project 1- masters thesis - bridge systems
TRANSCRIPT
Development Of Alternative Composite Concrete Bridge Systems For Short And Medium Span Bridges
By Dinesha Kuruppuarachchi (B.S.)
COLLEGE OF ENGINEERING AND SCIENCELOUISIANA TECH UNIVERSITY
July 2016
ABSTRACT• A total of four new bridge systems for short and medium span bridges are presented.
• These bridge systems are lightweight, efficient in flexure and shear and can be used in sites with stringent vertical clearance requirements while being able to accelerate construction by eliminating the need for site installed formwork.
• The proposed configurations are compared with traditional adjacent box beam • and decked bulb tee systems for spans that range from 80 ft to 150 ft.
• Both normal weight and lightweight concrete options are investigated.
• The comparison is made in terms of span to depth ratios, weight, number of strands, live load deflection and camber.
• It is demonstrated the two proposed systems (PS1 and PS2) that feature concrete topping are lighter than the adjacent box beam system for all spans considered. Additionally, PS2 requires fewer strands. Both proposed topped systems feature lower camber when compared to the adjacent box beam system. PS2 provides shallower superstructure depths for a given span compared to the adjacent box beam system.
Topped system means that the bridge uses a deck.Un-Topped system means that the bridge do not use a deck.
Proposed bridge systems
INTRODUCTION
Proposed bridge systems (Cross section of the bridge)
Traditional bridge systems
Traditional bridges1. Box beams• Positive - Strength, stiffness• Negative - failure of connections leads to
reflective cracking
2. Decked bulb tees• Positive – strength, stiffness• Caution – Need to be properly braced
before the installation
Introduction Methodology Results Conclusion
Traditional bridge systems
Objective
The goal of this project is to develop alternative composite concrete bridge systems for short to medium-span bridges, which are lightweight, efficient in flexure and shear and can be used in sites with stringent vertical clearance requirements while being able to accelerate construction by eliminating the need for site installed formwork
Methodology• Description od the bridge systems
• Live load distribution factors
• Validation
• Longitudinal connections
Figure 2.1 Proposed Bridge Systems
Description of the bridge systems
Dimensions of the traditional bridge systems (box beams and Decked bulb tees)
3D image of proposed systems
Dimensions of proposed bridge systems
• 2 different unit weights• 3 lanes• 48ft width• Different compressive strengths
General notes about the bridge
Description of the bridge systems
• Eg: Box bridge systems– 80ft span length : 33in in depth– 100ft span length : 39in in depth– 120ft span length : 42 in depth
And so the PS1 and PS2 used the same depth as Box bridge system.
The superstructure depth for the proposed systems is kept the same so that a comparison could be made in terms of weight, number of strands, live load deflection and camber.
• Eg: Decked bulb tee bridge systems– 80ft span length : 35in in depth– 100ft span length : 42in in depth– 120ft span length : 53 in depth– 150ft span length : 65in in depth
And so the PS3 and PS4 used the same depth as Decked bulb tee bridge system.
Description of the bridge systems
• Eg: Box bridge systems– 80ft span length : 33in in depth– 100ft span length : 39in in depth– 120ft span length : 42 in depth
And so the PS1 and PS2 used the same depth as Box bridge system.
The superstructure depth for the proposed systems is kept the same so that a comparison could be made in terms of weight, number of strands, live load deflection and camber.
• Eg: Decked bulb tee bridge systems– 80ft span length : 35in in depth– 100ft span length : 42in in depth– 120ft span length : 53 in depth– 150ft span length : 65in in depth
And so the PS3 and PS4 used the same depth as Decked bulb tee bridge system.
Description of the bridge systems
Live load Distribution factors
AASHTO (American Association of State Highway and Transportation Officials)code already provide information on how to find live load distribution factors (LLDFs) for traditional systems
Live load distribution factor is a quantitative value which indicates the percentage of live load that each girder can carry due to a wheel load.
But for the proposed systems we do not know how to get the live load distribution factors. But we can use an equation to find LLDFs.
Deflections in the mid-span of the bridge
Live load distribution factors for moment
Reactions at the edge of the bridge
One way to find deflections and a reaction forces of a bridge is by using finite element analysis. So we used this method. We use abaqus software for the analysis.
Live load distribution factors for shear
Live load distribution factors for moment
Finite element analysis
• Draw the model• Partition (for the loads)• Material properties (elastic modules, poison ratio) • Boundary conditions (pin and roller )• Loads (in terms of tire pressure)• Tie connections• Mesh (6in mesh)
Tie connections
6in Mesh
Loading positions
We used an AASHTO Truck and a tandem truck to explore the critical loading condition
AASHTO truck
AASHTO tandem loading
1 2
3 4
5
Loading positions
Light weight (0.12 kip/ft) , and normal weight (0.15 kip/ft) options were also considered. With the unit weight change the elastic modules of the material will change . So, that would impact for the model in abaqus.
6
7
Loading positions
Loads are placed at the edge of the bridge to get the maximum reaction force. That will help to get the maximum shear force.
All proposed and traditional systems are designed based on AASHTO LFRD Specifications using Mathcad.
The number of strands obtained from Mathcad calculations for the traditional bridge systems are compared with that obtained from the PCI Bridge Design Manual and Conspan software to validate the approach.
Validation
• Flexural stress at transfer at the mid span, at the edge• Flexural stress at service at the mid span, at the edge• Flexural stress at strength• Shear Strength• Deflection checks
Flexure is more critical in these kind of bridges than shear.
The next step is to determine how much shallower could the superstructure depth be for the traditional systems if LLDFs for moment computed from FEA are used instead of those calculated based on AASHTO LFRD Specifications
Once the shallowest superstructure depth for the traditional systems is obtained, the proposed systems are designed to maintain the same depth and a comparison in terms of weight and the number of strands required is performed.
Design Process
Perfectly bonded connections- due to the large contact surface between deck and precast components.
12in deep shear keys for the box beams using the tie constraints.
The connection between the flanges of adjacent decked bulb tees is simulated using a tie constraint for the full depth of the flange.
Longitudinal connections
Both are discrete connections spaced at 4 ft on center . The dimensions of this pocket are 6 in. in the transverse direction, 3 in. deep and 12 in. in the longitudinal direction.
The tension force in the longitudinal connections is calculated by recording the transverse normal stress in the 3D solid elements in the bottom precast flange and by multiplying it with the area of the elements that are part of the transverse connection.
Longitudinal connections in PS 1 and PS2
The transverse bending moment per one foot of length is calculated by recording the transverse compression and tensile forces in the top and bottom finite elements and multiplying them with the moment arm
Longitudinal connections in PS 3 and PS4
Results
To be able to perform a consistent comparison between the proposed systems and the traditional systems, live load distribution factors (LLDF) for moment for all systems are computed using finite element analyses
The traditional adjacent box beam system featured the lowest computed LLDFs compared to PS1 and PS2.The computed LLDFs for the decked bulb tee system are also lower than those calculated using AASHTO provisions.
Live load distribution factors for moments
Live load distribution factors
The computed LLDFs for shear are less than half of those calculated based on AASHTO provisions.
Therefore, if a more economical shear design is pursued, the computed LLDFs for shear presented in the report may be used in lieu of those calculated based on AASHTO provisions.
There is not a significant difference between the computed LLDFs for shear in the proposed systems and traditional systems.
Live load distribution factors for shear
Live load distribution factors
LLDFs for the normal weight and lightweight concrete options are similar.
When lightweight concrete is used the beams deflected more compared to the normal weight option, however this resulted in similar LLDFs for moment.
Superstructures with and without barriers are analyzed and it is found that when the barrier is omitted LLDFs for moment are higher
From the investigated load positions the ones that featured edge loading resulted in higher LLDFs.
It is determined that the case that featured a two truck loading configuration controlled over the other cases.
Other observations about Live load distribution factors
Mesh size Validation
A mesh sensitivity analysis is performed to determine whether the computed LLDFs are influenced by the size of the finite elements. This exercise is done for the 80 ft span decked bulb tee system. Four different mesh sizes are considered, 3 in., 4.5 in., 6 in., and 7.5 in.
Figure demonstrates that the mesh size did not make a difference in terms of LLDFs.
All bridge systems are designed using AASHTO provisions using Mathcad . The results obtained from Mathcad in terms of number of strands for the traditional systems are compared with those obtained from the PCI Bridge Design Manual and Conspan.
During this comparison the LLDFs are based on AASHTO provisions.
This results suggests that the design calculations used in Mathcad lead to reliable results
Validation
Topped systems
• Strand configurations for both light weight and normal weight box beams,
• The strands in the box beam and PS2 are harped due to their natural shape. PS1, however, cannot used harped strands because of its tapered web. It is more economical when the strands can be harped because they can control both mid-span stresses and other stresses at the end.
• Almost always, the controlling parameters are tensile stress at service, at mid-span.
• Light weight bridges always used less strands than normal weight bridges.
Light weight bridges uses less strands
Strand configurations for both light weight and normal weight box beams
Topped systems
PS1 cannot use harped strands. So the strand pattern in the mid-span is also same as the strand pattern at the edge
Strand configurations for both light weight and normal weight PS1 beams
Topped systems
Strand configurations for both light weight and normal weight PS2 beams
Light weight bridges uses less strands
PS2 can use harped strands. So the strand pattern in mid-span is not same as the strand pattern at the edge
Topped systems
Almost always, the controlling parameters are tensile stress at service, at mid-span.
Topped systems
Box beams, PS1 and PS2 use a deck. Without the deck it is called as non-composite section. With the deck it is a composite section.
Both normal and light weight options considered in here.
Weight wise PS1 and PS2 are better than Box beams.
Box beams are originally has a width of 4ft but all the proposed systems has a beam width of 6 ft. So the box beams 4ft was converted to 6ft width.
Material use - weight
Topped systems
Material use - strands
Strands wise PS2 is better than box beams
Topped systems
This slide shows a summary of the material use in box beam , PS1 and PS2 bridges in both normal and light weight options
Topped systems
Live load deflections and Camber
Live load deflections are lower for BOX beams, which is a plus point to box beams.
But the camber is lower in PS2 beams which is a plus point for PS2 beams.
Topped systems
Shallowest depthTopped systems
Un-topped systems
Un-topped systems
Strand configurations for both light weight and normal weight PS3 beams
Un-topped systemsStrand configurations for both light weight and normal weight PS4 beams
Un-topped systems
Almost always, the controlling parameters are tensile stress at service, at mid-span.
Un-topped systems
Material use - weight
Material use - strands
Un-topped systems
This slide shows a summary of the material use in DBT beam , PS3 and PS4 bridges in both normal and light weight options
Un-topped systems
Live load deflections and Camber
Ps4 has less live load deflections and camber, which means it is stiffer compared to DBT.
Un-topped systems
Shallowest DBT can get for 80ft is 32in and it uses 22 strands. PS4 can obtain that using 19 strands.
Transverse bending moments
ConclusionA total of four composite concrete bridge systems are developed for short and medium span bridges with spans ranging from 80 ft to 150 ft.
These bridge systems are lightweight, efficient in flexure and shear and can be used in sites with stringent vertical clearance requirements while being able to accelerate construction.
The developed systems consist of adjacent hollow precast concrete beams with and without concrete topping.
The comparison is made in terms of span to depth ratios, weight, number of strands, live load deflection and camber
PS2 and PS4 appear to be more competitive than Box, DBT, PS1 and PS3.
DBT needs more strong transverse connections
Questions
Thank you