Florida and Removal of Seismic Design 2

[abusementpark]

Does anyone know the history of how Florida decided to not require use of the IBC seismic design provisions? Did they do an rigorous study to prove that IBC was overstating their seismic risk? Or was it more of a quick "common sense" judgment?

Also, I know other jurisdictions have either removed or softened the IBC seismic requirements. Any information on this as well would be appreciated.

 

[JedClampett]

With the whole state suject to hurricane force winds and in a low seismic activity area, I'm guessing it is assumed that wind controls all lateral designs. I don't know this for sure, but it seems logical.
An interested fact is our company is concentrated in California, but is doing more and more work in Florida. Our CA structural folks find it impossible to believe that there is no seismic requirements in Florida.

 

[connectegr]

High wind forces in Florida far exceed the seismic forces.

http://www.FerrellEngineering.com

 

[Teguci]

I can't address the original post, but it is conceivable that a tall heavy structure (say a masonry lined chimney) could be controlled by seismic over wind even in FL.

You say no seismic at all?

 

[JLNJ]

I was just looking at Des Moines and they say to use SDC "A". Not really "no seismic" but definitely minimizes the work.

 

[JedClampett]

They take the sections regarding seismic out of Chapter 16 in the Florida Building Code.
So they're gone.

 

[Ron]

I'm sure there are exceptions and maybe I'll run into one at some point, but I've checked it in the past and it wasn't even worth the effort to go through...wind prevails.

 

[racookpe1978]

JLNJ (Structural)
16 Aug 11 8:30
"I was just looking at Des Moines and they say to use SDC "A". Not really "no seismic" but definitely minimizes the work."

That's probably because the New Madrid fault - slightly south of Des Moines but east towards the Mississippi river - shook the entire central US region significantly only 200 years ago: Those series of 7+ and 8+ quakes broke chimneys as far east as Charleston SC (who have also had quakes of its own) and as far north as Ohio and north Illinois.

But Florida? Nothing seismic. Well, really, nothing has fallen over due to known seismic activity. Everything sticking up that could have fallen down had already been blown over by hurricanes, or sucked down into sinkholes. 8<)

 

[hawkaz]

Des Moines is several hundred miles from the New Madrid fault. Per the ICC seismic ground motion tables- the impact of the New Madrid fault is negligible that far away. A significant quake would be felt- but minimal damage would occur as a result.

Seismic values in Florida, Des Moines, most of Texas... are all similar. Seismic rarely governs in these locations.

 

[msquared48]

So, the Hatian earthquake a few months ago was not felt in Key West or Miami?

Mike McCann
MMC Engineering

 

[wannabeSE]

Did Florida remove the seismic provisions for non-structural components inside a building where wind would not be an issue? The USGS has some historical information on the earthquakes in Florida

http://earthquake.usgs.gov/earthquakes/states/florida/history.php

 

[jhardy1]

Speaking from a Queensland (Australia) perspective - on coastal sites, the cyclone design wind loads will ALMOST always govern design over seismic requirements, so a common design approach is to first design for cyclone loads and then check for seismic. However, there are certain classes of structures and sub-structures where seismic loading will be significant, if not the governing design load case - e.g. inverted pendulum structures such as elevated water tanks, and components of buildings which are inside the building and therefore not subject to direct wind load.

I don't know the Florida codes, but it would be surprising to me to remove ALL requirements for seismic design - is this in fact the case?

 

[ron9876]

There are no seismic design requirements in Florida. I have never heard of anyone talking about even a minor shake. However we did have snow flurries once several years back.

 

[a2mfk]

The obvious has been stated regarding wind pressure from commonly occurring events such as hurricanes, tornadoes, and even regular thunderstorms that would control over any seismic lateral force that may be anticipated to occur here. True that this does not address interior issues such as heavy equipment anchoring, MEP and ceilings hung from roofs/floors, or the example of a slender but heavy URM smokestack.

I would assume the "no seismic design requirements in the FBC" is based on statistics which is based on history, and some pretty good geophysics knowledge of Florida. Though we only have about 150 years or so of very reliable history of our state in terms of seismic events, owing mostly to the lack of people living here and recording history, ie the factoid below. One could also state that our recorded history in terms of geology,weather, climate, etc. is a blink of an eye in terms of the history of the earth.

In the early 19th Century there were less than 35000 people living in Florida and they were concentrated in the ST. Augustine (north east coast) and Apalachicola River (Panhandle) areas. So if an earthquake happens in the Florida woods and nobody is around to write it down....

See the attached scans of a geology of Florida textbook that have probably the most amount of info I have ever found on the subject.

 

[abusementpark]

I'm sure there are exceptions and maybe I'll run into one at some point, but I've checked it in the past and it wasn't even worth the effort to go through...wind prevails.

Depending on building importance and soil classification, a structure in a high-wind region can still fall under Seismic Design Category C, which introduces a lot of detailing requirements, regardless of the seismic risk. So, I guess Florida doesn't think these requirements are important, since I would assume some structures in soft soils must fall in the SDC C.

Also, I practice in a hurricane prone region (V = 110 mph) along the gulf coast with no history of significant seismic events. We had a structure recently where IBC seismic loading actually governed various aspects of the lateral design, sometimes by a significant margin on elements that require the seismic over strength factor.

 

[abusementpark]

Also, it seems like the IBC seismic loading would govern the lateral load on any heavy, open platform structure with little wind exposure.

 

[racookpe1978]

a2mfk: You buzzard! Next time scan the paper the same way on both pages!!!! 8<)

So, this report (in early 1980's) lists only 5 events in the past 100 years in FL, and all below a cat 3 earthquake.

Nothing to worry, mates (realistically).

 

[Ron]

abusementpark...most of the populous areas of Florida are in a wind zone exceeding 110 mph. A 10 mph increase in wind speed (to 120 mph) results in a 19% increase in wind load, so that usually kicks it over.

 

[csd72]

There was an earthquake in the Australian city of Newcastle, a definately low seismicity area.

They found that virtually all structural failures were in buildings that would not meet the current wind code.

Yes, there are a few exceptions to the rule but usually buildings that are designed to resist high wind loads have a reasonable resistance to low level seismic activity.

You will also find that most countries in the world that have low sesmicity, do not check any of their buildings for earthquake loads (including the UK)

 

[a2mfk]

I will say this, if we were to ever have a earthquake of consequence, it would probably cause a lot of damage. We have a LOT of URM structures, most houses built in the 80s or earlier were lightly reinforced if at all. And brick on wood frame probably wouldn't do much better.

Since I obviously don't have to deal with seismic inside Florida, my exposure has been limited to a few out of state projects and that has been quite some time. I found the attached article on this subject but not much info on Florida except Orlando was mentioned in the one table.

Abusement- I can only assume that PERHAPS they took the worst case scenario for Florida, which would be the center of the State, where the wind speed is only V=100mph. Say Ocala, Florida, where by the way John Travolta can fly his 747 to his house, literally. Then look at the map (see my attachment above) of probable epicenters for Florida and note that "Continued subtle tectonic adjustments withing the Osceola complex are plausible, and are the probably sources of the few Florida earthquakes." The Osceola complex is fairly close to Ocala, judging from that map which is not meant to be highly accurate. Then which will govern design, V=110mph or a mild tremor that has a 5:150 year occurrence probability.

From a geophysics standpoint, there is just not anything under or near Florida that the experts believe is unstable to the point where it needs to be considered.

Look, Florida is hot as hell and disgustingly humid, there are gators, snakes and all kinds of bugs, we have all these annoying tourists and snowbirds, we can have major hurricanes and our thunderstorms are crazy, and the whole state is an old reef and sandbar that is barely above sea level. Oh, and we have sinkholes. The ground can just decide it has had enough and give up. God cut us a break on earthquakes.

Or the "code guys" may have just said, "Guys, in the snow and seismic part of the FBC do we really need to write down anything?"

And after some blank stares from the rest of the code writers they moved along...