Please Enlighten Me Here...

[msquared48]

Found this article on the web today dealing with the residential destruction in Oklahoma recently due to the Tornados as related to local construction practice, and a part of he article really bugged me...

http://news.yahoo.com/insight-tornado-alley-building-practices-boost-damage-110444131.html

In this article they are making the statement that nails driven at 90 degrees to the uplift force do not have the same uplift capacity as nails driven at 45 degrees, I.E., toenails. Toenails, mind you, toenails... Seems like a pretty generalized statement, devoid of specifics and justification to me that files in the face of experience and basic engineering practice. I mean, are we going to start putting thru-bolts for the shear wall end wall studs in at 45 degrees now? It's the same as toenails for the load application the article is describing to me. I'll be damned if I will specify toe nailing plywood sheathing for shear walls any time in this existence.

Am I missing something here? Or putting it plainer, where's the beef? And don't say in the Tornado... The guy who made the initial statement in the article is a Civil Engineer, apparently. The information on the nailing is much further into the article.

Mike McCann
MMC Engineering

 

[msquared48]

The only thing I can think of that might make a difference is if the nails were driven up at 45 degrees to resist the uplift. Certainly not down or horizontal... The nails would have to be longer too to get the proper penetration for shear into the studs, meaning more of a length to bend through.

But I would have to see the testing results... And..., I would have to see the same size nails use for the 90 degree and 45 degree situations compared.

Mike McCann
MMC Engineering

 

[BAretired]

Nails hammered into the wood at a 90 degree angle provide little resistance to the upward suction of a tornado, several construction and engineering experts said.

"We saw that in many places in Moore," said Randy Shackelford, Research Engineer and Code Specialist for Simpson Strong-Tie Company in McKinney, Texas. "Two nails sticking up where the stud (wall frame) once was."

The next best connection is what is called a "toenail" in construction jargon, which involves driving the nail into the wood at a 45 degree angle. This provides significantly more resistance than nails driven at a 90 degree angle.

Badly worded statement, I think. He means nails in withdrawal which are parallel to the direction of uplift and at ninety degrees to the bottom plate.

It would seem that the folks in Oklahoma would be wise to review their building practices before making the same mistakes all over again.

BA

 

[MikeHalloran]

The way I read it, an engineer for Simpson was contrasting the uplift resistance of three toenails at the base of a stud, vs. two nails driven straight up through the sole, as when prefabbing a wall panel.

Of course, Simpson makes simple products that could greatly enhance the strength of any nailed joint, and there's no great intrinsic expense in any Simpson product, including hurricane straps, so assertions that more robust construction costs a lot more ring hollow.

If the OK govt said every new unit needed hurricane straps and anchor bolts, the builders would complain about it, but after a little aggressive enforcement to make sure that everyone was actually doing it, I doubt you'd be able to measure a step in prices.



Mike Halloran
Pembroke Pines, FL, USA

 

[rowingengineer]

Mike H makes a good point, i have always found it intresting that for some codes they give lower risk levels to houses than commerial buildings.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Ron]

Mike...that jumped out at me also. I don't believe in toenailing...at all. Use a connector and keep that connection in direct shear on the fastener. Tension in nails is not good, but unavoidable for components and cladding....completely avoidable for main wind force resistance!

 

[woodman88]

The studs in wood walls are nailed at each end only to hold the stud inplace until the sheathing is applied. It is the sheathing (stucco/drywall/OSB/plywood/siding) that holds the wall framing together. When the sheathing fails the wall fails. By connecting the studs to the sill plate with metal anchor or straps. Rather than just having two nails sticking out the the sill plate, you would see studs sticking up with everything else still blow away.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[paddingtongreen]

If nails are to be used, toenails from both sides are better than straight up nails. You cannot put either in pure tension without moving the stud sideways, and then the other nail resists that.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[msquared48]

OK, but nails are not to be used in tension when driven in parallel to the end grain anyway, so I guess that toenails would be better in that circumstance. However, as Garth said, the plywood nailing, coupled with the foundation hold downs, are the major recognized, tested and rated, uplift mechanisms here, not any toe nailing.

If they are using only GSB or GWB for the sheathing elements of these residential structures instead of CDX plywood, I can see a problem, But I cannot see any appreciable increase of any uplift resistance from the use of these toenails at the location given. Certainly not enough to be the reason for a house still standing. I would attribute that to the whims of the tornado.

Mike McCann
MMC Engineering

 

[JAE]

All of this is bunk anyway - no wood framed house is going to stand up to 250 mph tornado winds no matter what sort of nailing or Simpson clips you use.

 

[MikeHalloran]

>>> no wood framed house is going to stand up to 250 mph tornado winds no matter what sort of nailing or Simpson clips you use. <<<

That's absolutely true, but becomes less so at less than 200mph.
Andrew trashed a lot of commercial developments, but nearby Habitat houses didn't even lose a shingle. I figure where an engineer specifies five nails, a commercial builder will use three, but an amateur will use twelve.



Mike Halloran
Pembroke Pines, FL, USA

 

[BAretired]

Maybe the folks in the tornado belt should be living in underground concrete bunkers covered with two feet of soil with a few peepholes to let the sunshine in.

BA

 

[JStephen]

I'm not sure exactly what all the "Tornado Belt" includes, but have spent much of my adult life living in places subject to tornadoes. Yet, I've never actually SEEN one in all that time.

There's no doubt you can do things to make houses stronger, and put in tornado shelters for personal safety in the event of a tornado. But it really gets down to probabilities and how much you're willing to pay for something that most likely won't ever be a problem. If you live down on the gulf coast, you can figure, sooner or later, a hurricane's going to blow through your area, and that gets figured into the engineering data, x% probability of being exceeded in 50 years, etc. With a tornado, it's just not that likely.

If individuals don't want to personally pay for better houses and storm shelters, there won't generally be a lot of motivation for their elected officials to be demanding more stringent building codes, either.

Keep in mind that mobile homes are fairly popular in Oklahoma, too.

Back in the olden days, storm cellars were fairly common out in the country- as in "Wizard of Oz". So it's not like the problem is unknown, or it never occurred to anyone to do things differently.

 

[JAE]

Attached is the USA tornado strike map:

 

[JAE]

And this is the FEMA zone map with recommended wind speeds.

 

[JStephen]

What are the probabilities associated with that map?
It looks like I've spent the last 49 years in Tornado Alley, and have yet to see one, by the way.
And it looks like every building in 2/3 of the country would have to be demolished and rebuilt as a bunker to comply with that map.

 

[frv]

JStephen. I don't think that's the point of the map. I think the point of the map is to determine the wind speed for design of an emergency shelter. I don't think anybody is advocating building bunkers. If the homeowner/business owner chooses to build a small emergency shelter, then that is the wind speed to which it should be designed.

 

[TXStructural]

The mean recurrence interval of the ASCE 7-10 maps is 1700 years for important buildings and covers EF-1 tornado winds.
The probability of reaching the FEMA 250 mph winds speed is very nearly zero, which is why the IBC doesn't require design and construction for tornado winds.

Also, the abrupt nature of tornado winds require significantly more strength of lightweight members (suddenly-applied loads are usually designed for twice the actual force), but heavy structures are not as easily accelerated by brief winds. But the 2.6 mile diameter tornado in Moore was not exactly brief.

As an example, the winds in Moore created a wind pressure that was five times that of code-level winds in Oklahoma (160 psf v 34 psf.) Those kinds of forces break wood members, so a well-connected wood home could probably withstand an EF-2 storm (135 mph), but once members start breaking, progressive failure will happen. I have been trying to get that message out, but people don't seem to care until they do.

Constructing an ICF home in these areas would cost about the same as a wood framed house with a storm shelter. Adding a bit more for a concrete attic floor results in a very robust house that will keep all the people and most of their belongings safe.