Hanging Sign 2

[medeek]

I have a small job to design a steel mount for a hanging sign (approx. 40 lbs) that is a 36" octagon shape. This is something new for me so I thought I might post a few questions.

1.) Calculated the wind loads using ASCE 7-10 eqn. 29.5-1 and Figure 29.5-1. Since the sign is simply hanging down I'm wondering if it flapping around in the wind like a flag might trigger some additional dynamic loading case or scenario that my current static approach is missing.

2.) Any suggestion on structural eye-bolts (McMaster Carr ?). Granted the weight of the sign is relatively light but the "Home Depot" (lag) eye-bolts are somewhat questionable at first glance, I'm thinking to treat the hardware (bolts, chains etc...) as lifting gear (ASME B30.20-2010) with its associated safety margins. I can't seem to find anything in the ASCE related to this sort of thing, probably looking in the wrong place though. Perhaps the steel manual...

Below is a quick mock up of the design concept. Initially I thought to use pressure treated lumber but I think the welded rectangular HSS will give me a cleaner look.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[EngineeringEric]

Do not use Lag Screws. Use through bolts only including your eyes. Lags can handle the load on paper but i have seen far to many failed lags in my short career. I have done some of those signs before and some larger but have never had any load that warranted much thought or concern. When I have a factor of safety of 10+ I usually don't give much concern for dynamic effects of wind, maybe that is wrong?

Another option for your main frame is angles, it isn't as pretty but is cheaper and easier to build.

Machine Eye-bolts are usually rated with working load that includes a huge factor of safety. i would probably use a HD galvanized shouldered eye bolt. A 3/8" diameter has a working load of 1300#

 

[medeek]

The sign itself is constructed of solid wood so my only option will be eye-bolts that are lags. I'm thinking along your lines though, enough of a safety factor both in diameter of bolt and embedment into the wood should take me into a zone where additional consideration for dynamic effects of wind on the sign are covered. Our wind speed here right along the coast is 155 mph (ult.) and some of the winter storms can get crazy here. In a really bad storm I would actually be interested to take a video of the sign's motion and response to the elevated wind gusts.

I'm actually surprised that the ASCE 7-10 does not have some sort of provision for "hanging" signs that differs from their other equations for signs, roof structures, lattices etc...

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[EngineeringEric]

Yeah, for the actual sign attachment i would put down my pitchfork against Lags. Barrel screws would be even worse

I suppose ASCE 7 has the sign loading which is baisically what you have. the second benifit of yours is when it gets a force applied it sways thus reducing the exposed cross sectional area and less lateral load.

 

[Guastavino]

Medeek,

I would rationalize that if you use the sign loads from ASCE7 they will be very conservative. Think about it, as the hanging size rotates/moves, the load is relieved in some capacity. If you design assuming it's rigid, I'd think you're good to go. As for dynamic increase, I wouldn't sweat it. If you have a 155 MPH wind, and that sign blows away, well, I don't think you'll get a call saying WHAT HAPPENED?

 

[medeek]

Given that this is a solid sign I'm wondering if Figure 29.4-1 might not be the more appropriate method for determining the wind force coefficient.


A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[3DDave]

Under high wind conditions it is the load of bashing into it's surroundings that's a problem, followed by natural frequency excited loads.

You might check around your area for similar signs that have been in place a while and consider that the more rigid the mount the more predictable the response.

 

[medeek]

I'm just about finished with the engineering and design of this sign mount but I'm waffling between two methods for mounting the eyebolts that the sign will hang from, see below:

I would like to use the lower option because I think it will look cleaner than the upper option but I'm worried that with the marine environment the threads will corrode over time (5-10 years) and the eyebolt could pull loose. I intend on using stainless steel bolts, washers and nuts (all hardware will be stainless) so the upper option should be pretty much fail safe.

I'm also wondering if anyone has encountered any galvanic reaction between structural steel (ie. HSS, W sections) and stainless steel fasteners?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[DBronson]

Medeek:

I'm not to keen on either scheme - the top one allows rain water to penetrate the HSS tube at the bolt holes. The second one is not very easy to fabricate -
that rectangular cope is difficult to make without machining (ie it's an expensive step). You might check out a Crosby S-265 Weld-On Pivot Link.
They are simply welded on. I think they could be adequately protected from the marine environment with careful painting, same as the HSS material will require.

Regards,

DB

 

[medeek]

@DBronson

I like the reasoning behind the use of the S-265 Pivot Link, I just wish they made one size smaller. I'll check McMaster Carr for a similar part but with a 3/8" link size.

I'm now revisiting the eye lag bolt issue that is currently installed into the wood sign (2 places). The ratings for these type of bolts is not good, at least not when compared to the machined eye bolts I was planning to install into the sign mount:

https://www.boltdepot.com/fastener-information/eye-bolts/Wire-Eye-Bolt-Working-Load-Limits.aspx

My main concern with this type of configuration is that the wood will eventually deteriorate over time and the lags will either corrode and snap, or fatigue and snap, or withdraw. The wood used for the sign is not pressure treated and will be exposed to a harsh rain/marine environment (washington peninsula, on the beach), everything made of wood rots out here. Painting helps but the rain just seems to turn it all into mush sooner or later.

I think the only way to reasonably suspend the sign is to weld a octagon frame of stainless steel channel (2x1x0.25) that clam shells the wood and has two eyebolt lifting points. Anything less than this will not be fail safe in this environment.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

The problem of water infiltrating into the HSS has got me thinking...

Why not just weld a couple of lugs to the HSS with eyes for shackles or chain links, problem solved.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[EngineeringEric]

Why not just weld a nut to the bottom of the HSS? then screw in a short eye bolt into that... I also think through bolting would work fine as long as HD Galvanized after it is drilled and a drain hole is provided somewhere else.

For your frame, maybe have a steel sleeve that goes around just the top and maybe sides would be sufficient. Or maybe just a plate border on both sides to through bolt the wood? To some degree you cannot make it last forever if it stays wood. may offer plastic, fiberglass, metal???

To be honest, i so appreciate the way you think your problems through but i think this one is small enough that anything will work for 10+ years (wood at 10, steel at 50+). But i am still curious where you go with it.

 

[medeek]

Stainless steel frame capturing the wood sign. The steel adds 32 lbs to the mass for a total sign weight of 72 lbs.

eye bolts replaced with welded lugs and shackles (SS), all of the connection hardware is now rated for 1000lbs+

DF No. 2 4x4 posts will be behind the two leg connection locations and the vertical leg.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[DBronson]

medeek:

Very bad detail - water will be trapped in the channel. Drilling holes in the bottom channel would help, but it won't prevent water from being held against the wood by capillary action. Also, the threads on the stainless fasteners will probably gall and seize making life difficult for everyone.

Since you seem to have a good handle on the loads and forces involved, I would recommend that you consider bringing in a sign contractor to look at the project and let them suggest how they might fabricate a sign like this. Every shop has certain preferences in how they go about making their products. The two of you could save the client some significant dollars by designing something that can be economically fabricated.

Regards,

DB

 

[medeek]

The real problem here is I am starting with a fabricated wood sign (non pressured treated) that will be exposed to a severe marine environment. Galling of SS threads is the least of my worries, in fact I am planning on adding loctite during installation to make the connections even that much more permanent. This sign is going to be a hang and forget, only when there is visible deterioration will any maintenance ensue.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[DBronson]

medeek:

"The real problem here is I am starting with a fabricated wood sign (non pressured treated) that will be exposed to a severe marine environment." Then why are you considering putting it in a frame that will ensure the wood edges are kept wet?
I'm only trying to point out some of the issues with the design and give you some ideas that have worked well for us on similar projects. You've come up with a expensive solution to a fairly simple problem.

Regards,

DB

NB: The SS fasteners are likely to seize going in; if they do, they'll have to drill them out. The locktite won't change this.

 

[medeek]

I did suggest to the client that he have a sign shop fabricate him a sign that is better suited for the environment. I don't think a wood sign is going to last very long out here. Which SS fasteners are concerning you the most? The lifting eyes are all stainless but I don't think they should give me too much trouble. The lag screws into the sign are into wood so they have nothing to gall against. I'm assuming you are worried about the frame bolts that lock the frame together.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[kingnero]

Depending on the thickness of the sign, here's another option...
excuse the crude sketch without details, but you'll understand the essentials without doubt.

 

[medeek]

After I suggested to the client that he might want to try a sign shop for a better solution I have had no word back. I'm guessing my services are no longer needed on this project. I gave it my best shot, but sometimes you can't win them all. I will say I am pretty happy with the sign mount itself, its just the actual sign that had me coming up empty.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com