Question
I would like to know the difference between 'Combined' and 'Independent' in the moving load conditions.
Answer
According to the design standards for bridges, since the DB and DL loads are not considered to act simultaneously, the 'Independent' method is generally used for load application in bridge design.
In Moving Load Cases, we set up moving load conditions such as minimum / maximum lane numbers per vehicle load group and combination method of vehicle load groups.
In the 'Loading Effect' of 'Sub-Load Cases', the combination method of vehicle load groups is specified, and there are two methods: 'Combined' and 'Independent'.
Combined method
The Combined method considers all possible combinations of Sub-Load Cases.
In cases where different vehicle loads are applied to each lane, which can result in more unfavorable results than when the same vehicle load is applied to each lane, the Combined method can output the most unfavorable result by considering all possible combinations of Sub-Load Cases.
Independent method
The Independent method considers each Sub-Load Case independently, meaning it does not consider the number of cases where different vehicle loads are applied to each lane. It calculates the results of each Sub-Load Case and then envelopes them to output the most unfavorable result. In the design standards for Korean road bridges, DB and DL loads are not considered to be simultaneously applied, so the Independent method is generally used.
In the case of defining two Sub-Load Cases as shown in the table below, the number of cases that occur depending on the Combined and Independent methods can be summarized as follows.