APA Portal Frames at Story 2-5

[jwhitmer]

The APA portal frame specifications give direction mostly on frames attach directly to concrete or to a raised floor on concrete.

I'm looking to see if I can get APA portal frames to work at level 2-4 of an apartment building.

https://www.apawood.org/Data/Sites/...ch/paper-2005-cib-w18---raised-floor-nwbm.pdf

 

[jwhitmer]

The FAQ's indicate that these can be used at a "second story of a 2-story structure" which leaves things somewhat vague about if it is limited to 2-story structures or if they are saying it is fine in general to us in "upper" floor scenarios.

They reference IRC R602.10.6.4, but i'm not really seeing much in there.

 

[jayrod12]

I imagine that the stiffness of the portal frame would begin to be an issue at much more than 2 storeys of portal frames. I unfortunately can't comment on APA directly, but just logically I feel you'd have drift issues.

 

[phamENG]

You'd likely be better off with FTAO - or are you dealing with sliding glass doors or other glazing that goes all the way to the floor?

I'm guessing it's a combination of 1) they didn't have the headroom to test up to 5 stories or gather sufficient data to extrapolate to that height so they aren't going to go out on a limb and tell you it's okay and 2) the stiffness is probably low enough that you start running into P-Delta instabilities in the stack.

 

[ChorasDen]

My understanding is that Portal Frames are an approach for prescriptive lateral load paths. If that's the case, prescriptive design allows 2-story construction for single family or duplexes. Prescriptive design is not allowed for larger projects.

I recall a number of years back that APA published a document listing the capacities of wood portal frames, but I don't recall the document number offhand. Maybe a search for that document would provide justification for it's use in your scenario?

 

[phamENG]

ChorasDen - I know that document well, and it's only meant for first floor applications with cast-in strap hold downs. Even the minor slip in HDUs invalidates the stiffness values.

Link

 

[ChorasDen]

Huh, that's interesting, thanks pham. I don't deal with portal frames on the west coast very much, but I occasionally work in Idaho, and portal frames are all the rage there, so I should review the provisions on them.

 

[jwhitmer]

Thanks everyone. This helps alot!

 

[phamENG]

ChorasDen - my pleasure. You've given some fantastic insight into the wood industry and wood design lately, so I'm glad I could share some small nugget in return.

jwhitmer - you're welcome. If needed, share some more details of the project and maybe we can help you puzzle out a solution.

 

[Celt83]

FYI TT-100 was revised in May 2020 Latest document can be found here: Link

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[JoshPlumSE]

Can anyone explain to me how this is different than what's described in the NDS document Special Design Provisions for Wind and Seismic related to Perforated Shear walls and / or Force Transfer Around Openings?

Is this document more geared towards "non-engineered" structures?

 

[ChorasDen]

Josh, not sure I understand your question? Perforated shear walls are shear wall segments with empirical reductions on wall capacity based on size and number of wall openings. FTAO is a different round-about method to justify openings in wall lines and apply a capacity to the modified wall segments. APA has a decent on-demand webinar that discusses some of the design capacity issues with FTAO analysis, including Deikmann method, cantilever beam, and other analysis methods. Portal frames are not shear walls at all, but are prescriptive methods used mainly at garage doors to provide a braced wall line where the wall thicknesses are too narrow to accommodate a braced wall section. Portal frames are allowed at 16" and 24" widths, but also have wall height limitations.

 

[ChorasDen]

To clarify, the document linked in this thread are published capacities per APA analysis for their prescriptive portal frame. I don't know the exact history of the portal frame method, but I assume it used to be published in IRC as a bracing method at garage doors, and the APA likely received sufficient requests from engineers asking for portal frame capacities that they published a document listing allowable design capacities for this prescriptive frame. As Pham indicated, this document is quite limited in use, in that it requires a slab or stem wall with cast in place hold downs.

 

[phamENG]

Josh - prescriptive braced walls have had the portal frame option for quite a while. These portal frames violate nearly every conceivable aspect ratio rule for any and all shear wall analysis method. So I'd bet a lot of engineers were getting push back from clients who can use it on prescriptive jobs but not on shear wall jobs. As a result, APA did testing to figure out true capacities so they could be used in engineered solutions the same way they're used in prescriptive structures - typically at garages. The OP's document is interesting, though, as it would expand it to second floor applications as well which could be beneficial. That's a research paper, though, and not really a technical note published for general design usage, so I'd have to consider it carefully.

FTAO vs. portal frame: I'd have to dig it up, but there was a research paper published recently that looked at the various FTAO methods, and of them Deikmann was the most accurate (which is why the APA excel sheet is based on it), but it falls apart when you lose the wall segment below the opening. Pretty much every analysis method was too inaccurate to make FTAO for door openings a reasonable design philosophy choice.