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.

 

[MotorCity]

Interesting observation. I think that as engineers, we can be our own worst enemies at times.....while solving one problem (roof uplift), we create another problem (cross grain bending). Honestly, I think we use hurricane straps to kid ourselves into thinking that we have taken steps to prevent the roof from blowing off our house in a hurricane. Those tiny little light gage straps are not going to be what keeps the structure intact in a 250 mph hurricane. The other issue here is cross grain bending. While it is frowned upon and makes us cringe, the reality is that there is SOME strength in tension perpendicular to the grain. I'd say its the equivalent of allowing tension in concrete...which ACI does recognize in the form of its rupture modulus. If I was not concerned about the backlash from contractors, I'd much prefer straps over the top of the roof joists instead of side attached clips. And more food for thought, this tension perpendicular to the grain is also common at the end of any/most truss tension members only the connections are made with nails, perforated plates, etc. It will continue to exist whether we permit it to or not.

 

[phamENG]

MotorCity - I agree that we can be our own worst enemy, and I agree that we'll never completely erase cross grain tension from our connections. But the code does address it (and gives just enough wiggle room for some to squeeze through):

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

And I disagree with on the efficacy of hurricane ties. When used as part of a properly designed and executed load path, they are significantly better than toe nailing the rafters. Of course if they aren't used properly or the load path is incomplete, the top plate will just go with the roof when it blows off.

So since we agree that tension perpendicular to the grain exists and will not cease to exist and wrapping straps is a non-starter for typical installations, where do you draw the line? At what point do you decide that some mechanical reinforcement is necessary or at least a longer strap should be used?

 

[driftLimiter]

Given that simpson tests these assemblies, I think that the published capacity would include cross-grain failures. If it doesn't then man what a shame. More than half of the arrangements indicated in the simpson manual and ESR report would induce cross-grain tension.

 

[phamENG]

driftLimiter - they certainly do not. Cross grain tension is not quantified anywhere that I'm aware of, and Simpson specifically points out to 'designers' to avoid it or reinforce for it (see the quote in my first post from their catalog about hurricane ties and cross grain tension)

 

[driftLimiter]

So then all the published test data is irrelevant? This is confusing me how can Simpson post these configurations with load values and not include this effect? Are they reinforcing every piece of lumber they test???

 

[phamENG]

I sent a technical inquiry to find out. We'll see...(well, we'll see if we see...my last technical inquiry went unanswered...)

 

[driftLimiter]

Lol I did as well. I am thinking it simply has to be included in the test. You can't avoid it.

Also NDS Dowel Type fasteners have an Allowable shear perpendicular to the grain Zperp. These configurations like shown in Figure 12G also put the member in tension across the grain but there is a capacity attached to it. I think the same type of thinking could easily be applied to the fasteners from the hurricane clip.

If they don't get back to US I will call the simpson rep who comes by our office from time to time and pester them. They usually get me what I need.

 

[phamENG]

I guess a better way of putting it is that "they don't specifically test for tension perpendicular to the grain." If it's there in the arrangement they're testing, it gets captured. If not, it doesn't. So part of my question was about their test setups. What size lumber, slope, etc. If they tested them all on 2x6 rafters, 2x8 or larger would have more cross grain tension.

I'm just trying to back up my 'judgement' on when reinforcement should be 'considered' with some empirical evidence.

 

[XR250]

My suspicion is Simpson has tested these products with rafters in this configuration. On my house I used their 6" SDWC truss screw in lieu of hurricane ties. Goes in a lot faster and is cheaper too.

 

[GeorgeTheCivilEngineer]

Does load duration factor into this? That is, tension perpendicular loading causes more creep for medium-long term loading?

 

[Craig_H]

I don't think there is too much to be concerned with here. Not sure what the NDS says, but CSA O86 says to design using the effective depth from the edge of the fastener group rather than for the complete section when calculating the shear resistance.

That would put your shear resistance somewhere in the order of 1000 lbf, which is more than the connector is rated for.

 

[phamENG]

Craig - thanks. NDS just has a footnote on the minimum edge distance table

Heavy or medium concentrated loads shall not be suspended below the neutral axis of a single sawn lumber or structural glued laminated timber beam except where mechanical or equivalent reinforcement is provided to resist tension stresses perpendicular to grain (see 3.8.2 and 11.1.3).

They don't define "heavy or medium." However the commentary states that:

. Concentrated loads less than 100 pounds and spaced more than 24 inches apart may be considered a light load condition.

So I guess anything greater than 100lbs is at least a medium load. If duration factor influences tensions perpendicular to the grain as it does for tension parallel (worth noting this is an unconservative assumption), then we can call that 160lbs for wind loads as the threshold between light and medium. 160 is usually about the minimum for uplift at the eaves on houses in my area. So for most applications, a connection geometry where most (and certainly where all) of the fastener group is below the neutral axis would violate the minimum edge distance requirements in the NDS.

 

[driftLimiter]

The response I got from Simpson was : "Our published loads do not take into consideration cross grain tension. Sometimes for certain applications when using the ties, cross-grain bending or cross-grain tension cannot be avoided. The designer will then need to take into account mechanical reinforcement. Please see below (attached).

This response is a somewhat expected deflection to addressing the question. Still not making sense how one could possibly test the hurricane clips on an actual piece of lumber without some cross-grain tension. The ESR report also deflects and says the connected wood member shall be designed by NDS.

Maybe we shouldstart speccing the SDWC like @XR250 said

 

[phamENG]

Better than the response I got, which was silence.

 

[Craig_H]

Does the NDS not provide a means to calculate perpendicular to grain splitting resistance? CSA O86 provides it, but only for bolts and dowels. I've always found it odd that it's not included for other fasteners, as wood's propensity to split seems pretty independent of the fastener type.

 

[phamENG]

Nope. It's more-or-less implied in the bolt design equations, but then they give you that rule about locating it such that it (mostly) prevents it in a qualitative rather than quantitative sense and in other places tells you it's bad, you shouldn't do it, but if you do, consider reinforcing it.

 

[XR250]

"Our published loads do not take into consideration cross grain tension. Sometimes for certain applications when using the ties, cross-grain bending or cross-grain tension cannot be avoided. The designer will then need to take into account mechanical reinforcement. Please see below (attached).

That is totally ridculous, Simpson - you can suck it!

Well at least 99.9% of the engineers don't consider this so the standard of care has been set.

 

[phamENG]

So it took a while, but I got a much more useful response from Simpson.

It was the same "we don't provide guidance on that"...but then he pointed out that the Truss Plate Institute has detailed guidance on cross grain tension for connections to girder trusses. Haven't dug into the roots of it to see what it's based on, but I have a feeling it's going to be at least "good enough."

If anyone's interested, check out 7.5.3.2 Design for Tension Perpendicular to Grain Forces. It gives equations for how much of the connection needs to be above the center line of the carrying member for various levels of loading and distance from supports. It will take some thinking to apply it to this application (since it's not identical), but easy enough to do.

 

[HouseBoy]

A very large percentage of connections will be putting wood into tension perpendicular to the grain.
I might be mossing something in the conversations but....I'm not sure that tension perpendicular to the grain is by itself the concern in NDS. MY understanding is that the concern is for cross grain tension DUE TO BENDING.