Load Spectrum

In CalME, truck passages are converted into passages of four axle groups: steering, single, tandem and tridem. Each axle group is made up of one or more axles. The steering and single axle groups each has one axle, the tandem axle group has two axles, while the tridem axle group has three axles. Each axle is made of a shaft with a hub attached to each of the two ends. The steering axle group uses single hubs (i.e., each hub has only one wheel), while all other axle groups uses dual hubs (i.e., each hub has two wheels).

As an example, one passage of an 18-wheeler is converted into one passage of steering axle group, and two passage of tandem axle group. This is illustrated below. As shown in the figure, the truck passage used in this example is further converted into the passages of one single hub and four dual hubs.

In CalME, truck traffic is distributed into the four axle groups, the 24 hourly periods for each day, and various group load brackets. An example of how the load spectrum is defined is shown below for Group1. As shown in the example, 0.528462% of the daily traffic comes in the form of 50kN steering axle group. Note that the load shown below is for the whole axle group. A group load of 80 kN for a tandem axle corresponds to a 20 kN load for each of the four dual hubs in the axle group.

The load distributions are derived from Weigh-In-Motion (WIM) stations at more than 100 locations in California (Truck Traffic Analysis using Weigh-In-Motion (WIM) Data in California). It was found that there is very little seasonal variation. It was therefore decided to use the same load spectrum for every day throughout the year.

An analysis of the load distribution data (Grouping of WIM Sites Based on Axle Load Spectra) showed that the load distributions fall into eight groups. The decision tree used to determine the group is shown below and in "corrected_flowchart".

The averaged traffic inputs from all WIM sites in each group were used as the traffic characteristics of the regions covered by the group. For CalME, the number of axles per truck and the hourly axle load spectra are averaged for each group. Other traffic inputs, including the number of axles per lane per year and growth rate are calculated for each highway section from the Caltrans annual AADTT report

http://www.dot.ca.gov/hq/traffops/saferesr/trafdata/

which is stored in the table "CaltransAADTT". The present number of axles in the design lane is calculated from the year of estimation in the table, using a linear growth rate.