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Marin County, CA
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Columbus, Mississippi
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 JLC article on wind-resistant construction
[#2] Posted: 02/17/2004 - 3:23:05 PM |  |  |
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[quote]Originally posted by Douglas Hansen
First, the author says he likes to use an architectural style "that includes a lot of jogs and bays. The corners and short wall sections add rigidity to the structure." That statement runs counter to everything I ever thought, unless he's talking about ways to deflect wind around a building.
Douglas,
As a carpenter, what the author says makes perfect sense to me. A long straight wall is not as rigid as a wall which is broken-up by "jogs and bays". Every time you turn a corner, even if only for a few feet, you create strength and rigidity. It probably helps to create turbulence in windy situations (like dimples on a golf ball), but I'm wholely unqualified to speak to that.
I'm not well-versed in the hold-down end of the pool either, but common sense says you have to be correct in thinking the 24 inch embedment is better.
Brian G.
Ex- and Part-time Carpenter
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Jacksonville, Florida
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JLC article on wind-resistant construction
[#3] Posted: 02/17/2004 - 4:02:34 PM | | |
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In the Northeast Fl. area epoxy tie downs seem to be the norm.The builders will place anchor bolts in the slab where there is a long window span.I agree that epoxied tie downs would not seem to be as effective as those floated into the slab.Something else to consider is the fact that when you drill a hole in concrete to pour some epoxy and place a tiedown rod that hole has got to be cleaned of all concrete dust for the epoxy to be effective.Many of the people in this area who do the tiedowns are subcontractors who I doubt would take the time to clean the holes properly.
Danny Pritchard
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| Danny Pritchard |
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Chicago, IL
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JLC article on wind-resistant construction
[#4] Posted: 02/17/2004 - 4:16:44 PM | | |
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It sounds like the Associate Editors @ JLC were reaching for filler material; must be a dry month for contributors. Since even a piece of crap mobile home on a hill can be anchored effectively against "average" wind loads, what are they thinking?
Wind force increases exponentially with wind speed. Windsurfers know this stuff; the increase of force on your sail between 15 to 20, and 20 to 25 is remarkable.
The (approximate) relationship of wind speed to wind force developed on 1 sf of flat surface set perpendicular to the wind follows:
5 mph = .12lbs of force
10 mph = .50 lbs of force
15 mph = 1.11
20 mph = 1.97
25 mph = 3.07
30 mph = 4.43
I could continue w/ the #'s, but as you approach 70mph, the chart starts going pretty steep; when you cross 90, it's straight up. Since destructive hurricanes are in the 110-180 mph category, you can imagine......... I've stood in the Columbia River Gorge measuring 70mph gusts; one cannot stand up to that. You can @ 50, 70 impossible.
If one is designing for hurricanes, one had better have a better design theory than "jogs & bays".
To understand the "jogs & bays" approach, think flying buttresses. These "extensions" beyond the primary envelope can provide lateral reinforcement against wind loads. Since the real danger in high wind is seperation from the foundation, roofing being blown off, or the house getting hit by flying objects, what the hell good do jogs & bays do? Not much in real world conditions. Get winds up around 80mph & hell breaks loose anyway. Jogs & bays aren't going to hold the roof on.
In earthquake country, I think they (jogs/bays) would weaken a structure, since shaking something from the foundation is WAY different than pushing against it w/wind. If I was building earthquake resistant style, I would want a nice monocoque shell w/ a minimum (or no) projections.
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Kurt in Chicago
"If I smell it, it goes in the report".............Phillip Smith...2012
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Indian Rocks Beach, Florida
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JLC article on wind-resistant construction
[#5] Posted: 02/17/2004 - 6:37:50 PM | | |
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| Quote:Originally posted by Douglas Hansen
I just read the article in February's JLC on wind-resistant construction and there are a couple of things I don't understand.
Damn you Hansen, once again you've distracted me from useful money-making work to answer one of your esoteric questions that only you, I and 4 other people care about. Just for that I'm going to insert one of those annoying smiley thingies you detest so much 
You're right about the jog and bay thing. If you think of the top plate of a wall as a beam, when you put large holes in it you reduce the strength. Now you can offset that by bracing it with the jogs and bays, but this takes some fancy engineering. The SSTD manual is comprised of generic details that can't accommodate things like introducing a bay into a wall. It's useful only for basic shapes.
As for the epoxied rods, you have to understand that the epoxy is stronger than concrete. Simpson high strength epoxy has a compressive strength of more than 12,000 lb! Once you get the rod in a certain depth, it ain't going anywhere.
The length of embedment is not really the limiting factor. Once you get beyond a certain length, increasing it doesn't increase the allowable tension load very much.
A Simpson 5/8" SSTB anchor bolt in 2500 psi concrete with 16 5/8" embedded has an allowable tension load of 4600 lb. Increasing the embedment to 25 5/8" only ups the load to 5175 lb.
A 5/8" threaded rod embedded 3 3/4" in 2000 psi concrete has an allowable tension load of 4,190 lb. Increase the depth to 5" and it goes up to 6,680 lb. At this point the limiting factor becomes the strength of the steel. You need to use higher grade of rod to achieve strenghts above this!
So you can see that the wimpy looking epoxied rod is probably much stronger than your 24" anchor bolt ever was. And who knows what kind of steel that bolt is made of anyway? They're all generic with no standards.
If you're worried about not cleaning out the holes, testing shows that even if you don't blow it out, you still achieve 63% of the design strength. So an 5/8" 8" rod epoxied in place isn't going anywhere. This would handle most any load you'll see in a house, even if you don't blow out the hole!
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Mark Cramer
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Chicago, IL
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JLC article on wind-resistant construction
[#6] Posted: 02/17/2004 - 7:05:17 PM | | |
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Shoot, just glue the sill plate to the foundation w/ the damn epoxy. That, & duct tape, would work wonders. Who needs anchors?
I have some experience w/ structural epoxy, & will testify to its amazing strength. Very similarly to wood, the glue bond is stronger than the material being joined. We have hammered on epoxy joined concrete & the concrete fails before the glue joint. I would have complete trust in a glued anchor rod; I think the wood frame would fail well before the anchor.
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Kurt in Chicago
"If I smell it, it goes in the report".............Phillip Smith...2012
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Columbus, Mississippi
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JLC article on wind-resistant construction
[#7] Posted: 02/17/2004 - 9:30:49 PM | | |
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[quote]Originally posted by mcramer
You're right about the jog and bay thing. If you think of the top plate of a wall as a beam, when you put large holes in it you reduce the strength. Now you can offset that by bracing it with the jogs and bays, but this takes some fancy engineering.
I'm not sure I'm with you here. If we're talking about horizontal wind loads on a frame wall anchored to a foundation, a wall with jogs and bays is going to be a lot harder to blow over than a straight wall. The top of the wall will be braced by the ceiling joists anyway, but if you just keep turning the wind speed up the straight wall is bound to go first...plain old leverage, no engineering required. What am I missing?
Think of a house going up, where the walls are framed but not braced, and the ceiling joists aren't in yet. If you found two walls of equal length where one was straight and the other had jogs & bays, and you tried pushing on both of them in various places I guarantee you the straight wall will not be as solid as the other one. Leverage, like gravity, is reliable and predictable.
Brian G.
Been There, Done That on My House
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Marin County, CA
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JLC article on wind-resistant construction
[#8] Posted: 02/17/2004 - 9:44:23 PM | | |
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Thanks guys, I appreciate the education. I didn't realize that wind force went up exponentially, but leave it to windsurfer Kurt to know that first hand.
One clarification - a hold down bolt and an anchor bolt aren't remotely similar here in earthquake land. I agree epoxy can actually be a superior method of securing an anchor bolt, but hold-downs here are used with these long rods with a double hook on the end.
From all you're saying, I guess the issue is that the engineer must think the overturning forces caused by wind aren't as great as that caused by earthquakes, but whatever they are, they aren't out of the range of an epoxied bolt.
I'd insert a smiley Mark, but I haven' learned how.
Download Attachment:  HoldDowns.JPG
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Douglas Hansen
www.codecheck.com
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Indian Rocks Beach, Florida
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