Plastic Moment of resistance of beam with welded tee at bottom 2

[ajk1]

Does anyone know of software that will calculate the plastic moment of resistance of a steel beam with welded Tee section at the bottom? I started writing a spreadsheet, but is a little laborious to cover all cases because the PNA can fall above the bottom flange of the beam, below the bottom flange of the beam or within the bottom flange of the beam.

 

[KootK]

I think that SAP and Risa both have section builder utilities. There are shortcuts that will make hand calculation pretty easy if you're interested.

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.

 

[ajk1]

Do RISA and SAP give the plastic section modulus?
I am almost finished my spreadsheet, but any shortcut as a check on it would be of interest.

 

[KootK]

Not sure about the software.

Shortcut:

- treat tension portion of section as just the two lower flanges.
- treat compression portion as top flange and whatever web is needed to balance the areas.

It's an approximation but a sound one. UK codes contain a similar method as an option.

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.

 

[BAretired]

Forget about the bottom flange of the beam altogether. Treat it like a deeper beam with a new flange further down. That is conservative and close enough for practical purposes.

BA

 

[ajk1]

To Kootk and BAretired:

I will try both methods and see how they compare with my spreadsheet answer. My spreadsheet still requires trial and error of the PNA location, but after that it does the calcs. I thought I could use the goal seek of Excel, but it does not always give me the right answer, so I did the trial and error manually.

They are both good ideas, BAretired's method really simplifies it the nth degree, and I suspect may give reasonably accurate answers in most cases. You guys are awesome.

 

[ajk1]

The beam has access one side only. See attached. Is the way I have shown the welding, with a backer bar on the blind side the way to do it?

 

[KootK]

I think that's doable.

I always wonder about fit up given that both members are likely to have natural camber and one is likely to have some deflection, even if jacked.

Perhaps:

1) make backer bar permanent structure and intermittent.
2) move backer bar to other side of stem.
3) connect backer to existing with fillet welds each side.
4) connect tee to backer with fillets one side.
5) throw in stiffeners at 1/4 points-ish

I've never done this but suspect that it might be rather field friendly.



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.

 

[KootK]

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.

 

[BAretired]

I don't understand KootK's (2) above.

I would be inclined to do it as ajk1 suggests except that I would prefer a small angle rather than a plate or bar. That would tend to preserve the right angle a little better.

BA

 

[ajk1]

ok, thanks. I take it that if I change the plate to a small angle, that it would still require a full penetration butt weld between the stem of the tee and the flange of the beam above.

 

[BAretired]

The small angle would serve as a backer which permits a full penetration weld. The angle is not structural. You need enough weld to develop the yield strength of the tee section. The weld does not need to be continuous.

It might be a good idea to add nominal stiffeners between the flange of the beam and the tee, although it could be argued that they are not required.

BA

 

[KootK]

A couple of additional reasons why I pitched a solution not requiring full pen welding:

1) It's kinda hard in the vertical position.

2) Depending on how much of a stickler you are for weld inspection, considerable inspection costs could be eliminated.

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.

 

[ajk1]

to Kootk: I am not sure that I am understanding your sketch. You seem to be welding on the blind side of the existing beam as well as on the accessible side. Are you saying that if the angle is shop welded to the new tee, that intermittent field fillet welds to the existing beam flange one side of the angle is sufficient?

What are the stiffeners doing, and what spacing are they at, or are they only at the ends. I don't see any mention of stiffeners in Newman's on-line notes.

 

[KootK]

You seem to be welding on the blind side of the existing beam as well as on the accessible side.

The angles would be shipped loose and I'm assuming that the heel side fillet weld to the existing beam is achievable. You'd need to do it anyhow for the backer bars although, admittedly, you'd be much less concerned about the weld quality in a true backer bar attachment. The tee would only be welded to the angles on the freely accessible side.

Are you saying that if the angle is shop welded to the new tee, that intermittent field fillet welds to the existing beam flange one side of the angle is sufficient?

I was proposing welding the angles into place and then, subsequently, welding the tee into place. As I showed in my crappy sketch, the angles would be welded to the beam in two locations. However, it would be the heel weld that would be doing all of the work.

What are the stiffeners doing, and what spacing are they at, or are they only at the ends.

The stiffeners would be stabilizing the Tee. They would be particularly relevant with my proposal as the tee would only be welded on one side which would introduce a small amount of eccentricity to things.


See figure 3 for an example: Link

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.

 

[ajk1]

ok, I think I understand.

 

[RWW0002]

Just FYI - I had a professor once tell me that "goal seek is for business majors." Solver is a much more sophisticated tool and will generally give much more reliable results when programming in excel.

 

[ajk1]

to RWW0002 (Structural)- that is very interesting information indeed. I will give it a try tomorrow. Thanks. Much appreciated.

 

[ajk1]

Where do I find "solver" in Excel?

I have a feeling that goal seek did not work (usually, for other spreadsheets that I have used it on it worked fine)is that perhaps it was using too coarse an increment and missed the number that would have satisfied the iteration. Seems strange though.

 

[wannabeSE]

Bentley's RAM Elements will calculate the plastic moment capacity of a built up section with a wide flange beam on top and WT on bottom.