Call out for Frames with tapered cross section

[DRW75]

Folks,
Is it standard to call out on drawings the section dimensions for a tapered frame, or is it more appropriate to call out the wide flange section to be used to form the shape, and the taper angle? I'm not sure how much info the fabricator will require

thanks for the input

DRW

 

[ajh1]

All depends on how you expect the fabricator to make it. If it is to be cut from a wide flange section I would call that out. If the fabricator has the option to make it as a three-plate section then the basic dimensions of each plate should be specified. I would question whether it is cost effective to cut a wide flange section to fit a taper versus making it up from scratch.

 

[DRW75]

It should be more cost effective (I would think) to make the tapered section out of wide flange, as all that is required to do is to cut the web along the desired taper angle (divided by two of course), flip the pieces, and then go with a square butt weld along the length of the web to re-attach.

To make a tapered section out of plate, the web still has to be cut on the taper angle, and then the welds to connect the flange to the web (even if fillet stitched with no prep). This would seem like more work, and more potential wastage of plate for the web & the flanges.

I wonder then, If it is left to the devices of the fabricator to choose the best method, then I would assume that they would design their own weld requirements (and of course send off the shop drawings for acceptance).

DRW

 

[UcfSE]

You would also have material considerations. If you cut the section out of a wide flange then your tapered beam will be A992 most likely. Otherwise you could end up building a section out of A36 plate unless you specify the more expensive grade 50 plate to match the strength that would have come from a wide flange. If you build-up a section from plates you also have the option of making a hybrid girder where you might have grade 50 flange and grade 36 web to save some money. It would seem that cutting the wide flange might also be easier because you weld back only the webs, causing less tendency for warping and requiring less labor-intensive care to avoid warping. If fatigue is a problem then I wouldn't let the shop choose from building the section from plate or a wide flange unless you really check both ways.

 

[DRW75]

Yeah, I think I will call it out a wide flange with the taper angle included. I don't want to get too dirty with the details to provide the fabricator... like I need to specify the end plate condition, and that it's an FMC bolted connection. I would prefer to let them do the job they are good at, and save me the time of designing all the little details.

For this case, fatigue won't be an issue - only wind loading present, and the stresses associated with that load case are low enough that it wont be past any fatigue threshold - high cycle stress is probably 5% yield at most. Snow governs the design of this member.

DRW

 

[UcfSE]

How about using WT's and cutting the web of one WT down? Is that any better that splitting a wide flange?

 

[DRW75]

It does seem to me that splitting the wide flange is the least cost/effort - unless it is substantially more difficult to split a wide flange than it is to trim a WT (it doesn't seem like it would be - but I’m not a fabrication shop savvy enough to say for sure). And, obviously with the WT case, there would be the wastage of the wedge of web that you would pay for regardless.

Having said all that, you could just use two WTs, and fillet them such that the webs are offset, that way you don’t have to trim anything, just form up the bevel angle - however you do have a lovely seam in your web then, an every so slightly eccentric section, and still some ineffective steel where the TEEs lap that you're going to pay for.

DRW

 

[ajh1]

Have you talked to any metal building manufacturers regarding your needs? This sounds like standard metal building construction and they spit out tapered sections every day very cost effectively.