How to calculate load distribution in a framed truss member?

Depends on the truss design. For some simple trusses with pinned ends and point loads it is possible to determine the force in each member by balancing forces at each connecting node using free-body disagrams. You posted this question under mechanical engineering - if you are taking an introductory course in statics I suspect that's all you need. Start at the point where the load is applied and work from there. But for more complicated structures, or for loads that are not simple point loads, the truss geometry may be indeterminate, in which cases you have to equate stresses as well as strains for each node. It can get really complicated if the joints are not simple pins - using welded joints (as in real-world construction) further complicates the analysis.

Using the equations of statics (Sum of the Moments = 0, Sum of the Forces = 0) you can solve for all the internal forces in a truss provided it is a simple pin jointed truss and it is a determinate geometry.

For some of the more common trusses used in residential structures try this calculator:

Medeek Design Inc. - Truss Designer (http://design.medeek.com/calculator/calculator.pl)