Wood Diaphragm Questions

[KBVT]

I have been designing some large scale multi-family wood residential projects and wanted some clarification on a few issues I am having with the lateral design.

1. The exterior of the buildings often have continuous offsets in the wall line which interrupts the continuity of the chord member (in this case, a double top plate with designed splice). Now the diaphragms are inherently deep in these type of projects, usually approx. 70'. But how do you resolve the tension on the far side of the diaphragm if there is no chord continuity from one end of the diaphragm to the other.

2. In all wood texts including Briar's 'Design of Wood Structures', the diaphragm is designed with a drag element if the shear walls do not extend the full depth of the diaphragm. In actuality, if I have a 70' deep diaphragm, and a 15' shear wall located somewhere in the middle of the building, can I design that shear wall as a single support for the 'deep beam' and design the diaphragm for the same story shear as the shear wall? Sometimes, a drag element is not possible as fire separations (and STC assemblies) prevent a continuous member from extending across the building.

I have been a lurker on this site for a while and have been really impressed with the amount of knowledge that many of you present. I appreciate any help.

 

[OldPaperMaker]

KBVT,
My solution to your first question has been to pick the "inside" line of offset shearwalls as the depth of the diaphragm (to get the load in the chord of the diaphragm), provide blocking between the joists and size a metal strap to transmit the load from the double top plate across the offset section to the next shearwall sections double top plate. This assumes that the joist are perpendicular to the shearwalls. I also call out nailing as a blocked diaphragm for the section of the diaphragm to the offset walls that are outside of the line that I picked as the chord. If the joists are parallel to the shearwall line I use the joist as the chord segment and just call out straps at the junction with the next shearwall segment.
I don't understand your second question so I will pass on trying to help with that question.

 

[JKStruct]

I think I understand the second question... it's been one that has been posed in our office before -- to paraphrase it in my own words "If you have enough shear capacity in your diaphragm to transfer the full shear into the resisting element, are collectors even needed?"

Personally, I think it ensures structural integrity to have a continuous line of resistance throughout the diaphragm depth -- i.e. provide collector elements into your shear wall, frame, etc. Even though you can justify that the shear from a 70' diaphragm can flow into a 15' shear wall, I don't know that it really will behave the way you think it does.

there maybe something in something in the codes about this... I'm thinking specifically of a california BC requirement that we once got caught on in peer review, but I
could be off. I'll check tomorrow, but I usually detail collector elements in this situation.

JK

 

[DaveAtkins]

I will give you my two cents, based on a lot of experience designing wood structures--

1. Another possible solution is to split the building in half (not literally) and resist the chord forces with shear walls that are parallel to these chords. In essence, you are assuming the building consists of two separate three sided diaphragms. I wouldn't go "hog wild" with this method, however--I believe the IBC even limits how large three sided diaphragms can be.

2. I DO think it is OK to assume only 15' of diaphragm can resist the shear--the other 55' is just "pushing" or "pulling" on the 15' deep diaphragm. Of course, this 15' deep diaphragm must check out.

DaveAtkins