K-Series Steel Joist Shear Strength 4

[jack36]

I am working on a project where we are designing a modification to shorten some existing K-series steel joists by 11'-4". I plotted my original shear diagram and new shear diagram and found that the shear in the modified joist exceeds the original shear diagram (based on allowable uniform loading) near the modified end.

The SJI specification states "The vertical shears to be used in the design of the web members shall be determined from full uniform loading, but such vertical shears shall be not less than 25 percent of the end reaction".

My question: How do the steel joist manufacturers design/fabricate the joist for variable shear capacity along the length? Do they vary the web member size or the length of weld from the web to the chords? I measured the webs on the joist in question and they are all 3/4" diameter bars.

Thanks for any input.

 

[JAE]

Probably, in your case with the same 3/4" bars throughout, it might be the weld between rod and chords that vary. Difficult to determine.



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[JAE]

This is a good reference:

https://steeljoist.org/product/td12...n-of-open-web-steel-joists-and-joist-girders/

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[KootK]

If it's bent truss bar webbing then there's a pretty good chance that the shear capacity actually is uniform along the beam. If it's individual hollow round bars, there's some chance that the thickness changes. I'd have to look to see if there actually is such a thing as a non-solid 3/4" round though. Not sure if it exists off of the top of my head.

As always, assumptions are dangerous. It would be good to have some positive confirmation of consistent webbing and welding.

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.

 

[jack36]

Thanks JAE for your input and the link to the document. I have seen that in the past and it has some good information in it.

The web bars are solid and are not bent. They extend between the double angle chords and are welded.

I believe I'm going to go back and see if I can verify any variation on the welds. What are your thoughts on adding weld from the existing web bars to the chords instead of adding angle reinforcement on each side of the existing webs (where the new shear diagram exceeds the original)? It seems to me that they wouldn't vary the welds because the welding doesn't look that great anyway. I would think they would need clean and precise welds if they were going to vary strength based on that.

Maybe these joists are actually KCS. There is not a tag, but I measured all dimensions and sent that info to the SJI using their online request form. By the way, that is a great service they provide. It took them less than an hour to send back the information. They told me what joist the dimensions were similar to, but they did not inquire about the webs being uniform.

 

[Triangled]

"
My question: How do the steel joist manufacturers design/fabricate the joist for variable shear capacity along the length? Do they vary the web member size or the length of weld from the web to the chords? I measured the webs on the joist in question and they are all 3/4" diameter bars.
"
It's my understanding that these two requirements are at play: a) SJI requirement and b) fabrication shop economy/quality assurance

It's my understanding that the manufacturer is only required to provide the minimum member and connection to meet the minimum required force. The 25% rule has not existed since the beginning but is currently in place...I'm sure someone can tell us the year of it's implementation.

Mathematically web sizes from end to middle of joist might vary by xx/100s of inches in diameter and weld lengths might vary by xx/100s in both throat and length....and their computer programs can likely calculate this to lotsa decimal places. But practically the shop has to order steel in advance and the laborer has to select the proper diameter steel from the shelf behind them and the welder has to supply enough weld and the manufacturer has to guarantee it all. So manufacturers tend to group web sizes together and install webs as Vs or Ws and they tend to provide minimum weld lengths.... all to economize actual fabrication without compromising on the minimum requirement.

 

[Deadblow]

Please help me understand the conditions. If the original joist was uniformly loaded and the modified joist is uniformly loaded but 11'-4" shorter, how can the shear be more in the modified joist? I'm sure I am missing something simple, so take it easy on me.

EIT

 

[jack36]

Deadblow, that's a good question. I'll take a shot at an answer...

The maximum shear in a beam with a uniform distributed load is at the ends, and it is zero at mid-span, correct? When you cut off 11'-4" of the beam your shear diagram does not stay where it was with only the 11'-4" cut off. It shifts so that maximum shear is at the end of the cut beam and zero is at the center of the cut beam. Therefore, the new bearing location can have a shear value that exceeds the shear value of the original beam at that particular location. It does not exceed the maximum shear of the original beam, just the shear in that location.

I attached the shear diagrams for the joist in question. The original shear diagram reflects a minimum allowable shear of 25% of the maximum shear. That is why the line does not have a uniform slope. Also, I have a couple of new point loads on the modified joist. You can see on the left end of the modified joist that the shear exceeds the shear of the original joist.

This is not a problem if you have a member that has a uniform shear capacity, like a KCS joist, but the K-series joist can be designed and fabricated with varying shear capacity.

 

[Deadblow]

Jack36, that makes perfect sense! Thanks for taking the time for the clarification! I wasn't thinking about the varying shear capacity!

EIT

 

[gravityandinertia]

As someone who worked at a joist manufacturer, I'd also share some insight.

Joist manufacturers calculate the minimum required member at each location to meet the design requirements for loading. They then reference the closest matching member size they have in stock (they only buy member sizes every 6 months, so common sizes get used up). If that minimum member size isn't available they will move to the closest size that is larger than the minimum member size. As a result, many times the joist will be stronger than it's rating, but will never be re-rated higher than that.

As for welds, a minimum weld is calculated, but where I worked that was only used to check that there was enough weldable length at the top chord and bottom chord to reach that. If there wasn't enough length, the chords may be increased which would give the additional weld length needed (again increasing the capacity of the joist, but not re-rated higher). In reality, the minimum weld was never used, the shop would weld the full length of the overlap between the members and the chord. Besides the fact no one wants to keep measuring if they achieved the 12 inches of minimum weld, it also reduces liability for the joist manufacturer by strengthening the connection.

Hopefully, that's useful information.