Sub-Diaphragms and Anchorage Forces
Sub-Diaphragms and Anchorage Forces
(OP)
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it doesn't take into account the 2.5 L/w ratio for subdiaphragms so I'm wondering if the example might be a little out of date as well.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.
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.






RE: Sub-Diaphragms and Anchorage Forces
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.
RE: Sub-Diaphragms and Anchorage Forces
RE: Sub-Diaphragms and Anchorage Forces
RE: Sub-Diaphragms and Anchorage Forces
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.
RE: Sub-Diaphragms and Anchorage Forces
RE: Sub-Diaphragms and Anchorage Forces
Atleast I think this would be an equivalent situation (minus gravity loads)
EIT
RE: Sub-Diaphragms and Anchorage Forces
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.