Bridge variable load

To optimise bridge loading based on an influence line analysis, any uniformly distributed load to be applied to a path needs to be specified either as a standard variable load or a user variable load.

A variable load comprises a variable uniformly distributed load (UDL) and knife-edge load (KEL).

A variable UDL is a uniformly distributed load that has a uniform intensity (force/length or force/area) for each path, which can depend on the loaded length or loaded area.

A KEL is a line load applied over the width of the Path. If CS454 KEL is selected this will also vary depending on the length loaded by the UDL.

A number of standard variable loads implement the requirements of specific design codes.

You can specify additional variable loads. This is most easily done using the Bridge Variable Load wizard.

Variable UDL types

5 curve types are available for variable UDL:

Nonlinear type 1 (e.g. BS5400)

The intensity of the load is defined by a series of segments. Transition points between each segment can be defined. Each segment of the VUDL curve between transition lengths has intensity per unit length per path. This is defined by power-law functions to give a loading intensity which varies with loaded length:

$$VUDL = A \times \left(\frac{1}{L}\right)^n$$

Where the variable UDL is in kN/m, $A$ is Factor in kN, $n$ is index, and $L$ is loaded length in m.

Up to three segments of variable UDL can be specified. You need to ensure the intensity matches either side of the transition length.

Nonlinear type 2 (e.g. EC1 UK Footway)

The load intensities are constant for the first and last segments respectively, with the intensity of the intermediate zone defined by:

$$W = 2+ \frac{120}{(L+\text{Transition Paramter})}$$

with $L$ and the transition parameter in m and $W$ in kN/m².

Nonlinear type 3 (e.g. AASHTO)

The load intensities are constant for the first and second segments respectively. The load intensity of the third zone is a curve defined as:

$$W = \left(30+\frac{3000}{L}\right)\times\frac{(55-B)}{50}$$

where: $W$ – lb/tf²; $B$ – ff.

Nonlinear type 4 (CS454)

The load intensity curve is a 4-segment curve defined in CS454 Table 5.19a. No user configuration is permitted.

Loaded length, (m) L	UDL (kN/m)
$L ≤ 20 \text{ m}$	$\frac{230}{L^{0.67}}$
$20\text{ m}<L<40\text{ m}$	$\frac{336}{L^{0.67}}-\frac{1}{1.92-0.023L}$
$40\text{ m}\leq L \leq 50\text{ m}$	$\frac{336}{L^{0.67}}$
$L > 50\text {m}$	$\frac{36}{L^{0.1}}$

Linear

The intensity of the load is defined by straight lines between transition points. The first and last straight lines are horizontal, joined by an intermediate line.

KEL Types

None

No KEL is applied

Constant

A constant KEL is applied across the path, regardless of the length loaded by UDL. A constant UDL of 0 is the same as type ‘None’

CS454

Varies with the length loaded by the UDL and is defined in CS454 Table 5.19a. No user configuration is permitted.

Loaded length, (m) L	KEL (kN)
$L ≤ 20 \text{m}$	$82$
$20\text{ m}<L<40\text{ m}$	$\frac{120}{1.92-0.023L}$
$40\text{ m}\leq L \leq 50\text{ m}$	$120$
$L > 50\text {m}$	$120$

Definition

Name

The name is used to refer to a particular variable load.

UDL curve type

Different linear and nonlinear variable UDL types can be selected.

Knife-edge Load (KEL)

Different KEL types can be selected, with a value specified where applicable.

UDL curve definition

UDL curve definition varies with curve type. Common options include:

Number of segments

The number of segments used to define the complete VUDL (with a maximum of three).

Transition point

The load length up to which the previous segment applies and beyond which the next segment applies.

Graph

This provides a view of the load intensity as a function of load length (or area). This is provided as visual confirmation of the VUDL definition.