INVESTIGATION OF A HOLLOW STRUCTURAL SECTION CONNECTION AND TRANSFER MEMBER FOR LOAD SHARING IN ANTI-RAM VEHICLE BARRIERS

ABSTRACT

As evidenced by various past and present events involving vehicle delivered explosive device

attacks on buildings, efficient and effective anti-ram barrier designs can be very beneficial in protecting structures. Anti-ram barriers put in place to protect structures from moving vehicles follow a different design, analysis, and construction approach than most structures due to the extreme nature of the vehicle impact load. In many situations, it is also important to consider aesthetic implications of anti-ram barriers when completing a design. Hollow structural section (HSS) steel members are very useful for the design of anti-ram barriers because of their efficiency in dealing with torsion or biaxial loads, and their aesthetic and architectural advantages. Certain types of load sharing configurations involving pierced HSS member connections and an HSS transfer member do exist as a nature of the HSS shapes, however their behavior under impact loading is largely undocumented. There existed a need to understand the use of these connections and transfer members parallel to the direction of impact in anti-ram barriers for load sharing purposes. A prototype structure using basic pierced HSS connections and an HSS transfer member was designed and its performance under vehicle impacts was examined parametrically. The prototype structure was designed using a basic plastic collapse analysis. Varied parameters included member sizes, piercing hole sizes, and connection orientation with respect to the piercing member. The prototype structure was examined using finite element (FE) analysis with modeling techniques selected from past modeling of full-scale crash tests and past literature. Additionally the FE models were validated against past full-scale tests of partially restrained moment connections. Results from the validated FE models that examined barrier behavior were presented in the form of member force time histories at important locations and via internal strain energy calculations. These results indicated that the use of a basic pierced HSS connection and HSS transfer member oriented parallel to the direction of impact effectively shared load between bollards in anti-ram barriers. In addition, recommendations for reducing member sizes when using the studied connection and transfer member are provided.

 

TABLE OF CONTENTS

LIST OF FIGURES ……………………………………………………………………………………………….vi

LIST OF TABLES ………………………………………………………………………………………………..ix

ACKNOWLEDGEMENTS …………………………………………………………………………………….x

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

1.1 Introduction ……………………………………………………………………………………………..1

1.2 Background ………………………………………………………………………………………………1

1.3 Problem Statement …………………………………………………………………………………….4 1.4 Objectives ………………………………………………………………………………………………..5

1.5 Scope ………………………………………………………………………………………………………5

1.6 Tasks ………………………………………………………………………………………………………6

1.7 Summary …………………………………………………………………………………………………8

Chapter 2: Literature Review …………………………………………………………………………………..10

2.1 Overview …………………………………………………………………………………………………10

2.2 Anti-Ram Barrier Design and Testing ……………………………………………………………10

2.3 Finite Element Modeling of Vehicle Barriers ………………………………………………….12

2.4 Finite Element Modeling of Partially Restrained Moment Connections ……………….15

2.5 Summary …………………………………………………………………………………………………20

Chapter 3: Relevant Testing …………………………………………………………………………………….21

3.1 Overview …………………………………………………………………………………………………21

3.2 Relevant Tests and Models ………………………………………………………………………….21

3.3 Summary …………………………………………………………………………………………………28

Chapter 4:  Prototype Structure Design, Parameters, and Evaluation Criteria ……………………29

4.1 Overview …………………………………………………………………………………………………29 4.2 Relevant Testing Design Basis …………………………………………………………………….29 4.3 Prototype Structure…………………………………………………………………………………….32 4.4 Design Load ……………………………………………………………………………………………..36

4.5 Member Sizing and Parameters ……………………………………………………………………38

4.5.1 Member Sizing ………………………………………………………………………………..38

4.5.2 Connection Geometry ……………………………………………………………………….45

4.6 Performance Evaluation Criteria …………………………………………………………………..46

4.6.1 Member Force Evaluation ………………………………………………………………….47

4.6.2 Bollard Internal Strain Energy …………………………………………………………….48

4.6.3 Collapse Mechanism Evaluation …………………………………………………………49

4.7 Summary …………………………………………………………………………………………………49  Chapter 5: Finite Element Model Details and Validation ………………………………………………50

5.1 Overview …………………………………………………………………………………………………50

5.2 Finite Element Modeling …………………………………………………………………………….50

5.2.1 Element Types and Discretization ……………………………………………………….50

5.2.2 Material Properties……………………………………………………………………………56

5.2.3 Component Interaction ………………………………………………………………………58

5.2.4 Boundary Conditions ………………………………………………………………………..60

5.3 Vehicle Model…………………………………………………………………………………………..61

5.4 Analysis …………………………………………………………………………………………………..62 5.5 Post Processing …………………………………………………………………………………………63

5.6 Model Checks and Validation ………………………………………………………………………66

5.6.1 Static and Dynamic Model Checks ………………………………………………………66

5.6.2 Full Scale Test Validation ………………………………………………………………….69

5.7 Summary …………………………………………………………………………………………………74

Chapter 6 Results and Discussion …………………………………………………………………………….75

6.1 Overview …………………………………………………………………………………………………75

6.2 General Observations …………………………………………………………………………………76

6.3 Connection Behavior ………………………………………………………………………………….83

6.4 Transfer Member Behavior………………………………………………………………………….89

6.5 Bollard Behavior ……………………………………………………………………………………….96

6.6 Summary …………………………………………………………………………………………………102

Chapter 7 Results and Discussion …………………………………………………………………………….104

REFERENCES……………………………………………………………………………………………………..108 APPENDIX A: Prototype Structure Member Sizes ……………………………………………………..115

APPENDIX B: Transfer Member Axial Force Time Histories ………………………………………117 APPENDIX C: Transfer Member Bending Moment Time Histories ……………………………….128

APPENDIX D: Bollard Base Bending Moment Time Histories……………………………………..139

Chapter 1: Introduction

1.1 Introduction

Recent attacks on buildings from vehicle delivered explosives, evidenced by many events within the past few decades, call to attention the need to design systems for the prevention and mitigation of these attacks. Two notable vehicle delivered explosive events in the United States are the attacks on the World Trade Center, New York City, in February of 1992 (Manning 1993) and the Alfred P. Murrah Federal Building, Oklahoma City, in April of 1995 (Oklahoma Dept. of Emergency Management 1995). Both of these events involved an explosive device delivered by a vehicle directly to the site of the attack. Internationally, and more recently, a vehicle delivered explosive device was used to attack the United Nations headquarters in the Nigerian capital of Abuja in August of 2011 (Murray 2011). The vehicle rammed through two gates surrounding the building before delivering an explosive device adjacent to the building and severely damaging it.              Structures designed to stop attacking vehicles, known as anti-ram barriers, are designed with the intention of keeping the vehicle at a safe standoff distance from a building. A standoff distance is beneficial for blast mitigation since the effects of blast loads on buildings decrease rapidly as the distance from the blast increases (Longinow and Mniszewski 1996). Anti-ram barriers can also prevent impacts from other moving vehicles near a building, such as civilian traffic, that may accidentally become a threat to the building and its occupant’s safety.

1.2 Background

Due to the extreme loading imparted on an anti-ram barrier by a vehicle impact, analysis and design of the barrier can be extremely complicated. The typical governing load used for design is that supplied by a vehicle impact. To account for this load, certain structural configurations could be utilized to simplify design and construction, while concurrently distributing loads throughout the barrier more efficiently. One possible example of this is a barrier that utilizes two rows of vertical members, or bollards, located in rows perpendicular to the direction of impact. Typically, one row of bollards, or even a single bollard, can resist standard load cases such as self-weight, sustained dead loads, wind loads, or other typical loads without the use of other bollards and connections. When the barrier is impacted, however, all bollards and connections within the structure work together to resist the extreme loading condition. With multiple barrier components sharing the vehicle impact load effectively, the load seen by each individual member is reduced. This load reduction is advantageous to the barrier design because it allows for the possibility of smaller members, which may lend themselves to less expensive, more aesthetic, or easily constructible designs. Figure 1-1 shows a possible above ground framing plan of an anti-ram barrier that would utilize two rows of bollards as described.

 

Figure 1-1: Possible Above Ground Framing Plan

Anti-ram barriers typically come in two main forms, (1) active barriers, such as those in an access passage, which can be temporarily removed to accommodate a passing vehicle, or (2) passive barriers, designed to be constructed and installed without plans for removal (UFC 2010). Since passive barriers typically remain around the protected area for a large portion of their life cycle, it is important to consider the aesthetic implications the barrier’s design and placement will have on the surrounding area. If this consideration is not included in the design process, the resulting barrier may be negatively received by building occupants or local residents, and possibly require replacement (Bush 2011). One way of accommodating aesthetic considerations is to integrate the barrier into other structures that the public would expect to see in the area (Hu et al. 2011). Designing the barrier to effectively share load, which allows for member size reductions as discussed previously, also allows the structure to be optimized. By optimizing the framing of these structures using stronger members, additional members, or modified connections, the frame could function as an anti-ram barrier with minimal aesthetic detriments.   These aesthetic considerations are part of the reason steel hollow structural section (HSS) members can be effective components in an anti-ram barrier design. HSS members are very useful for aesthetic and architectural considerations because they can be painted and left visible to the public, displaying smooth edges and rounded curves instead of the unfinished, jagged look of typical steel members (Fletcher 2012). They are also efficient structural sections, especially in spans that are unbraced and subjected to biaxial loads (Kloiber 2001). This can be particularly important when structures make use of irregular geometries or load cases, such as modern styled buildings or anti-ram barriers. Also, HSS members can be very efficient in resisting torsion or multi directional loads due to the behavior of their closed cross-sections (Fletcher 2012). The closed section of an HSS member under elastic loading remains planar during loading, mitigating warping torsion and leaving pure torsion to be resisted (AISC 2003). For multi direction loading, the closed cross section of HSS members places the material away from the section’s centroid, increasing the effectiveness of bending properties for all loading directions (AISC 2011).       One problem with using HSS members is that connections and their required detailing are often a challenging part of the design process. This is partly due to the inherent problems which come with connecting closed members that prevent access to their interior for bolting; most connection types involve welding plates to the HSS member (Scherman 2005). Also, certain HSS members and their corresponding connections sometimes do not undergo any load unless the barrier is impacted, which leaves the possibility for the connection and member to be overdesigned. Certain types of efficient, basic pierced connections do exist for HSS shapes, however their behavior under conventional and impact loading is relatively undocumented and hard to predict.

1.3 Problem Statement

Accounting for aesthetic considerations in anti-ram barriers can result in additional complications when attempting to ensure that the barrier effectively mitigates a vehicle impact. Using a basic pierced HSS connections and an HSS transfer member can help share vehicle impact loads and reduce member sizes to aid in optimizing the barrier. While this optimization helps take into account aesthetic considerations, there existed a need to determine the effectiveness of using this design to transfer load, determine preliminary design methods for selecting and reducing HSS transfer and bollard member sizes, and provide recommendations regarding connection piercing hole size and orientation.

 

 

 

1.4 Objectives

The objective of this study was to computationally examine load sharing in anti-ram barrier structures that utilize basic pierced HSS connections and an HSS transfer member under dynamic vehicle impact loads. The examination addressed the following objectives:

• Determined if the basic HSS pierced connection and HSS transfer member effectively shared the vehicle impact load between bollard rows.
• Determined if the design method used provided member sizes that could be reduced.
• Determined how member sizes should be adjusted, and if any changes to the basic pierced connection could be made.

1.5 Scope

The scope of this study involved computational investigation of a basic pierced HSS connection placed in a structure subjected to vehicular impacts using finite element (FE) models.  Figure 1-2 is a rendering of the studied configuration and shows the basic pierced connection with the vertical members, or bollards, being HSS 203.2×203.2×15.9 (HSS 8x8x5/8) sections and the transverse member an HSS 101.6×101.6×12.7 (HSS 4x4x1/2). A general view of the connection design considered in this study is shown in the figure, however, member sizes that were studied ranged between HSS 304.8×152.4×15.9 (HSS 10x6x5/8) and HSS

304.8×152.4×12.7 (HSS 10x6x1/2) for bollard members and between HSS 304.8×203.2×15.9 (HSS 10x8x5/8) and HSS 304.8×152.4×15.9 (HSS 10x6x5/8) for the transfer member, according to Appendix A, to facilitate the parametric study.

 

Figure 1-2: Example Basic Pierced HSS Connection

The scope of this study involved prototype anti-ram barriers having two bollards parallel to the direction of impact. Impact speeds and vehicle types were similar to those used for previous anti-ram barrier evaluations and consisted of a medium duty truck meeting the specified weight of 6,800 kg (15,000 lb) traveling at a speed of 50 km/hr (30 mph), both of which are indicated in the relevant standards discussed later.

1.6 Tasks

This investigation involved the following tasks:

1. Literature Review: The literature review served the purpose of collecting and analyzing existing information relevant to the objectives listed above. This included relevant studies regarding anti-ram barrier design, finite element (FE) modeling of vehicle barriers, and FE analysis of partially restrained moment connections. Information regarding anti-ram barrier design focused on the design standards, methods, and common practices used for barrier design. Information for FE modeling of vehicle barriers focused on modeling methods that have been used and validated in past studies of various types of barriers, including roadside safety barriers. Information for FE analysis of partially restrained moment connections focused on modeling and analysis methods that have been used and validated in past studies for determining the behavior of moment connections.
2. Relevant Testing: A review of relevant testing was included to help summarize antiram barrier designs and tests completed by the Larson Transportation Institute (LTI) at the Pennsylvania State University. This review also summarized the current state of antiram barrier designs at LTI as they pertain to the basic pierced HSS connection that was included in the current investigation.
3. Barrier Prototype Design: A simplified prototype structure was designed to investigate the basic HSS connection and HSS transfer member through computational models. The structure was designed based on past barrier designs and tests at LTI, and also incorporates parameter changes to create the necessary models to complete the study.
4. Model Construction and Validation: Simple FE models of the structure were created and compared to other common structural analysis methods as well as accompanying experimental data. These FE models were primarily used to validate selected methods used to mimic component interaction. Validation was based on qualitative or quantitative comparisons between models, simple calculation methods, and comparison to full scale connection testing indicated in literature. A description of each validation step is described in Chapter 5.
5. HSS Connection Computational Study : Once the models were validated, parameters of the model were changed to computationally assess the behavior of the connection in the prototype structure under different circumstances. Parameters that were varied include:
   • Member size and plastic moment capacity
   • Vehicle impact conditions
   • Connection orientation
   • Member hole size

The behavior of the connections and transfer member was assessed by observing select data from the model, which included:

• Bending moment in the bases of both bollards.
• Bending moment and axial forces in the transfer member
• Internal strain energy in the bollards
• Locations of plastic behavior and formation of plastic hinges in all members

6. Computational Results Comparison and Simplification:

After the analysis data was compiled, it was evaluated to examine the effects certain parameter changes had on the load sharing behavior of the connections and transfer member. These effects were summarized and simplified to show how changes in certain parameters do, or do not, affect certain behaviors. These simplifications could be useful to predict the behavior of these connections if they are implemented in design. Particularly, they could be used to provide information regarding transfer member effectiveness and member size refinement recommendations.

1.7 Summary

Due to the increasing possibility of vehicle delivered explosive attacks on buildings, antiram barriers are being increasingly implemented as one layer of protection for many structures. Anti-ram barriers are designed in many different ways that take into account vehicle stopping capabilities as well as aesthetic considerations. Anti-ram barriers can also be designed to transfer load between components, allowing for the structure framing plan to be optimized and member sizes to be reduced. For these reasons HSS members could be useful for certain barriers because they are highly efficient sections that also lend themselves to aesthetic designs. HSS members, however, are difficult to connect, and most existing connection designs have not been evaluated for anti-ram barrier implementation. Basic pierced connections between HSS members can be used which are efficient, simple to construct, and aesthetically pleasing, but a full understanding of the connection behavior under impact loading was needed. Computational models of simple HSS connections in prototype barrier structures were analyzed and compared to provide a basis for using them under vehicle impact scenarios.

INVESTIGATION OF A HOLLOW STRUCTURAL SECTION CONNECTION AND TRANSFER MEMBER FOR LOAD SHARING IN ANTI-RAM VEHICLE BARRIERS

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