RIICWD601E Manage Civil Works Design Processes Bridge Assignment Sample

Bridge Truss Design Calculations and Cost Risk Analysis

1. Constraints And Requirements

There are constraints and requirements that the bridge design must take according to safety, functionality and regulations. The bridge can't structurally exceed a maximum elevation above the high water level of 24 m, and must allow for enough clearance under the overhead power lines and so be a maximum height of 32.5 m. According to the rules, the deck has to be 10 meters wide to have enough space for two lanes of traffic, and transverse beams should not be spaced further than 4 meters apart (Kempton et al. 2021). The truss members are limited to steel and the deck to concrete, with deck thickness being determined by the selected grade of concrete (23 cm for medium strength versus 15 cm for high strength). The bridge must meet load and safety constraints, including supporting two vehicle loading scenarios: The H25 truckload and 480 kN permit load.

2. Functional Specifications

The key parameters for the design of the bridge are described in the functional specifications. The Warren truss configuration was chosen for optimal load distribution and bending moments resistance with verticals. An optimized truss depth of 5m is found such that there is a balance between structural efficiency and material use. Truss members are fabricated utilizing high strength steel to achieve durability and strength, and medium-strength concrete used for the deck to keep first cost low without substantial degradation to performance (Nguyen et al. 2020). The truss members are calculated to take on axial forces and maximum bending moments, while the bridge deck is designed to sustain a maximum uniformly distributed load of 89,953 N/m. Lateral bracing is designed and other provisions are included for inspection and maintenance.

3. Design Options 1: Warren Truss

Figure 1: Design of the Warren Truss

(Source: Self-created in AutoCAD)

The calculation has been done per AS 1170, AS 4100, and AS 5100 standards.

Bridge Parameters

As per the approach

Span is considered 44 meters

Width= 10 meters

Panel length (based on 4m spacing)= 4 meters

Number of panels= 11

Truss height= 4.4 meters (typical 1/10 of span)

Deck thickness= 0.15m (using high-strength concrete)

Load Calculation

Dead Load

For concrete deck= Volume of the panel=10m*4m*0.15=6m³

Weight of the panel=6*2400kg/m³=14400 kg

Hence load per panel= 141.12 kn

For steel members= Main truss members= 35 kn per panel

Floor beam= 12.0 kn/ joint(given)

For Future surface layer

5cm thick layer

Volume per panel = 10m * 4m * 0.05m = 2 m³

Weight = 2 * 2400 kg/m³ = 4,800 kg

Load = 47.04 kN per panel

Hence Total Dead load per panel=235.16 kn

Live Load Calcualtion

H25 Truck Loading (Yahya et al. 2020).

Axle loads: 178 kN (rear), 71 kN (front)

Dynamic load allowance (DLA) = 0.3

Design wheel load = 178 * (1 + 0.3) = 231.4 kN

Permit loading

Total load = 480 kN

With DLA (0.3) = 480 * (1 + 0.3) = 624 kN

Load per truss = 312 kN

Calcualtion Of Shear Force

Due to dead load

Vmax = (Total dead load*span)/2

Vmax = (235.16*11)/2 = 1,293.38 kN

Due to live load (controlling case - Permit load)

Vmax = 312 kN (when load at support)

Maximum Bending Moment

Dead load

Mmax = (w*L²)/8

Mmax = (235.16*44²)/8 = 14,227.18 kN⋅m

Live load (Permit):

M_max = (624*44)/4 = 6,864 kN⋅m

Analysis Of The Diagonal Members

Diagonal length = √(4.0² + 4.4²) = 5.94m

Angle with horizontal (θ) = tan⁻¹(4.4/4.0) = 47.7°

Maximum forces calculation (Huang et al. 2021).

Max sf=1,293.38 kN

Diagonal forces would be Vmax*secθ=1,293.38*sec (47.7)= 1,746.063 kn

Using high-strength steel

Material: High-strength steel (fy = 450 MPa)

Design factor (φ)=0.9

Effective length factor (K) = 1.0

Required strength=1,746.063 kN

Member sizing would be based on the required area

For tension members

Areq = P/(φ * fy)

Areq = 1746.063*10³/(0.9* 450)

Areq = 4311.26 mm²

The buckling condition (Kaewunruen et al. 2022).

Effective length = K*L = 1.0*5,940 mm

Maximum slenderness ratio = 200

Required radius of gyration = 5,940/200 = 29.7 mm

Selected section is square hollow section of Size: 250mm*250mm*12mm

The compression capacity of the section is

λ = KL/r = (5,940/98.2) = 60.5 < 200 (Satisfied)

Compression capacity = φN_c = 2,484 kN > 2,167.26 kN(Satisfied)

Tension capacity

φN_t = 0.9 × 11,500 × 450 = 4,657.5 kN > 2,167.26 kN (Satisfied)

Local buckling

b/t ratio = 250/12 = 20.8 < 35 (Satisfied)

Hence the Section is compact.

Minimum weld size 8 mm

Minimum Bolt diameter M24

Minimum
End distance 50 mm

Deflection check

Allowable 44000/800= 55 mm acceptable

With this design there is an approach of considering of the bridge with a Warren truss bridge

Cost Analysis

Category	Item	Quantity	Unit	Rate (Aud/unit)	Total Cost (Aud)
Material Cost	High-strength steel tubes (Chords)	15,892.80	kg	7	111,249.60
High-strength steel tubes (Diagonals)	10,727.64	kg	7	75,093.48
Carbon steel tubes (Floor beams)	8,500	kg	6.3	53,550.00
Subtotal Material	239,893.08
Connection Cost	Joints	42	joints	500	21,000.00
Product Cost	Products	15	units	1,000.00	15,000.00
Site Cost	High-strength concrete deck	11	panels	5,300.00	58,300.00
Excavation	150	m³	1	150
Abutments	2	units	6,000.00	12,000.00
Subtotal Site	70,450.00
Total Cost	346,343.08

8. Risk Analysis

Risk ID	Risk Description	Probability (P)	Impact (I)	Risk Score (P×I)	Mitigation Measures	Responsible Person
WR1	Diagonal Member Buckling	0.4 (Medium)	5 (Very High)	20	Enhanced member sizing, regular structural inspection, additional bracing (Mohammadi et al. 2023).	Structural Engineer
WR2	Joint Stress Concentration	0.6 (High)	4 (High)	24	Reinforced connections, stress distribution plates, regular joint monitoring	Design Engineer
WR3	Fatigue at Alternating Stress Points	0.4 (Medium)	4 (High)	16	Fatigue-resistant details, regular NDT testing, stress monitoring	QC Engineer
WR4	Uneven Load Distribution	0.4 (Medium)	3 (Medium)	12	Load balancing design, monitoring system, regular load testing	Bridge Engineer
WR5	Complex Fabrication Errors	0.6 (High)	3 (Medium)	18	Detailed shop drawings, enhanced QC, expert fabrication team (Sun et al. 2020).	Fabrication Manager
WR6	Health and safety: Free falling, burning or trauma during the execution of the work	0.6(High)	4(High)	24	Proper Usage of the safety planning and using PPE kits during the work (Sun et al. 2021).	Safety manager

4. Design Option 2: Pratt Truss

Figure 2: Design of the Pratt Truss

(Source: Self-created in AutoCAD)

Bridge Parameters

As per the approach

Span is considered 44 meters

Width= 10 meters

Panel length (based on 4m spacing)= 4 meters

Number of panels= 11

Truss height= 4.4 meters (typical 1/10 of span)

Deck thickness= 0.15m (using high-strength concrete)

Load Analysis

Dead load

Concrete deck

Volume per panel = 10m * 4m * 0.15m = 6 m³

Weight per panel = 14,400 kg

Load per panel = 141.12 kN

Steel members

Main truss members: 32 kN per panel (slightly less than Warren due to more efficient diagonal arrangement

Floor beams: 12.0 kN per joint

Future surface:

5cm thick layer (Kempton et al. 2021).

Load = 47.04 kN per panel

Total Dead load in panel 232.16 kn

Live Load

H25 Truck Loading with DLA = 231.4 kN

Permit Loading with DLA = 624 kN (312 kN per truss)

Diagonal Members Analysis

Geometric properties

Panel length (horizontal) = 4.0m

Truss height = 4.4m

Diagonal length = √(4.0² + 4.4²) = 5.94m

Angle with horizontal (θ) = tan⁻¹(4.4/4.0) = 47.7°

Force analysis

Maximum shear at support = 1,850 kN

Length = 5.94m

Design force = 1,850 * sec(47.7°) = 2,497.5 kN (tension)

Maximum shear near midspan = 425 kN

Design force = 425 * sec(47.7°) = 573.8 kN (compression)

Member sizing of end diagonal

Using high-strength steel (fy = 450 MPa):

Required area = 2,497.5*10³/(0.9*450)

Areq = 6,160 mm²

Selected section: SHS 250mm*250mm*12mm

Sizing of the Middle diagonal

Required area (with buckling) = 573.8*10³/(0.9*450*0.85)

Areq = 1,670 mm²

Selected section: SHS 150mm*150mm*8mm

Verification of the tension capacity of end diagonals

φNt = 0.9 × 11,500 × 450 = 4,657.5 kN > 2,497.5 kN Hence ok

Connection capacity:

Minimum bolt requirement: 6*M24 (8.8/TF)

Weld size: 10mm continuous fillet (Lazovic et al. 2022).

Verification of the tension capacity of middle diagonals

Slenderness check:

λ = KL/r = (5,940/58.5) = 101.5 < 200 Hence ok

Compression capacity:

φN_c = 1,250 kN > 573.8 kN Hence ok

Local buckling check:

b/t ratio = 150/8 = 18.75 < 35 Hence ok

Cost Analysis

Category	Item	Quantity	Unit	Rate (Aud/unit)	Total Cost (Aud)
Material Cost	High-strength steel tubes (Chords)	15,120	kg	7	105,840.00
High-strength steel tubes (End Diagonals)	3,218.40	kg	7	22,528.80
High-strength steel tubes (Middle Diagonals)	855.2	kg	7	5,986.40
Carbon steel tubes (Floor beams)	8,250	kg	6.3	51,975.00
Subtotal Material	186,330.20
Connection Cost	Joints	49	joints	500	24,500.00
Product Cost	Products	15	units	1,000.00	15,000.00
Site Cost	High-strength concrete deck	11	panels	5,300.00	58,300.00
Excavation	150	m³	1	150
Abutments	2	units	6,000.00	12,000.00
Subtotal Site	70,450.00
Total Cost	306,280.20

Risk Analysis

Risk ID	Risk Description	Probability (P)	Impact (I)	Risk Score (P×I)	Mitigation Measures	Responsible Person
PR1	Tension Member Failure	0.2 (Low)	5 (Very High)	10	Enhanced connection design, regular tension testing, strain monitoring	Structural Engineer
PR2	Vertical Member Instability	0.4 (Medium)	4 (High)	16	Additional lateral bracing, regular alignment checks, stability monitoring	Design Engineer
PR3	End Panel Overstress	0.6 (High)	3 (Medium)	18	Reinforced end panels, load distribution plates, regular inspection	QC Engineer
PR4	Secondary Stress in Long Members	0.4 (Medium)	3 (Medium)	12	Temperature consideration, expansion joints, deformation monitoring	Bridge Engineer
PR5	Web Member Connection Issues	0.2 (Low)	4 (High)	8	Specialized connection design, expert installation, regular maintenance	Construction Manager
PR6	Health and safety: Free falling, burning or trauma during the execution of the work	0.6(High)	4 (High)	24	Proper Usage of the safety planning and using PPE kits during the work.	Safety manager

Conclusion

This civil works design assignment demonstrates the application of structured design processes in accordance with RIICWD601E Manage Civil Works Design Processes and relevant Australian Standards. By evaluating both Warren and Pratt truss bridge options, the study addresses regulatory constraints, functional requirements, load actions, member design, cost efficiency, and risk management. Detailed calculations confirm structural adequacy under dead and live loading scenarios, while comparative cost analysis highlights the economic advantages of the Pratt truss configuration. The inclusion of risk assessment further strengthens design reliability and constructability considerations. Overall, the assignment reflects sound engineering judgement, compliance-driven decision-making, and practical design outcomes suitable for real-world bridge infrastructure projects.