California 18' garage door question 1

[pttcc]

I am in the middle of a disagreement with my structural engineer. For a 18 feet by 8.5 feet door opening, my structural engineer instructs me to frame it just like a 2 feet wide window except a bigger header on double studs. I think it's wrong, but I don't know what to show him. Can anyone point me anything to prove he is wrong?
Thanks.

 

[msquared48]

Our purpose here is not to prove anyone wrong from the get go, but to get to the truth of the matter. I assume that you are a Civil Engineer?

With the very information you have provided, I can give no definitive answers here, and there are many reasons why he is probably correct.

Mike McCann
MMC Engineering

 

[IRstuff]

Don't know if this helps or hinders; these are pictures from a house with 2-car garages on either side of the main entrance. The second story front bedrooms sit directly over the garages, so there may be additional factors.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[XR250]

Garage doors are horizontally spanning structural elements. When the wind blows on them, the load is transferred to the king studs which transfer the load to the foundation and floor or roof framing above. The jack studs typically are only counted on to support the gravity (vertical) loads. As such, more king studs are required on a wide garage door than a narrow window. Don't know if wind is a problem for you in California. For seismic, it probably does not make much difference as a garage door is so light.

 

[ztengguy]

I dont think you need to show him anything. If you are paying him to do the design, ask for set of signed and sealed calcs, review them, and if errors are found, send them back.

You should not have to educate someone working for you.

It very well might work per the numbers. If so, you should be satisfied.

 

[AELLC]

Even with lightly loaded garage door headers 16' and wider, framers like to provide at least 2 jack studs (trimmers) because the header shrinks in length, and they have had problems with those headers just barely bearing on a single jack.

 

[jdgengineer]

I would be surprised if the header shank much in length. Typical shrinkage parallel to grain is around .1% of the length. This would be less than a 0.25 for a 18' header.

With that said I would definitely but 2 jack studs beneath the header. With 2 jack studs you would be capable of supporting a reaction say ~4,000 lbs (off the top of my head)

I'd be most concerned with the king studs for out of plane loads on such a large opening.

 

[AELLC]

jdgengineer,
Isn't that data dependent on average humidity? In very dry climates (Southwest US), shrinkage would be higher?

 

[woodman88]

From General Technical Report FPL–GTR–190 page 4-5
a free download from

http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

"Longitudinal Shrinkage
Longitudinal shrinkage of wood (shrinkage parallel to the
grain) is generally quite small. Average values for shrinkage
from green to ovendry are between 0.1% and 0.2% for most
species of wood. However, certain types of wood exhibit excessive
longitudinal shrinkage, and these should be avoided
in uses where longitudinal stability is important. Reaction
wood, whether compression wood in softwoods or tension
wood in hardwoods, tends to shrink excessively parallel
to the grain. Wood from near the center of trees (juvenile
wood) of some species also shrinks excessively lengthwise.
Reaction wood and juvenile wood can shrink 2% from green
to ovendry. Wood with cross grain exhibits increased shrinkage
along the longitudinal axis of the piece.
Reaction wood exhibiting excessive longitudinal shrinkage
can occur in the same board with normal wood. The
presence of this type of wood, as well as cross grain, can
cause serious warping, such as bow, crook, or twist, and
cross breaks can develop in the zones of high shrinkage."

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.

 

[jdgengineer]

AELLC shrinkage of wood is very dependent on the humidity it will be exposed to in its final use. However, the majority of the shrinkage occurs tangential (ie width/depth) of the beam and not in the length of the beam. For example, with a sutdwall the majority of the shrinkage occurs in the plates and not in the studs. However, with a very long beam, the shrinkage in length may not be negligible.

I could be wrong but I seem to remember that wood shrinks tangentially somewhere in the neighborhood of 10x the amount it shrinks in length. I'm sure Garth's reference would have better info.

 

[jdgengineer]

Just looked it up. Seems that the it's closer to 40x not 10x. Same point though.

 

[southard2]

Not running any loads on this thing so bear that in mind. But I would be more concerned that there are only two full height studs adjacent to the garage door. I see that you have metal panel shear walls. So you are probably either in a earthquake zone or a high wind area. Whether the cripple studs are adequate or not really just depends on how much axial load they are carrying. But in my mind only two full height studs probably isn't enough for the out of plan wind loads that the garage door will be transfering on to them. However I don't even know what wind zone you are in and unfortuneately this is an all too commone problem with stick framing around the garage door area. Side entry garages also have a lot of advantages by getting rid of the need for the those short shear panel return walls.

John Southard, M.S., P.E.

http://www.pdhlibrary.com/

 

[TDIengineer]

Although the header is at the very top, I am only seeing a single full height stud carrying any out-of-plane loading. I would check that stud for the point load. Also, I would question the toe nail connection of the single jamb stud delivering that wind load to the top plate.