Collector Straps along Blocking

[333OnlyHalfEvil]

If I'm designing a collector strap that's going along blocking along the top of the wall, do I have to design the strap to take the entire force the shearwall is resisting in tension? Or can I get it in plf and size it that way since the particular strap I'm designing is really long?

 

[KootK]

Or can I get it in plf and size it that way since the particular strap I'm designing is really long?

Definitely not this.

If I'm designing a collector strap that's going along blocking along the top of the wall, do I have to design the strap to take the entire force the shearwall is resisting in tension?

Similar to this but probably not quite so bad. Some of the shear might be coming into the wall from:

1) A collector on the left side of the wall.
2) Shear transfer right on top of the wall.
3) A collector on the right side of the wall.

Depending on how that is arranged, your peak strap tension can be substantially less than the entire shear resisted by the wall. To figure it out, I'd recommend constructing an axial load graph for the collector along it's entire length. If you post an elevation sketch showing the lengths of the wall and it's collectors and the applied diaphragm shears, I'd be happy to help with that.

With very long strap collectors, it may also be wise to consider strap elongation as that may govern your strap design.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Design the strap for the full force. If you want to take the chance with the contractor, you could gradually decrease the nailing to the blocking as you move away from the shearwall, but I would not do that. Chances are it would be installed wrong.


If you did want to vary strap sizes, you would have to worry about splice lengths and possible additional blocking. To me, it would not be worth the effort, again with more of a chance of an installation error.

Mike McCann, PE, SE (WA)

 

[333OnlyHalfEvil]

Thanks for the replies. Now, if I add in clips from the blocks to the framing members, can I assume that the clips capacity is helping the strap?

 

[msquared48]

Yes, but then the framing members below the blocking (double top plate or beam) become the drag strut and must be continuous, with other strap ties as needed across any joints. And there would need to be a connection of the other framing members to the shear wall.

If you did that, the strap tie above could become superfluous.

Mike McCann, PE, SE (WA)

 

[333OnlyHalfEvil]

Also, what about adding in more nails than what's required in the manufacturer's catalog? Can you increase the capacity at all that way? or doubled up the strap one on top of the other?

KootK: how do you go about considering strap elongation with long straps?

msquared: I'm having a little trouble following how the strap could become superfluous and also about the drag strut. Would you mind re-explaining it with a lengthier description of how this happens?

THanks again

 

[KootK]

Thanks for the replies. Now, if I add in clips from the blocks to the framing members, can I assume that the clips capacity is helping the strap?

I agree with Mike's assessment if you are in fact utilizing the approach that he's suggested. I don't believe that to be the case however. Consequently, I would say that the answer is no for two reasons:

1) Stiffness incompatibility. You'll probably snap your strap before engaging the capacity of the clips in a meaningful way.

2) Most blocking clips that I'm familiar with would have to mobilize fasteners in withdrawal in order to transfer tension. While that's not a show stopper technically, it's not a great detailing choice in my opinion.

Also, what about adding in more nails than what's required in the manufacturer's catalog? Can you increase the capacity at all that way? or doubled up the strap one on top of the other?

No doubt this would help but, frankly, I'm not sure how you'd go about quantifying that help in a way that would be acceptable to code reviewers etc.

KootK: how do you go about considering strap elongation with long straps?

Step 1: construct that axial load diagram that I mentioned above.
Step 2: axial displacement at the far end of the strap will be INT(P/AE)dx|0->L.
Step 3: If the displacement is 1/2" at the end, you're good to go; if it's 3", not so much.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

Quote (OP)
Or can I get it in plf and size it that way since the particular strap I'm designing is really long?

Definitely not this.

KootK - when you say definitely not, are you referring to the steel strap itself, the nailing, or both. I understand the steel being sized by the total load. But, the load going into the strap is plf, so it seems reasonable to design the nailing with the plf.

As far as clips go, I don't like the idea of a lateral connection depending on nails in withdrawal.

 

[KootK]

when you say definitely not, are you referring to the steel strap itself, the nailing, or both.

Just the steel strap.

But, the load going into the strap is plf, so it seems reasonable to design the nailing with the plf.

You'll get no argument from me. The strap tension is developed as distributed shear (plf) delivered to the strap via the nails. That's mathematically implied by the INT(P/AE)dx|0->L that I mentioned in my last post. In retrospect, perhaps INT(q(x)/AE)dx|0->L would be less likely to cause confusion. q(x) representing a variable, distributed plf shear coming in through the nails.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Also, what about adding in more nails than what's required in the manufacturer's catalog? Can you increase the capacity at all that way? or doubled up the strap one on top of the other?

I could get behind using a double strap if it were installed like this, from the top down:

---------------------- Strap
========== Plate
---------------------- Strap
========== Plate

I don't imagine that you're contractor would look upon this solution with much fondness however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Scrap that last solution. I forgot that you're using blocking as the collector rather than top plate. If your framing is at least as shallow as your blocking, you may be able to use two straps concurrently, one on top of the blocking and one below. Detailing the force transfer out of the lower strap into the shear wall could be a constructability challenge however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[333OnlyHalfEvil]

Also, what about adding in more nails than what's required in the manufacturer's catalog? Can you increase the capacity at all that way? or doubled up the strap one on top of the other?

No doubt this would help but, frankly, I'm not sure how you'd go about quantifying that help in a way that would be acceptable to code reviewers etc.

What's not to accept? Twice as much steel + 100x as many nails = 2x the strength min.

 

[KootK]

An important failure mode is the nails splitting the wood substrate. If you're adding nails in excess of the holes provided by the strap manufacturer, that could be a problem. It's hard to imagine that you'd be picking up distributed shear that quickly however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.