Bridge Loads and Design Principles - Examples & Applications

Examples and Applications

Practical problems covering dead and live load calculations, the application of influence lines for moving loads, and LRFD load combinations.

A simply supported bridge span has a length of

L = 25\text{ m}

. The superstructure consists of four steel plate girders spaced at

S = 2.4\text{ m}

on center. The deck is a reinforced concrete slab with a structural thickness of

t_s = 200\text{ mm}

and a monolithic wearing surface of

t_w = 15\text{ mm}

. A future wearing surface (FWS) of

75\text{ mm}

of asphalt is planned.

Assume the unit weight of reinforced concrete is

\gamma_c = 24\text{ kN/m}^3

and asphalt is

\gamma_a = 22.5\text{ kN/m}^3

. The cross-sectional area of one steel girder is

A_g = 0.05\text{ m}^2

, and the unit weight of steel is

\gamma_s = 78.5\text{ kN/m}^3

Calculate the total unfactored uniform dead load (in

\text{kN/m}

) acting on one interior girder, broken down into DC (component dead load) and DW (wearing surface dead load).

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A bridge deck component is subjected to a maximum static vehicular live load moment of

M_{LL} = 450\text{ kN}\cdot\text{m}

caused by the design truck. The design lane load causes an additional maximum moment of

M_{Lane} = 120\text{ kN}\cdot\text{m}

According to AASHTO LRFD specifications, the dynamic load allowance (IM) for deck components is

33\%

(or

0.33

Calculate the total unfactored live load moment (

LL + IM

) acting on the deck component. Note: IM is applied to the design truck, but not to the design lane load.

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A simply supported beam has a span of

L = 20\text{ m}

Determine the maximum ordinate of the influence line for the bending moment at midspan ( $x = 10\text{ m}$ ).
If a point load of $P = 150\text{ kN}$ moves across the beam, calculate the maximum absolute bending moment at midspan using the influence line.

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For an interior girder of a bridge, the following unfactored maximum bending moments have been calculated at midspan:

Component Dead Load (DC): $M_{DC} = 400\text{ kN}\cdot\text{m}$
Wearing Surface Dead Load (DW): $M_{DW} = 80\text{ kN}\cdot\text{m}$
Total Live Load with Impact ( $LL+IM$ ): $M_{LL+IM} = 650\text{ kN}\cdot\text{m}$

Using the AASHTO LRFD Strength I limit state combination, calculate the factored design moment,

M_u

Assume the following load factors:

(Assume the load modifier $\eta_i = 1.0$ for simplicity).

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