Fences greater than 6 ft wind design, Florida 1

[dianium500]

Anyone have any suggestions of the required wind design for a fence greater than 6 ft? The Florida building code allows for fences up to 6 ft tall to be designed at 75 mph winds, or 115 mph with 3 sec gust but there is no mention of the of higher fences. I could go w/ ASCE's wind recommendations for a risk category I structure, but it just seems like that would be overkill. Bit of background, it's for a middle school.

Thanks for the input in advance!

 

[TehMightyEngineer]

It's not overkill; fences, facade, signs become huge parachutes in hurricanes; and if they break free, can get moving pretty fast before being slammed into something. Florida takes wind-borne debris seriously.

I'm still new to Florida practice but I'd design it as a freestanding wall per ASCE 7 fig. 29.4-1 unless there's something in the local code that states otherwise.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[SlideRuleEra]

...any suggestions of the required wind design for a fence greater than 6 ft?

Review recommendations in Chain Link Fence Wind Load Guide for the Selection of Line Posts & Line Post Spacing (2018) published by the Chain Link Fence Manufacturer's Institute. Fences up to 20' high with wind speeds up to 170 MPH are addressed.

http://www.SlideRuleEra.net

 

[spats]

dianium500, Where in the Florida Building Code do you see the 6 ft tall wall numbers you reference?

I was recently asked by a local contractor to design a 6'-8' tall (sloping grade) CMU fence wall on the beach in St. Augustine, FL. Using ASCE 7-10, Figure 29.4-1, Case C for Risk Category I (121 mph), Exposure D, the ultimate wind pressure 0-S (end of wall) calculated out to 94 psf. RIDICULOUS! Using strength design, I was coming up with #6@32" o.c. for the 6 ft wall + 4" cap, with the footing only buried 8 inches. At that point I didn't even want to see what the 8 ft tall wall reinforcing would be, but the footing for the 8 ft wall was calculating out at 4'-6" wide x 1'-6" thick, and still wasn't quite large enough.

The contractor had already priced the wall for the owner expecting #4@48" o.c., and a much smaller footing. I had to tell him that I couldn't help him, because he wouldn't be able to afford to build my design.

 

[Ron]

Where in the Florida Building Code do you see the 6 ft tall wall numbers you reference?

dianium...not sure where you are located in Florida, but oddly enough, that provision only applies to HVHZ, which is Dade and Broward Counties. Another of many quirks in the FBC. One would think it would actually be outside the HVHZ that you could apply that (spats....see Section 1615 of the FBC). Most other fence provisions up to 6 feet are prescriptive.

Since your application is for a middle school, you have to comply with Chapter 4 of the FBC also, which loops you back to ASCE 7 provisions, without regard to the height.

Anything over 6 feet is not prescriptive and must be designed accordingly. I consider them to be designed like signs; however, it also depends on the material used. As an example, masonry fences over 5 feet tall are essentially considered walls of an open building.

 

[EZBuilding]

The fences I have designed in South Florida, I followed the methodology outlined by TehMightyEngineer. I have also generally received push-back from Contractor's and Clients when my design comes in looking like spats. Their expectations are typically not in line with my design, and I expect that others are finding shortcuts or alternative methodology to reduce the wind pressures on their design.

 

[TehMightyEngineer]

Using ASCE 7-10, Figure 29.4-1, Case C for Risk Category I (121 mph), Exposure D, the ultimate wind pressure 0-S (end of wall) calculated out to 94 psf

Hmmmm, that seems high. I did a half-second back of the envelope calculation and came up with 36 PSF for case A & B. I agree you get something like 100 psf for the 0 to s section in case C but it tapers off as you go down the wall. If you design horizontal reinforcement in the wall to get it to act as a single unit I would think it wouldn't require extra heavy reinforcement and footings? Sounds like you looked at that, though.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/