New and would like help

[jfmann]

Outward thrust (in PLF) for symmetrical gable frame roof with uniform load is calculated as..........(wL/4) x (12/s)............where w is uniform load in psf, L is total span of the gable roof (from wall to wall) in feet, and s is the vertical rise (inches) per 12 inches (standard way of describing slope).

See "Design Example - Outward Thrust Force" on the following web page; http://www.structural101.com/Structural-Ridge-Beam.html

For the data by Don P.........w = 35 psf..........L = 24 feet..........s = 5................Horizontal thrust (per linear foot of roof; PLF) = (35 psf x 24 ft /4) (12 in / 5 in) = 504 lbs / ft (PLF)

For rafters spaced at 16 inches.........Horizontal Thrust (lbs) = 504 PLF x (16 in /12 in/ft) = 672 lbs

You can of course eliminate the second step by starting with uniform load (w) in PLF, which would be 35 psf x 16/12 = 46.33 PLF

Basic formula can be derived by the usual analysis methods.........taking moments about the peak equal to zero since joint at the peak of gable roof is a hinge. You solve the moment equation for the horizontal thrust force............and then substitute value for ridge height from relation between ridge height and roof slope (simple proportion).

Basic formula can also be derived by considering the gable as a simple truss (which it is). For such analysis, point loads are applied to the three joints. 

[jfmann]

Many (probably the majority) of gable roof frames do not have adequate connection between low ends of rafters and attic floor / ceiling joists (rafter ties) to satisfy current design requirements using building code provisions. There are two main reasons that such roof remains standing;

(1) Full design load has not been applied (yet)
(3) Failure capacity of whatever connection has been made is much greater than design capacity
(2) Roof sheathing acts as a diaphragm to resist outward movement of the rafters

Over time however, connection capacity is reduced (for various reasons including cyclical loading from wind and snow & roof leaks that damage wood at low ends of rafters) and ability of roof sheathing to act as diaphragm is also reduced (due to loosening of connections). When design loading (something close) is finally applied, movement can then occur much more easily.

However.......based on my experience, most cases of gambrel roof spreading occur during construction, either before roof sheathing is installed or (more likely) as roof sheathing is being installed.

[Don_P]

Can't be... I think I see the problem, my oddball notation. I called the steep pitch a 12/5, your math is for a 5/12 pitch. Let's convert it to standard notation... it's a 28.8/12 pitch. Doing it that way I'm coming up with ~116 lbs thrust.  
(35 x 6) (.416) (1.33)=116.

Exactly half what I figured... ah, half each span's load is supported by the wall.

http://windyhilllogworks.com/Calcs/RafterThrust.htm

[jfmann]

My calculation is for a 5 on 12 slope.........which is what I thought the slope is for upper (triangular, gable) part of the gambrel roof assembly. The basic formula is correct for any gable roof with rafter tie across low ends of rafters.............or without rafter tie but supports that can resist the outward thrust force.

As noted in the link referenced in my previous comments...........the force calculated per the formula is consistent with required nailing (rafter to attic floor / ceiling joist) listed in tables in the standard Wood Frame Construction Manual (WFCM). Derivation of the formula is based on standard structural engineering principles. When using the formula......understand that it is only for the stated conditions........uniform loading on a symmetrical gable.

The formula is not applicable at low end of the "wall" part of the gambrel roof assembly (with 12 on 5 slope or 28.8 on 12 slope, not sure now which it is; diagram would help)..........only the upper gable (triangular) roof part (on top of the sloped "walls").

Without any rafter tie (ceiling joist)........the relatively large (672 lbs per rafter) outward-acting horizontal thrust force would be applied (by low ends of rafters with 5 on 12 slope) to top of the "walls"...........which (in general) do not have capacity to resist such force. Of course, without snow on the roof, the outward force is much less since we are dealing only with "dead load" of the roof........not full design snow load. Typical result for a gambrel without rafter tie is for some horizontal movement (spreading) at the slope-change points. Such movement can be relatively large.......on the order of 2 inches..........resulting in noticeable sag in the ridge. This condition also sets the roof up for total collapse in the event of heavy snow load later.

[glenn kangiser]

Once again -thanks for the clear explanations and discussions,  jfmann and Don_P. 

It helps all of us who don't get around the engineering end much.

[Don_P]

Thanks to all of you

jfmann, this is a quick sketch from memory. The upper tie is somewhat unconventional and dictated the width of the upper roof... 16' long tie. The joints are all in one flat plane with 1/2" ply gussets nailed over them, Ive shown the peak gusset, the pitch break and tie is also gusseted. It might be good to use this as something to dissect but I'd also like to know how to figure the forces and build this as a moment resisting frame... a more conventional hayloft.

[John Raabe]

I will throw this into the mix for what it is worth. This is a revised version of a site built gambrel truss I first posted to the freee Download section in 2002.

As it says on the plans, consult a local expert for appropriate loading and engineering for your area (or have a truss company build it).

http://www.countryplans.com/Downloads/Gambrel.pdf

[Don_P]

LOL, it seems I'm defending a balloon framed untied kneewall and John R has drawn his rafter feet down on the floor using it as a thrust resisting tie. This might be a good exercise in finding some limits  

30/60 is a more typical gambrel, if we want to shift the discussion of forces to those pitches it might be easier. 6.93/12 and 20.8/12 would be the pitches.