Sub-Diaphragms and Anchorage Forces

[palves]

Ok, I'm having a little debate with a co-worker regarding sub-diaphragms. Our point of contention rests on provision 12.11.2.2.1 of the ASCE7. Briefly, this says that continuous ties need to be added between chords in order to distribute the anchorage forces to the diaphragm.

We've both read it and see slight differences. In my mind when the ASCE7 says "continuous ties" that makes me think that I need to start the anchorage connection at the wall and continue that tension load path all the way until we anchor into the next chord. My co-worker thinks something very similar, but doesn't agree with me that we need to anchor the connection into the next chord. He is content with stopping the strapping one bay before the chord and calling it good.

Take a look at my link and you'll see a good illustration from an example the APA put out. I agree with them that if the sub-chord is out at the 3rd purlin line then we need to provide a strap at that purlin in order to fully finish the anchorage connection. My co-worker would just stop the strapping at the 2nd prulin line.

The trouble is that he found his own example in the Breyer wood book that says the strapping at the 3rd purlin line isn't needed so now we both have evidence for our own way of thinking. His Breyer book is pretty old because it doesn't take into account the 2.5 L/w ratio for subdiaphragms so I'm wondering if the example might be a little out of date as well. Might this be something that has changed or is one of our examples wrong?

Anyway, what do people think? After going through both of our methods I'm curious as hell to see which is correct.

 

[jittles]

That strap/joist line essentially is collecting/distributing the shear force across the diaphragm. The shear force collected/distributed at the 3rd line is going to be 0.

I guess academically, there would be a pittance of force there still, as your diaphragm ends *just past* the end of that strap/joist line.

So, no, I would not strap at the 3rd and final line.

 

[jittles]

Also, looking at the diaphragm as a beam, shear in a beam is maximum in the center and minimum at the top and bottom. That last strap would be at the top/bottom of your "beam" and the strap force would be the shear in the beam.

 

[palves]

Right. Each strap/joist line is distributing shear force across the diaphragm. But, why don't we just find the development length that we need to take that tensions force out within the diaphragm and stop it at the appropriate length? Then we could have situations where the anchorage strap stops well short of the chord because it has sufficient nailing into the diaphrahm. Unfortunately I've never seen any calcs. that do this. They always either connect directly to the chord or stop one bay before. It's that provision in the ASCE7 that says to have "continuous ties or struts between diaphragm chords" and I'm confused on what exactly they mean by "continuous".

 

[jittles]

By "continuous" I just read that it needs to be able to transfer the forces required.

Now don't take my word for this, as I don't have a source, but what I've been told by my mentors is that you cannot rely on wood sheathing to transfer tension/compression forces.

So you check the subpurlin for tension/compression and then at splices you need to add the strap to take the tension. You only need to develop the needed capacity of the strap at the splice, provided the subpurlin itself can carry the tension elsewhere.

You are relying on the diaphragm to transfer ONLY shear forces.

 

[palves]

I agree with you that we shouldn't count on the sheathing to transfer any sort of forces. This is why I feel like we should strap across the sub-diaphragm chord itself in order to ensure that the anchorage forces travels from one diaphragm chord all the way to the other. If we left it one bay short like I have seen in other examples then we would essentially be leaving the sheathing to take the tension force at the point where it meets the sub-diaphragm chord. Thoughts?

 

[RFreund]

Hmm... this is interesting because the continuous ties or subpurlins (right?) seem to be required to act as mini drag struts. Basically taking the force perpendicular to the wall (parrallel to the subpurlin) and 'dragging' it into the subdiaphragm. Therefore I could see making the argument that all you need to do is develop this mini subdrag strut. However I see this being the problem - if the subdiagphram was a full sized diaphragm. The mini purlin would be equivalent to your roof joist. So it would be similar to cutting your joist x feet short of the shear wall and having the sheathing extend all the way to the shear wall. Does this diaphragm still work?
Atleast I think this would be an equivalent situation (minus gravity loads)

EIT

 

[palves]

That's an interesting way to look at it, and I would say that situation doesn't work. If I were stamping the drawings I would run that connection all the way to the other side and connection to that opposing wall. However, we never really deal with that situation since we always look at loads from both directions. If you building is 100' long and you need 80 to "develop" an anchorage connection then you won't leave it short because that same connection will also have to be developed in the opposite direction. In that case the connections would overlap and you might as well just strap all the way across. However, if the development length is less than half the building length there could arise a situation where the straps from each end will not meet because they don't even need to cros the midpoint of the building.

Hypotheticals aside, what gets me with all of this is that anchorage forces are never talked about as being "developed". The ASCE7 states that diaphragms need "continuous ties between chords" and I'm still unsure if that means we need to strap across an interior sub-diaphragm chord or can leave it one bay short.