The best way to accomplish this is to use the “Independent” element referencing options in the Member Shape Description command. These options are at the end of the Locate By list: and are called:
If the element referenced is a support, then the element will adjust to changes in support location or span length. If the element (for example a web plate) is in the interior of a member then referencing the left and right ends to adjacent elements will automatically adjust the web length if the adjacent elements change.