Open web steel joist fabrication question 2

[CLT49er]

I have a project that may require reinforcing the chords of an existing open web steel joist (K series from the late 1980s). I am adding a concentrated load of approximately 600 lbs about 1.5 ft from the end of a 35 ft long joist. I developed a plot of the allowable moment and shear capacity using the allowable uniform load capacity from historical joist load tables. When comparing the moment diagram from the new loading to the allowable moment diagram I get an overstress of the top and bottom chord of around 16% between the support and the concentrated load. The overstress decreases back below the allowable capacity a few feet past the concentrated load (towards midspan).

Going through the process above got me to thinking about how the joist is fabricated, specifically what makes the moment capacity vary along the length of the joist? I could only think of two things that would make the capacity vary. The first being that the joist manufacturer changes the size of chord angles periodically along the length (I have never seen this in the field before, but I haven't really looked that closely). The second would be increasing the compression capacity of the top chord at midspan by reducing the unbraced length through the use of intermittent connectors between the chord angles or by decreasing the panel point spacing near midspan. Does anyone more familiar with joist fabrication have any idea how this is done? Does it have something to do with the way the web members are welded to the chords?

For my specific case the required moment capacity at 1.5 feet from the end of the joist is nowhere near the required capacity at midspan (which is not overstressed). So, if the chord angles are the same size along the length of the joist I would think that moment capacity at the end would be the same as at midspan (assuming unbraced length for compression is the same). Does anyone think my logic is flawed?

Shear reinforcement will still be required.

I have not yet gotten a chance to take some field measurements.

 

[KootK]

Does it have something to do with the way the web members are welded to the chords?

You're knocking on the door there. I believe that what you're missing here is recognition of the complimentary nature of shear in this situation.

A joist's moment capacity is limited by the capacity of the webbing and connections to deliver axial tension and compression to the chords. That capacity, is essentially horizontal shear capacity. And, taking it back to 2nd year mechanics of materials class, horizontal shear and vertical shear are equal (complimentary).

In summary, by improving your vertical shear capacity, you'll also improve your horizontal shear capacity and, therefore, your moment capacity. Practically, if your chord size and panel point spacing are constant, you can consider your possible moment capacity to be constant at the peak value. You just have to beef up the shear capacity in order to take advantage of that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Once20036]

CLT,
I think you're spot on. The moment capacity of most joists is constant along the length of the joist. I have never seen a chord change size (I would be nervous if I did) and I have seen blocking installed near midspan to cut down the length and provide an increased capacity over a limited length. I would not worry about a 16% increase at the end of the joist.

The moment capacity of the joist is based on the top chord area, unbraced length, the bottom chord area, and the distance between the chords. Vertical and horizontal shears are important to get the forces into the chords, but increasing your shear capacity does not increase your moment capacity. Think about it this way - if you double the size and connections of all your web members, your moment capacity would be the same. If you decreased all the web members by 50%, you would never reach your moment capacity because you would have a shear failure, but still, the moment capacity is the same.

 

[CLT49er]

These joists have double angle top and bottom chords with rod webs. The seat is another inverted double angle. The end diagonal connects directly to the seat, not the top chord. Unfortunately I don't have a picture where the joist seat is exposed (most are covered by spray on fireproofing). The concentrated load falls between the panel points so I will be adding struts to eliminate the chord bending at the concentrated load.

 

[Triangled]

kinda deja vu ish....
anyways, OP, your logic seems sound