Stiffener Design for Web Crippling

[Once20036]

AISC 13th Sect J10 has a number of requirements that need to be checked when you have point loads on flanges, including flange local bending, web local yielding, etc, etc.

Most of these can be strengthened with the use of transverse stiffener pairs (perpendicular to the web). Per section J10.8, "...compressive forces... design in accordance with the requirements of Sctions E6.2 and J4.4. The member properties shall be determined using an effective length of 0.75 h and a cross section composed of two stiffeners and a strip of web having a width of 25tw..."

I'm designing a plate girder supporting some pretty serious concentrated forces. In lieu of using one really thick stiffener - I'd like to use several smaller stiffeners, say 5 pairs, spread out over the width of the applied force. I had been running calculations ignoring the "strip of web" but feel like I`m missing out on some strength here, but using a width of 25tw would overlap, or double count, some web sections.

I'd like to use a strip of web equal to 1/2 the stiffener spacing in each direction. Does anybody see any problems with this procedure?

Alternatively - I could use member properties for the entire 5-stiffener pairs with the web extending 12.5tw past the end of each stiffener pairs, but I believe this will provide much more strength than the above and doesn't intuitively feel right to me.

 

[nutte]

I would use 12.5*tw each side of the outer stiffeners, plus the web between the outer stiffeners, and check it all as one combined section. (This assumes that the spacing between adjacent stiffeners is not greater than 25*tw.) It may not feel right, but think about the case for a single stiffener pair. You're counting on the stiffener, plus 12.5*tw each side of the stiffener. That's the same thing you're doing with the 5-pair configuration.

You could do the other, count on the stiffener plus web length not to exceed stiffener spacing. This will be conservative. But there's no reason you shouldn't take advantage of the full 12.5*tw each side.

 

[Once20036]

To clarify my original post:
If you check the compressive strength of the (1) 5 stiffener pair system, the capacity will be significantly higher than if you check the compressive strength of (5) individual stiffener pairs.

Nutte,
I agree that regardless of which of the above you choose, it is appropriate to use 12.5tw past the outermost stiffeners.

Thinking about it a little bit further, there are no slots cut in the web separating individual stiffener pairs, such as Option 2 implies (5-individual pieces). Everything is tied together and checking it as one system is starting to feel more reasonable.

 

[JAE]

I would use the 12.5tw on each side as well.

 

[ToadJones]

Are you wanting to use multiple stiffeners for one concentrated force?
Or are you wanting to use multiple stiffeners to support multiple concentrated forces?

In the past when I have designed a single pair of stiffeners, the column shape being checked turns out to be a cruciform shape.
When designing a double pair with a single force, the shape I used to check buckling looks more like an I-shape. The radius of gyration for the I-Shape being more effective in preventing buckling.

I think it depends on how the load is applied to the stiffeners.

 

[Once20036]

JAE,
I assume you mean 12.5tw on each side of the outside stiffeners designing as (1) 5 stiffener pair system? and not 12.5tw on each side of exterior pairs and (5) individual stiffener pairs?

ToadJones,
It's a concrete column landing on the plate girder. There will be a flat plate welded to the top of the plate girder with reinforcing steel for the concrete column welded to that. So it's multiple stiffeners resisting one "concentrated" force. outside stiffeners will align with the outside of the concrete columns with intermediate stiffeners distributed between.

You lost me a little bit the the cruciform/I-shape.
I would expect one stiffener pair to look like a lower-case t.
|
=== === web
|
I would expect two stiffener pairs to look like two lower-case ts.
| |
=== === === web
| |
By designing for an I-shape, are you ignoring the web outside the two stiffener pairs?
| |
=== web
| |
How would including this steel hurt your radius of gyration?
(And how's that for some ASCII art? lol)

 

[Once20036]

That lined up much better typing it than it did when I posted it.
Guess my ASCII art isn't very good after all...

 

[nutte]

Structural20036, I believe that's what JAE meant. It is also what I meant.

It sounds like you're thinking each stiffener pair will take 20% of the load. You don't have to check it that way. Take it as one combined section, with 5 stiffener pairs and the web going past the outer pairs.

 

[ToadJones]

Structural-
Yes, I agree with you 100% that "I would expect two stiffener pairs to look like two lower-case t's"
But the web's outside of the stiffeners dont help the radius of gyration as much as the stiffeners. I would often start by ignoring the extra web and design myself a little "I" shape column is all I was saying.

Cruciform = lower case "t".

 

[dhengr]

Structural:
Keep in mind that the cost of that plate girder is in good part a function of welding and fabricating costs. So, more stiffeners may not be the most economical solution. Why not look at bringing the max. allowable amount of web material into play, per pair of stiffeners. Maybe that means 3 or 4 pairs, not 5 pairs. Maybe fewer huskier stiffeners optimizes this relationship btwn. web material and stiffener material. Toad’s cruciform shape is just the effective web and a single pair of stiffeners showing up as a cross shape when a horiz. section is cut through them. His I or W shape col. shape is two pairs of stiffeners with only the web material btwn. the stiffeners showing when a horiz. section is cut through them. I have also seen pairs of split S shapes or WT’s used as web stiffeners, or bearing stiffeners.

The 12.5tw or 25tw is an effective width limitation, isn’t it, and is a function of the way the load is applied to the stiffener, its height (length), the relative thickness of the stiffener and web, and shear flow and shear lag; as this relates to plate crippling or buckling. If you had a space of 26 or 27tw btwn. two pairs of stiffeners I would expect that web to be pretty much fully effective, and might stretch things a bit, allowing this; while a web area more than 12.5tw might be stretching the effective web width a bit too much outside the last pair of stiffeners.

If you are welding heavy rebars to a thick base plate or to a thick girder flange plate, you might pay some attention to through plate tensile stresses. This might involve UT’ing. those plates in those areas to be sure there are no internal inclusions or laminations in those areas where you have high tensile through plate stresses. These inclusions or laminations will usually show up when cutting the plates to size if you cut through them. I believe there are couplings which could be welded to the top flange of the plate girder. They accept threaded rebars to start your conc. col., and would eliminate the base plate and its welding to the top flange. This welding could be problematic in that you have high tensile loads, plate bending and prying on the weld root in the vicinity of each large rebar.

 

[JAE]

dhengr - well yes you are right about the added expense of stiffeners, but if you have an 80 ft. plate girder with a special case having very serious concentrated loads right over one spot on the girder; adding thickness to the web all down the length of the girder when you only need it at the support/load point might be uneconomical. You'd have to look at where you can splice/change the web thickness to see if that is helpful cost wise.

In other words - 80 feet of thicker web might cost more than a few extra stiffeners.

For the above posts - yes - I meant 12.5tw beyond the outside stiffeners.

 

[Once20036]

dhengr,
I`m inheriting a project that another engineer began, so my flanges and webs are already bought and fabricated. The previous engineer showed stiffeners (so they're already bought) but didn't provide sizes, welds, or spacing so I`m working on the details.
Everything is going to be thoroughly inspected/UT'd/etc by a (very good) CWI
I don't believe that high tensile loads are expected and the welds were added (by the previous engr) for continuity/peace of mind.

As always, I`m very appreciative of this website and everyone's thoughts.

 

[dhengr]

JAE:
Wow! You had to work pretty hard to read that into my post. It would have been so easy for me to say increase the web thickness, full length of the girder, and forget the stiffeners altogether if that was what I intended. Since the OP’ers. sketch and description are sorta lacking in loads, material sizes and his assumed stiffener spacing, we are all guessing to some extent.

Please read my post completely for the first time, for its full intended meaning, and tell me were I suggested increasing the web thickness over the full length of the girder, or suggested not using stiffeners. The OP’er. was talking about fairly closely spaced stiffeners (less than 25tw apart) which did not bring the full web length (for whatever the thickness might be) into play, i.e. 12.5tw per side or 25tw btwn. each pair of stiffeners. I was suggesting maybe using fewer stiffeners (maybe somewhat huskier stiffeners) at the load location but spacing them to take full advantage of the 25tw. Maybe even trying to make a spacing a little more than 25tw work btwn. stiffener pairs if that would reduce the number of stiffeners.

 

[JAE]

dhengr - relax. I read your statement [blue]"Why not look at bringing the max. allowable amount of web material into play, per pair of stiffeners. "[/blue] and read into it that you were suggesting more web material. Sorry I misunderstood you.

 

[Once20036]

Thanks again everyone.

My next question, which may belong in a new thread, has to do with the next couple sentences after the appropriate web width is determined.

"The weld to the flange shall be sized for the difference between the required strength and the applicable limit state"

So far so good.

"The weld connect full depth bearing stiffeners to the web shall be sized to transmit the difference in compressive force at each stiffener to the web"

This is where I get a bit lost. Is this the same design force as the welds to the flange? So if the difference between strength and load is 100kip, the welds to the flange & the weld to the web are both designed for 100 kip?
For a partial depth stiffener, this has to be true (you get the force in, so you have to get the force out), but what about for a full depth stiffener?