H Series Joist Capacity 1

[SteelPE]

I am investigating an old open-web steel joist roof to support a new RTU. Based upon measurements taken onsite I am fairly certain that the existing joist is a 22H6. The joist in questions spans 34’. From my 75 year manual I have determined the following:

Resisting Moment = 422,000 in-lbs
Maximum End Reaction = 5,400 lbs
Allowable safe load in pounds per linear foot = 243plf

So now I need to draw a shear and moment diagram for the joist and determine where reinforcement is needed (where the new loads create an over stress in the joist). I know a H series joist has a minimum shear capacity of 50% of the end reaction, however, since this joist is controlled by moment, I am not sure how to determine the shear capacity of the joist and any given location (how the shear diagram will look). As far as I can tell I have 2 options:

1) Maximum End reaction – distance * allowable safe load > 50% of the maximum end reaction otherwise 50% of maximum end reaction
2) 1/2*Allowable safe load * joist length-distance*allowable safe load > 50% of the maximum end reaction otherwise 50% of maximum end reaction

I believe #1 is correct; however I am having an issue coming to terms with this method. Does anyone have any opinion on the matter?

 

[JoshPlumSE]

I'd say that 1) is probably the way to go.

You might want to check out (if you haven't already) the procedures described in the following publication:

http://www.vulcraft.com/catalogs/designing-with-vulcraft

 

[Once20036]

Personally, I like #2.

Shear and moment are related. If you draw the shear diagram for #2 (shear capacity based on maximum end reaction), it will not produce the moment diagram that you're expecting.
In addition, #2 is more conservative and seems more appropriate, given the uncertainty.

I expect that the maximum end reaction is based on the strength of the joist seat and not the shear capacity.
The 75 year manual you used should have the specifications used to manufacture the joist. I`d check there for insight.

 

[haynewp]

I would also vote #1. I wouldn't see the point in them providing the maximum end reaction in the tables otherwise. It looks like they have gotten away from listing maximum end reactions in the newer catalogs. For what it's worth, the 1934 table from the 75 year manual gave this explanation related to the maximum end reactions listed: