Que on Length of Free Gusset Plate Edge 1

[learning2banengineer]

When designing brace to gusset plate connection for seismic, a large gusset plate is often resulted when the brace come towards the adjacent members at a steep angle while satisfying the 2t distance for ductility. This results in long free gusset plate edges. When stiffener is added to re-enforce these edges, the plate will get into the 2-t zone in order to decrease the free gusset plate length to be within the limit as suggested by Astaneh-Asl, 1991, please refer to the attached sketch. The length of the free edges was also resulted from the weld length required between HSS brace to gusset, since the recommended weld length = 2w of the HSS (CISC). It seems to me that the requirement to decrease free gusset edge length and providing 2t clearance cannot be always satisfied at the same time, unless by increasing the gusset plate thickness to an unreasonably thick gusset plate. Anyone has any commendation on this?

 

[nutte]

I wouldn't think the free edge stiffener can connect at the 2t free zone. Perhaps you could skip this area, putting in two free edge stiffeners, one on each side of the bend line.

I'd be inclined to simply make the gusset thicker. You're using 5/8" now. 3/4" isn't unreasonable for seismic detailing.

 

[learning2banengineer]

Thanks nutte for the response, you are right, maybe skipping the zone can help, but it would look kind of awkward (just my opinion). yeah even it I increas the plate to 3/4" it still does work due to the geometry with the weld length required. 1" will do, but seems a little excessive for 140kips. It seems to me that if the free edge is put into consideration, then the gusset thickness almost is dictated by the length of brace to gusset weld and the free edge length requirement.

 

[Gumpmaster]

I agree with nutte, I wouldn't feel bad at all using a 3/4" gusset plate, that will help with the free edge buckling.

Check out the Steel Tips from May 2004, Design of Special Concentric Braced Frames, page 112. It says that the brace can be considered to stiffen the free edge until the free edge is a perpendicular distance b away from the wall of the HSS where b/t=52/Fy^.5. This should solve your problem. You may need to decrease your flare angle to less than 30deg, but it shouldn't be a problem.

Is your column an HSS? I've never been able to get the horizontal gusset plate force to work on an HSS column. Did you check that?

 

[learning2banengineer]

Hi Gumpmaster, I will look tht up, I am just wondering are steel tips for free or is it only for AISC member? Since I am in Canada, company didn't join AISC.
Yes the Column is a HSS column. For this particular project, structural engineer on record specified the workpoint to to be on top of the cap plate. I was thinking that the only force then required to be transferred along the horizontal edge of the gusset is only the horizontal component of the brace load, there is almost not moment since the work point is only thickness of the cap plate away from the horizontal edge. Is that what you were talking about?

 

[sdz]

I would ask why the brace is so far away from the column? Move it in as close as practical and you reduce some of the problems.

 

[Gumpmaster]

It depends how you distribute the forces. If you use the Uniform Force Method (UFM), you'll have a horizontal force on your column. This is needed to balance out all the moments. See Part 13 in the 13th edition AISC manual if you have that. If not, there are several examples floating around the web of the UFM. Another option is the KISS method. In that method all of the horizontal force goes into the beam and all of the vertical force goes into the column, but you need balancing moments on the beam and column so that your overall moment is zero. Those moments must be handled by both the gusset plate weld and the beam/column iteself.

The article "Designing Compact Gussets with the Uniform Force Method" by Larry Muir in the 1Q 2008 AISC Engineering Journal gives a good overview. Also, "Bracing Connections to Rectangular HSS Columns" by N. Kosteski may be a good one. Both are available for free on the internet.

Bottom line is you have to use some method of distributing the force to the beam and column. I've seen quite a few people say that the work point lines up, but that doesn't quite cut it. There is no moment about the work point, but there may be moments on the gusset plate, beam or column.

Steel Tips are available free for residents of California. They are available for a small fee ($30 or so) to the rest of us. They're well worth it. There are several other Steel Tips on SCBF's.

 

[nutte]

sdz, these seismic connections require a bend line (the dashed line on the left in the original sketch, shown as 2" from the end of the brace), so that the gusset can buckle clear of the gusset-to-column and gusset-to-beam connections. Your configuration doesn't allow for that.

 

[learning2banengineer]

Hi Gump,I was actually using the KISS method. Using that method, if the gusset slots into the column, and if the workpoint is at the top of the cap plate, then there is no moment to the colmun, is there? Please refer to the sketch attached. I guess there will be moment to the beam since the centre of the beam is now 0.5 d to the work point.

 

[Gumpmaster]

You're right. I didn't realize you were slotting through the HSS. There shouldn't be a moment on the column.

I think beam will feel a moment of your horizontal force *d/2, but that moment won't be felt at the beam/gusset interface. The compression/tension force in the beam is still taken out at the centroid of the beam, but applied at the beam flange.

I may be missing something because I didn't take the time to draw out a free body diagram, but you need to show both the applied loads and the reactions to figure out what acts on each member. It helps to draw an exploded free body diagram where the forces on the beam, column, and gusset are all drawn seperatley, if that makes sense.