Sign Wind Loads 9

[dik]

I have a bunch of little signs to design... different sizes on gravity bases.

I would normally use the projected area and use a force of design wind pressure x exposure factor (0.7), x wind pressure coefficient (1.3, for windward and leeward), x gust factor (2.5 rather than 2.0). They are temporary structures and I'd use an overturning factor of 3.

Does the ASCE or IBC have another prescribed approach? I don't have access to either document.

Thanks, Dik

 

[WARose]

No windward/leeward approach. Just a directly applied load. For solid freestanding signs ASCE gives the design force as (in Chap. 29 of ASCE 7-10):

F = q[sub]h[/sub]GC[sub]f[/sub]A[sub]s[/sub] (lb)

Probably all of those variables you've seen before....the only tricky one is C[sub]f[/sub]. It can vary from 0.55 to 4.3 depending on load case, aspect ratio of the sign, "region" of the sign, etc, etc. But for a 1:1 aspect ratio of a sign.....a C[sub]f[/sub] of 2.25 should suffice for all cases.

They have a "Case C" where the wind load is hitting it at a angle....and the load there depends on the "region" of the sign.

 

[JAE]

dik - if this is a temporary structure then ASCE 36 has diminished wind speeds for shorter term situations.

What sort of "temporary" is it? 1 month? 6 months?

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[BAretired]

Unfortunately, some "temporary" designs end up being permanent.

BA

 

[JoelTXCive]

On little signs, we just apply 40 psf uniformly to the sign and support. To my knowledge, the firm has not had one blow over yet.

 

[dik]

Thanks gentlemen...

I'll see if I can scrounge a copy of the ASCE document. Seems the sign is not so simple... my approach gives about 30 psf... maybe a tad lite.

I have no idea of how temporary is... my experience is that it can extend for years.

With the near side and far side being so close, I can see that there is no windward and leeward...

I'll have to find out what factors Cf has if it can vary so much. Is there a *.pdf file that explains it or has a design example. I haven't had anything 'blow over yet'... and it would be a shame for a silly little sign to break the record.

Dik

 

[Lomarandil]

Unless you're talking very temporary (days to maybe weeks) the decrease in loads permissible under ASCE 37 doesn't help you much. Probably not worth the trouble of tracking it down for this project.

If you have a copy of ASCE 7, the procedure is pretty straightforward. From your responses, it sounds like ASCE 7 is not in your library? If so, I'd also be pretty comfortable with an upper bound around 40psf and going from there.

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

 

[robyengIT]

ASCE 7 2010 is easily available on WEB

 

[Shotzie]

Would Figure I-24 "Free-standing plates, walls and billboards" from the Wind Commentary in NBCC work for what you're doing? It gives parallel and perpendicular forces to design for.

 

[dik]

Shotzie: that's what I was going by...

Dik

 

[Bobby46]

I recently did some roadside DMS signs according to AASHTO LRFD Spec for Highway Signs, Luminaries, and Traffic Signals. Wind loads ended up being 40 psf (Extreme I)and 21 psf (Service I).

You're state DOT (or similar) probably has a standard detail based on sign area with prescriptive designs.

 

[dik]

Shotzie... surprising little has changed. ICYC. from way back when... the Canadian Code guys used data from Swiss engineers done 20 years earlier.

Dik

 

[Shotzie]

Thanks Dik, that's interesting to know. I believe Sentence 63 in NBCC 2010 Commentary I makes a few remarks about it as well. Always good to know where these things come from.

 

[dik]

Ya... we copied them from the Swiss... I've got a scan(and original) of the 65 document. I don't know if copywrite is valid after 50 years... I can post it or can drop it to SRE's site. It was given to me by Malcolm Anderson, the best contract administrator, I ever met.

Dik

 

[JAE]

dik,
I should have referred to ASCE 37...not ASCE 36.

In 37, the wind loads are adjusted for shorter term durations.
I know you stated that the signs may end up being permanent but just FYI here is what 37 does to the wind speeds:
<6 weeks - 0.75 x typical wind speed
6 weeks to 1 year - 0.8 x typical wind speed
1 to 2 years - 0.85 x typical wind speed
2 to 5 years - 0.9 x typical wind speed





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[Ron]

dik....I use ASCE 7...in all of its varieties! Cf has, in my opinion, always been an unresolved shot in the dark that you have to use your best judgment to resolve. I've done a lot of plain signs....from low monument signs to high billboard signs. In most cases, I've used Cf= 1.5....for no good reason sometimes other than you have direct wind load on the windward side (1.0) and you get some negative pressure on the leeward side (0.5). I practice in a relatively high wind area and sometimes the pressures exceed 40 psf, depending on height and area.

 

[dik]

Thanks Ron:

To have a variation from 0.55 to 4.3 is almost unfathomable... I managed to scrounge a copy and will look at it this weekend. The Canadian code is quite simple... it doesn't seem to have the same reach. I know that signs can be installed in all sorts of environments... but, seems pretty simple to me. I was looking to see if there was something I may have overlooked... Is the IBC just as complicated, or, do they reference the ASCE?

Dik

 

[Ron]

IBC references ASCE 7. That document increases in complexity and unintelligibility (?) with each new edition.....or maybe I'm getting older with lower BS tolerance!

 

[dik]

Thanks, Ron...

Dik

 

[azcats]

The cf range from 4.3 to .55 isn't really representative of the 'global' wind loads you'll be encountering. Those numbers represent the extremes of localized pressures for oblique winds (Case C loads) on signs with width to height ratios exceeding 45. That range gets tighter as the ratio shrinks. Case C is not applicable to ratios less than 2.

Case A & B cf factors range from 1.3 to 1.95 depending on aspect ratio and clearance ratio.