Channel Toe up or Toe Down? 1

[LowLax]

I have an existing runway beam with a cap channel. The runway needs to be reinforced to carry a new higher capacity crane, so I am adding a channel to the bottom flange to get the extra capacity.

When I did the calcs I had the bottom channel toe up so I had a doubly symmetric section (top and bottom channels are the same size). Now I'm thinking it's going to be much easier to weld with the bottom channel toed down.

Maybe I haven't had enough coffee this morning, but is there a problem with doing this? The I value would increase slightly, but there is such a small area at the extreme bottom fiber that I'm worried I am going to reach yield sooner.

 

[LowLax]

hokie66, I like the idea from FLCraneBuilder in the other thread, but I don't have control over the crane being purchased. The client is getting a deal on a used 30ton crane, and wants to put it on an existing 10ton runway.

 

[cntw1953]

If the rated crane capacity has increased 3 folds, you may need to check on columns, braces, and backup truss, which could suffer more load increase than the beams.

 

[dhengr]

Stillerz: RE: your 10DEC09 15:53 post;

After all the trouble you have caused, all the man-hours consumed on one topic, you should never be allowed to utter the phrase ‘shear flow’ in public again. You can count on that subject being on your final exam. I’m still lookin to buy some shear flow, seems you have caused a world shortage of shear flow with your antics.

 

[weab]

One thing that I don't see addressed here is related to the purpose of the original top channel, lateral load strength. If you are using the bottom channel to get a symmetric section, keep in mind that you have lost the use of the channel for lateral load.

If this combined section does happen to have some reserve strength after accounting for strong axis bending, then the top flange/channel will provide some strength for lateral loads, but I'm guessing that it is little.

I hope this works for you.

 

[apsix]

I see now it's the bottom the channel is being added to; please ignore my comment about crane ploughing thru dust.

 

[BAretired]

Perhaps, instead of simply adding a channel or a plate to the bottom of the existing runway beam, you should be adding a W shape so that you significantly increase the section modulus of the beam. You are going from a 10 ton to a 30 ton crane so this is not an unreasonable solution.

The flange width of the added beam should be greater than the flange width of the existing so that you can use down-hand welding. The cost of the additional material is not going to be significant compared to the labor cost.

BA

 

[apsix]

Is 30 t capacity required?
Just because the crane is rated for 30 t, it doesn't mean that the total system has to be.

 

[hokie66]

apsix,

The system has to be rated the same as the crane. If the crane is downrated to 20 t, then the runway, etc. could be rated 20 t as well, with the actual mass of the crane used in the runway design.

 

[Stillerz]

dhengr-
I hope like hell you are jsut being a smart ass (assuming with the smiley face that you are). I can appreciate it.

I will say this however: There are people who posted a lot more on the subject than I did. Obviously it was a good discussion as 14 million posts were made on the subject. If you need some shear flow here is how to calculate it....
Shear Flow = VQ/I, where Q is the first.....

 

[miecz]

I'm intrigued by the notion that from an allowable stress standpoint, you are better with the toes up, whereas from an allowable strength standpoint, you are probably better with the toes down. I'm trying to determine if, in fact, the strength of the section goes up by pointing the toes down, according to AISC360, but it's turning into an involved process, and, well, I've got to get back to work. If anyone has the time to take this on, I'd like to see the results.

 

[SteveGregory]

From a practical viewpoint, what kind of environment does this bridge crane operate in? If it is in a foundry or a place that has material that could collect in a toes up channel, then I would suggest a cover plate instead.

 

[dhengr]

Stillerz:
You guessed it, I most certainly was just pulling your leg. Your post generated an amazing amount of discussion on a topic which we studied in our strength of materials and theory of elasticity classes in school, but some people still seem to have some trouble with. There were a number of interesting variations on explaining the subject. The subject of shear flow used to be one of my std. questions during job interviews, for engineeers, because we did so much fabrication of unusual built-up sections. See the posts after 2DEC09 on that thread for my comments in the same vane as above, again just trying to inject a little levity. I thought I understood the subject after years of dealing with it, the discussion got pretty involved and intense, and time consuming, but I do want to read it all when I get time to try to make sense out of each post and the best explanations of the topic.

On this thread: I would use added plates; the bottom one being wider than the existing bot. flg. to allow down-hand welding. Use min., but sufficient (VQ/I), sized continuous fillet welds at each flg. tip. They are easier to make, much less fatigue sensitive than stitch welding, and require much less layout time, maybe only one size change in span length. I dislike plug welds for the same reason I dislike stitch welds from the fatigue standpoint. Top plate could be added, atop the channel, to balance the cross section, approx. same width as top flg. on main beam. But, must now design weld btwn. plate/channel and check increased shear flow (weld size) btwn. channel and main beam. Also, check larger shear flow in main beam web just below the radius btwn. its flg./web, that pretty well addresses the beam. As others have said, brackets, columns, etc. may come into play too. The deal on the used crane may turn out to be an expensive deal.

 

[cntw1953]

dhengr:

Seems you have finally got abundant supply of "shear flow"

"...and check increased shear flow (weld size) btwn. channel and main beam. Also, check larger shear flow..."

 

[Stillerz]

dhengr:
I definitely appreciate the sarcasm...I was laughing pretty damn hard at your post. I declare you the Shear Flow King.

Let me be clear here...the plug welds I advise are put in place prior to welding the beam flange to the channel web. This does help control heat distortion in the channel web and keeps it in good contact with the beam flange directly under the crane rail. We have done many runway surveys where stitch welds had cracked and upon investigation the channel cap was cupped allowing the channel wed to "bounce" up and down.

 

[Stillerz]

"wed" = "web"
shear flow

 

[dhengr]

While VQ/I = 0 near the center of the built-up beam, may seem to indicate no need for weld btwn. the flg. and web on a uniformly loaded simple beam; there are at least two reasons why that is not so. The flg. is highly stressed and could buckle if not welded to and stiffened by the web; and in this case a moving wheel load must be transmitted through the flg. to the web, through bearing in the welds, same would apply to any point loading on the flgs. Without large point loading the min. weld size will be governed by relative plate thickness; max. weld size vs. thinnest pl. and min. size vs. thicker pl., to assure enough heat input to the thicker pl. to provide a sound weld without HAZ cracking problems.

Stillerz: RE: the plug welds; they are an intermittent stress raiser with many of the same start and stop and weld root and heat input problems as exist with stitch welds, as these matters relate to a fatigue sensitive structures. This is somewhat less a problem when the welds occur in a compression region. I just have an aversion to plug welds and stitch welds if I can avoid them on anything but a static structure. I do see your concern for channel web distortion, but that is an argument for min. sufficient fillet weld size at the WF flg. tip to C web. I can see how the fillet welds would tend to cause the channel web to belly up, particularly if they are heavy welds. Doesn’t bolting the track down to the WF eliminate most, or all, of the channel web distortion? Should you bolt the track down before you make the fillet welds. Wouldn’t that movement under the rail cause compression at the fillet root, and thus be a less signif. fatigue situation? But, you say they were stitch welds, that bothers me more than the flexure of the channel web. I am not arguing with success or the considerable evidence you say you have, but just offering food for thought. I worked on some crane rails in our own shops, and a few more otherwise, so I don’t claim to be an expert on the subject. But, I don’t recall thinking that the top channel web distortion was an issue. A good guy with a rosebud torch could heat the top of the channel web and draw that belly out of it. If building new, would you roll an intentional, reverse belly into the channel web, clamp the channel down out at the flg. tips, do fillet welds, and the welds would draw most of the reverse belly out of the channel web?

cntw1953: after these couple of posts, I’ve run out of shear flow again, for my own project. I still want to just buy that stuff buy the lineal foot or in 1000 lb lots so I can eliminate all the calcs. and headaches. Ideally it would come in tubes, like caulking, then you could just flow however much of that there shear into the joint as you thought you needed.

 

[cntw1953]

dhengr:

Just Google it, you will get plenty off the shelve of program vendors

 

[Stillerz]

For fear of getting into another 8 year long posting, shear-flow-type session here...this is my final attempt to explain my thought process here.

The weld procedure I spec is:

First, Plug weld Channel web to beam flange; say every 2 feet to ensure the to surfaces are flat together and to MITIGATE, not ELIMINATE, cupping.

Secondly, CONTINUOUSLY weld the beam flange tips to Channel web.

I do not advocate the use of intermittent welds nor the use of ONLY plug welds.

 

[Stillerz]

dhengr:

I you make one more reply to this, I will fire the shear flow discussion back up and keep it going for years!!!!

 

[FLCraneBuilder]

At risk of perpetuating another lengthy debate, I'd like to weigh in on intermittent welds for cap channels..... In my world of Lower capacity, light duty, moderate duty and even somewhat heavy duty bridge cranes (ie steel service centers), we have not EVER continuously welded a cap channel. Always been stitch welded. The firm has been doing it for nearly 50 years and thousands of ft of beam. I recognize this does not make it right.... But we've experienced no problems with this practice.
One of the runway design programs we use even sizes and quantifies the intermittent welds. The software won some kind of award from AISC

http://www.ezsdc.com/software1.aspx

From a cost perspective, I'm quite sure that other crane & runway builders use intermittent welds for this application. In a competitive environment, bidding continuous welds would cause one to lose the job…
Of course, continuous weld costs more and in the case of top channel will put negative camber in the beam which few shops have the equipment to remove. I am quite sure most owners do not know or care about the effects of negative camber or reduced fatigue resistance. In these economic times, they are buying cheapest available products & services they can find.
In contrast to all of this, my design preference is to use heavier weight beams as opposed to adding cap channels. This doesn’t apply to original post though.

In summary, it appears I need to study the fatigue issue, and, being a cost guy, I’d like to discourage general statements that these welds should always be continuous. I hope I have not ruffled any feathers with this – but both sides of the issue should be considered.

Oh and thanks much to Hokie66 for referring to one of my previous posts. It’s nice to see my thoughts are appreciated.