Applied Loads

Vertical Loads

The Dead Load on the top slab consists of soil weight plus the weight of the concrete slab. The program is capable of analyzing additional uniform dead load as well as accepting up to three (3) extra concentrated dead loads.

The program complies with STND Articles 16.6.4.2 and 16.7.4.2 (Modification of Earth Loads for Soil Structure Interaction) for embankment installations (see also LRFD 12.11.2.2). These Articles state that the earth loads described in STND Article 6.2 may be used if they are multiplied by a soil-structure interaction factor, F_e, that accounts for the type and condition of installation.

The soil-structure interaction factor, F_e, is not applicable if the Service Load Design Method is used.

For CHBDC, the program uses the Vertical Arching Factor from Table 13 in Section 7.8.4.2.3, which depends on the installation type, B1 or B2.

Horizontal Soil Pressures

The program will not only investigate the maximum lateral earth pressure but will also investigate the minimum lateral earth pressure acting on the outside walls. This is an AASHTO loading condition to check for maximum positive moments in the top and bottom slabs.

For the Standard specification, the software uses an equivalent fluid pressure method to calculate the horizontal soil pressure.
For the LRFD specifications, users have the option of selecting how the horizontal soil pressure is calculated,
- Equivalent fluid pressure
- Active earth pressure
- At-rest earth pressure
For the AREMA specifications, the software uses the soil weight multiplied by the minimum and maximum lateral pressure coefficients.
For the CHBDC specifications, the program uses the unit weight of the soil in conjunction with the Horizontal Arching Factors from Table 7.11, again depending on the installation type.

Internal Water Pressure

Water pressure inside the culvert barrel can reverse the wall moments and add to slab positive moments and may be checked. The program is able to use full or partial depth of water pressure and no water pressure as two loading cases. Water weight is taken as 62.4 pcf. The input is the height of the water table above the centerline of the top slab for exterior water pressure, and the elevation head above the bottom of the top slab for the interior water pressure. So, for example, an interior pressure head input of 0-ft indicates a triangular pressure load resulting from the culvert being filled with water, but no additional pressure.

Load Combinations

Load combinations are created based on code requirements. Generally, they are broadly grouped into service and strength categories.

Service Combinations

Service combinations are primarily for the Service Load Design Method and for serviceability checks

Strength Combinations

Strength combinations are checked for shear and flexural strength.

For STND and AREMA, we only have one load combination to check:

Loading = 1.3(DL) + 2.17(L+I) (STND)

Loading = 1.4(DL) + 2.33(L+I) (AREMA)

In the above equation, DL includes concrete, soil and water dead loads.

For LRFD, Eriksson Culvert checks four load combinations:

Maximum vertical force on the roof, and maximum inward force on the walls:
Minimum vertical force on the roof, and maximum inward force on the walls:
Minimum vertical force on the roof, and maximum outward force on the walls:
Maximum vertical force on the roof, and minimum outward force on the walls:

For CHBDC, five load combinations are checked:

Maximum vertical load on the roof and minimum horizontal inward on the walls:
Maximum vertical on the roof and maximum horizontal inward on the walls:
Minimum vertical on the roof and maximum horizontal inward on the walls:
Maximum vertical on the roof and maximum horizontal inward on the walls:
Minimum vertical on the roof and maximum horizontal inward on the walls: