential shortcomings in modeling techniques and/ or understanding of structural behavior during progressive collapse could have devastating consequences.  Therefore, every aspect of the modeling procedure should be carefully considered to accurately capture the behavior of interest.

In this study, preliminary investigations are used to evaluate the impacts of several modeling parameters, including: element composition, element discretization and load application.  Two different beam models and two different building bay models are used to investigate these parameters on single components and on space frames, respectively.    These preliminary investigations are necessary to understand the implications of these modeling parameters and how to consider them in the full-building models.

To maintain a holistic approach in the building model investigations, the response of the building to the state-of-the-art collapse design load is considered at four different failure locations.  The failure locations are: the corner column at the lobby level, the middle column on the short side of the building at the lobby level, the middle column on the long side of the building at the lobby level, and the interior corner column at the top parking level.  At each location, several building model variations are considered to investigate the impacts of the modeling parameters discussed above.  The pedestrian load case will be evaluated in models with a column removal location at the corner column on the lobby level and the interior corner column at the second parking level.  The behavior of these models will be compared with the behavior of corresponding models subjection to the United Facilities Criteria (UFC) design load combination.

1.3 Scope and Limitations of Research

The study will be conducted in correspondence to the analysis and design progressive collapse provisions of the Department of Defense (DOD) and General

Services Administration (GSA).  The investigated structure is a 10-story, 152′-0″ × 192’0″ (46.3 m × 58.5 m) steel moment framed building with a composite concrete slab, designed in compliance with the ASCE/SEI 7-10: Minimum Design Loads for Buildings and other Structures and the AISC Steel Construction Manual, 14th Edition.   The building features 8 above grade office levels and 2 underground parking levels.  It is considered an Occupancy Category III structure because it is has more than 500 occupants and it has unrestricted public access to three floors.

The finite element software, Abaqus, was used to model this structure.  The collapse performance was analyzed with a Riks, “push down” procedure.  Load vs. deflection graphs were used to evaluate and compare the performance of the building models.  These plots present the load acting on the structure with respect to its load definition and the corresponding structure deflection measured at the location of column removal.

Three significant assumptions govern the focus of this study.  The first assumption is in regard to initiating local failure. Historically, progressive collapse events are initiated by vehicle impact and/or explosion, compromising one or more critical structural components.  This study does not consider events causing local failure.  Structural components assumed to be subject to local failure are completely removed from the model, as is conventional.  Therefore, this study does not consider elements with partial sections or damaged resistive strength.  Likewise, this study does not consider flying or falling debris caused by extreme loading events.

The second assumption is that the building connections are not explicitly modeled; rather, behavior at connections is defined with nodal degree-of-freedom constraints.   This is an acceptable for this study because it is assumed that the connections are sufficiently stiffer than structural components, where plastic behavior is of primary interest.

The third assumption regards material definition.  The steel material definitions incorporate nonlinear plastic response, however, although the concrete material definition is linear-elastic and does not incorporate any cracking, crushing, or other material failure.

1.4 Objectives

This study has two objectives.  The first objective was to investigate how three modeling parameters: element type, element discretization, and load application impact the collapse performance of the building models.  These parameters were investigated in response to local failure at four critical column locations.

In order to consider the effects of these modeling parameters on the building model, several sub-assemblage models were investigated.  These models were used to anticipate which parameters will impact the building model performance and which parameters can be dismissed.  The cantilever beam models investigated the model sensitivity to element discretization subjected to a single load.  The fixed-fixed beam models were investigated to consider the impacts of two different load applications (distributed or concentrated) on models of varying element discretization.  The bay models were used to investigate the performance of these modeling parameters in a 3D model.  Results from these models were incorporated into the building investigations.

The second objective was to compare the building performance subject to a critical pedestrian load and to the state-of-the-art collapse design load.  This comparison provides a more comprehensive understanding of building response through the entire collapse event history, as well as challenges the state-of-the art collapse design ideology.

1.5 Task List

The following major tasks must be completed:

  1. Select/ design an appropriate building model.
  2. Review current progressive collapse analysis procedures and design guides.
  3. Investigate occupant egress analysis.
  4. Review Abaqus modeling capabilities.
  5. Construct numerical models in Abaqus.
  6. Investigate sub-assemblage models for preliminary study.
  7. Investigate building models subject to collapse design loads.
  8. Investigate building models subject to egress loads.
  9. Present results and discussion.
  10. Present conclusions and recommendations.
  11. Cite specific future research.

1.6 Organization of Thesis

This thesis is organized into six chapters:

Chapter 1: Introduction provides the reader with a background of the significance of progressive collapse research, as well as the problem statement, scope and limitations, objectives of this thesis, and the tasks necessary to complete it.

Chapter 2: Literature Review introduces the reader to the governing organizations and structural design guidelines, collapse design approaches, modeling techniques, occupant egress considerations and modern research development pertaining to progressive collapse.

Chapter 3: Preliminary Investigations presents several preliminary investigations.  Two beam and two building bay models are used to investigate the modeling parameters thought to be the most influential on building model behavior.

Chapter 4: Description of Studied Building and Numerical Modeling discusses the building geometry, structural design, member selection, Abaqus model assembly and numerical model variations.

Chapter 5: Building Model Investigations presents the building model investigations and results.

Chapter 6: Conclusions presents conclusions with respect to the objectives of this thesis and proposes areas of future research.

MID-RISE BUILDING PROGRESSIVE COLLAPSE FINITE ELEMENT MODELING WITH CONSIDERATION OF OCCUPANT EGRESS

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