AASHTO Stantard Specification LFD Live Load Impact for Truss Member

[everestking]

I am doing Load Factor Rating for Truss Bridge. The section 3.8.2.1 of AASHTO Standard specification defines Live Load Impact as 50/(L+125), where L is the length in feet of the portion of the span that is loaded to produce the maximum stress in the member. For a truss member (compression or tension) how can L be calculated? In my understanding as an example For HS-20 Truck, the length of the truck 14'+14'= 28ft(using the minimum length from 14ft to 30ft ) is portion of span that is loaded to produce maximum load on truss member.Am I correct or is there any other way of determining 'L'?

Thank you for your help.

 

[bridgebuster]

L for the impact factor is the length of the truss, not the lengths of individual members.

 

[yakpol]

You may need to construct influence line to find L (length of positive or negative area).