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Major Hurricane Irma


NJwx85

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Well, this is great news. I hope Irma keeps weakening. The best news a Floridian could receive is that a major hurricane coming their way is losing its strength. Although it doesn't seem likely, maybe there's a slim chance that it weakens to a Category 2 before it hits FL.

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3 minutes ago, TAP said:

Sorry but this is totally untrue.  I am a structural engineer (licensed in FL, grew up in south MIA, and was at north edge of eye wall when Andrew hit).  Building codes including the FL building code require us to design for winds in multiple directions.

I have only researched back 60 years to the 1957 South Florida Building Code but see attached.  Requirements have been even more strict since then.

So if a building is rated for 140mph, does that assume higher winds above 30 feet? For instance a storm could be blowing at 140mph at 30ft, and 170mph at 200ft. Is the likelihood of 170mph at higher levels part of the calculation for taller buildings? Or would 140mph be the max for such a building regardless of height?

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8 minutes ago, TAP said:

Sorry but this is totally untrue.  I am a structural engineer (licensed in FL, grew up in south MIA, and was at north edge of eye wall when Andrew hit).  Building codes including the FL building code require us to design for winds in multiple directions.

I have only researched back 60 years to the 1957 South Florida Building Code but see attached.  Requirements have been even more strict since then.

59b2a8e0dca5b_1957SFBCWind.JPG.64b4e4afd110b323d8052b9a7fc6798d.JPG

I've lived in Miami and can state that a significant fraction of residential homes have un-permitted modifications, remodels and additions.  Even roofs are installed over weekends to avoid permitting fees and inspectors.  So while commercial structures are compliant i wonder about much of the residential inventory.   Also at any one point you have a large number of "in process" construction and cranes.  Who knows how well those will fare.  

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10 minutes ago, TAP said:

Sorry but this is totally untrue.  I am a structural engineer (licensed in FL, grew up in south MIA, and was at north edge of eye wall when Andrew hit).  Building codes including the FL building code require us to design for winds in multiple directions.

I have only researched back 60 years to the 1957 South Florida Building Code but see attached.  Requirements have been even more strict since then.

59b2a8e0dca5b_1957SFBCWind.JPG.64b4e4afd110b323d8052b9a7fc6798d.JPG

I'm going to take you at your word sir. My original post was made from first person accounts that were in the greater Miami area during Wilma.

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41 minutes ago, NJwx85 said:

As I posted earlier, most of the structures in SE Florida are built to handle strong Easterly winds, when you flip them around to Southwesterly, not so much. It's one of the reasons why Wilma was one of the most damaging hurricanes in recent history.

Come on man.  This isn't remotely true.  

 

Structures have to be designed for wind in all directions and from various angles of attack.  I'm a licensed structural engineer.  It has been this way for as long as I've been designing structures. 

 

Wilma caused so much damage because not all structures are designed to the same codes.  The codes and research have gotten significantly better in the last 20 years, but some of those buildings are older than that.  

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9 minutes ago, catdaddyfalcon15 said:

Thank you. The NAM has screwed me so much for so many winters. 

 

It also did well with some recent big ones in the NE.  But you cant compare how a piece handles your backyard  snow amounts to how it can handle a tropical system in the Atlantic. Its not apples to apples here. 

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8 minutes ago, Papa Joe said:

Thanks for the informed feedback, but if they get 20' storm surge (I'm going to "predict" 15') there will still be significant damage. It sounds like the new codes will certainly help with wind damage.

While the storm surge may not be as high as the '35 storm (for a variety of reasons), I expect it will be wider and affect more islands.

Codes are also quite strict on building elevations.  Alot of the work I did was determining a property's  elevation ASL because homeowners need to get elevation certificates before they can occupy a structure.  (I was once determining a property's ASL and the new homeowner stopped by, and asked if the house had well water.  I looked at him weird and informed him that his property was 4' ASL ... aint no well there, my friend).

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6 minutes ago, Michigander said:

So if a building is rated for 140mph, does that assume higher winds above 30 feet? For instance a storm could be blowing at 140mph at 30ft, and 170mph at 200ft. Is the likelihood of 170mph at higher levels part of the calculation for taller buildings? Or would 140mph be the max for such a building regardless of height?

Well, a building is designed for wind pressure.  To calculate pressure, you use several factors including the wind speed of an area, as well as exposure category and height above the building. The higher above ground level you are, the higher the velocity pressure factor (and the higher the pressure)

 

 

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8 minutes ago, csnavywx said:

FL wind is up to 145-150, but still an inner max there. Eye is trying to clear out, but there's still some IEW remnant there.

hlgMAXw01a.gif

 

yeah you can tell on radar there is still some remnants of the IEW hanging on but it is very close to clearing out. the new eyewall looks very good structurally already and once the old one finally decays we could see irma off to the races again.

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2 minutes ago, tdp146 said:

Thank you for this. Do you have any similar information on what more recent codes are?

Yes.  With location and date built you can find out the code requirements.

This site also provides info for recent codes by location:  http://windspeed.atcouncil.org/index.php?option=com_locationfinder&view=location&Itemid=10

Please do not get focused on design wind speed and relate that to this or any other storm/wind event.  Engineers can design something for a very high wind speed but if it is not built correctly or something small fails it can easily turn into a catastrophic failure.  Do the stricter building codes help?  Yes...but they are not foolproof.  Besides wind....flood and surge are a whole different topic with even more variables and minimal testing and guidance in the code.  The flood/surge code is a very poorly written document in my opinion.

I just put a brand new 180 mph metal roof on my house in the Keys last month.  If one fastener fails the roof is gone.  This failure could be from improper installation, poor substrate, etc.  At that point, the fact that it was designed for 180 mph does not matter.

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Hurricane Irma Discussion Number  38
NWS National Hurricane Center Miami FL       AL112017
1100 AM EDT Fri Sep 08 2017

Recent data from both NOAA and Air Force reconnaissance planes
indicate that Irma's intensity remains at 130 kt, although this
value could be 5 kt higher or lower. The central pressure has been
oscillating around 927 mb.

The environment continues to be favorable for Irma to maintain its
category 4 status, and only unpredictable eyewall replacement cycles
could result in intensity fluctuations during the next 48 hours.
Nevertheless, the NHC forecast brings Irma near south Florida as a
category 4 hurricane. Thereafter, interaction with land and an
increase in shear should induce gradual weakening.

Plane, satellite and radar fixes from Cuba indicate that the eye of
Irma is moving toward the west-northwest or 285 degrees at about 12
kt. The hurricane should continue on this track for the next 24
hours.  After that time, Irma will reach the southwestern edge of
the subtropical high and begin to turn north-northwestward and
northward. This turn will occur, but the precise moment is still
uncertain, and that is why NHC emphasizes that nobody should focus
on the exact track of the center. The new NHC forecast was adjusted
just a little bit westward and is on top the latest ECMWF model and
the HFIP corrected consensus. In fact, these two aids are also very
close to each other.
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