Large Detached Garage Raking 3

[Simba13]

Greetings all,

Finishing up the design of a largish detached garage (64'x30' w/ 14' tall walls, roof peak at 19'). I've gone through the tall wall calcs, designed some 'shear walls', foundations etc. But my boss is concerned that since the long dimension is fairly long there could be raking away from the gable walls since there are no interior walls (trusses frame front to back with the garage door side being the front). I've been using RISA 2-d for most of my non-hand calc analysis at this firm but I feel like this is a 3-d problem. The trusses with sheathing on them will form a diaphragm and so it should be fairly stiff in that direction right? For those that have designed wide garages, is this something you considered to be an issue and if so how did you solve it?

Thanks,

 

[phamENG]

Think of it like an open web steel joist. You don't design the bridging for that, do you? For standard joists you can select it from the SJI/manufacturer's table, but ultimately the location of bridging is shown by the fabricator on the shop drawings. For special joists, you provide loading - point loads, moments, axial loads, etc. It's just a specialty building component requiring specialty engineering.

 

[r13]

Think of it like an open web steel joist. You don't design the bridging for that, do you?

I don't, but chose a bridging pattern, size, spacing per data sheet, and specified on the purchase order, and the construction drawing. For special truss, we have to select type of joist, profile and depth, provide load/load point and support reactions. The "special" is usually due to geometry oddity, loading irregularity that require special arrangement and upsize the members, which we have no control, but relying on the manufacturer's expertise. Without specific instruction/request, the manufacture won't offer bracing for lateral load, but stability braces for construction.

 

[phamENG]

the manufacture won't offer bracing for lateral load, but stability braces for construction.

Same thing with prefab wood trusses. The bracing is for stability. The diaphragm is for lateral loads. And in either case, the joist or truss can be identified by the EOR as a collector (drag truss/strut) and designed for axial load. When that happens, special considerations for bridging and bracing are required by the manufacturer.

 

[dik]

You might check here:

https://tpic.ca/wp-content/uploads/2019/06/tpic_2019.pdf

http://www.concordtruss.com/wp-content/uploads/2015/07/Bracing-Metal-Plate-Wood-Trusses.pdf

file:///E:/Downloads/eMail/WWTA-Bracing-Guide.pdf

https://files.engineering.com/getfi...b8d6-6a18b80f721a&file=WWTA-Bracing-Guide.pdf

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

Yeah, I need time to digest the difference, as for lateral load concerns that are out of ordinary, we (the engineer) need to design/request special bridging/bracing, the manufacture is required to response to our demand only. You can see in our practice, the manufacture has very limit role in the design, only occasionally run a truss analysis to strengthen the truss/joist to meet the special needs. Otherwise, the manufacture's role is purely a supplier of spec and the truss/joist, including bridging and bracing that specified by us. We may never have any contact with the manufacture, if the data sheet covers everything adequately, then only the manufacture's name, truss/joist model number, and a note on bridging/bracing will be shown on our drawing.

 

[r13]

Quote from the second document provided by dik. Note the last sentence. I am not sure the stance of the first document though.

 

[jayrod12]

There is a document put out by BSCI that very clearly delineates the requirements of the truss supplier versus EOR for bracing.

Essentially the final building bracing must all be shown by the EOR. That includes top and bottom chord bracing, note that sheathing and a rigid ceiling would satisfy the requirements for both. And getting the loads from the web braces up to (or down to) the required resistance locations (diaphragms). Therefore an EOR at a minimum should have a few diagonal braces going from bottom chord to top chord along a few of the webs to take the stability web bracing loads to the sheathing.

 

[phamENG]

Well thanks for enlightening me some everyone. All the prefab trusses I've done have been plywood sheathing on the top chord and gyp on the bottom, which certainly helps. The suppliers around here also provide bracing drawings - they are primarily for erection stability but they also direct the contractor to leave them in place unless the DOR directs them to do otherwise. I've seen enough gable end wall failures that I do make a point of specifying something there.

Next prefab truss job I have I'll be sure to pay a little closer to the final bracing design and see if it makes much of a difference. And I'll certainly keep it in mind for any exposed truss jobs I come across.