Live Load

Owned by Becca Alford (Unlicensed)
Last updated: Nov 01, 2021
6 min read

The live load vehicle definition of Eriksson Culvert is very flexible and therefore very powerful. It permits most load types encountered in highway and railroad bridge design to be modeled. Several predefined vehicle types are included as default trucks in the vehicle library. The primary vehicle types for all specifications are discussed below:

 

H and HS Loadings

Eriksson Culvert can model all H and HS loadings. These vehicles consist of an axle load portion or a lane load portion. The axle load portion of the H series consists of a pair of axles that are fixed in spacing. However, the HS series axle load consists of three axles: the front pair of axles is fixed and the rear pair is variable in spacing.

Separate analyses are performed for the axle load and lane load analyses. The results of each analysis are compared for moment and shear at each location under consideration along the structure and the governing value is adopted.

 

HL-93 Load

The HL-93 load consists of the following three distinct parts, as defined in Art. 3.6.1.2 of the LRFD Specifications:

  • Design Truck: Three axles of 8 kips, 32 kips, and 32 kips spaced a 14 (fixed) between the front axles and variable between 14 and 30 ft between the rear axles.

  • Design Lane: Uniform load of 0.64 klf.

  • Design Tandem: Pair of 25-kip axles spaced at 4 ft (fixed).

As with all vehicles, separate analyses are performed for each vehicle component. However, what distinguishes a live load analysis performed under the LRFD Specifications as compared to the Standards Specifications is the manner in which the components are combined. Under LRFD, the results of the design truck are compared to the results of the design tandem, and the governing values are adopted. These combined results are then added to the results of the lane load analysis.

Note that whether the results of the truck analysis and the lane load analysis are to be combined in an “AND” or an “OR” fashion is specified by clicking the respective radio button in the Combine group at the bottom of the Vehicle Properties dialog box.

Live Load may be neglected when the depth of fill is greater than or equal to 8 feet, and exceeds the span length of a single culvert, or exceeds the total width of multiple culverts.  Additionally the lane load may be turned off, as lane loads are typically not applied to many types of buried structures.

 

Cooper E80 Load

As per AREMA 2.2.3, the recommended live load for each track the Cooper E80 loading with no lane or tandem load.  The longitudinal distribution of the axle loads are to be limited by the axle spacing.  The transverse distribution of the axles loads are to be limited by the extent of the structure.  The program assumes only one track on a structure, and the direction of the track must be placed parallel to the main reinforcement.

 

CL-625/CL-625-ONT Load

Per CHBDC 7.8.4.3, the live load shall be the trucks in Section 3, and shall include the dynamic load analysis as specified in Section 3.8.4.5.2.  The truck definition file shipped with the program includes one large axle (Axle 4), and 2 smaller axles in the tandem (Axles 2 and 3).  Of course, the user may define their own CHBDC truck that includes all of the axles.

 

Fill Depths < 2 Feet

When the culvert has traffic running directly on the top slab, wheel loads are distributed as in ordinary bridges.  This is also done when the height of fill on the culvert is less than 2 feet.

When the traffic direction is parallel to the main reinforcement, the program will distribute wheel loads over a slab width, E, equal to 4 + 0.06S, where S is the perpendicular distance between wall centerlines.  See STND 3.24.3.2.  For LRFD, the equation is the same (see 4.6.2.10.2), but here S is defined as equal to the clear span.  Note that the program calculates this width for one wheel only, for both STND and LRFD.  Also, note that the program will only use one traffic lane, with the appropriate MPF.  For CHBDC, we use the same live load distribution as is found in the 6th Edition of LRFD.

When the traffic direction is perpendicular to the main reinforcement, the program calculates a distribution length based on the desired specification, and then places as many axles as possible on this width.  Again, this distribution width may be limited by the culvert length.  Additionally, more than one lane may be placed on the culvert, depending on the magnitude of the clear span.    Due to complex placement of wheel loads for the perpendicular case specified in the AASHTO specification each lane is assumed to be 10 feet wide.  This conservative assumption may result in increased flexural and shear demands on the structure depending on the clear span.  For example Figure 5-3 shows the maximum live load distribution used in Eriksson Culvert for a 40 foot clear span when the number of lanes is allowed to be computed by the program.  However, if the user feels that this is too conservative, they may limit the number of lanes placed on the culvert in the Loads dialog box. 

 

The user may elect to not limit the distribution width to the culvert length.  In this case, the user is responsible to insure proper shear transfer between adjacent culvert sections.  Note that in STND there is an absolute limit of 7 feet for the distribution width.  This absolute limit cannot be overridden by the user.

 

Fill Depths >= 2 Feet

When the height of fill is greater than or equal to 2 feet, wheel loads are distributed over areas having sides equal to 1.75 (for STND and CHBDC), and 1.15 for LRFD, times the depth of fill.  This is outlined in STND 6.4.1, CHBDC 6.9.6, and LRFD 3.6.1.2.6.  When these wheel loads overlap, the entire truck load is distributed over the length of the distributed truck.  As with fill depths less than 2 feet, the user can force the program to limit the total width of distribution to a maximum of the supporting slab, or they may allow the distribution width to be exceed the width of the supporting slab.  Note, however, that limiting the distribution width equal to the width of the supporting slab may be overly conservative, and in any case is not required by any of the specifications supported by Eriksson Culvert.

The method of analysis is similar regardless of the traffic direction (with one exception, as noted below).  For all specifications, the program considers two, three and four adjacent vehicle lanes and selects the critical case.  Appropriate lane reduction percentages are used for the three and four lane loading cases.

Under the last paragraph of LRFD 3.6.1.2.6, if the live load and impact moment, based on the distribution of the wheel load through earth fills, exceeds the live load and impact moment calculated according to LRFD 4.6.2.10 (equivalent strip method for fills < 2 feet), the latter moment shall be used.  Therefore, when the fill depth is greater than or equal to 2 feet, then Eriksson Culvert runs the moving live load analysis twice, one with the assigned fill depth, and once with a fill depth equal to 0.

 

Live Load Distribution in AREMA

In AREMA, all fill depths are treated the same for live load distribution.  The live load distribution factor is taken as 1.0 for the transverse direction.  For the longitudinal direction, the program assumes that each axle load is distributed over a length of 5 feet. 

In AREMA, the fill depth must be at least 18 inches in depth (AREMA 16.1.1).

 

Skew Effects

Both specifications mention the need to account for the effects of skews, but do not provide any details for implementation.  For STND, if the design span exceeds 6.0 / sin(θ), where θ is the skew angle, the distribution width is reduced by multiplying the cosine of the skew angle for fill depths less than 2 feet.  Culverts with skews in excess of 60 degrees are designed for a skew of 60 degrees.  For LRFD, when the skew angle is greater than 15 degrees for all fill depths, the program places the traffic lanes in the perpendicular direction.  Note that these effects may be turned on and off via the Analysis Options dialog box.

 

Live Load Surcharge

Live load surcharge is automatically applied by the program for fill heights less than 2 feet.  Surcharge should be applied by the user whenever the culvert is under live load influence for fills ≥ 2 feet.  Surcharge need not be applied when live load may be neglected in accordance with AASHTO Article 6.4.2.

 

Impact Factors/Dynamic Load Allowance

For STND, the Live Load Impact is in accordance with Article 3.8.2.3.  For the LRFD Specs, a value of 0.33 is assumed for the dynamic load allowance in accordance with LRFD Table 3.6.2.1-1. This value is only applied to the wheel load component of the live load.  For AREMA, the impact factor is assumed to be 60% at a fill depth of 18 inches decreasing linearly to 0% at a fill depth of 10 feet, see AREMA 16.4.4.  For CHBDC, the dynamic load allowance for box type structures shall be the value obtained from Section 3.8.4.5.3.