80' unbraced steel columns

[DETstru]

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.

 

[KootK]

Bloody cool. I see the argument but vote for no eccentricity. Even with smaller columns, I feel that the rotation of the joist and seat effectively put the reaction pretty close to the edges of the column. In summary, if concentric is good enough for little columns, I feel that it's good enough for big columns too.

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.

 

[canwesteng]

Wouldn't moving the reaction closer to the edge of the column increase eccentricity? Based on your sketch, I think you should consider some eccentricity, but it probably won't make much difference. The moment due to eccentricity is still just P*e, regardless of column height.

 

[JStephen]

P-delta consideration or seismic loading would introduce some moment as well. If exposed to the wind, you can get some vibration issues as with stacks that you might not expect. Check if the column can be handled with its own weight. Pipe/round tubing might be cheaper than rectangular tubing.

 

[MotorCity]

I would not worry about the moment due to eccentricity, I would be more concerned about the moment due to wind or seismic when combined with the axial load. And then you have drift......

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.

 

[cliff234]

Yes, you should consider eccentricity, including accidental eccentricity due to tolerances, etc. Also be sure to consider "P-little delta" second-order effects. You might also consider using W14 columns with plates welded to the sides to create box sections.

 

[jayrod12]

Jumbo HSS sections are becoming more economical and readily available. I would at least give them a look before I started fabricating box sections out of W14s

 

[TehMightyEngineer]

Don't forget the old rule of thumb that industrial facilities love to attach things to building columns down the road. This tall of a column I'd definitely have some extra capacity for when someone does attach something to the side of this.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[calvinandhobbes10]

Splices---it sounds like it's a design-build effort, if so talk to the contractor. You are asking helpful questions that can direct them toward an approach. Field splices, probably CJP flange welds with backing bar all around, will likely be high off the ground, labor-intensive, require special inspections, and I suspect there will be a lot of them. Arranging to ship 80' columns may be advantageous.

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.

 

[DETstru]

Thanks for the insight guys.

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.

 

[KootK]

One interesting facet of this is that the steel column design provisions that we use embody a Shanley style imperfection which is, analytically, a function of column length. That has good and bad associated with it:

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.

 

[SlideRuleEra]

I agree with cliff234 about using W14 columns, with reserve capacity (per TME's comments), but without side plates. Since industrial application usually have a long useful life, maintenance is important. Neither an HSS nor a boxed column will allow routine inspection and painting inside for corrosion protection. Also, bolted connections to a W section are more practical. Welds are good, but for heavy loads we preferred bolts for our electric generating stations located in high wind / high seismic locations.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[calvinandhobbes10]

kl/ry < 200, assuming we are abiding by that, dictates section options for steel.
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.

 

[hokie66]

I think a little more information about the lateral system is needed before selection of the column type is made.

 

[jayrod12]

I estimated a drift value for the braced frames to check the impact of P-delta on the gravity 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.

 

[structSU10]

If this building has a girt system for the cladding, your exterior columns with wind loads from girts will be a large driver for the size you may want to use.

 

[KootK]

SRE's comments got me thinking that, one way or another, any eccentricity could be ironed out by providing a wide flange shape to attach the joist girders to. That could take the form of a WF column, a partially plated WF column, or an HSS cap detailed as shown below. The stub would impact the column buckling capacity not at all.

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.

 

[DETstru]

Braced frames for sure. I'm not sure if it'll end up as an SCBF or maybe BRB's.
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.

 

[JStephen]

For interior corrosion, if you can cap the top and bottom so the column is airtight, that pretty well eliminates the concern.

 

[racookpe1978]

Splice is to allow shipping of the two 40 foot sections, right?

Or do yo propose trucking an 80 foot column as a special traffic load?