2024
Design, Construction, and Evaluation of a
Truss Bridge Model: Structural Efficiency
and Embodied Carbon Analysis
The Civil Engineering Project Report
Group Truss Annihilator
Xichen Luo (2665907)
Xinye Xie (2665942)
Yitong Lin (2665936)
Junhao Pan (2665923)
Songyu Yang (2665952)
Xuting Li (2665956)
Date: 17/12/2024
Contents
1.Introduction ............................................................................................................ 3
1.1. Background ................................................................................................... 3
1.2. Overview....................................................................................................... 3
2. Embodied carbon analysis ...................................................................................... 4
2.1. Embodied carbon ........................................................................................... 4
2.2. Symbols ........................................................................................................ 4
2.3. Make the assumption of the steel specification ................................................... 4
2.4. Calculation .................................................................................................... 4
3.Model demonstration............................................................................................... 5
3.1. Design concept ............................................................................................... 5
3.2. The design of the model bridge ........................................................................ 6
3.3. Construction process ....................................................................................... 7
3.4. Testing procedure ........................................................................................... 8
4.Comments ............................................................................................................. 10
4.1. Advantages .................................................................................................. 10
4.2. Disadvantages .............................................................................................. 10
5.Suggestions for improvement ................................................................................. 10
6.Conclusion ............................................................................................................ 11
7.References............................................................................................................. 11
1.Introduction
1.1. Background
To Improve Dundee’s transportation development, a new truss bridge is planned to be built
across the River Tay in Dundee, according to the city administration committee. This initiative
is designed to ease traffic congestion and improve the quality of life of residents. Truss bridge
is a bridge with its load-bearing structures composed of a series of wooden or metal triangles,
known as trusses. Given that a triangle cannot be distorted by stress, a truss gives a stable form
capable of supporting considerable external loads over a large span. [1]
1.2. Overview
The purpose of this report is to present to the Dundee Municipal Administrative Committee our
design concepts, technical solutions and practical results. The work we have done includes in
Figure 1:
1. Designing and making a model. Our team used CAD software to complete the conceptual
design of the truss bridge and then made a model.
2. Calculating the embodied carbon. Our team used real-world dimensions and steel as the
assumed material to calculate the embodied carbon.
3. Model testing. Our team carry out model testing to verify the performance of the design.
4.Comments and Suggestions for improvement.
5.Conclusion.
Figure 1. The Process of the Project
2. Embodied carbon analysis
2.1. Embodied carbon
Embodied carbon is the total account of the greenhouse gas emissions associated with the
production stages of a product’s life.[2] To meet greenhouse gas (GHG) emission targets at
global, national, and sector levels, it is imperative to explore reduction opportunities in both the
embodied and operational carbon of the built environment.[3] Given the UK's commitment to
achieving an 80% reduction in GHG emissions relative to 1990 levels by 2050, reducing
embodied emissions represents a strategic approach to meet the goal.[3]
2.2. Symbols
The Table1 introduces the significant symbols and simple description in this report.
Table 1: Significant symbols in this report
Symbols
Description
Unit
𝑴ⅈ
ρ
𝒍ⅈ
𝑬𝒄ⅈ
Weight of the bridge members
Steels of linear density
Length of the bridge members
Embodied carbon of the bridge
kg
kg/m
m
kgCO₂e
2.3. Make the assumption of the steel specification
The actual material is assumed to be steel. Ignoring the differences in steel used in actual
construction, it is assumed in this project that all steels have the same linear density (37.0 kg/m)
Table 2: Embodied carbon factors of bridge martial
Bridge member
Type
ECF (kgCO₂e /kg)
Top and Bottom Chords
Vertical Members
Girders
Bridge Deck
Structural sections
Structural sections
Structural sections
Reinforcement bars, Plate
2.45
2.45
2.45
0.76, 2.45
2.4. Calculation
The total weight of the bridge is calculated with the following formula:
𝑴ⅈ = 𝝆 ∗ 𝒍ⅈ
∑ 𝑴ⅈ = 𝟐𝟏𝟏𝟏. 𝟔𝟎 𝒌𝒈
The total embodied carbon of the bridge is calculated with the following formula:
𝑬𝒄ⅈ = 𝑴ⅈ ∗ 𝑬𝑪𝑭
∑ 𝑬𝒄ⅈ = 𝟒𝟖𝟔𝟗𝟏. 𝟐𝟔 𝒌𝒈𝑪𝑶₂𝒆
3.Model demonstration
Figure 2. Model Demonstration
3.1. Design concept
The bridge is designed with Pratt truss. This structure is suitable for situations where loads are
concentrated at the bottom, such as bridge decks, and efficiently manages vertical forces. The
Pratt Truss design of slanted diagonals resist tension forces, suitable for handling tensile
stresses in the structure. The vertical members resist compression forces, providing stability
and preventing buckling.
The bottom of the bridge is provided with beams and bracings, which increase the stability of
the structure through the mechanical characteristics of the triangle, form a strength support
structure, and ensure the bearing capacity of the bridge.
3.2. The design of the model bridge
The bridge model design is illustrated in Figure 3.
Figure 3. The bridge model design
The bridge model design is illustrated below, comprising a front view in Figure 4, a side view
in Figure 5, and a top view in Figure 6.
Figure 4. Front view
Figure 5. Side view
Figure 6. Top view
3.3. Construction process
The construction of the truss bridge model was carried out in four steps, following the
specified design and utilizing the materials provided, which included balsa sticks, glues, rules
and so on. Below is a detailed outline of the construction process:
1. Planning and design verification
Before proceeding with the model construction, the design was reviewed and finalized in
AutoCAD, ensuring that all dimensions were accurate. This step included calculating the total
length, width, and height of the bridge, and ensuring the truss structure is correct according to
the Pratt truss.
2. Material preparation
The balsa sticks were cut into required lengths based on the dimensions derived from the
design.
Glues were used to combine balsa sticks.
Rulers were used to measure and mark accurate dimensions for each balsa stick.
Knives were used to cut the balsa sticks into the truss elements: vertical members, diagonal
members, and horizontal members.
Abrasive papers were used to smooth out rough edges after cutting.
3. Assembling the truss bridge model
The truss bridge model was assembled according to the final design. The assembly process
followed these steps:
First, position the horizontal bottom beams to establish the basic framework.
Add crossbeams and diagonal braces (diagonal members) to the bridge deck section.
Glue the top and bottom chords and vertical members into their designated positions to form the
basic structure of the Pratt truss, and the angles between the members should be approximately
45°.
Once the truss framework is fully assembled, additional crossbeams should be applied to
connect the top surface of the bridge.
4. Final checks
After the bridge structure was assembled, a final inspection was conducted to ensure: All
members were securely glued and aligned according to the design; there were no gaps or
misalignments that could weaken the structure; the dimensions of the assembled model were
consistent with the initial design calculations.
3.4. Testing procedure
The testing process aimed to evaluate the structural efficiency and failure point of the truss
bridge model under loading conditions. The steps are outlined in Figure 7.
Figure 7. Testing Procedure
1. Measuring Dimensions and Weight of the Model
Before testing, the final dimensions and weight of the bridge model were measured.
2. Prepare for the load
Once the dimensions and weight of the bridge had been recorded, a loading plate was placed
centrally on the deck of the bridge to ensure uniform distribution of the applied load. The
plate was placed on the bridge deck, aligned with the central axis of the truss to prevent offcenter loading that could introduce additional stress concentrations.
3. Adding weight until failure
The following steps were performed:
Sand was gradually added to the loading metal bucket. The weight was added in small
increments to ensure the stress on the bridge increased steadily.
Each time a weight increment was added, the bridge deflection (any visible bending or
warping of the structure) was carefully observed. Notice any changes in the height or
alignment of the bridge, especially at key stress points such as the joints and diagonal
members.
The process continued until the failure point was reached, defined as the moment when the
bridge structure could no longer support the applied load.
Failure could occur in the complete collapse of the truss (e.g., the breaking of a balsa wood
member) or structural deformation such that the bridge could no longer bear the load.
At the failure point, the maximum load the bridge could sustain was recorded by a tension
balance. This value was used to calculate the structural efficiency of the bridge, by dividing
the maximum load by the weight of the bridge.
4. Final evaluation
Once the failure point was reached, the results were summarized and analyzed:
•The maximum load sustained by the bridge and the corresponding failure point were
recorded.
•The failure modes were evaluated to understand the weaknesses of design.
•The structural efficiency was calculated, and the findings were discussed in the context of
design improvements and the overall performance of the bridge.
5. Test results:
Table 3: The Data of Results
Maximum load
Bridge weight
18415 grams
105.9 grams
The structural efficiency is calculated with the following formula:
𝑺𝒕𝒓𝒖𝒄𝒕𝒖𝒓𝒂𝒍 𝑬𝒇𝒇ⅈ𝒄ⅈ𝒆𝒏𝒄𝒚 = 𝑴𝒂𝒙ⅈ𝒎𝒖𝒎 𝒍𝒐𝒂𝒅 / 𝑺𝒕𝒓𝒖𝒄𝒕𝒖𝒓𝒆 𝒘𝒆ⅈ𝒈𝒉𝒕
𝑺𝒕𝒓𝒖𝒄𝒕𝒖𝒓𝒂𝒍 𝑬𝒇𝒇ⅈ𝒄ⅈ𝒆𝒏𝒄𝒚 = 𝟏𝟕𝟑. 𝟖𝟗
4.Comments
4.1. Advantages
The bridge structure demonstrates symmetry, ensuring that loads are evenly distributed across
all components. This design not only enhances structural stability but also contributes to its
aesthetic appeal.
The adoption of the Pratt truss design provides significant structural stability, as loads are
evenly transferred to each member through its stable triangular configuration. This allows the
bridge to effectively resist deformation and damage under external forces.
Additionally, the design maximizes the mechanical properties of the materials used, with the
inclined members utilizing the material's tensile strength while the vertical members take
advantage of its compressive strength.
4.2. Disadvantages
Despite its strengths, the single-layer truss structure may also prove insufficient for supporting
excessive loads, particularly under concentrated forces. During model testing, only one side of
the bridge failed, indicating an uneven distribution of loads.
Furthermore, the insufficient strength of the bottom horizontal members resulted in excessive
force concentration, compromising performance.
Stress concentration at the nodes was also observed, likely due to inadequate connection
strength.
5.Suggestions for improvement
To address these disadvantages, incorporating a second layer of trusses beneath the bridge
would improve its ability to withstand deck loads, increasing load-bearing capacity and
structural resilience.
Simplifying the structure could further improve maintainability and disassembly while
reducing embodied carbon, as suggested by Pomponi, De Wolf, and Moncaster.[4]
To enhance performance, thicker materials should be used for vertical members to better resist
compressive forces and minimize the risk of buckling during testing.
Strengthening the node connections would mitigate stress concentrations, and reinforcing the
bottom horizontal members would ensure they can withstand the applied forces more
effectively.
6.Conclusion
The project demonstrated the design and construction of a Pratt Truss bridge model, achieving
a structural efficiency of 173.89 and an embodied carbon value of 48691.26 kgCO₂e. The
experience highlighted the importance of teamwork, as effective collaboration helped
overcome design and construction challenges. Testing provided critical insights into load
distribution and structural behaviours, identifying areas for refinement. Recommendations
include reinforcing vertical members and adding lateral bracings to enhance stability. Overall,
the project showcased the functionality and potential of the Pratt truss design, offering valuable
lessons for continued improvement.
7.References
[1] “Truss bridge,” Britannica. Accessed: Dec. 17, 2024. [Online].
Available: https://www.britannica.com/technology/truss-bridge#ref1.
[2] U.S. Environmental Protection Agency, "What is embodied carbon?" EPA, Dec. 2024.
[Online]. Available: https://www.epa.gov/greenerproducts/what-embodied-carbon. [Accessed:
Dec. 17, 2024].
[3] D. Densley Tingley, S. Cooper, and J. Cullen, “Understanding and overcoming the barriers
to structural steel reuse, a UK perspective,” Journal of cleaner production, vol. 148, pp. 642–
652, 2017, doi: 10.1016/j.jclepro.2017.02.006.
[4] F. Pomponi, C. De Wolf, and A. Moncaster, Embodied Carbon in Buildings: Measurement,
Management, and Mitigation. Springer eBooks, 2018.