IRC Portal Frame Modification 4

[XR250]

For Portal Frames with pony walls on top, the code requires a 4,000 lb. strap on the interior face of the wall. The contractor would rather sheath the inside face of the wall with OSB - identical to the exterior. Would that serve the same function as the strapping? Seems like it would.
Thanks

 

[KootK]

You make some good points. At the least, I think that it probably makes sense to focus on the one sided situation as that is wholly rational way to strip out some unnecessary variables and isolate what matters. Clip below for the benefit of anybody following along.

 

[KootK]

Further to phamENG's latest, I think it worthwhile to explore two questions:

1) For the closing joint case, what mechanism does keep the joint from pulling apart in tension? What's different that eliminate the need for a strap on the outside to match that on the inside? I've shown a proposal below.

2) What is the gaggle of fasteners for at the joint? Is it for moment transfer? I'm skeptical of that as the lever arm on most of the fasteners would not be advantageous. I propose that those fasteners are there to prevent shear buckling in the sheathing the panel zone, similar to how we plug weld steel column doubler plates in panel zones.

 

[KootK]

I'm not sure that there's anything new here but, having started to think about the analogous steel situation, this is now how I'm envisioning the mechanics of the opening joint. The shear arrows are backwards.

 

[phamENG]

Here's a nugget of wisdom from APA:

It accompanies a recorded webinar here.

Sorry, XR - looks like my agreement with the scheme was in error.

 

[KootK]

Sorry, XR - looks like my agreement with the scheme was in error.

I'm not so sure. If the strap is performing that function when the wind pressure is inwards, does that not mean that the exterior sheathing is performing it when the wind pressure is suction? And if that's the case, is it not reasonable to assume that interior sheathing could replace the strap?

 

[phamENG]

This is true. Though you'd have to be careful about joint placement, but you have to be on the exterior face anyway. I guess that response above has a trace of CYA in it since that probably wasn't specifically tested.

 

[KootK]

I guess that response above has a trace of CYA in it since that probably wasn't specifically tested.

1) A position statement from APA has evidentiary weight, for sure. More so than some random nutball on the internet obviously.

2) I used to be the wood truss version of the APA / AISC help desk folks. Writing little FAQ's, putting together presentations... Having seen the wizard behind the curtain so to speak, my expectation getting truly definitive technical answers from such sources is rather soured. 19 times out of 20 they will be right. But, in my experience, on the real deep dives these folks are often no better that we practitioners. Sometimes worse as they tend to have limited practitioner experience themselves. Over the years here, we've had a few questionable responses from AISC etc.

3) Regardless of it's other roles, I still am not convinced that the strap plays no important role in the in-plane behavior of the moment frame. For me to buy that, I would need to see either:

a) Testing done without the strap. If somebody knows of the existence of such a thing, I would love to be made aware of it. It's been a long time since I really dug deep on this stuff. For all I know, maybe the testing never did have straps to begin with...

b) Some explicit statement indicating that the strap plays no role in the primary lateral performance.

 

[phamENG]

Well I've gone from circling the time suck that is full on portal frame research to a rapid descent. This paper appears to have a bibliography ripe for the picking.

Now I have 4 reports to write before I go to bed tonight, so perhaps I can dive deeper tomorrow...

 

[KootK]

Pace yourself... the thread will still be here next week.

That paper get's me a least part of the way:

1) All of the testing had the strap.

2) They analytically calculate moment capacity without the strap. [Correction: this seems to be one component of moment resistance that is then added to the strap's contirbution].

I bet your average carpenter would get a kick out of the precision implied.

 

[KootK]

This table is from pham's latest post. Takeaways:

1) It appears that the calculated moment capaicities are the SUM of the sheathing moment capacity and strap moment capacity (see footnote b).

2) The correlation between calculated and tested capacities is astoundingly good.

I'm getting the impression that a 4000 LB strap demand might really be:

4000 LB = 1000 LB for the in-plane moment frame action + 3000 LB for the transverse wind business.