ASCE7-05 Gust effect

[WBUW]

Hello all,

I'm working on running the numbers on nothing more complicated than a fence. I'm trying to figure out, using the asce method II what to do for the gust effect factor. Its not really a building, I would say its more like a flexible structure described in 6.5.8.2

In looking at the calcs described to determine Gf...what would one consider the n1, the natural building frequency to be? Or is there a graph I can pull to get around this?

Thanks,
Will

 

[twinnell]

Why wouldn't you design it as an "Other Structure" and find F based on a solid wall (conservative) or latticed framing?

 

[csd72]

As twinnell said it is under other structures and is covered in the same table as signs.

I wouldnt imagine this is a vibration sensitive structure so you could probably take Cf as 0.85.

csd

 

[UcfSE]

Calculate the natural frequency of your fence. If it qualifies as rigid, then use 0.85 instead of doing all that extra work.

 

[WBUW]

Yep, I was planning to use eqn 6-28. For the Force coefficient Cf, I was going to use 6-20.

That seems pretty clear, the part I'm a little confused on is 'G'. Looking at 6.5.8.1

UcfSE - Sounds like a nice easy approach. The fence is so short it probably is less than 1 Hz. When you calculate the frequency its the inverse of the period...so you have to calculate the period its been a while since I thought about that. Does ASCE give any direction for that or is it back to physics 121 for oscillations?

 

[WBUW]

Okay...so to get the period, I see ASCE 12.8.2.1 for an approximate period.

So...Ta = Ct*Hn^x = 0.02*7^0.75 = 0.086

then v = 1/T = 1/0.086 = 11.63 Hz

So, looking at that (I misspoke earlier per ASCE 6.2 of its greater than 1 Hz you can consider it rigid) I agree with UcfSE that for GCf just use 0.85.

Anyone disagree?

 

[WBUW]

Rather G = 0.85...Cf I still need to figure out.

ASCE states to use figs 6-21 through 6-23 for Cf. This is a slatted fence.

To be an open sign at least 30% of the area should be open, in a slatted fence its more like 90% is not open. So I would say 6-22 is no good.

A fence is most certainly not a trussed tower so 6-23 is gone.

I would say the closest approximation would be 6-21, a chimney...solid walls square with wind normal to face, but as far as the height to depth ratio goes the pipe is 4" diameter the fence is 84" high so I'd say the h/D is 21. With linear interpretation that goes to 1.865.

End result 1143 lbs force with method II. With method I, I calculated 1204 lbs force.

 

[UcfSE]

I would find the period based on a uniformly-distributed load for a cantilever. The equations out of chapter 12 are approximate and meant for buildings.

See figure 6-20 notes 1 and 2.

 

[WBUW]

Sounds more accurate, where do I look for a reference to find the period based on a uniformly distributed load for a cantilever?

I could use 6-20. I'd have to use 6.5.14 in lieu of 6.5.15 but thats doable too.

 

[UcfSE]

Well, it's not about "could" use. It's not like you can call a fence a chimney, tank or RTU for the sake of finding a pressure coefficient. If you have a freestanding wall, which a solid or mostly solid fence matches, then you use freestanding wall coefficients. 6.5.14 is the same as 6.5.15 essentially, there's not much difference, so it's not like you'd be doing a totally different procedure.

You can check your structural dynamics book or use software. Some internet may help. Anil Chopra wrote the standard text used in many classes. You can exercise judgment and assume the wall is rigid, but if you don't know or haven't ever checked, it would be better for you to check first.

 

[WBUW]

Yeah, I agree. Its best to use the most applicable design within the code. However, sometimes this is not very clear on which direction one should take.

I actually re-ran the numbers using the free standing wall from ASCE. The applied load actually came out lower than the other two. 876 lbs force.

Hmm, well I have the Hibbler structural analysis book...doesn't look promising, I have my old kinematics and dynamics book (Beer + Johnson) maybe that will shed some light...