The Variable Spacing Method places the live loads at specific positions along the transverse direction of the bridge based on the value of the Minimum Spacing Between Locations. The algorithm used for the generation of individual live load positions is explained after the Minimum Spacing Between Locations (for Variable Spacing) section.

Minimum Spacing Between Locations (for Variable Spacing)

This is a user-defined spacing value that specifies the minimum center to center distance that should be maintained between any two consecutive live load (LL+I) positions that are considered by the program while auto generating the various (LL+I) cases.

In other words, Substructure will first generate all the possible (LL+I) positions internally (according to the live load generation algorithm discussed later). Then, based on the Minimum Spacing Between Locations value specified by the user, it will combine only those live load positions, which are separated by at least that minimum distance from the last considered position (going from left side of the bridge to the right). The remaining positions are not considered for the purpose of combination. This option is provided to reduce the total number of (LL+I) cases used for analysis when the generated positions are very close to each other. Thus, by specifying the value for Minimum Spacing Between Locations, the user may reduce the total number of live load positions used for combinations and consequently the analysis time.

Default values: 0.5 ft or 0.15 m.

Range of values that can be input: 0 to 20 ft (or 0 to 6 m).

The above definition may be illustrated with an example taken from Tutorial 1. In Tutorial 1, if the user does an auto live load generation with HS20 truck (one-lane loaded), the live load positions generated by the program corresponding to Minimum Spacing Between Locations values of 0.5 ft, 1 ft and 2 ft are shown in the following table.

This algorithm for live load positions is further used by Substructure as follows.

In the Auto Load Generation: Live Load screen, there are several loaded lane options (depending on the number of lanes entered by the user) in the Loaded Lane list (e.g., 1 lane loaded, 2 lanes loaded, All combinations, etc.). For example, if you select 1 lane loaded, the program will investigate the effects of every live load position created by the algorithm. If you select 2 lanes loaded, the program will only investigate the effects of any two live load positions simultaneously. In this case, Substructure will select any two live load positions that are not overlapped. If you select All Combinations, the program will consider the effects for both the 1 lane loaded and 2 lanes loaded and report the maximum cases. The final live load cases for any specific number of loaded lanes are determined by the maximum load effect in each member. The maximum load effect means that the position of the specific loading(s) produces the maximum or minimum internal forces/moments in any of the frame members.