what header is best

[Guest]

We got bets on this one. What is the best method for framing an opening in a bearing wall.
1. put header up against double top plate cripple down to rough opening.
2. header at rough opening and cripple up to top double plate.

Thanks

[Guest]

Re: what header is best

Hope you put your money on #2!

#1 is weird and requires more wood for the header at the ro

And.... don't forget to adequately support the load before cutting into the wall...unless it's less than 3ft

[Guest]

Re: what header is best

Junior,

I'm with Steve on this one. Sometimes on Custom Homes or Additons cripples wind up being 3-4", and you sometimes have a girder above the header and this header is a 3-1/2" x 9-1/2" or 11-7/8" micoLam we'll put the header against the top plate.

I've seen some framing crews put the headers to the top plate with 6' or 8' openings for doors or windows and then cripple down but I don't like it. I'd rather have an 8' door slam up against a 2x12 header or microlam header then double 2x4's with cripples.

I have done jobs befor where the headers were nailed to the top plates because the customer wasn't sure what height doors they were using.

[Guest]

Re: what header is best

Junior,
I'm with Joe on that one -
I don't like the idea of a hillbilly style header over my door. with cripples up to the real one.....

Mps

[Guest]

Re: what header is best

Those darned engineers.....

From a structural standpoint, it's better to install the header up against the top plate. Beams will carry more weight if they are restrained from flopping over on their sides. While the typical nails from the king-stud into the ends of the header will stabilize short headers adequately, it is possible that long headers carrying heavy loads will need to have their tops stabilized by framing attached to the top plate.

Take for example a garage door at an eave wall. For a header installed tight against the top plate, the roof trusses or rafters will brace the top of the header every two feet. But if you install the header at the top of the rough opening and then cripple up to the plate, it would be easier for the top of the header to flop over like a wet noodle.

Design standards for all beams (wood, steel, concrete, masonry, aluminum--anything) have pretty serious reductions for the allowable strengths of beams that are not stabilized along the edge of the beam that's in compression (for a header, this is the top edge).

For shorter headers, you don't even need cripples--you can just install a horizontal 2x across the top of the rough opening. As long as you have no more than 24 inches from this member to the "real" header above, you meet code for attaching drywall. (At least it did 20 years ago, when I framed my last opening like this.) Granted, for heavy exterior doors, you would want something beefy all around the opening.

So, to settle your bet you just need to define "best". If you mean "strongest for carrying vertical loads" then Method #1 wins.

Thor


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[Guest]

Re: what header is best

Thor

Thanks for your input and advice. I have a few comments/questions

I don't know that I have ever seen a garage opening with cripples, (or any other large header for that matter) - wouldn't the 7'0 min opening would usually make this impossible due to the 4x12 header? personally, I frame almost all my openings with 4x12 as it eliminates the tedious installation of cripples and waste of different sizes of headers.

Wouldn't the "flopping" of the header only be a concern with a secondary side load pushing against the header?

Thanks
SJP

[Guest]

Re: what header is best

Now that both sides have support ...

I don't think it matters. Engineers usuallyignore the contribution of the sheathing. I think structural sheathing stablizes the wall enough to make the differences between #1 and #2 immaterial.

I was going to look at shear strength issues, but I just don't want to.

[Guest]

Re: what header is best

Steve,

For 8-foot walls you are correct: a 4x12 header frames right up to the top plate. I am thinking of my old garage, which was two steps lower than the main house but had the same top-plate height. The header spans 16 feet and has 12-inch cripples above it going up to the top plate. (Note: The header has sagged noticebly, but has not collapsed. Also note that I would not sign my name to plans for that same installation!)

The compression "flange" of a beam (the top half of a header) acts similarly to a column. As a column gets longer, it will carry less and less load--at some point it will even buckle under its own weight. At what is called "critical load" for a column it takes no sideways force whatsoever to produce a buckled shape in the column. Once the column begins to buckle, if you increase the load on it at all it just crumples.

The above applies to "theoretical" columns. If the load is applied off-center, this will have the same effect as putting a small sideways load on the column. Ordinary knots and splits in a header would be enough to introduce this off-center force effect at "critical load".

Engineers must design for vertical loads occurring at the same time as wind loads, so for headers in exterior walls you would have a sideways loading besides what is discussed above.

As a structural engineer, I definitely pay attention to this issue for longer headers. "Long" is a relative term, but a double-car garage header is certainly "long". A single-car garage header might be "long" if it is a 4x12, but "short" if it is a 6x8--but this is not a class in engineering.

George, take a look at the NDS strength reductions for unbraced beams. There is definitely a material difference between Method #1 and Method #2. I would give you specific sections of the NDS, but its Friday night....oops, now it's Saturday morning!

Thor


Shear Wall Construction Guide still waits for you

[Guest]

Re: what header is best

Thor

Thanks for the info

Steve

[Guest]

Re: what header is best

Thor..thanks for your info its not often that I get advice for free from a structural eng. I personally have always framed with #2 method. The job that I am on now (300K addition) they frame with #1 and it makes sense. So in your opinion I should use method #1 for all bearing openings 24" and up..What about IB code do they have any preferences?
Thanks to all for your follow up
junior

[Guest]

Re: what header is best

Junior,

I don't know of any specific requirements in the "conventional light-frame" sections of any of the codes. I just looked in the 2000 IBC, Section 2308.9.5, "Openings in exterior walls", and there is no mention of exactly where to install headers. But there are tons of things that are not specifically called out in the codes.

Glad to provide advice--give me your address and I'll send you a bill (just kidding!). Really, this forum is very educational for me, learning about different building methods and regional variations, and I am happy that I can give something back now and then.

Thor


Shear wall mysteries laid to rest soon...

[Guest]

Re: what header is best

Thor ---

I looked at the NDS (I think section 3.3 or 4.3) and I was going to work thru an example to make myself believe you, but I had some painting to do so I just thought about it.

The first problem I have with your analysis is that it applies to beams not plates. While the perscriptive methods applied to beams provides safe estimates for design purposes, it is not clear that the errors in the perscriptive methods are similar enough to allow drawing your conclusions.

The second problem I have is in detemining where the compression side of the beam is. If we consider the beam to be a box beam with the header at the bottom, plywood on the sides and a pair of top plates at the top, the compression side of the beam is the top of the plates and not not the header. For both cases the compression side of the beam is fixed.

The third problem I have is the load distribution. While one can believe that the load due to floor joists is uniform for case #1 (header at the top), it is more difficult to believe the same for case #2. In case #2 load is transfered toward the ends of the beam by shear transfer in the plywood. This changes teh problem analysis.

The fourth problem I have is in the actual construction. I build headers with a piece of 1/2" plywood between 2x stock. For a dropped header I extend the plywood up to the top plate and use 2x2's as cripples on both side. I suppose others use other construction methods. Again this brings the compression side of the beam up to the top plate.

I wish this simple question had a simple answer, but I just don't see it.

[Guest]

Re: what header is best

Ok ...

I botched up an example.

A 4x12x20' (E=1.6x10^6, Fb=1000) has Fb reduced to 920.

A 6x12 (no stress reducion for bucking) loaded at 400#/ft will span 15 or so. (Note the increase in beam size and the decrease in assumed span.)

A 6x12 with the stress reduction will span 2-3" less. (Might only be 1-2" if I had used the right sizes above.)

I am sure that 2-3" is often a lot, but with all of the issues I raised above I just don't see a real difference between case #1 and case #2.

[Guest]

Re: what header is best

I don't know about all the engineering stuff but I always used #1 and I like it the best.

[Guest]

Re: what header is best

George,

You seem to be making a complicated issue out of the original question. The original post did not mention plywood or prescriptive versus engineered designs.

There are many redundant elements in the typical house. While there could be some composite action of the header, plate, and sheathing material, the original question did not state what sort of sheathing material was used, if any.

For vinyl siding and drywall interior, you will not get any effective "box-beam" action. For plywood or OSB on only the outside, you will only get lopsided composite action, and then only if you are VERY careful about nailing the sheathing edges.

I have never heard of anyone framing headers the way you say you do. The deeper your plywood is in the sandwich, the plywood will be carrying more of the load than the 2x stock (depending on a lot of things that I don't have time to go into.) Production framers in California typically use 4x or 6x header stock to match the wall framing, saving the labor that nailing two pieces of 2x stock together, let alone framing on both sides of a layer of plywood.

My response was to the original "simple" question.

Thor


Shear Walls de-mystified