Gambrel truss (arch?) design questions

[IHDiesel73L]

I am planning a small pole barn (20' x 24') and I'd like to incorporate a gambrel roof in order to accommodate a good sized storage loft.  What I'm trying to figure out is whether or not I can scale up a simple gambrel truss (more like an arch) that I've seen used in smaller buildings.  My father has a stick built shed on his property with a gambrel roof.  The truss consists of 2 x 4 rafters joined together with 1/2" plywood gussets.  The span is 14' and the trusses are spaced 24" OC-they are toenailed into the top plates of the walls.  They are gusseted on both sides, but are not glued as far as I can tell.  The gusset plates are also small, and they are stapled on.  That said, by my calculation, there is about 3500lbs of snow on the roof right now and it has seen much more than that over the last 20 years.  The truss has not sagged a bit, nor have the walls bowed out at all. 

I am hoping that I can use this design to create a large overhead space for a storage loft on top of a 20' x 24' pole barn frame.  The plan would be to use 2 x 6 x 8s for the rafters (using a traditional gambrel design each rafter would be about 7'-8") joined together with 1/2" plywood gussets, glued and nailed on each side.  The gussets would extend 18" out from each joint.  The biggest concern is that the truss will transfer too much horizontal force to the walls, pushing them out.  I suppose I could add a collar tie near the top, but then I start to lose headroom.  The floor joists for the loft (I plan on using 2 x 12 x 20) will serve to hold the walls together, but I plan on putting that in later as funds are available.  Plus, the loft will only occupy about half of the building-the other half will be open to the ceiling.  It will look like this when done:



I was also able to find a paper online detailing the construction of these kinds of trusses and it doesn't seem to mention collar ties-just gussets:

http://www.csbe-scgab.ca/docs/journal/21/21_1_47_ocr.pdf

I'd be interested to hear thoughts from others who have built similar trusses or have more design knowledge than I do.

[Don_P]

That said, by my calculation, there is about 3500lbs of snow on the roof right now and it has seen much more than that over the last 20 years.  The truss has not sagged a bit, nor have the walls bowed out at all.

remember when using that kind of logic that if your new span is about double, the load is going to double AND the lever arm is twice as long. The bending moment... the torque on those rafters and joints, would quadruple... 2 times the load and 2 times the breaker bar length. You're not proposing quite doubling the size but there's a concept I see most folks miss.

I haven't studied the paper deeply, it looks interesting once I convert to measurements I can wrap my head around. Notice the length og gusset BL is 3/4 the length of L1. For mine I just tied across the joint of the upper pitch with a level ceiling.

The thrust produced in an untied rafter is a function of the ratio of rise to run, at 12/12 pitch 1 lb vertical gravity load is one pound horizontal thrust.

another common way I've seen it done in old barns was to run beams lengthwise under the pitchbreak to support the roof. basically a series of H frames with beams running atop the upper legs of the H, usually in 30' and wider center aisled barns with stalls along each side.

[MushCreek]

I suggest studying the trusses used at barnplans (dot) com. That's how I built my barn. A little less space in the loft, but strong. Even they use a loft to brace the roof, though- it has to be at least half the length of the barn, and they use steel rods in the open area to prevent spread. You could probably use 1/2" threaded rod for a temporary brace- they're cheap in the electrical supply area of building supply places. I would worry, though, that you need the loft diaphragm to prevent the building from racking as you look straight down on it. I'm not sure a gambrel roof would brace against that. If so, you might add some temporary diagonal braces as well.

[IHDiesel73L]

remember when using that kind of logic that if your new span is about double, the load is going to double AND the lever arm is twice as long. The bending moment... the torque on those joints, would quadruple... 2 times the load and 2 times the breaker bar length. You're not proposing quite doubling the size but there's a concept I see most folks miss.

I haven't studied the paper deeply, it looks interesting once I convert to measurements I can wrap my head around. Notice the length og gusset BL is 3/4 the length of L1. For mine I just tied across the joint of the upper pitch with a level ceiling.

The thrust produced in an untied rafter is a function of the ratio of rise to run, at 12/12 pitch 1 lb vertical gravity load is one pound horizontal thrust.

another common way I've seen it done in old barns was to run beams lengthwise under the pitchbreak to support the roof. basically a series of H frames with beams running atop the upper legs of the H, usually in 30' and wider center aisled barns with stalls along each side.

Thanks for your comments.  Since posting I've also found these old designs from one of the extension services:

://www.lsuagcenter.com/NR/rdonlyres/B460DBFF-4DA0-4B02-BB34-1523FBB6FD6A/12245/5625bracedRafterFraming2.pdf

Most of the older designs I found prior to this were for barns with spans of 36' and greater, but this details spans of 16' to 24', perfect for my application.  The one question I still have is whether or not this type of construction will work with no floor/ceiling joists to tie the walls together.  These smaller plans don't offer any guidance as to the amount of horizontal thrust transferred to the walls, but this plan for much larger spans does:

http://www.lsuagcenter.com/NR/rdonlyres/456F1339-9D1F-48BF-9199-2E802C275994/12277/6411BracedRafterDesignData3650Spans1.pdf

If you follow the table on the bottom to column 19 it shows the horizontal reaction of each rafter in pounds per rafter (max snow load plus dead load), given a set span and assuming a rafter spacing of 24" OC.  For a 36' span, 6lbs psf dead load, and 52lbs psf snow load (quite high for my area, but it could happen), the horizontal thrust per rafter is 1013lbs.  If you wouldn't mind taking a look at this document I'd appreciate it-since it's English measurement instead of metric it's probably a little easier to interpret than the first one I posted.  It's clear that this type of construction works, it's just a question of scaling it properly, and and whether or not it will work with no floor joists in place.  You mentioned using a collar tie across the top of the arch.  Perhaps that will be enough?  Also, it's interesting to note that the larger span plan is a true gambrel (all rafters the same length) wheres the smaller span plan has a 9' lower rafter and a 6' upper rafter.  I wonder how this affects horizontal loading if at all?

I suggest studying the trusses used at barnplans (dot) com. That's how I built my barn. A little less space in the loft, but strong. Even they use a loft to brace the roof, though- it has to be at least half the length of the barn, and they use steel rods in the open area to prevent spread. You could probably use 1/2" threaded rod for a temporary brace- they're cheap in the electrical supply area of building supply places. I would worry, though, that you need the loft diaphragm to prevent the building from racking as you look straight down on it. I'm not sure a gambrel roof would brace against that. If so, you might add some temporary diagonal braces as well.

I thought about using steel cable and turnbuckles in order to keep the walls together before the loft is put in.  I had planned on only doing the loft over half of the garage, but it would probably be a good idea to brace the other end somehow as well.  Let's just say for the heck of it that a 20' span imparts 800lbs of thrust per rafter.  In my application I'll have 13 rafters-that's a force of 10,400lbs pushing outward on each wall.  Before the loft goes up, three lengths of 1/4" wire rope would probably work if properly installed.

[IHDiesel73L]

Sorry to be bumping my own thread, but did a little more Googling and found a 20' version of the truss referenced above:

http://pbis.okstate.edu/images/documents/6410_high.pdf

I've run across these old USDA plans before but this was the first time I had found a design that fit my needs.  The drawing contains dimensions to build a truss or braced rafter system spanning anywhere from 20' all the way up to 34'.  The 20' truss is built from  four 2 x 6 x 8s, two 2 x 4 x 8s, fourteen gusset plates of 1/2" plywood, and a lot of nails.  Glue is not mentioned, but I will definitely be gluing mine with a high strength construction adhesive in addition to nailing.  Once I build a jig they should go together fast. Here's a section view of the new barn plan incorporating the braced rafter design:



As it turns out I wasn't too far off with the horizontal thrust figures either-it's 777lbs per rafter for the 20'. 

[Don_P]

I looked at the link in your last post thus far, cool. These look to be good links to try to digest.

One thought for resisting the thrust is to use beams as top plates on the outer walls in the open area. If I'm reading right you have 5 rafter sets producing 3885 lbs horizontal load at the plates. To resist the horizontal component of the roof load the plates would be laying flat and would need to be well attached to the bearings which are the floor diaphragm and the end gable wall. A triple 2x10 top plate/girder would accomplish that. This would not be adequate for the to support the vertical component, 1332 lbs/rafter at that 12' span so more posts, a wall, or a beam oriented vertically under the plate girder, or some combination to handle that load direction., anyway, one possible way to handle the thrust.

In older barns I've also seen a "kicker" from the center of the hip brace to the inside of the joint where they have the gusset, just one more element to keep things rigid. In those barns the braces were usually 1x material on each side of the rafters and the kicker was sandwiched between and supporting the joint. I'm guessing this was an pre plywood ag extension method.

[umtallguy]

do you really love the look?

a full second floor and commercial truss ceiling may well be cheaper and easier.

[IHDiesel73L]

do you really love the look?

Most of all I need the space.  I live on a .25 acre lot, so I don't want something with a large footprint that's going to take up my already small backyard.  That's why I'm trying to go up and maximize space that way. 

a full second floor and commercial truss ceiling may well be cheaper and easier.

Easier, yes, but certainly not cheaper.  I shopped around a bit at local lumberyards and saw some very nice trusses, but between engineering fees, delivery fees, and materials and labor, they were going to end up being 80% of the cost of the project 

I may have come up with a pretty simple solution.  In the links I posted above, there is a plan for an 18' wide braced rafter design.  I had thought about going 20' wide, but doing that with a clear span is simply too difficult without resorting to exotic solutions, like engineered trusses.  Instead, I'm thinking I'll go with the 18' wide barn with the rafters spaced 24" OC, as specified in the extension plans.  In the notes, it mentions that if the center girder is omitted, 2 x 12s spaced 24" OC may be used for the floor.  I'll use 2 x 12 joists in conjunction with my rafters to create two 8' wide lofts at either end of the building.  That will only leave 3 rafters not tied with joists in the middle.  This way I'll end up with more loft square footage and hopefully, won't put too much stress on the walls.  I'll be running this by the code official, but wanted to get some feedback here as well.   

[umtallguy]

they do not have standard size trusses available?

I think around here a 20 foot truss is like $100

[IHDiesel73L]

Another set of interesting links:

http://www.jlconline.com/coastal-contractor/design-making-gambrels-work.aspx

http://www.jlconline.com/coastal-contractor/design-gambrel-roof-structures-and-wind-uplift.aspx

I found these because I had found some old APA literature on rigid frame construction.  By combining the terms "rigid frame" and "gambrel," a Google search yielded these links which detail how a gambrel arch can be constructed from sawn lumber and plywood gussets.  The author makes no mention of gluing the gussets in addition to nailing, but I certainly will be gluing mine.  Another important detail is the addition of a birds mouth at the bottom of each rafter leg inboard of a 2x8 plate nailed to the floor in order to keep them from spreading.  The author used 2x12s to span 28' which meant the rafters (all equal length) were nearly 11' long.  For my 16' span I think 2x8s will be plenty (rafters will be 6'-1") with a 2x6 plate. 

[Don_P]

The author does mention gluing the gussets in the first article. Glue in the field is engineeringwise worth zero, doesn't mean it doesn't help stiffen things but it is under uncontrolled conditions. Subfloor elastomeric type glues creep to failure. Rigid structural glues are generally expensive and fussy but the way I would lean. I believe these still have a full floor.

A couple more resources to research would be using the term moment resisting frame or moment frame to replace rigid frame in your search.

There is a section on the design of three hinged arches in the AITC manual, American ijnstitute of Timber Construction, expensive manual but probably available thru interlibrary loan, there are also some tech articles on their website.

I don't believe I've ever had a 4x6 post remain straight, go 6x6.

Enjoying your research.