Single Column Supporting Small Structure - Lateral Design 3

[Charred]

I have a project that includes a look out, or viewing platform. It is a small 14'x14' structure supported on a single tube column (see attached sketch). I am looking for some guidance on which methods to use for seismic & wind design. Seismic design category D and wind speed is 120mph - exposure C.

For seismic, I first looked into using ASCE7-10 chapter 15 for non-building structures since there is an entry in table 15-4.2 specifically for inverted pendulum type structures. However, section 12.2.5.3 states that inverted pendulums need to be designed with section 12.8. I can use steel special cantilevered column systems since the height is less than 35'. What are your thoughts and/or suggestions on this?

For wind, I could look at this several ways and looking to get some consensus on this. My thoughts are listed below:

1). Use Ch. 27 Part 1
2). Use Ch. 29 - section 29.4.1 for signs
3). Use Ch. 29 - section 29.5 for other structures

 

[Lomarandil]

I'd consider it a building structure, particularly for seismic design. If you have (non-industrial) human occupants, even temporary, I think that's the reasonable standard of care. Is there a reason you wouldn't want to use ELF for seismic design?

That said, I don't know that I'd call this a "regular building" for wind design by Chapter 27. So I'd be very tempted to consider or even envelope the wind provisions of Ch. 29 as though this were an elevated tank or sign.


----
just call me Lo.

 

[phamENG]

I agree with Lo on the seismic - especially if you're design category D. Consider it a building structure and design it as a cantilevered column system.

For wind, it looks like you have a partially open structure. Use the directional procedure, and define h and z accordingly. I'd also apply the wind loads to the projected area of the stairs below based on their z.

 

[Charred]

Thanks for the responses.

Lo, the reason why I was looking into using a non-building structure for seismic design and not use ELF was to keep from having to increase the loads from the upper system per section 12.2.3.1 (vertical combinations) since this is already going to be a large column and footing.

Let me throw another thought out here - one I am completely unfamiliar with and looking to be educated on...

This project is for an organization that mentors young girls and the project consists of 6+ actual buildings and this stand-alone viewing deck. Could this structure be classified as some sort of playground assembly or something of that nature? If so, what kind of requirements would that entail? I can't seem to find much information in the IBC or ASCE in that regard. I may be way off path here. I would rather be conservative in my design and ELF may be the correct way to go, but I was looking into other options to make sure I didn't get too out of hand with my design loads if there was a more practical and generally accepted method.

 

[r13]

was looking into using a non-building structure for seismic design and not use ELF was to keep from having to increase the loads from the upper system per section 12.2.3.1 (vertical combinations) since this is already going to be a large column and footing.

I think the very important thing is the connection between the lookout play house (not a open shade platform) and the column. Agree with all the above.

 

[KootK]

Lo, the reason why I was looking into using a non-building structure for seismic design and not use ELF was to keep from having to increase the loads from the upper system per section 12.2.3.1 (vertical combinations) since this is already going to be a large column and footing.

I believe that this is not a Cantilevered Column seismic system but, rather, it's more punitive cousin the Inverted Pendulum. As such, you'll be stuck with R=2 and no 25% overturning reduction in your footing design. That is, of course, unless you can persuade the authority having jurisdiction that these things are not truly "occupied" structures. I see the logic that but, then, these are often emotionally charged conversations, even when had by engineers. "A very low probability of a handful of little girls getting injured in a collapse represents a very low hazzard". It is true but, for many, unpalatable. That, especially, against the argument that you may only be spending a few hundred dollars extra per installation to ensure that doesn't happen.

 

[KootK]

For what it's worth, I imagine that your highest risk "failure" here is wind vibration, lateral and torsional. A little vortex shedding at the cap might get the thing oscillating torsionally. Of course, so long as collapse doesn't ensue, maybe that just adds to the fun.

 

[JStephen]

Wind vibration came to my mind also, it's hard to imagine it would work as drawn.
Also consider eccentric live load. And the possibility that kids in that thing would intentionally get it rocking as much as they could.

 

[BAretired]

The column, as drawn, appears to be about 5 or 6 inch in diameter. I don't think that is even close to what is required. A 10 or 12 inch diameter column would seem more reasonable but probably would not appeal to the architect.

BA

 

[Charred]

I don't know the size of the column they have in their sketch-up model, but I did tell them it was going to be much larger than they have shown. Wind is definitely proving to be the problem here.

 

[Charred]

KootK, would the spiral staircase not act as a strake to suppress any vortex shedding?

 

[JLNJ]

I don’t have ASCE 7 handy - is an inverted pendulum even permitted for this type of building? At 21’ tall this thing is going to move all over. You are going to need a big ‘ol pipe.

 

[Charred]

35' is the max height for cantilever columns in SDC D

 

[msquared48]

Use a pole footing too...

Mike McCann, PE, SE (WA, HI)

 

[azcats]

What size column are you ending up with? My gut says 20" or 24" based on my sign experience. But we don't control deflection that tightly.

 

[KootK]

35' is the max height for cantilever columns in SDC D

I feel pretty strongly about this being an inverted pendulum system rather than a cantilevered column system. Conveniently, there is no height limit for inverted pendulums.

KootK, would the spiral staircase not act as a strake to suppress any vortex shedding?

It would. My concern was not with dynamic response to wind on the post but, rather, on the box at the top of the post. Obviously, it won't make sense to make a wind vibration science project out of this structure where the consequences of vibration issues would be minor. Maybe just keep your natural frequencies for lateral and torsional vibration to a reasonable range and leave it at that.

 

[Charred]

What size column are you ending up with? My gut says 20" or 24" based on my sign experience. But we don't control deflection that tightly.

20" right now and deflection is a little higher than I'd like. The unbalanced live load is the killer. I started off with a 100psf, but have since backed that up to 60psf. Haven't found a good justification yet, just playing with the design until I get something I feel comfortable with and then I'll verify/justify all of my loads.

How practical, or available, is this size of pipe column? Keep in mind I have to have fixed cantilevers at the top of this thing to support the small structure above it. I like the interesting and challenging, but starting to think I may want to switch gears and talk them out of it and put their money to better use elsewhere on the project.

I feel pretty strongly about this being an inverted pendulum system rather than a cantilevered column system. Conveniently, there is no height limit for inverted pendulums.

This is what I have circled back to myself even though seismic shouldn't control here.

ASCE 12.2.5.3 says to use ELF for inverted pendulums, but you have to get the seismic coefficients from Chapter 15. This has always been confusing to me - why is this?

 

[human909]

I feel pretty strongly about this being an inverted pendulum system rather than a cantilevered column system. Conveniently, there is no height limit for inverted pendulums.

For those not familiar would you mind elaborating on the difference?

Everything about this seems to be what I'd describe as an 'inverted pendulum'. All in all I would expect it to be a pretty flexible structure so seismic forces would be less likely to dominate than wind or live load.

 

[Deker]

Since the primary function of the structure is to provide occupancy, I would classify it as a building structure. That's open to interpretation, and ultimately the AHJ has to sign off on however you choose to classify it, but it's what I've done on structures similar to yours. Accordingly, the following advice will be tailored to a Chapter 12 approach to the design.

Your lateral system is a cantilevered column. If you are in SDC D, Table 12.2-1 allows you to use a special cantilever column (max height 35') or an ordinary cantilever column if you meet the height and weight limitations listed in footnote i and Section 12.2.5.6. Each have different seismic compactness requirements in AISC 341, and if you use a round HSS it shouldn't be an issue. But if you use a square HSS and are forced into using a special system, you'll likely end up needing to fill the HSS with concrete to satisfy the compactness criteria.

Since you have a cantilevered column, you'll need to satisfy Section 12.2.5.2. Axial load limit is probably not a big deal since your column will be huge to meet drift requirements. But you also have an inverted pendulum structure, which is treated in Chapter 12 as a subset of the cantilever column system. To test if you have an inverted pendulum, imagine the deflected shape of your structure. Does the mass at the top of column rotate to match the slope of the column? If so, you've got an inverted pendulum. As such, you'll also need to satisfy Section 12.2.5.3. This section requires that you design the top of your column for half the base moment to account for rotation of the mass transferring some moment to the top of the column. If you plan on using the same column section all the way up, this shouldn't be a problem.

ELF is the most appropriate way to analyze your structure, and really, what else would you use? Perhaps the confusion is stemming more from which seismic coefficients to use rather than the analysis procedure itself. As mentioned above, you would use the Table 12.2-1 coefficients.

 

[KootK]

For those not familiar would you mind elaborating on the difference?

I'll give it a go.

1) It's an ASCE7 terminology thing. I've no idea if the differentiation matters in other jurisdictions.

2) The formally stated criteria for inverted pendulums are:

a) 50% or more of the structure's mass is located on top of the column and;

b) The supported mass derives it's overturning resistance from its vertical connections to the column.

3) Some frou-frou KootK elaborations:

a) Inverted pendulums are super bad because, as one can imagine, they suffer from a hyper-terrible P-Delta problem. They laterally stabilize a ton of mass at the same time that they are the vertical support for that mass AND the mass is imposing direct moments on the top of the post in addition to lateral forces. Lastly, any lateral displacement of the mass center of gravity as a result of mass rotation serves to exacerbate the P-Delta issues even more.

b) Most cantilevered columns are effectively just alternate forms of moment frames within buildings. Much of the stabilized mass is not vertically supported by the cantilevered columns or does not impose direct moments on the tops of the columns.

c) I believe that a cantilevered column is considered less redundant because so much of the vertical load is supported at the columns. In many cantilevered column systems, there are numerous, more distributed load paths for the vertical loads. This lack of redunancy translates into increased risk and, therefore, a need for greater conservatism.