help with modification of existing barjois

huangyhg

help with modification of existing barjoist
good afternoon all
i am writing to ask about modifying an existing bar-joist system.  the building in question has bar joist resting on masonry walls. what the owner is thinking about request us to do is replace a section with glass wall system.
this will require us to move the bearing point of the structure back from the masonry wall and on to a beam that is preliminarily 18" farther into the existing bar-joist support members.  i am looking for any suggestions or comments on the course of action that should be followed in redesigning the joist in place and moving the bearing point. i realize that this is a truss and can be modeled as such. i appreciate your time on this issue.
please note this also appears in the demolision and remodaling sections.
ewing

it is usual for a bar joist to have the same top chord angles along its length, especially near its end.  you can move the bearing point back, add a couple of new angles to form the seat, and then size a web   
you also need to check the shear capacity across the joist as you now have shifted the "bow tie" shear diagram over...granted that it is now a little smaller.
analyze the current joist condition to get a shear diagram across the full, original span.  note that most joists provide web capacities in the center section (where the shear diagram goes to zero) of a minimum 25% of the max. end reaction.  this shear diagram with the 25% minimum is your theoretical capacity across the full span.
now with your new, shorter span, you will have a different, applied shear diagram that will be based on the smaller span and lower end reaction.  this diagram can be superimposed over the original shear capacity diagram to see if you are forcing higher shear onto webs that weren't designed for it.
if so, add additional, small steel angles welded to the top and bottom chords, that run diagonally with the current web   
don't forget proper shoring and if required bracing to keep safety at every moment in the work.
thank-you all for taking the time to respond to this question.  you were very helpfull  have a great day.