Load Path to Roof Diaphragm w/ Overhang Trusses - See Detail

[eng003]

Interested in how others typically transfer load from exterior wall to roof diaphragm when dealing with trusses overhanging a porch. Please see attached detail, initial plan is to use the jack trusses as drag struts and sheath the porch ceiling with 3/8" plywood. Even with the high wind it is exposed to, the force per truss doesn't seem that much <200#, do you think in this scenario it is even needed to specify this load to the truss manufacturer? wouldn't a typical spike plate more than take care of this load?

If this is not a reliable load path any suggestions on bracing or other means of getting the wall load to the roof diaphragm?

 

[woodman88]

Per your detail, I would call for the truss sitting parrallel and on top of the wall to be designed for 200 plf drag load. Then connect it to the top of the wall for the shear.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[masterthenight]

I am likely misunderstanding this. If you are talking about loads parallel to the exterior wall you show, I typically specify full depth pre-engineered truss blocking designed for the correct amount of design shear. The blocking sits over the exterior wall and receives edge nailing from the roof diaphragm. The blocking is then toe-nailed or attached with clips to the wall below to transfer the shear load.

My confusion comes from you talking about using the jacks as drag struts. Drag forces act in line with the member, so that comment implies you are transferring load perpendicular to the exterior wall. If that is the case, ignore what I said above.

 

[eng003]

There are two forces that need to be transferred between the roof diaphragm to the wall. The first is bringing roof diaphragm forces down to the shearwall below. These forces which would be in line with the truss sitting on the wall and the shearwall itslef. The question with this force is whether it is appropriate to specify to the truss engineer to design the truss as a drag collector for the design shear force? Alternatively, I could have them sheath the truss to act as a wood shearwall? The second foce that needs to be considered is delivering the out-of-plane wind load on the wall up to the roof diaphragm. This is what I referred to above using the jack trusses as drag struts tied to the plate on the wall by the 3/8" porch roof sheathing?

 

[eng003]

Well its made its way to the truss designer and I have specified for the jack trusses and girder truss sitting on wall to be design for my calculated drag loads. He has called me up and gave me I don't know what your talking about...he is a rural truss plant and admittedly told me I need to keep it real simple for him. Am I off base here?? how common is it to make the designer incorporate a drag load? I guess I will have to make the contractor sheath the truss on the wall and someone brace the wall out of plane back to the diaphragm?...

 

[bigmig]

The truss guys typically have no clue what a drag collector even is. Eliminate the truss over the CMU wall (the one you are thinking about using as a drag strut) and just frame a wood framed wall that goes from your masonry to the plywood on your roof. Sheathe or x-strap this wall (i.e. typical shear wall design) and you've got it.

The concept that woodman88 suggested would work, except I would sheathe that truss so that it is more 'wood wall' than truss.

When specifying special truss conditions, anything beyond load type, location and magnitude and the truss jockeys go to lunch. Conversations about shear, drag etc. are way over their heads generally speaking.

 

[a2mfk]

bigmig "nailed it", and I like truss jockeys, that is a good one. I worked for about a month or so for a truss company right out of school before wising up. There were three truss jockeys from various construction backgrounds, none of them engineers (actual one had a degree in electrical, still can't figure that out). They are very efficient at using the software and coming up with fast and efficient standard truss designs. Then some PE, maybe in another state (this was done), signs and seals it. So forget about getting them to do any engineering or out of the box design. I think most building codes and the Truss Plate Institute now specify the responsibilities of delegated truss engineers and the engineer of record. It is your building, you should design all the MWFRS and load path stuff.

I also like Bigmig's design, it is what I have done in the past many times for shear transfer from the roof diaphragm into the shear wall. Now for out-of-plane lateral loading of the wall, you can use your concept of using the porch roof as a diaphram or each bottom chord of the truss will really collect that load at 2' o.c. Make sure that connection at the T.O. CMU to the truss is designed for that force also. The problem is at the left side of your detail, at the truss heel, that truss plate is not designed for this drag force to get it up into the top chord and into your roof diaphragm, and per above conversations you will not get them to design for this. I think if you are only dealing with 200lbs I would put a plywood gusset just to the right of the heel with 3-5 12d in the top and bottom and call it a day.