How to Calculate Support Reactions in Pratt and Howe Truss Bridges

Support reactions in Pratt and Howe truss bridges are essential for understanding how loads are distributed across the structure. Calculating these reactions helps ensure the bridge’s stability and safety. This article provides a step-by-step guide to determine support reactions in these types of truss bridges.

Understanding the Truss Structure

Pratt and Howe truss bridges are common types of truss designs used in bridge construction. The main difference lies in the orientation of the diagonal members. In Pratt trusses, diagonals slope towards the center of the span, while in Howe trusses, diagonals slope away from the center.

Steps to Calculate Support Reactions

The calculation involves applying static equilibrium equations to the entire structure. The main steps include:

• Identify the total load on the bridge, including dead loads and live loads.
• Determine the span length and support positions.
• Apply the equilibrium equations: sum of vertical forces and moments equal zero.
• Solve for the reactions at the supports.

Example Calculation

For a simply supported Pratt or Howe truss with a uniform load, the reactions at supports can be calculated using the formulas:

Reaction at support A: R_A = (Total load × distance to support B) / span length

Reaction at support B: R_B = Total load – R_A

For non-uniform loads or more complex configurations, moment equations around one support are used to solve for reactions.