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, Fe, that accounts for the type and condition of installation.
The soil-structure interaction factor, Fe, 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 STND and LRFD, the program used an equivalent fluid pressure method to calculate the horizontal soil pressure, not the soil weight. For AREMA, the program uses the soil weight multiplied by the minimum and maximum lateral pressure coefficients. For CHBDC, 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 height of water pressure and no water pressure as two loading cases. Internal water pressure is taken as 62.4 pcf.
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, we check three load combinations:
For maximum vertical load on the roof and maximum outward load on the walls,
DCmax + DWmax + EVmax + EHmin + (LL+IM)max + WAmax
For minimum vertical load on the roof and maximum inward load on the walls,
DCmin + DWmin + EVmin + EHmax + LSmax
For maximum vertical load on the roof and maximum inward load on the walls,
DCmax + DWmax + EVmax + EHmax + LSmax + (LL+IM)max
For CHBDC, we check five load combinations:
For maximum vertical load on the roof and minimum horizontal inward on the walls,
DCmax + DWmax + EVmax + (LL+IM)max + EHmin + WAmax
For maximum vertical on the roof and maximum horizontal inward on the walls,
DCmax + DWmax + EVmax + (LL+IM)max + EHmax + WAmin
For minimum vertical on the roof and maximum horizontal inward on the walls,
DCmin + DWmin + EVmin + EHmax + WAmin
For maximum vertical on the roof and maximum horizontal inward on the walls,
DCmax + DWmax + EVmax + LSmax + EHmax + WAmin
For minimum vertical on the roof and maximum horizontal inward on the walls,
DCmin + DWmin + EVmin + LSmax + EHmax + WAmin