Built Up Box Section Supporting Moving Equipment, Slender Elements 1

[Boiler106]

I have a 4'x4' built up box section pedestal standing 10' tall supporting moving equipment (robot) on top that I am evaluating, however, it is not currently in service, nor has it been.

The walls are made of 1/4 inch plate which obviously qualify as slender elements per AISC Chapter B. The top of the column/box section sees downward axial force, horiz shear, overturning moment and torsion in one loading condition.

Being a building structural engineer, the slenderness makes me uneasy. I have checked by hand the combination of forces using AISC 360-10 and it appears to work, however, I am unsatisfied that i can use such a slender element without stiffening. I am limited to RAM elements for analysis.

Ignoring vibration and fatigue, are there code limitations on width-to-thickness slenderness for axial/shear/flexure of these elements that i am missing? Can anyone provide guidance on stiffening?

 

[WARose]

AASHTO has put out manuals on steel box girder design. So that might help.

 

[SAIL3]

loads?...

 

[Boiler106]

Service level loads
axial=10k
shear=7k
moment=70k-ft
torsion=40k-ft

 

[WARose]

By the way, if you don't have the AASHTO reference i mention above.....another good one is 'Guide to Stability Criteria for Metal Structures', 5th ed., by: Galambos. He's got a whole chapter on it (i.e. Chapter 7).

 

[SAIL3]

first cut...assume axial/moment loads are resisted by the corners.......
16t=16x.25=4"....4x4x1/4 equivalent angle in each corner....
total axial in corner P=10/4 + 70/(2x4)=2.5 + 8.75 = 11.25 kips/corner
max total shear per side V= 7/2 + 40/(2x4) = 3.5 + 5 = 8.5kips
I would add a ring of stiffeners @ mid-height to reduce the z-unbraced length and brace the corners in the z-dir...assume the pl provides a continuos bracing in x & y dir....
I would also add a comp brace(corner to corner) @ this ring of stiff to prevent racking and for shipping & handling.
the ring of stiffeners also act as an intermediate stiff on a girder for shear...
assumed top and bottom of structure is torsionally restrained

 

[dhengr]

Boiler106:
There is an awful lot of relevant engineering design info. missing in your description of your problem. Sketches of the whole system, dimensions, loads and their application points, etc. are all needed for a meaningful discussion. The devel’s in the details on a structure like this and you don’t say a word about any of the details. 10k is not a big vert. load, but if it is concentrated at the mid point of one of the sides, as might be the case while the robot operates, it could be a big deal. These types of design problems are really interesting/fun, but they do require some engineering imagination, judgement and experience. They are not well codified, and you won’t find a lot of worked-out example problems. The design involves a lot of Strength of Materials, Theory of Elasticity, Machine Design, etc., plus the AISC code, and its design intent and thinking, if not its exact code paragraphs and formulas.

 

[SwinnyGG]

As someone with a lot of experience putting robots in weird places...

1/4" is very thin for a box section robot riser that tall.

How thick are the flanges, and what class of robot?

As a point of reference, there exists an organization called NAAMS, which is a joint venture organization between Chrysler, GM, and Ford, which maintains standards for shared manufacturing fixturing components so that their suppliers can work to a common standard and reduce cost.

One of the specific families of parts covered by NAAMS is robot risers, and there is a long list of typical configurations available here:

http://www.naamsstandards.org/Standards/chapters/assembly/D.pdf

Judging by your values for service load (these are unfactored, yes?) I'd guess you're talking a robot in the 500-700kg class; this would be covered by pages D-30 thru D-33 in the NAAMS document above. You'll notice that 700kg class is the largest covered; there are robots much larger than this (over 2,000 kg payload) but they are installed in such small quantities, and for such specific applications, that the mounting schemes are typically engineered on a case-by-case basis.

Again, just as a point of reference, the tallest riser which does not require special engineering per NAAMS is 1800 mm, and that riser has a 42" OD round column with a 1" wall and 2" flanges.

If I were the lead engineer on a robot install and I had a 700kg-class robot on a 10 ft riser that was a welded box section only 1/4" thick with no other reinforcement... I would be assuming that riser is nowhere near close to adequate.

 

[SAIL3]

on further thought my comments did not consider the affect of deflection would have on robot operation and I also have no experience in robot operation....in that light, I would defer to the comments by jgKRI who has this experience.....