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

 

[Charred]

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.

I've never heard of an inverted pendulum being defined this way before. It makes sense though. Do you have a reference for this or is it just logical?

 

[KootK]

It is, somewhat opaquely, built into the ASCE7 definition of an inverted pendulum structure.

 

[KootK]

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.

Logically, I feel that this requirement ought imply that the connection between the supported mass and the column also be capable of transmitting half of the base moment. That's likely to cause a great deal more difficulty than the design of the column itself.

 

[JLNJ]

Maybe you can do something 4-legged with the legs (moment frames) just outside (and supporting) the spiral stair. It won't look as sweet as the sketch, but dreamers aren't dreaming about EQ loading.

 

[Deker]

I've never heard of an inverted pendulum being defined this way before. It makes sense though. Do you have a reference for this or is it just logical?

No reference, just my attempt at elucidating the language KootK posted above.

Logically, I feel that this requirement ought imply that the connection between the supported mass and the column also be capable of transmitting half of the base moment. That's likely to cause a great deal more difficulty than the design of the column itself.

Not required by code, but I agree with the logic. Considering that Charred will be designing and detailing the top connection for unbalanced live load anyway, it shouldn't be difficult to accommodate half the base moment.

 

[KootK]

Frankly, I only know what the definition means because it's come up here often as a point of confusion and, at some point, I'm pretty sure that I saw an article on it (SK Ghosh, Structuremag etc). Just reading the text verbatim, I feel that one could misapply that a half dozen different ways.

I get my kicks rewriting badly written code so...

INVERTED PENDULUM-TYPE STRUCTURES

Structures in which:

a) more than 50 percent of the structure's mass is concentrated at the top of a slender, vertically cantilevered structure and;

b) that mass is itself vertically cantilevered from the structure below.

If feel that this would leave much less room for misinterpretation.

 

[r13]

I suggest to eliminate the spiral staircase for simple climb ladder to provide room for adding a few kickers to stabilize the playhouse.

 

[JoshPlumSE]

Sounds like an inverted pendulum to me.

Also, a viewing platform can still be a non-building structure, I believe. The majority of the time it is unoccupied. Right?

All the industrial platforms I've worked on were (for the most part) non-building structures even though there were maintenance people up there on a semi-regular basis.

 

[azcats]

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.

20" and 24" are available in a wide variety of wall thicknesses certainly up to 1/2". Might not be structural HSS (A500), but something like API 5L. If your local fabricator can't find it they need to talk to someone in the sign business.

Looks like a fun project.

 

[dauwerda]

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.

Also, a viewing platform can still be a non-building structure, I believe. The majority of the time it is unoccupied. Right?

All the industrial platforms I've worked on were (for the most part) non-building structures even though there were maintenance people up there on a semi-regular basis.

I tend to agree with Josh on this. ASCE 7 defines a "Building" as, "Any structure whose intended use includes shelter of human occupants." Quite frankly, this structure isn't intended to provide or be used as shelter - it is intended to provide a higher viewing area. Are there human occupants involved? Yes. Is the intent to provide them shelter? No.

I get my kicks rewriting badly written code so...

INVERTED PENDULUM-TYPE STRUCTURES

Structures in which:

a) more than 50 percent of the structure's mass is concentrated at the top of a slender, vertically cantilevered structure and;

b) that mass is itself vertically cantilevered from the structure below.

If feel that this would leave much less room for misinterpretation.

Per chapter 11, an Inverted Pendulum-Type Structure is defined as:
"Structures in which more than 50 percent of the structure’s mass is concentrated at the top of a slender, cantilevered structure and in which stability of the mass at the top of the structure relies on rotational restraint to the top of the cantilevered element."

I feel as though the code defines it fairly well, and essentially the same as you.

 

[MR_E30]

I'll agree with dauwerda above as I have used that definition in the discussion of certain projects I have worked on in the past. I have convinced the AHJ that this was an acceptable path forward with the design.

However, under no circumstances and with no AHJ (have successfully completed projects in over 30 states), have I been able to convince them that a non-guided (non-access controlled) viewing deck or non-building structure can be designed to anything less than 100 psf LL. I have reduced LL in certain circumstances, but these involve restricted/controlled access to a certain number of occupants. Just throwing this out as food for thought.

 

[r13]

I think the importance of design live load is next to how it is distributed, whichever governs.

 

[MR_E30]

retired13, I agree completely. I'd even consider a concentrated live load near one of the windows, as people have a tendency to pile up in an area to catch a glimpse of something happening down below.

 

[Charred]

MR E30,

I was concerned with that - a 100psf LL is hard to accommodate with a slim profile - in this case. I've got a few options that I will present, but I feel like reality is too far off from the original conception that this one will be scrapped from the project.