Method to install strap steel diagonal shear restraints on wood frame building

[fastline12]

We are working on a wood post frame construction building with corrugated metal sheeting. Though the corrugated sheeting functions to provide shear resistance, we want to supplement this with strap steel diagonal members in key areas. We want to install this low profile product directly to the wood columns, on the outside, and under the metal sheeting, with of course a protective membrane between the sheeting and straps.


The issue is how best to attach these straps directly to the columns? This will become the weak link in the system and we need the lowest profile possible. We had considered a nailing schedule right through the straps with a requirement of predrilling the straps but this still seems a marginal method at best. Also, due to this method pretensioning would be rather difficult and though this system is considered a secondary load path, we want to ensure it provides reasonable strength.


Any ideas on how to attach these?

 

[MikeHalloran]

The steel that's used for shipping straps is pretty hard, i.e., not easy to drill by ordinary means. There exist 'hardsteel' drills that basically melt their way through, but they leave a heat affected zone around the hole, of uncertain metallurgy. Strapping seems optimized for use with tools that stretch a loop tight through one or more steel sleeves, then crimp the sleeves onto the strap, severely deforming both, while the strap is under tension, then snipping off the free end, all in a few seconds. To use such a thing diagonally between columns would require providing mitered surfaces on the columns, with rounded corners, so the strap wouldn't be subject to uneven stretch or too tight a radius. The strapping can develop significant force during application, so you'd probably want a compression member between the columns, aligned with the strap and basically coplanar. .... and if you're going to put in a diagonal compression member, maybe you don't need the tension member.

It should also be noted that the strapping is not intended for long-term use, so it would need some corrosion protection, and you probably couldn't get a warranty anyway.

Maybe you have in mind some other kind of strapping, with test reports and such, which I would not be likely to have come across.

Speaking of long-term use, I saw an application of steel strapping that was new to me, reinforcing wooden telephone poles in situ. ... wherein, a channel cold formed from ~1/2" steel plate, with a web of ~6" and legs of ~3", with large bend radii suggestive of pretty hard plate, is driven into the ground alongside the pole. The part exposed above ground is ~5 feet long, suggesting that the channel is maybe twice that length. The channel is then bound to the pole with two pairs of super heavy duty steel straps, maybe 16 gage x 2" wide, and the whole assembly is coated with semigloss brown paint that makes it almost invisible against a creosoted pole. I remember these poles as having been installed at least ten years ago, in eastern PA, with the channels appearing last summer-ish.



Mike Halloran
Pembroke Pines, FL, USA

 

[Retrograde]

You can get galvanised steel strapping specifically for this purpose that has holes for nailing.

 

[txeng91]

I have only done kicker bracing or galvanized wire rope bracing on wood post frame buildings. I usually attach the wire ropes to the column/beam and column/foundation joints w/ steel angles and bolts. If you were to use steel strapping you can get coil straps from Simpson w/ nail/bolt holes already in them, and they have decent tension values but they need to overlap the framing by a certain distance to get full capacity, so you would need to make sure you have an adequate column dimension to achieve the desired load into the straps. I think the main issues would be the pretensioning as you mentioned.

 

[XR250]

Why do you need to supplement the corrugated sheathing?

 

[rdubs]

Have you considered adding structural sheathing to the inside surfaces of the columns?

 

[MotorCity]

Will nails or screws not work for attaching the strapping? (I assume we are talking about strapping of a gage thickness and not 1/4" thick steel plate). If you need more capacity from the connectors, you can probably use lag screws or through bolts with the heads sunk in recessed holes. The heads may even fit below the corrugations and no recessed hole required. I am curious, since you are partially relying on the corrugated metal siding for lateral resistance, where did you find the lateral resisting capacity of the siding?

 

[fastline12]

Thanks so much guys. I will try to answer a few questions.

1. We are building the structure in a unique way in which there is some indication from university testing that flush mounted purlins in joist hangers may reduce the stiffness of the diaphragm roof. If we allow too much movement there, we get things a little hot on the columns mid span. If we can keep the roof tight, load distributes better. Yes, we could probably improve this in many ways but we have specific reasons to target the steel strap to solve some issues. I know most builders reach for wood but we are trying to optimize the stud openings for insulation, eliminate any need for unsightly sheathing on the interior, etc.

2. As far as why we even need to supplement the corrugated metal? Well, I guess we don't want all our eggs in one basket. While the skin might get us covered on loads in a worse wind condition, it is also possible that it will wrinkle the skin, damage or distort the mounting screws, etc. We just want to take some of the responsibility away from the skin. As well, our overall design goal is to try to limit frame failure in a high wind condition in which the skin would be blown off. Without additional bracing, the second a panel comes off, it may weaken the structure enough to cause catastrophic failure. What we want to see if the skin ripped off and the frame left standing. This is all considering a tornado wind condition to exceed 130mph. Obviously there comes a point where God will take that structure regardless.