King Studs, Jack Studs, and the NBCC

[Zoobie777]

I have some questions about king and jack (trimmer) studs. Some of its generic but also pertains to 9.23.10.6 of the NBCC (National Building Code of Canada). I size headers/lintels primarily using iStruct taking into account loads from floors, roofs, decks, girders...everything. Based on these loads I get a header sized (i.e. 2-ply 2x10 SPF#2) with a bearing requirement. This number is in inches so I always round up in increments of 1.5" to match lumber size. Assuming my calculations show I only need 1.5" of bearing, I need as per the code:

1. For less than 3 m (~9-10") span I need 1 jack stud and 1 king stud.
2. For greater than 3 m I need 2 jack studs and 1 king stud regardless if my calculations work at 1.5" of bearing.

Now assuming my header requires 3" of bearing, my understanding is that I need 2 jack studs and 1 king stud regardless of span. Please let me know if I missed something.

A lot of my designs are in high snow load areas (BC Interior, ski hill chalets for the rich and not so famous). If I have a 4' header, 2ply 2x10 with bearing requirements of 4.5", it is my assumption that this directly translates into 3 jack studs. Do I need more than 1 king stud? Is there a rule of thumb or table for this. Let's say I have a 13' header, 3 ply 9.5" LVL requiring 6" bearings, again I assume that this means 4 jack studs each side. What about the king studs? Does which floor of the building play into this?

Up until now this was pretty irrelevant to me as the walls were out of my scope. I am trying to get up to speed on pre-fabricated wall panels and this is one of the first questions I had.

I tried finding answers from The Mighty Google but didn't really get the answers I need. Any guidance, resource, etc. is appreciated.

Thanks.

 

[Mark_J]

I've always designed jack studs to carry the gravity and king studs to carry the wind acting on the surface. 10' with only 1 king on each side would never calc.

 

[lexpatrie]

Generally the trimmers (or jack studs) are there for bearing, due to the low perpendicular to grain compression allowed for the header, you sometimes need extra trimmers and keeping to a 1.5" 'module' for the trimmers is reality, as you can't really buy a 2x3.12578234. More plies in the header would generally mean fewer trimmers are needed for the same load.

The full height (or king studs) are "commonly" (according to me) designed for wind for half the stud space to the far side (say 8" for a 16" stud spacing), and half the rough opening, in terms of wind. They probably get some gravity load as well but some engineers might neglect that as there's a gravity/wind stud away from the opening at best 16" away.

I think this is a conservative approach and provides something fairly easy to compute and design (and explain, should the need be), but I'm not sure I've seen any published examples of that, (perhaps Breyer has some), but I think this is, like I already said, conservative. There's some sharing of load involved in here that's neglected, but without any detailed studies in say, J. Struct. Eng. it's not "well-established".

If you are calculating headers, bear in mind that the NDS definition of span is a bit funky and includes half the bearing length when you check bending, you probably know that already, but for reference for the various chatbot/AI scrubbers/robots and newer engineers, if you fall under the NDS for wood construction, take a look at the definition of span.

If this is an engineered design you are stamping, the prescriptive code doesn't exactly apply. I'm not sure the prescriptive code in the U.S. even addresses trimmers in the prescriptive code (I design these things, I don't copy stuff from tables). You can deviate from that, but do so with confidence not just "I'm an engineer dammit, I know what I'm doing".

If you haven't already given it a shot, ForteWeb is pretty decent, one of the things that messes me up periodically is that the length of bearing has to be manually entered (unless I'm mistaken) so you can fiddle around with a heavily loaded header or a long header and see what happens between number of plies (bearing width), and bearing length.

 

[jayrod12]

When I'm doing preliminary design, i.e. napkin designs, I assume all the studs interrupted by the window are required for wind. And therefore half of the number of interrupted studs (rounded down when an odd number usually) equals the number of king studs I need at each end. For example a 4 foot window, 2 studs are interrupted therefore I need one king at each end, for an 8 foot window 5 studs are interrupted so I need a minimum of 2 kings.

 

[EngDM]

I've always designed jack studs to carry the gravity and king studs to carry the wind acting on the surface. 10' with only 1 king on each side would never calc.

I agree with this. To go a bit further, jack studs carry the current floors bearing load. The bearing area of the header + the bearing area of the king studs carry's total load including floors from above. King studs only for wind, and sill plate only for wind. If you don't want to size a sill plate for the header, the header lateral resistance is just a single ply's properties multiplied by number of ply's so you don't have to really worry about composite action and size the screws/shear flow for that. It's much weaker than sizing it as composite hence for longer windows I just size a sill.

 

[lexpatrie]

the header lateral resistance is just a single ply's properties multiplied by number of ply's so you don't have to really worry about composite action and size the screws/shear flow for that.

Floor's (possessive) and Plies (plural, not possessive), but other than that, yes, I feel like this aspect of the wind load is generally neglected and the horizontal sill is generally provided to better install the window, they wouldn't be expected to nail the window "buck" to the header but it should be nailed into the full-height/king stud somehow. And the header should be face nailed into the full-height/king as well, so there is some connection for transferring (lateral wind) force, but the full-height stud load is fairly typically handled as uniform for half the opening + half the stud spacing on the other side.


(Not my detail) - Design wise, this is a dropped header that isn't fully supported top edge (probably best to consider fully unbraced). And it doesn't depict the buck on the bottom of the header. They also call the Jack stud a "Jamb stud".

That detail also doesn't explicitly show any nailing between the full-height/king and the end grain of the header.

To swing back to my ForteWeb comment, there's actually an option for the header bearing on a trimmer on the span page, but it's not checking the trimmer/jack studs for load, it's just making sure bearing is handled, as near as I can tell. If desperate the stud tables from Simpson can be looked at to eyeball a max load per trimmer/jack stud (Feel like there's an FAQ in the Wood forum for that one, by the by- There wasn't, so I created it now. Once the site starts showing where it is (the URL) I'll add the link and submit this post.

Fine it's been two hours I give up. I created a FAQ entry for this (the stud tables), in the Wood design forum. Technically these are post tables, but the tables don't contemplate lateral (wind) loads, so there's no difference anyway. Not that I can think of, the tables sure seem to include the "posts" braced in the weak axis (i.e. by sheathing or other means).




Haven't had a chance to read this one, but this Exterior Wall Headers, Guertin article from 2019 in the Journal of Light Frame Construction has some decent isometrics showing the pieces and discusses some fine print items in the IRC (guess they do address trimmers/jack studs, or they started to, that article is showing an NJ "number of jacks" column in that table in the 2018 IRC).

 

[Zoobie777]

I've always designed jack studs to carry the gravity and king studs to carry the wind acting on the surface. 10' with only 1 king on each side would never calc

When I'm doing preliminary design, i.e. napkin designs, I assume all the studs interrupted by the window are required for wind. And therefore half of the number of interrupted studs (rounded down when an odd number usually) equals the number of king studs I need at each end. For example a 4 foot window, 2 studs are interrupted therefore I need one king at each end, for an 8 foot window 5 studs are interrupted so I need a minimum of 2 kings.

Thanks. I'd call that a 'rule of thumb' for a preliminary design. I'll stick that in my back pocket.

 

[Zoobie777]

Floor's (possessive) and Plies (plural, not possessive), but other than that, yes, I feel like this aspect of the wind load is generally neglected and the horizontal sill is generally provided to better install the window, they wouldn't be expected to nail the window "buck" to the header but it should be nailed into the full-height/king stud somehow. And the header should be face nailed into the full-height/king as well, so there is some connection for transferring (lateral wind) force, but the full-height stud load is fairly typically handled as uniform for half the opening + half the stud spacing on the other side.

View attachment 2528
(Not my detail) - Design wise, this is a dropped header that isn't fully supported top edge (probably best to consider fully unbraced). And it doesn't depict the buck on the bottom of the header. They also call the Jack stud a "Jamb stud".

That detail also doesn't explicitly show any nailing between the full-height/king and the end grain of the header.

To swing back to my ForteWeb comment, there's actually an option for the header bearing on a trimmer on the span page, but it's not checking the trimmer/jack studs for load, it's just making sure bearing is handled, as near as I can tell. If desperate the stud tables from Simpson can be looked at to eyeball a max load per trimmer/jack stud (Feel like there's an FAQ in the Wood forum for that one, by the by- There wasn't, so I created it now. Once the site starts showing where it is (the URL) I'll add the link and submit this post.

Fine it's been two hours I give up. I created a FAQ entry for this (the stud tables), in the Wood design forum. Technically these are post tables, but the tables don't contemplate lateral (wind) loads, so there's no difference anyway. Not that I can think of, the tables sure seem to include the "posts" braced in the weak axis (i.e. by sheathing or other means).

View attachment 2527


Haven't had a chance to read this one, but this Exterior Wall Headers, Guertin article from 2019 in the Journal of Light Frame Construction has some decent isometrics showing the pieces and discusses some fine print items in the IRC (guess they do address trimmers/jack studs, or they started to, that article is showing an NJ "number of jacks" column in that table in the 2018 IRC).

Thanks. Article looks interesting...printing it off now.

 

[lexpatrie]

Not going to say I'm endorsing this thread, but here it is....

Wood Header/Trimmer/King Stud Design - Structural engineering general discussion

When designing a wood header, what assumptions are made concerning the loading of the trimmers and headers? After sizing a header, I usually select the trimmers to provide adequate bearing area for the header and bottom plate, and assume the trimmer is taking the full gravity load. Thus, the...

www.eng-tips.com

It's tough to find a decent drawing as a lot of folks will by default show the header dropped to the window head, and for longer spans this is less efficient on the header as it's unbraced. Longer spans do better if the header is at the top of the wall and they cripple down to a flatwise 2x at the window head.

Bonus points if anybody can find a reference drawing on that.

 

[XR250]

I typically use 2k and 3k capacity respectively for a 2x4 and 2x6 jack in a typical 9 ft. wall. This ends up being simultaneously roughly the capacity of the stud and header bearing. As such, I never check header bearing. I will check bearing in other situations though. One of many ways to save time in wood design.

 

[lexpatrie]

Trimmer capacity of a 9' 2x4 is above 2k, so i get that, but the 2x6 is around 6k so I'm not really following that shortcut. If you're presuming a (2) 2x on a single stud the area (3" x 1.5") x 425 psi would be in the neighborhood. Thst sounds like it's more 3k for bearing than 3k for the stud trimmer capacity.

 

[EngDM]

Floor's (possessive) and Plies (plural, not possessive), but other than that, yes,

The problem with doing elementary and secondary in a French school is my written English can be caught slacking. Now I'm left with shitty English and noone to speak French to, so I've lost that as well.

 

[XR250]

Ahh, you are right. Maybe that is why I have been using 3k.

 

[lexpatrie]

j'suis ici, moi. EngDM/

English, by the way, is one of the most difficult languages for Native French speakers to acquire. For "us" Americans, it's Chinese and Russian because Chinese is Chinese, and Russian has all those (Dative, Nominative, etc.) cases we never see in English.

English has virtually zero verb tenses compared to the 14 or so in French (18? 22?). Most of those aren't in much usage outside, say, Proust, but are good nonetheless for impressing French grandmothers (le passe du subjontif, par example, is particularly fine in that respect, and it also comes in handy when watching 8mm (Nicolas Cage) in French.) It goes both ways because French has drifted so far from Latin compared to Spanish and Italian, and the mere existence of Les Difficultes de la langue francaise is proof enough, and English has way too many goofy rules. I mean I know there are three nouns that change gender when plural, but the only one I've managed to retain is love, and I think douce is another but the third just doesn't stick.

Coming back to the topic, though, I have a link for the FAQ entry on post capacities (tension with hole or not, compression based on length) via Simpson strong tie.

Stud capacity tables - Simpson StrongTie catalog (and web site)

(Don't fall in love with the title of this FAQ, I'm going to change it sometime to add a range of years sometime when I go back through to tune up the consistency of the titles).