80' unbraced steel columns
80' unbraced steel columns
(OP)
I am proposing on a single story industrial project that will have 80' tall unbraced steel columns. The roof structure will be metal deck supported by joists and joist girders on 50' bays. Top of roof will be approximately 80' above top of slab. No mezzanines.
I want to get a sense of what these columns will be before I send my fee. I know I'll likely need to deal with splices and fairly large sections.
The joist girders will sit on top of column cap plates. I don't worry about eccentricity when you have a joist girder bearing on top of the usual HSS8x8 or 10x10 column since it's a small section. In this case, I'm wondering if it's warranted to assume some amount of eccentricity (with unbalanced loading situations) since the column sections will probably be 14x14 or larger and they're just so darn tall. The joist girder will still be on top of the column but maybe assume a few inches of eccentricity? See attached sketch.
Any opinions on whether I should assume some eccentricity for unbalanced load situations or thoughts on tall columns would be appreciated.
I want to get a sense of what these columns will be before I send my fee. I know I'll likely need to deal with splices and fairly large sections.
The joist girders will sit on top of column cap plates. I don't worry about eccentricity when you have a joist girder bearing on top of the usual HSS8x8 or 10x10 column since it's a small section. In this case, I'm wondering if it's warranted to assume some amount of eccentricity (with unbalanced loading situations) since the column sections will probably be 14x14 or larger and they're just so darn tall. The joist girder will still be on top of the column but maybe assume a few inches of eccentricity? See attached sketch.
Any opinions on whether I should assume some eccentricity for unbalanced load situations or thoughts on tall columns would be appreciated.






RE: 80' unbraced steel columns
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: 80' unbraced steel columns
RE: 80' unbraced steel columns
RE: 80' unbraced steel columns
My gut is telling me HSS 16x16 are on the small side for this building. You may want to consider precast columns. They won't look like toothpicks 80' in the air and would get erected pretty quick.
RE: 80' unbraced steel columns
RE: 80' unbraced steel columns
RE: 80' unbraced steel columns
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: 80' unbraced steel columns
As MotorCity pointed out, lateral is critical. If you have 80' tall braced frames, chord columns will see higher loads, and the 80' out of plane buckling length still applies. May want to locate these at building corners where you can reduce that buckling length.
RE: 80' unbraced steel columns
I estimated a drift value for the braced frames to check the impact of P-delta on the gravity columns. It's in a high seismic region and drifts will be significant.
I've never inquired about rolling or shipping 80' long sections. I think the trucking could be done at night with the correct permits. I've seen those large wind turbine blades shipped at night and they're much longer than 80'. I'll have to discuss with the client if I get the project.
There are going to be A LOT of these columns so I'll be pressured to skinny them as much as possible.
RE: 80' unbraced steel columns
1) A taller column implies a larger imperfection which will tend to dwarf the impact of the load eccentricity at the cap plate. Good.
2) Your splices will potentially be sources of abnormal imperfection. Bad. This might warrant some consideration, particularly with regard to splice locations (40'/40' versus, say 20'/40'/20').
I'm confused by the Big P-Delta concerns expressed here. Do we not only have Little P-delta to worry about for pin ended gravity columns? In my opinion, Big P-Delta only comes into play for the lateral system and any columns where end restraint is being considered. And I would hope that these 80' columns are not the lateral system (moment frame).
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: 80' unbraced steel columns
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: 80' unbraced steel columns
Per jayrod, jumbo HSS probably lightest section. Those manufacturers will know shipping constraints too. Avoid splices.
sliderule makes a good point about corrosion. You'll have to balance this consideration with a shape that gives you the ry needed. Open shapes unlikely to work for ry, and built-up shapes add lots of labor cost. Consider HSS with "weep" holes or special treatment near the base.
RE: 80' unbraced steel columns
RE: 80' unbraced steel columns
Hokie, that's not enough info? Braced frames I would expect would be the only way to go on something this size. I can't even get 20ft moment frames to work reasonably. I can't imagine what the columns for an 80 ft moment frame would look like.
RE: 80' unbraced steel columns
RE: 80' unbraced steel columns
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: 80' unbraced steel columns
The geometry will be interesting in an 80' tall by 50' wide bay. I have some ideas though.
structSU10, you're definitely right on that. The exterior wall wind loads will be significant. I'll probably need a lot of wide flange wind columns to handle the load.
RE: 80' unbraced steel columns
RE: 80' unbraced steel columns
Or do yo propose trucking an 80 foot column as a special traffic load?
RE: 80' unbraced steel columns
I'd prefer the columns be one piece but I've never specified a member that long. I don't know how long they can make them at the mill or if shipping them to the fab shop/site at that length is easy to do.
Even if they can make them that long and ship them that long, whoever the fabricator/erector/contractor is may not want to deal with an 80' long column. I'm really not sure. If I get the project I'll definitely recommend getting a contractor/fabricator on board early to discuss the best way to build this.
RE: 80' unbraced steel columns
Well, if you are insistent on using HSS, you need to find out. After all, the internet is at your fingertips... I looked into it (at least from one manufacturer) and it took less than 5 minutes:
60 feet, max. But I also found out that you are not "stuck" with choosing from "wimpy" standard sizes or thicknesses... of course the cost goes up, up, up. Don't rule out W sections. Read this: Link
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: 80' unbraced steel columns
RE: 80' unbraced steel columns
If I get the project this will be the first thing I tackle. I'm sure any fabricator I contact would love to get their hands on this project so they'd probably be willing to help me hash out the fabrication and constructability questions.
RE: 80' unbraced steel columns
www.SlideRuleEra.net
www.VacuumTubeEra.net