How do you size single window headers for Advanced Framing?
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The code tables call for double 2x’s for headers, advanced framing suggests one. How do you size headers using a single member? Do you do the engineering calculations? or is there a revised table with values? And how much trouble is the building department going to have with this?
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Advanced Framing was primarily developed for small houses, small bearing spans and small loads. In most cases you need more than 1 ½” of bearing, see Table R502.5(1) and Table R502.5(2) of the IRC; in many cases you need a 3-play or 4-play header. Truss details and manufactured beams sometimes require 3”+ of bearing on the ends as well.
Table 602.3.1 of the IRC will also tell you that 24” o.c. does not apply to all walls. Single top plate walls are also for smaller houses, heights, spans and bearings. Angled and circular walls need to be framed with double top plate.
You really need to understand the limitations and proper application of Advanced Framing before you use this technique or hire an engineer.
Advanced framing, or what was originally called Optimum Value Engineering, is appropriate for any size of house up to two stories. Bearing surface is not an issue since there is a full-width top plate over the headers and all structural members (studs, joists, rafters) are aligned on the same centers for direct load paths to ground.
To determine appropriate header sizing, you can consult an engineer (which your building inspector may require for any variation from prescriptive code), or you can use this rule of thumb:
The strength of a beam is determined by its section modulus (BD²/6), so it's proportionate to the breadth (B) and proportionate to the square of the depth (D). In other words, if a certain depth of doubled header is specified, you can replace it with a single header that's 1.4 times as deep (√2). To be safe, make the single header 1.5 times as deep.
For example, if the code specifies a double 2x6 (5½" deep), you would need a single 1½" x 7-13/16" header. Or go up to the next size and use a single 2x10 (9¼" deep).
Alternatively, you can go to the APA website for instructions on building plywood box beams and use hollow, insulated headers. In either case, make sure the headers are adequately supported by steel brackets if the trimmers/jacks will be eliminated.
That's a newone on me, I guess I need try to explain "rule of thumbs" to inspectors next time!!!.... must be in hickville...
Structural engineer is your best bet - typically advanced framing/green building programs push for roof trusses in lieu of convention stick framed roofs - a truss roof will typically put more load on the exterior walls since most of the time the interior walls are designed to be non-load bearing - need to be careful and take that into consideration.
Typically if you are going through the trouble of doing advanced framing you are going to increase to a 2x6 wall from a 2x4 wall anyway - you can still use the double header and have room to add insulation. The other option is to put the header above the top plate and hang your floor joists/ceiling joist on it and eliminate the header above the window in the wall - will allow for more insulation in the wall this way.
The IECC recently accepted single headers as an alternative - there will more than likely be tables in the 2012 code.
Tex,
What I offered is the same formula that you can pay an engineer to work out your headers. If you went to your inspector sounding like you knew what you were talking about, then you wouldn't have to explain anything.
Thanks for the information! It's very helpful!
Most beams are sized for deflection, which is determined using the cubed value of the height, not the squared value. The width of the member times the height cubed gives equivalent bending capacity.
To change a (2)-2x6 header, with a dimension of 3" x 5.5", to a single header with a base dimension of 1.5" I would say: 3 x (5.5)^3 = 499. 499/1.5 = 333. 333^.333 = 6.9. Round that up to 7.25 and you find a single 2x8 header has the same bending capacity as a double 2x6 header.
In other words, for any rectangular, uniformly loaded joist or beam, the member's base times the height cubed needs to stay consistent.
Short beams, such as some headers, are sometimes sized not by bending but by shear, or crushing at the ends. I think that's one reason why the code does not provide tables for single headers, because it's a hard thing to nail down prescriptively.
Headers are sized for maximum bending moment, not deflection. The only code limits for deflection are for repetitive members and beams supporting floor, ceiling and roof membranes.
Robert, most headers DO support floors, ceilings and roofs. They are beams which just happen to be over window and door openings. Why wouldn't you calculate them like any other beam?
What size headers do I need for a 8' span is it 2x8 or 2x10 ?
Randy,
Is that to support an elephant or a donkey?
I am not affiliated with Bruce... but here's another plus to his designs. As... all his insulation is outsulation, so the headers is less of a concern. Also timber framers mostly outsulate and gain the same advantage. The only disadvantage is if you prefer to use cellulose, then forget outsulation and go with the Riversong Truss wall.
http://www.aaepassivesolar.com/ for Bruce's site.
The last home that I built did have single headers for single windows and double for double windows all insulated and in 24"OC framed 2x6 walls, and no one complained. I have known my inspectors for decades and inspections at this point are just signatures on pieces of paper to lose or file... mostly lost it seems as when I have asked for old files... well... they never can find them. We are hicks up this way for sure. Happy hicks at that.
Oh and thanks for the formulas gents.
Old question I never noticed to answer:
"Robert, most headers DO support floors, ceilings and roofs. They are beams which just happen to be over window and door openings. Why wouldn't you calculate them like any other beam?"
Meaning, why wouldn't I engineer them for deflection?
Because for most window or door headers, bending moment (fiber strength) is the limiting factor. Loaded to the maximum allowable bending moment, the deflection is insignificant (about 0.03") and the horizontal shear is well below design values.
If you're installing window headers that approach the span of a typical center support beam, then you're exceeding the limit for a doubled lumber header, if it's supporting a floor, a ceiling, a second storey wall and a roof. Basement girders spanning 8' oc posts typically have to be made of 4-2x12s because of deflection limits as well as frequency of vibration.