SUSPENSION PEDESTRIAN BRIDGE NUMERICAL MODEL DEVELOPMENT AND LOAD CASE SIMULATION

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SUSPENSION PEDESTRIAN BRIDGE NUMERICAL MODEL DEVELOPMENT AND LOAD CASE SIMULATION

ABSTRACT

During rainy seasons, rural communities around the world become isolated from health care, education and other essential facilities due to flooding. Under this circumstance, suspension pedestrian bridges are built to provide rural communities with access to their basic needs.

However, due to low stiffness, mass and damping of these bridges, vibration problems may occur. Therefore, research needs to be conducted to improve bridge dynamic performance and

serviceability to ensure that those intended to be served are not fearful of using the bridge.

The present study refined one available scaled physical model in order to obtain more accurate tested dynamic responses that were utilized to calibrate numerical suspension pedestrian bridge models. By achieving a better numerical modeling methodology, the present study refined currently employed numerical models, which enabled the completion of more reliable dynamic analysis simulations under more complex load cases than have previously been studied.

The present study set a reasonable serviceability limit, considering that people in those rural communities of developing countries may lower their expectations of bridge performance. A nonlinear, direct-integration, time-history analysis was conducted to determine the dynamic responses under each defined load combination. The analysis evaluated the responses on the basis of vertical velocity, vertical acceleration, lateral acceleration and compare the responses to the serviceability limits. Simulation in the present study includes twenty pedestrian load combinations, five animal load combinations and three handcart load combinations. At last, an investigation into the effect of a bystander on bridge response was conducted through modeling a human bystander as mass or a damped system.

The present study found that the peak response induced by a jogger reached the double response of a walker, while a cycler produced a 77 percent smaller response than the walker. The most critical case for two pedestrians is that they start from the same end of the bridge, from the same time, and keep the same pace. The existence of a bystander decreases the vertical response near the bystander’s location while the lateral response is minimally affected.

 

 

 

TABLE OF CONTENTS

LIST OF FIGURES ……………………………………………………………………………………….. vii

LIST OF TABLES …………………………………………………………………………………………. x

ACKNOWLEDGEMENTS …………………………………………………………………………….. xii

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

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

1.2 Problem Statement ……………………………………………………………………………… 2

1.3 Scope of Research ………………………………………………………………………………. 2

1.4 Objectives …………………………………………………………………………………………. 3

Chapter 2 Literature Review ……………………………………………………………………………. 5

2.1 Vibration Sources ………………………………………………………………………………. 5

2.1.1 Pedestrian Walking Footfall Loads ……………………………………………… 6

2.1.2 Animal Footfall Loads ……………………………………………………………….. 8

2.1.3 The Handcart Load ……………………………………………………………………. 9

2.2 Receiver of Vibrations ………………………………………………………………………… 11

2.2.1 Human-Structure Interaction ………………………………………………………. 11

2.2.2 Serviceability Limits …………………………………………………………………. 14

2.3 Previous Relevant Study ……………………………………………………………………… 17

Chapter 3 Improvement of Physical Model ……………………………………………………….. 18

3.1 Available Physical Models ………………………………………………………………….. 18

3.2 Model Improvement Design ………………………………………………………………… 20

3.2.1 Improvement of the Suspenders ………………………………………………….. 21

3.2.2 Improvement of the Hand Cable and Fence ………………………………….. 21

3.3 Model Construction ……………………………………………………………………………. 22

3.4 Summary …………………………………………………………………………………………… 24

Chapter 4 Physical Model Testing and Calibration of Numerical Model ………………. 25

4.1 Test on the Physical Model ………………………………………………………………….. 25

4.1.1 Data Collection and Processing …………………………………………………… 26

4.1.2 Results …………………………………………………………………………………….. 27

4.2 Numerical Model Design …………………………………………………………………….. 29

4.3 Calibration ………………………………………………………………………………………… 31

4.3.1 Boundary Condition of Deck-Abutment ………………………………………. 32

4.3.2 Calibration Results ……………………………………………………………………. 34

4.4 Summary …………………………………………………………………………………………… 35

Chapter 5 Load Case Simulation ……………………………………………………………………… 37

5.1 Method of Evaluation …………………………………………………………………………. 37

5.2 Multiple Pedestrian Loads …………………………………………………………………… 39

5.2.1 Load Patterns of Pedestrian Force ……………………………………………….. 39

5.2.2 Design of Load Combinations …………………………………………………….. 41

5.2.3 Results and Discussion of Multiple Pedestrian Load Combinations …. 48

5.3 Other Load Cases ……………………………………………………………………………….. 59

5.3.1 Load Patterns of Animal Force and Handcart Force ………………………. 60

5.3.2 Design of Load Combinations …………………………………………………….. 61

5.3.3 Results and Discussion of Animal and Handcart Load Cases ………….. 62

5.4 Effect of Bystanders …………………………………………………………………………… 66

5.4.1 Parametric Study Design ……………………………………………………………. 67

5.4.2 Results and Discussion of Bystander Effect ………………………………….. 69

5.5 Summary …………………………………………………………………………………………… 78

Chapter 6 Summary and Conclusions ……………………………………………………………….. 80

6.1 Conclusions……………………………………………………………………………………….. 81

6.2 Recommendations for Future Research …………………………………………………. 82

Appendix  Time-History Data …………………………………………………………………………. 84

BIBLIOGRAPHY ………………………………………………………………………………………….. 113

Chapter 1  

 

Introduction

1.1 Background

There are two principal criteria in the design process of a structure: strength and serviceability. The strength criteria is always the primary focus, causing serviceability to be the secondary. However, for a suspension pedestrian bridge, serviceability is of much significance because a serviceability failure may result in pedestrians choosing not to use the bridge.

A suspension pedestrian bridge is a bridge designed exclusively for pedestrians, and in some cases cyclists and animal traffic, rather than vehicular traffic. Several organizations around the world construct suspension pedestrian bridges in rural communities using simple and inexpensive local materials. Those over impassable rivers provide isolated communities with access to health care, education and other essential facilities during rainy seasons. If a pedestrian bridge suffers from a serviceability failure, people may fear crossing the bridge and seek other routes, sometimes more dangerous, to cross the obstacle. This betrays the motivation for constructing the bridge. Therefore, it is clear that serviceability is an important design criterion.

Recently, structural engineers have become more aware of the problems of human-induced vibrations. However, little research has been conducted to evaluate bridge dynamic response induced by pedestrians on suspension pedestrian bridges.

1.2 Problem Statement

Despite growing interest in the dynamic responses of pedestrian bridges after the infamous swaying of the new Millennium Bridge in London during its opening day, little research has been conducted on other types of pedestrian bridges. Bridges in the present study differ from the London Millennium Bridge in that they are located in rural communities of developing countries and are constructed to provide people with access to basic needs like medical care and education. People may lower expectations for comfort under this circumstance. Therefore, the existing guide specifications for the design of pedestrian bridges, published by AASHTO (AASHTO, 1998) is likely too restrictive, especially for serviceability of the pedestrian bridges in the present study. Furthermore, most of the existing design specifications and manuals do not require a detailed dynamic evaluation as long as the natural frequencies are within specified limits.

To better simulate the behavior of dynamic responses of a suspension pedestrian bridge, the present study will modify an existing, scaled, 40 m suspension pedestrian bridge model and test it. Based on the scaled bridge model, a numerical model built in SAP2000 will be calibrated and more complex load case simulations will be conducted using the calibrated numerical model.

1.3 Scope of Research

The suspension pedestrian bridges studied here are bridges with wood decks, steel wire cables and rebar suspenders. The scaled physical model used in the present study is a 40-meter span bridge with five percent sag with a scale factor of 18. The studied numerical models are limited to a 40-meter bridge with five percent sag and an 80-meter bridge with 7.5 percent sag.

The modal damping ratio is defined as 0.01 percent and 1 percent, which is recommended for outdoor pedestrian bridges in AISC Design Guide 11. The Rayleigh damping frequencies are defined as the first and tenth modal frequencies for each model.

SAP2000 is used to complete simulations of load cases. Load cases will be studied, including multiple pedestrian loads, animal load, and handcart load. The number of pedestrians modeled simultaneously is no more than two. Pedestrians are modeled as walking, jogging, cycling across the bridge or standing stationary on the bridge. Two pedestrians begin movement from either end of the bridge at any time. Bystanders are stationary at the midpoint or quarter-point of the bridge span.

Simulations of animal walking load with different walking speeds are also studied. The animal body weight is equal to a human body weight, making these two load cases directly comparable. Studied handcarts have one axle or two axles and carry 22.7 kgf cargo. Handcart load cases include walking footfalls following the handcart, simulating a person pushing the handcart.

1.4 Objectives

The primary objective of the present study is to determine the critical load case that results in the most severe dynamic response within the defined scope. In order to achieve this goal, a significant step is to advance the numerical modeling methodology to achieve higher accuracy and to match more closely with observed bridge behavior. Detailed objectives are as follow:

  1. Obtain a scaled, physical pedestrian model that matches more closely the

construction of a real, full-scale bridge;

  1. Obtain a more accurate numerical modeling methodology using the dynamic test data obtained from the refined physical model and apply to numerical models that will be used for the subsequent load case simulation;
  2. Determine the critical load case within the defined scope by evaluating dynamic responses under load cases within the defined scope; and
  3. Observe the effect of a bystander on the dynamic response of a suspension pedestrian bridge.

SUSPENSION PEDESTRIAN BRIDGE NUMERICAL MODEL DEVELOPMENT AND LOAD CASE SIMULATION

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