Thread: Rafter nailing

What do you guys think of this.

Thanks,

Bill

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Bill,

Looks to me like some kind of field modification.

What are all of those bent over nails and the added strap for?

Obviously not designed by a truss engineer.

Can't tell too much more from the photos.

Looks like a mess. I would recommend the seller (or builder as the case may be) verify the trusses were installed per the truss manufacturer's plans. (I doubt he can.)

Other than the wedge seat which may slide down, that is typical of trusses setting on other trusses. The other thing which looks lacking to me is that there is no vertical or diagonal below where the trusses is bearing on the other truss. This is not always required by the truss engineer, but it is always a 'good idea' anyway.

The strapping is to hold the upper trusses to the lower trusses during high wind events (Bill knows ... hurricanes), and the nails were bent over because that is what was required.

Typically, though, I would have expected the lower trusses to have been sheathed over to form a more rigid diaphragm onto which the upper trusses are set. Not always done and not always required.

What I especially don't like is the coincidental location of the upper truss and strap in the second photo being right at the double knot location in the truss it is bearing on and strapped to. During high wind events, that truss top chord is likely to snap right there, right where it is needed the most (and one of the reasons it is a 'good idea' to have a vertical or diagonal meeting at that point).

I would raise the issues and recommend a structural engineer "design appropriate repairs". If the structural engineer comes down and says 'no repairs are needed', then that is the 'appropriate' repair - none (but it still does not look good) and the engineer bought the failure should it fail.

Tell your client to save that engineer's letter and keep it handy to give to their insurance company (if it says 'no problemo').

While I agree that the double knot location should not be a strap point-- I would have liked to have seen the strap-point closer to the elbow. Also, I do not see the wind-strap looped over the top section of truss that is sitting on the lower. These should be tied together-- maybe they are, but I just can't tell from the photo.

Richard

First, understanding the strapping requirements is necessary.

For years, no, make that decades, both Broward County and Miami-Dade County (and Palm beach county, on-again-off-again) required and enforced the following strapping requirements:
1) Up and over the top cord with at least one nail on the other side, three nails total in the top truss.
2) Three nails into the bottom attachment, truss or other bearing.
3) Nails bent over. Not "clinched" mind you, just "bent over".

Then along comes Hurricane Andrew and the reason for the up-and-over strapping technique both passed and failed the test.

Sure, going up and over with at least one nail on the other side takes a lot of the uplift force off the top truss nails and onto the strap, but the bottom three nails ...

Okay, let's get to those three nails ... ummmm ... those three nails were not as strong as the straps, not even "bent over" (but I will get to the "bent over" part soon enough). Nails, in those installation, are in "shear", i.e., the strap is trying to "shear" the head off the nails, and when it does so off just one nail, the straps go through the rest like butter because they do not present enough shear force to resist the uplift.

Okay, sooooo ... three nails justa-ain'ta-no-gooda.

Now for the "bent over nails". First, the nails have two values: 1) "shear" (see above); 2) "pull-out resistance". Now, "pull-out resistance" is a known factor (within ranges) for a certain size nail in a certain species of lumber, i.e., 16d in SYP.

Now, those "pull-out resistance" numbers are based on the full length of the nail being embedded into the wood. As is obvious, you cannot fully embed a 3-1/2" long 16d nail in a 1-1/2" thick piece of wood ... BUT ... you need a 16d for its "shear" value. Sooo ... you drive the 16d into the 1-1/2" thick piece of wood and ... you are left with 2" of nail sticking out, so, hey, we will bend them over to get more "pull-out resistance" - it has to straighten the nail before it can pull the nail out. As any of us who have ever worked on houses know, it is much harder to pull out a bent over nail then a nail not bent over, which is why we 'un-bend' the nail so we can pull them out.

Right?

Dang straight that's right. For us and our limited pulling out power, anyway.

What "bending the nail over" did, though, as to *enlarge* the hole the nail is driven through, thereby *reducing* its net "pull-out resistance" factor. Get that nail started and it will straighten right out .. in a now larger hole ... and *Zip!* ... it pulls right out!

Now, *IF* the nails had been clinched, anyone know what "clinching" and how it's done? It sure ain't easy in the best of conditions, try doing it in a production nailing situation and when you are bending over and teetering on a ladder or while standing on a tie-beam or the trusses. Just is not going to happen, and it did not happen.

Three nails? Insufficient shear resistance.

Bent over nails" Insufficient pull-out resistance.

Simpson came up with the solution (don't they always?). While testing showed that three 16d were insufficient in most cases, more 12 or 10d could be installed with less splitting or damage to the wood, and that would hold.

Put those pesky nails still stuck out through the other side of the truss, damaging the truss hangers, straps, etc. So Simpson made special Truss Nails, available in 10d and 12d diameter and only 1-1/2" long! NO MORE PESKY nails poking through!

The end result was testing which showed that then number of nails (and sometimes there are A LOT OF NAILS) required to resist the uplift to attain full strap uplift rating without driving unnecessary nails in which would just increase the splitting of the wood and achieve nothing in uplift (sure, more nails would not allow the strap to shear the nails off, but now the straps were failing because of the limits of the strap used).

Need more uplift resistance? Don't "Bubba, drive a few more nails in that strap, will ya?", no, use a different strap! Wider, thicker, stronger metal, whatever is needed, then use the CORRECT amount of nails (NOT TOO MANY, and NOT TOO FEW - the *correct* amount of nails).

So, what is wrong? Lots, it *looks like*.

So, what is wrong? I don't know for sure, not without looking at the truss engineering.

There are other things I see in there which 'just do not look right', but 'might' be.

'Might be.'

"Might not be.", also.

Thanks Jerry. A definite eye opener for me.

Bill

Thank you for the explanation.

Much appreciated.

It was a long and much appreciated explanation. That's why im here.

Thank you.