Lateral wind thrust on fence

[Teach Me]

I have no engineering skills, but I have 30 years experience with fence. We are bidding a 16' high sound wall. Three sides, two short ends are 12' long and the long middle is 80'. We will be attaching a 1/4" thick sound abatement material that weighs 1 lb per square foot. Two questions: 1) How do I determine the bending moment at the base of the fence posts in a 60 mph wind? 2) How do I convert that number into what size posts I should use? I will have an engineer calc it when I get the job, I just want an idea of what I can bid the job with.

Any help?

[RWOLFEJR]

Hi and welcome to the forum...
That's quite a kite you're dealing with there. Seems the better idea might be to consult your engineer before you bid...? What if some unknown on the internet whose credentials you have not a clue... tells you to use fifty posts this big by this big and it turns out they were full of it? Then you get the job and find out you'll have to use 100 of those posts...

The cost for the engineers service prior to bidding would be minimal and well worth it... It's called the cost of opportunity.

Good luck...!
Bob

[PinkertonD]

How do I determine the bending moment at the base of the fence posts in a 60 mph wind

Gotta say I am with Bob (RW..) 100%. I know it is the most often intoned reply to things like this, and I know it is not what you want to hear, but 16' high x 80' long is a huge load. The end pieces will not count for much support unless the entire 80' wall is structurally rigid and self supporting and that would be a total and excessive waste of materials.

I would also check with the local County on wind and snow load parameters for buildings as 60-mph sounds way too light to me. We get gusts to 55 regularly and I am guessing Code for your area, (wherever it is) will be 90-mph. If they design houses for that then it would be folly to use 60-mph for the fence.

Get an Engineer to do you a ballpark assessment and then double it.

[Teach Me]

Ok I am willing to be realistic and constult and engineer, however, two more questions: 1) What kind of engineer? 2) What is the going rate for this type of inquiry $200 or $1,000...I really have no idea.
I'm just looking for a competent person to say "Use 6 5/8 schedule 40, set 6' deep, 8' on center, etc & so and it should calc."

[zeke]

Since it looks like you have no experience with sound walls, I would respectfully recommend that you no bid this job.
The cost of a consultant versed in this discipline would not be worth the investment.
This is not your garden variety engineering problem but embraces acoustics, earth structure mechanics, and structural engineering.
The people who bid this type of job would have to demonstrate a track record and are usually staffed with inhouse engineering people.

[RWOLFEJR]

Ok I am willing to be realistic and constult and engineer, however, two more questions: 1) What kind of engineer? 2) What is the going rate for this type of inquiry $200 or $1,000...I really have no idea.
I'm just looking for a competent person to say "Use 6 5/8 schedule 40, set 6' deep, 8' on center, etc & so and it should calc."

1) Civil Engineer.
2) Can't answer that accurately but you can find out quick enough with a phone call. And like Zeke said... might as well get one with experience in sound barrier wallls and have hime go full boat on the design? Might be able to see if he'd sell you a bill of materials for quoting purposes and if you win the bid you buy the details?

3) Without seeing the site or knowing what the area is like it's pretty much impossible to say what you need. I can say for absolute certain that 6-5/8 sch 40 won't come close. Ballpark calculations... based on not nearly enough information. I can tell you that you most likely won't be using tube due to cost. The loads your looking at would require tube that would cost far more than the cost of BIG wide flange I-beams that'd handle the load. And that was figuring 8 foot spacing... You'd need some really big diameter and heavy wall tube to handle the load. Beam shape, and manufacturing process, allow them to be both stronger and less money. (Stronger in this sort of application... always less $ per pound...)

How deep will depend a lot on what you're sinking them into. Gumbo... sand... dirt... bedrock? Can't guess there but rather than sinking them in the ground I'd consider pouring concrete with J-bolts and bolt these posts to it. Posts with big plates welded and gusseted to base with plenty of holes spread out to anchor them.

Grab a phone book and ask around.
Good Luck,
Bob

[PinkertonD]

In a addition to Bob and Zeke's comments, are you limited to a maximum width for wall and supports? If not then triangulating the support structure on a concrete foundation much like a coarse-toothed comb, might minimize the steel quantity.

Maybe check the Annually-prevailing wind direction and support the wall with diagonals on the leeward side, or both sides if winds are flukey. This is a pretty major undertaking and I lean more towards Zeke's comments than Bob's as you may be opening a huge can of worms getting into an area of wall building expertise you are not yet familiar with.

There may be myriads of issues involved with the fixing of the sound deadening material to the wall, it's maintenance, the chemical interaction or reaction between materials or a whole host of other things as yet undiscovered by a neophyte in sound-wall building.

This is a huge dollar project to get involved in as a first for a new (to you) technology for wall building. I'd be running away and not just walking. But -- I'd sure be watching and asking a lot of questions as it is being built by whoever gets the project.

[Teach Me]

It looks like the posts will be 8 5/8 Schedule 40, set 8' deep on 6' centers, so you guys were on the right track. As for bailing on the project, it is a bit less complicated then I implied. We are basically installing chain link fence which is a milk run for us, then we are attaching this pre-engineered sound material to it. So the actual sound engineering is assumed, and the giant posts set 8' deep should keep it up and plumb for a long long time.
My next thread will be how to convert the bending moment (strength) of round tube into "I" or "H" beams.
All of you were very helpful and generous with your responses. I do appreciate it. If you want, I will let you know how the installation goes.

[RWOLFEJR]

Just plug your numbers into this...
http://www.engineersedge.com/calcula...re_case_12.htm