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Lintels: Wind face loads, to be or not to be

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Greenalleycat

Greenalleycat

Structural
Jul 12, 2021
508
Hey team,

Coming at you live from another office technical discussion...do you include wind face loads (out of plane loads) in the design of your steel / timber lintels? This is in a residential context but I suppose would be fairly comparable in commercial too.

Common sense says yes, wind face loads should be included, with tributary equal to half the jack stud wall above and half the window/door below. However, here in NZ (probably like other countries too) we have a 'pre-engineered' code that builders, architects, and homeowners can use to size common house items, including lintels, without specific engineering involvement. This code has a technical basis that we can review, and this doesn't include any wind face loads on lintels. So, as soon as something falls outside the 'pre-engineered' limits, we have to do a specific design, which inevitably leads to a huge jump in lintel size just to make the out-of-plane loads work.

A huge portion of the buildings here are built off the 'pre-engineered' code and seem to perform without issue, so it raises the question as to how exactly this all works. Is it luck? Secondary systems? Greater stiffness inherent in the linings?

What approaches do you guys take in your offices?
 
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Luck and secondary systems.

Nzs3604 has a few holes in it from an sed perspective.

There are off the top of my head:

-Footings under bracing walls on timber stumps. Footings for m and h class sites.
-Wind loading not in sync with nzs1170.2 2011 or next year some time 2022. Brand website is also out of date
-Bracing details, the lack of proper hd at the end of some bracing walls makes me cringe.
- bracing design in general, the bracing testing has a reliance on redundant systems, hard to justify as an sed design

They way I see it is that the government is happy based on historical data, rather than probability's that engineers use for design. Maybe our safety factors roughly 60 percent ontop is too high for a house, maybe this code is happy with 20 percent extra or less.

 
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What, you mean that this doesn't constitute a solid engineering basis? [glasses]

"The provisions for diaphragms were introduced in the 1978 version of the standard, and have changed little since then. Their engineering basis is not known, but is probably based on practice of the day and sound engineering judgement."
 
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If I'm doing a full design of the header, then yes - I consider out of plane loading. If it's a mostly prescriptive design from the IRC (our residential code in the States), I'm good with using their tables. Unless you're looking at something over 4' wide, most of the load is going to go laterally to the jambs anyway. The area below and above the window is usually small enough to hardly matter, and there's a good chance the sheathing will spread it out. So I don't lose sleep over those. Big windows, though...those can get dicey. And ribbon windows are the worst.
 
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@phamENG, we generally only get involved with lintels that are getting over ~3.5m (11-12ft), so they're carrying a reasonable amount of load
And, of course, big windows/doors are often on high walls to maximise the light yadda yadda architectural things...
Then, when you consider the combined deflection of the supporting trimmer studs, the whole thing bows like a mother and next thing you know there's an ungodly amount of steel involved or a very thick end wall
 
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My personal experience is similar to Pham's. On short windows I don't worry about it, once I exceed about 6 feet, then I start to provide horizontal sill plates above and below the windows with adequate connections to the king studs on each side of the opening.
 
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I've always tried to consider it for SED lintels and even longer span non SED lintels.

There is a lot in NZS3604 that doesn't calc out, try justifying their number of trimming studs and you'll find you need up to double the number. Based on the draft released in 2018 for the long awaited NZS/AS1720.1 the successor to NZS3604 you're about to feel a lot more pain. The changes to bearing strength (halved), and the changes to column/stud axial capacity basically give the middle finger to 90x45 SG8 timber. Basically you'll need LVL11 to be equivalent to the current SG8 capacities for axial loading. Proposed bearing capacity changes basically mean bearing goes from almost never governing to almost always governing.

When it is finally published (if the changes stick), the divide between SED and NZS3604 design will be huge.


On a related note, what are folks, doing for the end connections to transfer the face loads bing talked about to the doubling/trimmer studs?

 
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Face loading of brick facades under seismic parts loads is another area where NZS3604 is basically silent. But it will almost always govern if you actually check.

 
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Yea, 3604 is a mare, it always leads to me feeling like I'm a dumb engineer when everything I SED is way heftier than the 3604 equivalent.

RE: face load end connection. For a horizontal trimmer plate, CPC40/80 can work

We've tried specifying long end-grain screws for lintels on occasion...doesn't work, the pre-nailed framing always stuffs it up
The nearest stud will be too close for them to get the screw + driver in so they will half arse it or not do it
Otherwise, end nails are an option for relatively low loads


 
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Most likely redundancy (i.e. frames designed as pinned are not fully pinned) and probability of wind design event being very low.

However, I do check lintels for horizontal wind particularly if they are more than 3m long. Usually, people don't make a fuss about bigger timber lintels (or upgrading it to steel lintel) as long as its not hard to build (builders) and satisfies aesthetics (architect), so I make sure to tick those boxes.

 
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Connections - usually either end grain or toe nails, if those don't work going up to proprietary screws, and if needed Simpson has some nice header brackets that can handle a fair amount of out of plane loading. Not sure how available they are in NZ, though. I played with some steel brackets once, but once I let the architect in on that he found a new window arrangement that gave me some opportunities to break up the span.

If I can't make one of those work, then wood/EWP probably isn't a viable material for either strength or serviceability anyway, and steel is coming into play.
 
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