Design of Bridge Superstructures - Examples & Applications

Examples and Applications

Practical problems covering the determination of effective deck width in composite action, calculation of load distribution factors, and the flexural check for steel bridge girders.

Example

Problem 1: Effective Flange Width for Composite Action

A bridge superstructure consists of a reinforced concrete deck over steel I-girders acting compositely. The simply supported span length is

L = 24\text{ m}

. The structural thickness of the concrete deck is

t_s = 200\text{ mm}

. The steel girders are spaced at

S = 2.5\text{ m}

on center. The top flange width of the steel girder is

b_f = 300\text{ mm}

According to typical design codes (e.g., AASHTO LRFD), determine the effective width (

b_{eff}

) of the concrete deck that acts compositely with an interior steel girder. The effective width for an interior girder is usually taken as the lesser of:

One-quarter of the effective span length ( $L/4$ )
$12$ times the average thickness of the slab, plus the greater of web thickness or half the width of the top flange of the girder ( $12 t_s + b_f/2$ ) (simplified for this example as $12 t_s + b_f$ )
The average spacing of adjacent beams ( $S$ )

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Example

Problem 2: Lever Rule for Load Distribution Factor (Exterior Girder)

The Lever Rule is a simplified method used to determine the live load distribution factor (LDF) for an exterior girder when complex empirical equations do not apply.

Consider a bridge with steel girders spaced at

S = 2.4\text{ m}

. The deck overhang distance from the center of the exterior girder to the edge of the curb is

0.9\text{ m}

. A truck wheel line (half of a truck's weight) acts as a point load. To maximize the load on the exterior girder, the wheel line is placed

0.6\text{ m}

away from the curb (the minimum allowed distance).

Calculate the Load Distribution Factor (in lanes/girder) for the exterior girder using the Lever Rule. Assume the first interior girder acts as a hinge support.

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Example

Problem 3: Conceptual Girder Selection Logic

A design firm is tasked with selecting the primary superstructure girders for a new highway overpass over a busy interstate. The required span is

35\text{ m}

. The clearance below the bridge is severely restricted, allowing very little structural depth. Furthermore, closing the interstate for construction must be minimized to a few night shifts.

Evaluate the use of Cast-in-Place (CIP) Concrete Box Girders, Precast Prestressed Concrete I-Girders, and Steel Plate Girders for this specific scenario. Choose the most appropriate option.

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