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