Tension Perpendicular to the Grain - Hurricane Clips

[phamENG]

In a recent thread, RontheRedNeck mentioned how hurricane ties are often installed improperly. It got me thinking about some of the ways this is done, and one jumped out at me as being as potentially bad as it is ubiquitous.

I have never seen a hurricane tie specified for any reason other than the capacity and installation geometry. So I got to thinking about this condition that I see pretty frequently:

A 2x12 rafter on a relatively low slope roof (drawn at 4/12) and a Simpson H2.5A hurricane tie 'properly' installed. This is common in attics that will be occupied so it gives the architect room for insulation. The 2.5A is the most common hurricane tie I see. But...it's located entirely below the neutral axis of the rafter. In fact, I'm not sure of any hurricane tie except those that wrap around the top edge that would not induce some amount of cross grain tension in this connection. The load table footnotes in the catalog include this:

5. When cross-grain bending or cross-grain tension cannot be avoided in the members, mechanical reinforcement to resist such forces shall be considered by the designer.

So what do you all think about this? Do you think about it? What is your threshold for worrying about it? If all contractors were compliant and did exactly as we say without a fuss, what would you do? Since the previous question is about as far from reality as flying pink unicorns, what do/will you do?

My thought is this: Wind loads are not sufficiently sustained to cause catastrophic failure so long as the strap and the sheathing fasteners overlap. Unfortunately, that means that the length of the hurricane tie above the plate needs to be just shy of the depth of the rafter. That will get loads of push back from contractors. I do think the image I posted above is likely inadquate or, at least, doesn't meet code or the intentions of the manufacturer.

 

[dhengr]

PhamENG:
I suspect that what DriftLimiter meant was that the tension perpendicular to the grain ( T⊥ ) would have had to have been included in meaningful/representative models for testing, and the T⊥ was not the limiting testing/failure condition, for the limiting allowable tabulated load. I would think that all good Wood Design textbooks, Design Guides and Bldg. Codes have essentially the same admonition (NDS 3.8.2) which PhamENG showed. Fastening into end grain and T⊥ to near a loaded edge are just too variable and indeterminate to put any meaningful design values on, so it becomes a matter of good, sound, engineering judgement, and is generally discouraged. The problem today is, that we have way to many people pretending to do this kind of design work without having much idea what they are really doing, or why, or how, certainly no good engineering experience or judgement to make these determinations.

Discussions of edge and end distance for various loading conditions and orientations to the grain should be studied with a good understanding of the variability of material conditions and properties, and an understanding of the grading rules, and some understanding of the low quality level of much of the bldg. trades these days. We see the same general admonition about T⊥ and the top edge (loaded edge) of ledgers for decks and the like, and the preference for bolts/lags below mid-depth of the ledger.

DriftLimiter:
I would have asked Simpson (or whoever) the question in a slightly different way, and maybe you did. How many tests did you run, something statistically meaningful for the design values, I assume? Did you use typical rafter, birds-mouth, top plates and studs details/models to run your tests on, so that T⊥ failures would have shown up, if they were the limiting failure mechanism? At your tabulated design values, was T⊥ ever the failure mechanism for the connection? What were your limiting failure mechanisms for these connectors? We are having a discussion in the office about whether T⊥ is ever an issue with these connectors, at your design values, and how worried we should be about this. I am not trying to put you on the spot, but rather to understand how you and we (engineers) should look at these issues. I would want to talk with one of their engineers involved in this testing work, not some sales rep. or phone/question answering monkey. Over the years, I have found several of these mauf’ers. helpful, informative and willing when you can get to the right person, so you are actually talking engineering, not just the memorized code and corp. lingo. In their footnotes and corp. lingo, they are being intentionally vague, until you drag it out of them, with the right question and good engineering discussion, just because we have so much trouble putting real numbers on this issue. Alternatively, they wouldn’t be selling that connector, that configuration with its dimensions and nail hole locations, if T⊥ was regularly cropping up as a problem with their use.

 

[XR250]

MY understanding is that the concern is for cross grain tension DUE TO BENDING.

That was my understanding as well. Then again, nobody is condemning the typical bottom plate which is ripe for cross grain bending.

 

[phamENG]

Sorry - I vanished for a while. Computer went kaput. Amazing how much work I've gotten done between setting up my new computer and logging into Eng-Tips...

and the T⊥ was not the limiting testing/failure condition

According to the Simpson rep I spoke to, rafter splitting IS the most common failure mode. And the published values tend to have a factor of safety of about 3. So that did make me feel a little better. They declined to share any details on their tests, though. I found that to be a bit disappointing.

Houseboy: I disagree.

Tension Perpendicular to Grain[/b]]Designs that induce tension stress perpendicular to grain shall be avoided whenever possible. When tension stress perpendicular to grain cannot be avoided, mechanical reinforcement sufficient to resist all such stresses shall be considered

There is no mention of bending at all, and it is specifically titled "Tension Perpendicular to Grain."

Then again, nobody is condemning the typical bottom plate which is ripe for cross grain bending.

I disagree with this one, too, though I grant you it's not often talked about. SDPWS 4.3.6.4.3 requires plate washers installed within a 1/2" of the edge of the sole/sill plate at all anchors unless they've been designed for shear only and meet other requirements to ensure there's no net tension at the anchor bolt. Not sure what they'd be preventing with the 1/2" to the edge other than perpendicular-to-grain-tension caused by cross grain bending. I require them at all wall anchorages where the anchor bolts are resisting uplift.

 

[XR250]

I disagree with this one, too, though I grant you it's not often talked about. SDPWS 4.3.6.4.3 requires plate washers installed within a 1/2" of the edge of the sole/sill plate at

There is still cross grain bending between anchor bolts unless they are spaced close together.
What about a typical slab-on-grade prescriptive, 1 story house with anchor bolts @ 6 ft. O.C. No one is installing plate washers
there (at least in my neck of the woods)

 

[phamENG]

I agree - most tract home builders around here don't do it and the inspectors don't know to look for it - and it's a problem if the building ever sees any serious loads. It's a well documented failure mode in hurricanes, and I imagine in earthquakes, too, since Seismic Category C and above requires them on all anchors.

 

[Aesur]

I typically only see the plate washers actually installed in "higher" seismic (SDC C) or high wind areas (hurricane), it is rare that they get installed here on any project, be it commercial, multistory mutifamily or single family homes. Our plans call for it but I rarely see it actually done as it isn't the "local standard of care". It's been a while since I reviewed that section of the NDS, but doesn't it talk about if you can show that bending between the bolts isn't an issue then you don't need to place the washers? I can't remember if it was the code of elsewhere that talked about that.