Ice and Wind load on netting

[RFreund]

I am analyzing wind load on netting used along a baseball parking lot. The netting is 0.12in thick (diamter) and it has a 1.75"x1.75" square mesh. The total area is 65'long x 40' tall with 5 poles. If I consider build up of ice per ASCE7-10 the thickness is 1.5" which would mean the net is now a solid sign. And I come up with some pretty large drilled piers. Ice thickness if for midwest, td=2*I*Kzt*t

Basically I=2, Kzt=1, t=0.75, fz=1.01
Seems like alot as I knew EIA-TIA-222-F used to recommend 1/2" however this may have changed in revision G.

Also the system uses 6" diameter aluminum poles which if the calculated loads were reached I'm pretty sure the poles would fold first.

Is the ice thickness correct and should I be applying it the entire net? Basically looking for some reassurance or correction before going to the boss.

EIT

 

[boo1]

Ice exposure coefficient is an arbitrary value that may be assigned based on the judgment of the designer, considering the probability of an event occurring where maximum ice accumulation and peak wind velocity occurs at the same time in the locality the fence is installed

Chain Link Fence Manufacturers Institute has a design guide

 

[RFreund]

Thanks for the reference, however does this manual apply here?
The manual does not reach heights of 40'
This is a flexible netting
The opening explanation of wind load scares me a little in that they say the wind considers a solid sign and then subtracts the voids. That would be unconservative because the wind must flow around the mesh which increases pressure. They may account for this but I have not checked.

I feel in reality a solid sign design seems like overkill however I believe if it is designed per ASCE7 I have no choice?

EIT

 

[paddingtongreen]

The NESC places a much reduced wind velocity on iced up transmission lines and the full velocity on ice-free wires.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[RFreund]

hmmm... so your saying a 36" diameter x 15 deep would be overkill?

EIT

 

[RFreund]

Also I'm assuming your referring to the Design Guide for Rural Substations? (found here:

http://www.usda.gov/rus/electric/pubs/1724e300/1724e300.pdf

)

Page 165 shows 0.5" radial ice and associated wind loads.

Do you think these loads would be suitable for netting?

Also the specs call for the loads/calcs to follow ASCE7 for loading.

EIT

 

[JAE]

What happens when the local team puts up a huge banner on the netting? Solid sign then right?

 

[RFreund]

JAE - Touche, problem solved!
The foundations seem large but we'll see what happens.

EIT

 

[SAIL3]

I have not read the referenced material so the following would be my own engineering judgement:
1. using a solid sign with design velocity does also seem like
overkill to me.
2. I would keep the solid sign and try and get a feel for what
would be a reasonable wind velocity to use.
3. I would try 75% of design wind with solid sign....
the probability of max wind and max icing occuring at the
same time is pretty remote in my judgement.
Now, I would go back and try and get the capacity of the netting for max wind, 75% of max wind, etc.
I suspect that the netting would fail in trying to transfer the wind loads, at the levels we are considering, to it's support...so this may give me and upper bound on a pracical wind velocity to use.
I know this was not the original intent of the OP....but, so what if the system fails?..I don't see an immediate danger of loss of life....only material loss...this factor would also play into my engineering judgement...

 

[RFreund]

Sail -

Thanks for the input as I would tend to agree with your points and I believe you have addressed the overall intent of the post.

I have learned that this system has pulleys and will only be used during summer months so I may have to make some modifications based on this.

EIT

 

[paddingtongreen]

Following on from SAIL3, the net would form a catenary between the poles, if the net is tight, the tension will be very high. How much "sag" will you allow at installation?

The NESC puts 8psf on 1/2" of radial ice, but the wires are relatively stiff compared to the netting. They are also thin where the face of the net is large and will be simultaneously subject to varying loads from place to place. Even if the installation is tight, the net will ripple in the wind, discarding the ice.

you may have to put struts between the tops of the poles to make them work together.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[SAIL3]

also the need for 1.75x1.75" mesh all the way up to 40' may not be warrented....could possibly increase the mesh size,say,above 15' and this would give you a higher confidence factor in reducing the max wind or completely soid assumption...
you also design in weak links or "blow-out " panels to limit the max loads....

 

[RFreund]

The actual netting system (poles, nets, connections) are by the manufacturer.

All good points though.

EIT

 

[paddingtongreen]

If the manufacturer controls the materials, he should also provide foundation loadings. Absent the loadings, I would use the failure load for a pole.



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[racookpe1978]

OK then - If the sign that we expect to be placed on the "netting" can be "assumed" to be attached at to the fence at a limited number of places, then you'd only have to reinforce those posts - not all of the posts.

But, you'd have to verify each time a sign or banner goes up that it was put in the right place. Hard to do.

Change the problem: Attach the netting (which can be replaced cheaply if it ever fails) with low-strength wraps/attachments that will fail if there is ever a combination of ice/snow loading AND high winds. Then, the "plastic-tie-wrap-substitute" rips, the netting falls down and sheds its wind load, and the rest of the posts stay up. (In theory, at least.)

Above assumes people are NOT using the facility during times of ice buildup and high winds.