How to design for Bridge: Structural and Dynamic Load Calculation

Bridge load calculations form the core of civil engineering; ensuring the ends of the structure to be both safe and functional, it calls for proper know-how on the types of loads that a bridge is subjected to. The present article reveals key concepts on structural and dynamic loads, making use of formulas and to illustrate the process with an example load calculation problem. We simplify these concepts through breaking down terms to the simplest level so that the information may reach the desk of civil engineers and students alike.

Bridge Load Calculation: Structural and Dynamic Loads

1. Bridge Load Calculation
Bridge load calculation is very important for establishing the forces through which the bridge structure will pass at all stages of its lifetime. Loads can be broadly categorized into two types:

• Structural Loads (Static Loads): Loads that do not change with time. Example: dead load of the bridge.
• Dynamic Loads: Loads that change with time. Such loads include traffic, wind, and seismic forces.
Knowledge and proper calculation of these loads ensure the lifespan and safety of the bridge.

 

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2. Components of Bridge Loads
To dive into the calculation aspect, it is essential to know the kind of loads that occur on a bridge:

Structural Loads (Static Loads): • Dead Load (DL): The permanent load due to the dead weight of the bridge structure, including beams, deck, pavement, or permanent installations.
• Superimposed Dead Load: These are other permanent loads, such as barriers, sidewalks, utilities, etc., over the bridge.

Dynamic Loads:
• Live Load (LL): These include the weight of vehicles, pedestrians, etc. These are time-varying loads. The magnitude of the live load varies with traffic intensity.
• Wind Load (WL): Wind flows laterally over a bridge, especially over long bridges.
• Impact Load: This is a shock load due to moving objects as a result of dynamic effects.
• Seismic Load: The type of load due to earthquakes or any kind of vibratory motion of the ground.
• Thermal Load: Thermal expansions and contractions in the bridge materials due to temperature variation.

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3. Structural Load Calculations
There are two parameters to be defined at the onset of the process for calculating the load of a bridge, the dead load (DL) and live load (LL).

Dead Load Calculation (DL):
Dead Load = Weight of the bridge deck + Weight of beams + Weight of pavement

For a bridge whose: • Deck width = 12 meters
• Deck thickness = 0.25 meters
• Density of concrete = 2400 kg/m³

To calculate the dead load: • Volume of the deck = Length * Width * Thickness = L * 12 * 0.25 m³
• Weight of the deck = Volume * Density of concrete

Assume the length of the bridge is 30 m:

• Volume = 30 × 12 × 0.25 = 90 m³

• Weight of the deck = 90 × 2400 = 216,000 kg = 216 metric tons

For this example, we consider only the deck. If the static loads are accommodated by beams and pavement, then the overall dead load will increase.

Live Load (LL) Calculation:
Live Load usually is calculated in respect of the expected traffic flow. For example, an automobile bridge with the design live load of 10 tons per lane: • For a two-lane bridge, the Total Live Load = 10 tons * 2 lanes = 20 tons

The total load that acts upon the structure, dead and live load together is:

• Total Load (T) = Dead Load (DL) + Live Load (LL) = 216 tons + 20 tons = 236 tons.

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4. Dynamic Load Calculations

Wind Load Calculation (WL):
Wind load is calculated based on the wind-exposed area of the surface of the bridge and its velocity. Wind load can be approximated by:

• Wind Load (WL) = Wind Pressure * Area exposed to wind

For this example: • Wind pressure = 1.5 kN/m²
• Bridge height exposed to wind = 10 meters
• Length of the bridge = 30 meters

Area exposed to wind = Length * Height = 30 * 10 = 300 m²
• Wind Load (WL) = 1.5 kN/m² * 300 m² = 450 kN

Seismic Load Calculation:
Seismic loads are computed according to the seismic zone of the bridge location. In most cases, the seismic force (F) is determined by:

• Seismic Force (F) = Seismic coefficient (C) × Total structural weight (W)

Given:
• Seismic coefficient = 0.1 (for a medium-risk zone)
• Total structural weight = Dead load + Live load = 236 tons

Convert tons to kilonewtons (kN): 1 ton = 9.81 kN → 236 tons = 2315 kN
Therefore, Seismic Force (F) = 0.1 × 2315 = 231.5 kN

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5. Example Calculation
Assume the above calculations for an example:
• Bridge Length: 30 meters
• Dead Load (DL): 216 tons
• Live Load (LL): 20 tons
• Wind Load (WL): 450 kN
• Seismic Force (SF): 231.5 kN

Total Load on the Bridge:
• Total Load = Dead Load + Live Load + Wind Load + Seismic Load
• Changing Dead Load and Live Load to kN: (216 tons + 20 tons) * 9.81 = 2315 kN + 196.2 kN = 2511.2 kN

Adding Wind Load and Seismic Load:
• Total Load = 2511.2 kN + 450 kN + 231.5 kN = 3192.7 kN

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Conclusion
Bridge load calculation is necessary since its structure has to be safe and sound. Engineers consider the static loads such as dead load plus live load, and dynamic loads such as wind forces and seismic forces, to ensure that a bridge can withstand stresses from varied environmental and operating conditions. Precise calculations ensure that bridges will continue to function safely over time.