Program now enables users to define their own combination by defining the load factor for each load type or simply request program to generate default combinations. If users choose to use default combinations, then these are generated using the definitions of load groups specified in AASHTO LFD Art. 3.22 and LRFD Art. 3.4. Substructure uses two methods to generate the load combinations.

When program generates load combinations from load groups specified in library, it uses maximum, minimum factors, the dependency and the reversibility into account. Program creates separate combinations for all variations of maximum and minimum load factors which may result in more than one combinations. The reversible loads including generated wind loads when used in a combination generate two combinations. One with positive sign and the other with a negative sign. The actual load positions are not changed. This might be different than anticipated for wind cases with overturning load. If you would like wind overturning load treated differently, then you may add additional case to cover all scenarios.

The first method uses a comprehensive algorithm that provides all the possible combinations, to include so-called cross grouping. For example, assume a combination requires dead load, live load, and wind load. If you have one dead load (D1), two wind loads (W1, W2) and two wind loads on live load (WL1, WL2), then W1 will be grouped with WL1 and also with WL2. This results in comprehensive load combinations.

The second method uses a dependency concept that distinguishes all loads into independent and dependent loads. Inclusion of an independent load in a particular load combination does not depend on the existence of any other load. Wind load is an example of an independent load. Inclusion of a dependent load in a particular load combination is predicted by the presence of a related independent load. For example, wind load on live load is a dependent load because it is dependent on the wind load. The following table lists the dependent loads used in Substructure for AASHTO LFD and LRFD:

User can view/modify the dependent load table on the A/D parameters screen. This provides a great tool so that customized dependent/independent relationships can be specified.

The program checks that the same number of dependent load cases is provided as the load case on which it depends with the same indices. For example, if two wind load cases are provided and the non-cross combinations option has been selected, Substructure allows either no wind on live cases or two winds on live cases. It no longer allows any other number of winds on live cases for the analysis. One further check is made to ensure that the load indices are the same. This means if the user has specified wind load on structure cases, W1 and W2 (in LFD) and wind on live load cases are needed, those must be WL1 and WL2. If the user has WL2 and WL3 along with W1 and W2, the program give an error message and stops running.

Frame analysis results of the load combinations include the impact factor for the live load and load multipliers, while individual load cases provide results without either the impact factor or load multipliers