axis_strange bridge project report(gp 6).pdf

AXIS ENGINEER Analysis and Design of an Unusual Pedestrian Bridge

Analysis and Design of an Unusual Pedestrian Bridge

C V E N 7 5 0 F I N I T E E L E M E N T

A P P L I C A T I O N S

Z a c h r y D e p t o f

C i v i l E n g i n e e r i n g

T e x a s A & M U n i v e r s i t y

3 / 3 1 / 2 0 1 4

Group # 6

The analysis and design of an unusual pedestrian bridge to connect

the clients driveway to the other side of the gorge where the tree

house is located.

AXIS ENGINEERS Analysis and Design of an Unusual Pedestrian Bridge

Group Members: Abhinav Prashant Mohanakrishnan

Insiyah Juzer Lightwala

Steven Chee

Xin Zhou

Honor Statement:

On my honor, as an Aggie, I have neither given nor received unauthorized aid on this academic work.

Abhinav Prashant Mohanakrishnan Insiyah Juzer Lightwala

Steven Chee Xin Zhou


TABLE OF CONTENTS

Executive Summary 1

Problem Description 1

What is Unusual? 3

Construction Material 3

Type of Bridge 3

Tools for Design 3

Preliminary Design 3

Bridge Model 4

Modelling the Bridge on SAP2000 4

Loads on a Bridge 5

Member Sizing 5

Cost Estimate 5

Construction Timeline 6

Analysis of the Bridge 7

Appendix A-Wind Load Vibration 8

Appendix B-Newfoundland Puppy Analysis 10

Submittal To The Architectural Control Committee 11

Bibliography 12


LIST OF FIGURES

Figure 1: Circular Pedestrian Bridge in Lujiazui, China 2

Figure 2: Pythonbrug Bridge in Amsterdam 2

Figure 3: Plan and Elevation of the Bridge 4

Figure 4: Site Layout 4

Figure 5:3-D Model of the Bridge 5

Figure 6: Newfoundland Puppy 11


LIST OF TABLES

Table 1: Dimension of the Bridge 3

Table 2: Calculation of Weight of Structure 6

Table 3: Calculation of Total Cost of the Structure 6

Table 4: Joint Displacements due to load case 1 7

Table 5: Joint Displacements due to load case 2 7

Table 6: Modal Periods and Frequencies 9

Table 7: Modal Participating Mass Ratios 10

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EXECUTIVE SUMMARY Axis Engineers were approach by an undisclosed client in Galveston to plan, analyze, design and

construct a pedestrian bridge connecting her residence to the parking space across the gorge. The client wanted to have an aesthetically pleasing structure that is convenient for her family members and also wanted it to be structurally sound. The primary purpose of the unusual bridge was to connect her driveway (where cars are parked) to the other side of the property (where the 1200 square foot tree house residence is located). PROBLEM DESCRIPTION

This project delves into the world of bridge design with the help of finite element software packages. It involves the selection of most appropriate software package to facilitate the design process smoothly and its application. The primary concerns for the design were

Minimum center span = 50 feet (no piers may be placed in this span). Total span across gorge= 150 feet (there is a 40 foot drop in the 70 feet between the north bank and the

north edge of the deep gorge & a 60 foot drop in the 30 feet between the south bank and the south edge of the deep gorge) .

The ground under the outside spans is too steep to hold water. Given the disaster of the Tacoma Bridge, resonance due to a set of pedestrians walking in harmony was

an important factor. The client lives with two puppies and they are expected to run across the bridge to meet her when she

drives up. Therefore the bridge had to be dog friendly. SELECTION OF BRIDGE

The first step in the project was the determination of a precise type of bridge that would serve all purposes for the owner. There were a number of unusual bridges to base our design on but due to cost and constructability issues, only two bridges made the final cut. They were the Circular Pedestrian Bridge in Lujiazui, China and The Pythonbrug Bridge in Amsterdam. The circular bridge required a lot of space and its geometry was not compatible. Also the primary purpose of the bridge was to connect the driveway to the tree house. Though a circular bridge would be considered unusual, it unnecessarily complicates the design and extends the construction period. Since the client required a relatively fast construction time period, this design was determined not be an apt solution. The lateral stiffness of the bridge was also a critical factor in eliminating this bridge design. The Pythonbrug Bridge, as the name suggests resembles the shape of a Python. The bridge is a well braced structure. The images of the bridges are shown below.


Figure 1: Circular Pedestrian Bridge in Lujiazui, China

Figure 2: Pythonbrug Bridge in Amsterdam

WHY IS IT UNUSUAL? The unusual nature of the bridge is due to its shape. It is composed of a double curvature resembling the shape of a python.


CONSTRUCTION MATERIAL Steel is one the most extensively used construction material in the United States. In the domain of

pedestrian bridge construction, this material offers an array of lucrative advantages.

It reduces the time period of construction.

It has a superior height to weight ratio and this is crucial in pedestrian bridge design. This allows the bridge to withstand greater loads for shallow depths.

Also the low self-weights of beams warrants an easier transportation and placement of the structural element.

Another important advantage with steel is that, if designed with architectural aspects in mind, a steel bridge can be a visual delight. Some of the world famous bridges known for their structural magnificence are also aesthetically pleasing due to the utilization of steel sections.

TYPE OF BRIDGE

Selection of a bridge type often serves to be a very cumbersome process but since we drew our inspiration from the Pythonbrug bridge, our structure is a truss bridge. Truss bridges have early roots in Europe and they were an integral part of Americas expanding rail network in the 1800s. A truss bridge basically involves a series of elements forming triangular units as only this geometry maintains the angle sections in place when the elements are connected.

Newtons Laws usually suffice for a simple analysis of a truss bridge system but they are rarely statically determinate. Hence the application of software packages becomes inevitable. TOOLS FOR DESIGN We explored SAP2000 and CSI Bridge and found that SAP2000 is a relatively convenient way to model the structure and run the analysis. SAP (Structural Analysis Program) is an extensively used software program for the analysis and design of buildings and bridges. It is very user friendly and includes all the structural codes required for design. PRELIMINARY DESIGN Keeping in my mind the primary concerns for design as mentioned in the problem description and the limitations that the site presented, we modelled the structure on SAP2000. The bridge is 150 long with a single support at 50' from the left end. The side with cars is 15 higher than the side with the house (though the floor of the house is 12 higher than where cars are parked. The AASHTO specifies that the width of a pedestrian bridge should be between 8 and 156. Our bridge is 12' wide. The dimensions of our bridge are summarized in the table below.

Table 1: Dimension of the Bridge

DIMENSION MINIMUM MAXIMUM PROPOSED

Length No Limitation No Limitation 150

Width 8 156 12


BRIDGE MODEL The plan and elevation were drafted on AutoCAD and are shown below. As you can see on the model, the left side of the bridge is at a higher level relative to the other side of the bridge where the house is located.

Figure 3: Plan and Elevation of the Bridge

Figure 4: Site Layout MODELLING THE BRIDGE ON SAP2000 The three dimensional model of the bridge was developed on SAP2000. The modeling process follows as well established order of precedence. The various steps are elucidated below.

Grid Definition: The modelling process begins with the determination of an appropriate grid system. The grid definition is a key aspect in 3-D modelling. Accurate referencing of the required geometry results in a more efficient design.

Element Definition: The grid definition is followed by inputting elements that constitute the bridge and frame section assignment. This step only involves a preliminary member sizing.


Application of Loads: This is followed by the application of all necessary loads and the analysis is conducted.

Modification of Member Sizing: Based on the results of the analysis the member sizing is modified to arrive at the lightest possible sections. Element forces and demand ratios were taken from the model to appropriately size the final design.

The 3-D model of the bridge is shown in Figure 4.

Figure 5: 3-D Model of the Bridge

LOADS ON A BRIDGE

Any structure designed and constructed for use by the general public has to withstand dead and live loads at a bare minimum. In addition to these loads, wind is a very important criteria for design. Interestingly, unlike buildings dead load is not as important in the design of bridges. The most critical load in the case of a bridge is an army marching in harmony as it causes resonance through the length of the bridge leading to failure. A real life occurrence of this phenomenon is the Tacoma Bridge in Pierce County, Washington. Our client was well aware of this structural failure and requested us to account for this phenomenon in our design. Also considering that the clients Newfoundland puppies are expected to bound across the bridge to meet her when she drives up, we included the load caused by the dog in our design. MEMBER SIZING

Determining the appropriate member sizing is vital as it controls the weight of the structure. Selecting the lightest structure that fits all requirements is the goal of every structural engineer. The main portion of the structure consists of WT 6 x 36 and WT 8 x 50 sections while the braces are all L4 x 4 x sections.


COST ESTIMATE The cost of construction is one of the driving forces in any engineering project. The unpredictable

fluctuations in construction costs makes proper planning and budgeting essential elements for construction. The total weight of the structure was calculated and based on the cost of material, other costs like labor,

transportation, architect fee, engineer fee and consulting charges were estimated. The total cost of the structure including markups was found to be $1,311,385.31. The excel sheet used for the calculation of the weight of the structure and cost estimation are shown below.

Table 2: Calculation of Weight of the Structure

Table 3: Calculation of Total Cost of the Structure

CONSTRUCTION TIMELINE As mentioned earlier, one of the primary client requirements was a relatively short construction time. Given the unorthodox nature of the project, it was indeed a challenge to optimize construction time. After a number of meetings with the architect, engineer and contractor arrived at an anticipated completion time of 18 months. The elaborate timeline for different components of the project are shown below.

Architectural o Consultation o Design : 3 weeks

Engineering o Preliminary Design : 3 weeks o Shop drawings and RFIs : 3 weeks


Construction o Formwork Construction: 4 weeks o Steel Welding: 5 weeks o Structural Construction: 6 weeks

Overview o Preliminary: 4 weeks o Design and Construction: 12 weeks o Final Inspection

ANALYSIS OF THE BRIDGE As mentioned under the section Loads on a Bridge, dead load, live load, wind load and a moving load

were applied on the model and analyzed.

CASE 1: Dead+ Live+ Wind Table 4: Joint Displacements due to load case 1

The bridge was analyzed for the

load combination dead+ live + wind

load and the results are tabulated

below. It is evident from the table

below that the magnitude of

deflection is very small. The results

obtained are within the bounds of

expectations.

CASE 2: Dead+ Live+ Wind

Table 5: Joint Displacements due to load case 2

The lateral deformation of the

structure was also checked by

applying the wind load. Given the

extent to which the structure is

braced, the magnitude of lateral

deflection is also very small.


APPENDICES


APPENDIX A-Wind Load Vibration

Modal Analysis

Vibrations of a pedestrian bridge are a serious threat to human comfort. The degree of discomfort

varies with different human beings as some people are very sensitive while others can easily adjust to such

unexpected vibrations. A typical human response can be classified into physiological and psychological. When

the frequency of vibration approaches the natural frequency of the internal organs of the human body, the

response is physical. On the other hand, the psychological response is a mental response resulting from

unexpected motion.

AASHTO specifies that the natural frequency of the bridge in the vertical mode has to be a minimum of

3 Hz. As you can see from the results of the modal analysis shown in the table below, our bridge meets this

requirement.

Table 6: Modal Periods and Frequencies


Table 7: Modal Participating Mass Ratios

As this table shows, the first mode will dominate with a participation factor of 85%. The next highest

percentage is the second mode with 1%, meaning it is highly unlikely to occur, though even if it does, the

frequency of this mode is 5 Hz, above the minimum requirement.


APPENDIX B-Newfoundland Puppy Analysis (Moving Load Analysis)

To account for the behavior of the clients dogs, a moving load was modeled in SAP. A large factor of safety

was assumed by modeling this load as one of either a 1 k/ft distributed load over 10 feet or as a concentrated

point load of 2 kips. The load was modeled as crossing from one side to the other and back to ensure the

bridge did not deform too much. As shown in the analyses before, the deflections were incredibly small. This is

due to the rigid nature of the arch design, in conjunction with the bracing provided. The bracing was designed

to prevent the puppies from falling through the sides and to provide vertical stability to the bridge.

Figure 6: Newfoundland Puppy


SUBMITTAL TO THE ARCHITECTURAL CONTROL COMMITTEE

A video was conceived to be presented to the architectural control committee. The link to the video is

given below.

https://www.youtube.com/watch?v=SXLryzIfXwE&feature=youtu.be


BIBLIOGRAPHY

1. AASHTO. (2009). LRFD Guide Specifications for Design of Pedestrian Bridges. New York: AASHTO.

2. http://www.amusingplanet.com/2012/12/circular-pedestrian-bridge-in-lujiazui.html

3. http://7zero-fa.blogspot.com/2011/11/blog-post_2748.html

4. http://www.wpi.edu/Pubs/E-project/Available/E-project-031013-103455/unrestricted/WPI_-

_Pedestrian_Bridge_Study.pdf

5. http://www.structuremag.org/article.aspx?articleid=1148

6. Looney Tunes Thats all Folks Video

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axis_strange bridge project report(gp 6).pdf

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