Gust Effect Factor - Monument Sign

[Nulukkizdin]

Was hoping to get some input regarding calculating wind loads on a freestanding steel monument sign. The proposed design from the manufacturer is fairly simple, with some decorative tubes surrounding a 6'x7'x½” piece of corten steel. Due to the nature of the construction, we are basically only relying on the ½” plate steel as the structure to resist wind load. Although design wind loads aren’t anything out of the ordinary, we do know for a fact that this area sees significant sustained and gust wind loads, giving us some pause about the current design.

In the grand scheme of things, we’re a fairly short sign, which would many times lead us to a rigid structure and thus the assumption of G = 0.85. As our sign is so thin, I have a hard time believing we’d be in this category and thus would want to confirm that we are, in fact, flexible (if our frequency is < 1 hZ). Looking in the commentary of ASCE 7-10, I see estimates for cantilevered poles (C26.9-11) or towers, but am not sure if there is an alternate equation that is more suited for our application. (As a point of note, if I use the equations for the cantilevered poles, I get a frequency of approximately 0.2 hZ). Does anyone have any guidance on what to use there? As a follow-up question, does anyone have a good idea of what we want to limit our deflections to for this situation, and does this get treated any differently in the case of a highly increased gust factor?

 

[JAE]

Can't you utilize 26.9.5 (ASCE 7-10) for flexible structures derivation of Gf?


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[Mjkkb2]

Not much direction in ASCE 7. I have to admit I never thought twice about the very few sculptures I did - always assumed G=.85 but they were mass monument type things so rigid for sure.

One approach is to just be very conservative with the design to account for it (overestimate the design wind load). I would just use an arbitrarily high G, say double .85 to account for unknown characteristics of the system.

AS far as deflections, I don't think you're limited. You may want to make sure whatever ornamental items are attached to your main sculpture element are not affected by excessive deflections. IF in doubt, just treat is a structure and use what you would typically use H/'whatever'.

 

[Mjkkb2]

JAE - the problem is one needs a natural frequency to plug in to the equations and there's no direction of calculation such frequency for a non building element. - at least this how I understand it....

 

[JAE]

Since I don't have a visual on the configuration of the sign, I would also say that the derivation of frequency is not understood by me - but the OP did say that they had calculated a frequency of 0.2 Hz.
We use RISA and other things to determine natural frequency of structures that aren't otherwise well defined in the books.

And section 26.9.5 lays out a procedure for determining Gf.



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[Nulukkizdin]

Thanks for the responses!

JAE, to clarify, the 0.2 hZ frequency was for reference but calculated using the equations intended for poles or masts. The first part of my question really stems from trying to find a good way to actually calculate the frequency of my rectangular sign. As Mjkkb states, we don't have a good direction for calculating this frequency (besides for masts, poles, or towers).

For reference, see the link below - this is not our project but a similar idea. In the picture they have HSS columns behind the sign, where our designer is proposing having the thin sign itself cantilever off of a base-plate (which we are trying to stiffen up).

https://i.pinimg.com/originals/69/05/b3/6905b33b8775bc7f2bcd386c4042847f.jpg

 

[Lomarandil]

Hmm... I seem to remember calculating the frequency of 1D cantilevers in a structural dynamics class in college. I wouldn't be surprised if google turned up a closed form solution for that (which I don't think would be too far from your 2D solution).[sup][/sup]

----
The name is a long story -- just call me Lo.

 

[Nulukkizdin]

My only concern with using the 1D cantilever equations from dynamics is the statement in the ASCE 7-10 commentary. Referring to the empirical relationships for building periods available in the earthquake chapters of ASCE 7;

"it is noteworthy that these expressions are based on recommendations for earthquake design with inherent bias toward higher estimates of fundamental frequencies. For wind design applications, these values may be unconservative because an estimated frequency higher than the actual frequency would yield lower values of the gust-effect factor and concomitantly a lower design wind pressure."

If it comes down to it, we could probably use something of the sort as an estimate, but per the statement in the commentary, it's probably not our best choice.

 

[JAE]

Do you have RISA or other structural software that can model plates etc. and get you modes/frequencies?

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[RWW0002]

AASHTO Spec. for Structural Supports for Highway Signs and Luminaries recommends a minimum gust effect factor of 1.14 for wind sensitive structures and has a decent commentary on the historic use and derivation. Absent any better data this is what I would use.

 

[JAE]

I think the 1.14 factor is for something more like this:

https://www.youtube.com/watch?v=O53a2yn-BI4

The sign indicated by the OP is perhaps more stable - but they'd have to determine the frequency to know.

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[dhengr]

Nulukkizdin:
You say..., ‘ some decorative tubes surrounding a 6'x7'x½” piece of corten steel, for a free standing steel monument sign.’

Can you even get small HSS tubes in corten, or one piece of .5" pl., 7'x8' (final size 6'x7') for that matter? These are not usually stock items in a steel warehouse. Most likely not for the HSS tubes, maybe for the pl., so you will have two different material surfaces as the corrosion takes place, one may be needing regular painting. You might talk to a few fabricators or steel suppliers, who might find some left over bridge steel which would then/better dictate the matr’l. sizes and thicknesses. Like the sign you’ve shown, it looks like they found some 30 - 48" wide material intended for bridge girder webs, and worked the splice welds into the design aesthetics. Then, you might find some girder flange mat’l., 1" - 2" thick which you could rip down into 2"x3" (or 2"x4"), (or 2" thk. by 4" at the top & 6" at the bot.) pcs. to take the place of the edge tubes. Can what you called “decorative tubes surrounding...,” now ripped bar stock, be used to frame the sign and work as end posts? Can some huskier posts be worked into the design in some way, without killing it? I don’t have the last few Eds. of ASCE 7so I don’t know exactly what they say, but this is a free standing sign someplace, not a sixty story bldg. which might fall on people. What kind of a FoS can you achieve w.r.t. yielding at the base, under the normal wind loads, and call it a day. Are both sides show sides or is one more significant than the other, so there are some options there?