Angle as Diaphgram collector on wood structure

[sponton]

Hello fellow engineers,

I'm in a bit of a pickle. I'm designing a 4 story wood structure, very regular in shape, not so much in wall continuity. As usual, due to architectural constraints, plumbing walls and other headaches, I am not able to use most of the walls as shearwalls. So the way the diaphragm has been subdivided it has caused some areas to have large shear concentration.

Since I cannot really use a single truss spanning across the whole width of the bdg as a collector and attach it to a ledger in the CMU wall, cause frankly the connection would be a bit cumbersome, some coworkers pointed out a similar project in which they had built a collector using flat blocking and an angle, granted that the direction of the trusses was spanning in the opposite direction to the one depicted in the attached pdf, so the blocking was tightly fitted between the trusses, assuring continuity using simpsons tie straps and the angle went underneath the top chord until it connected to the wall w/ expansion bolts.

Anyways, my question is, do you think there might be any problems with buckling of the built up section consisting of an angle + (2) 2x6's screwed at probably 6" o.c. ? I haven't input the information at least for the (2)2x6's because automatically due to the length [32'] it's going to tell me it buckles under a ridiculous stress, it tells me that under the 4.9k load it's overstressed 218% [buckling on the weak direction]. Anybody has any pointers or ideas, if there's a better way to do this.

 

[MotorCity]

I assume the cmu is acting as a shearwall for your lateral system? Can you use a larger steel member as your collector? Maybe bury a small wide flange or HSS within the depth of your joist framing?

Another option might be decreasing the width of your diaphragm (use subdiaphragms) and make use of the steel framing to resist the load. You will have to consider the stiffness of a cmu shear wall and steel brace/moment frame.

 

[KootK]

I do think that your collector may buckle. In the gravity direction, it's only really braced by the sheathing. You might consider running 2X cross members between the trusses on either side of the collector in order to brace the collector intermittently. Alternately, perhaps you could lap splice your collector to one of the floor trusses and use that as the collector for much of the distance. Obviously, you would have do deal with whatever eccentricities arise from that arrangement.

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.

 

[sponton]

@Motorcity

The problem is that the larger the steel member gets the more it will sag, and then comes the problem with connecting it. Truth being told they haven't even provided sections [you know architects..] The problem with using the steel structure and adding moment frames I see as twofold, the upper level have non-aligned, non-matching walls (-_-) and plumbing walls. I'm also concerned with deflections, that you might end up with a section of the building that is too stiff while the other deflects too much and causes tearing of the diaphragm.

@KOOTK

If I do run flat blocking perpendicular to my collector, should I take precautions by cross bracing a number of trusses to guarantee restraint or the sheathing is good enough [3/4" plywood]
I liked the idea of using the truss but the splicing and special nailing seems to be an area where I am certain they will mess up during construction -_-'

 

[KootK]

I see no need for extra truss bracing. The sheathing will brace your collector in the horizontal plane and the 2x's and trusses should do the job in the vertical plane. And yeah, the truss splice probably would have a high probability of getting messed up.

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.

 

[sponton]

@Kootk

I guess time to run some numbers, I appreciate your help!