Author Topic: Need advice, ideas, for roof design  (Read 457 times)

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Offline kreyszig

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Need advice, ideas, for roof design
« on: June 06, 2019, 07:06:54 AM »
Hi,

I have been working on a chicken coop design for a while. It is mostly done, but I am having some issues with the design of the roof. The issues arise from the chicken run part of the building, where there are no walls.

The roof of the run is supported by 4x4 posts every 4'. The trusses are also every 48" and are sitting right above the posts. Currently, the design I have uses braces made of 2x4's that go in both directions (not for very post though). Here is a picture of how it looks like:


A better view without the purloins:


The building is 8' wide, plus 1' overhangs and 20' long, plus 1' overhangs. The posts are 7' tall. The transversal braces attach at 2/3 on the posts. The trusses use a king post design and are made of 2x4's. Slope is 12/12 (45 degrees). I assume Douglas Fir select structural for everything

I like the look of this design, but there are some issues related to the braces:
  • They hang a bit too low for my liking in the run. I would have to put bird netting around them if I don't want them to become roost bars for the chickens
  • They are not very strong in tension. If I use 2x4's with two 1/4" bolts for the connection to the posts, I compute 308 lbs in mode IV (tension) with the AWC calculator, which means only 73 lbs in side load at 7' height. In compression it is much better, as expected. The maximum compression force in the brace is 1200 lbs, which means 292 lbs side load at 7' height. A possible solution for this would be to use 2x6's with more than two bolts, but it would not look too good with the 2x4 posts unless I slim them down to 3.5" in their center.
  • To attach the transversal braces to the trusses, I will need to notch the bottom chord (so it does not become a limiting factor in compression) and use side plates (when in tension and to prevent warping). Maybe the bottom chord will need to be replaced by a 2x6 so the notched piece can handle the bending stress.

So given these issues I started to think about another design. It has been a bit challenging though. Here are some design constraints which I don't want to deviate from, which the above design was satisfying:
  • I want 12" overhangs for the roof to extend the life of the posts
  • I want to be able to replace the posts when they rot
  • Snow load is 60 psf
  • The structure needs to be strong enough to support solar panels covering one side of the roof (should represent an extra 5 psf dead load approximately)
  • I want to avoid the use of soffit


An idea I had was to cut a 1.5" slit at the top of the posts, and attach them to both the top and bottom chords of the trusses. For this to work the bottom chord could be extended by about 2' total, so the distance between the top and bottom chords above the posts is about 12". With this design, the maximum roof side load is limited to 127 lbs in both directions for a bottom chord at 7' height. So it is comparable to the design with 2x6 braces in tension, but much lower than braces in compression. This 127 lbs side load generates a bending stress of 2621 psi in the posts, which is equal to Fb' when I use C_D=1.6, C_M=0.85, C_F=1.5 and I computed C_L=0.894. For this design to work I would need to do the following:
  • Notch the bottom chord to attach the trusses to the posts
  • Provide lateral support to both sides of both post segments on the top chord, as a 127 lbs side load at 7' will generate a 892 lbs force at the top of the post, so a connection using a couple of 1/4" bolts would fail miserably. I was thinking of fastening four pieces of 1x4's to the top chord. The outward pieces could be glued and thus could be shorter. The pieces towards the center of the building would need to only be fastened and thus to be longer because they would need to be removed to change a post if it needs replacement.

So here are the possible issues I see with this solution:
  • Because the lower chord would reach outside the building up to the edge of the overhang, I am not sure what I would do for the overhangs in the four corners of the roof so we don't see the lower chords when looking at the roof from either end of the building
  • I might need to stabilize the posts in the other direction (I have yet to do all the calculations). I would at least fix a vertical 2x6 to the posts over the length of the building to replace the top plate shown in the original design

So these are the ideas I have had for the roof so far. Do you have ideas to go around the issues I have identified for either design or another better design idea that would satisfy the constraints I have?

Thanks a lot!

Offline akwoodchuck

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Re: Need advice, ideas, for roof design
« Reply #1 on: June 06, 2019, 08:05:30 AM »
Yeah that's kind of an awkward design....it was me, I'd run a 4x or built up beam down each long side over the posts, then frame the roof conventionally with a ridge board, rafters, and rafter ties, with knee braces from posts to beam....
"The lyf so short, the craft so long to lerne."

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #2 on: June 06, 2019, 09:22:09 AM »
Yeah that's kind of an awkward design....it was me, I'd run a 4x or built up beam down each long side over the posts, then frame the roof conventionally with a ridge board, rafters, and rafter ties, with knee braces from posts to beam....

In the first design I described (the one pictured), I do have a built up beam from stacked 2x4's along each side over the posts and there are knee braces as I think you describe between the posts and that beam. The only difference I think is the use of trusses rather than ridge board, rafters and rafter ties in my design. But the main challenge I have is not with the longitudinal braces, it is with the transversal knee braces that attach to the truss lower chords/ tie beam, to prevent the building from raking when side wind hits the roof. These are the knee braces that take up space in the run.

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #3 on: June 06, 2019, 09:30:10 AM »
Regarding my comment about the lower chord showing up with the second design, after more thoughts I don't think it would be much of an issue, because I would cover the side of the last trusses with siding. If I covered the other side of the overhang for these two trusses with siding as well, it might not look too bad.

I do have some concerns about the bending stress generated in the top chord of the trusses by the side force from the top of the posts though. I should make sure that it is not an issue when combined with the compression stress from the snow load + dead load. It would be possible to beef up these top chords to counteract this though.

Offline Don_P

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Re: Need advice, ideas, for roof design
« Reply #4 on: June 06, 2019, 04:55:54 PM »
Just... winging it. I've increased the overhang to 2' and used it as the brace outboard, raised the tie, cleaning up the inside.

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #5 on: June 07, 2019, 06:27:51 AM »
Just... winging it. I've increased the overhang to 2' and used it as the brace outboard, raised the tie, cleaning up the inside.


Thanks, this is a good idea to provide support this way. I will crunch the numbers. Unfortunately I will have to stick with 1' overhangs though, as larger overhangs will create issues with the coop design (windows, doors, etc). An option could be to combine that type of brace with shorter knee braces. Your braces could also be fixed to the top chord with metal plate and a mortise in the post

Offline JRR

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Re: Need advice, ideas, for roof design
« Reply #6 on: June 07, 2019, 03:53:13 PM »
What if the horizontal element was a small diameter steel cable ... it could easily meet the tension need,  would the fowl find it fine footing?

Offline Don_P

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Re: Need advice, ideas, for roof design
« Reply #7 on: June 08, 2019, 03:46:42 AM »
That would work for the roof thrust part of the problem but the gorilla in the room is bracing.
 Raising that brace to 1' is going to negate it. Its a case of wanting the cake and eating it too. Steel moment connection?

Offline JRR

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Re: Need advice, ideas, for roof design
« Reply #8 on: June 08, 2019, 03:25:38 PM »
If it just for chickens;  why not just drop in center posts from the roof apex to the ground, and use a ridge board ... and do away with all roof trusses and chords?   Just ridge board and rafters for the roof.  If you install a post in the front end wall, the door will have to shift offset to one side.  All anti racking braces can be included in side walls and end walls; and removed from the roof structure.  Could simplify structure quite a bit, and I doubt the hens will mind.  I see, perhaps five, posts in your sketch to do the job.

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #9 on: June 08, 2019, 06:00:50 PM »
If it just for chickens;  why not just drop in center posts from the roof apex to the ground, and use a ridge board ... and do away with all roof trusses and chords?   Just ridge board and rafters for the roof.  If you install a post in the front end wall, the door will have to shift offset to one side.  All anti racking braces can be included in side walls and end walls; and removed from the roof structure.  Could simplify structure quite a bit, and I doubt the hens will mind.  I see, perhaps five, posts in your sketch to do the job.

Well the thing is that we get quite a bit of snow in eastern Ontario and I want to roof to be sturdy enough to be covered by solar panels in the future as well.

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #10 on: June 08, 2019, 06:42:52 PM »
 So I was thinking about that... The top plate I have in the current design might not be the best idea, because it sits over the posts and the trusses sit over the plate. When the transversal knee braces will get in compression, this will generate quite a bit of stress in these weak areas above the posts. It would make a lot more sense to have 4x4's on top of the posts transversally and have the transversal knee braces attaching to them (post and lintel with braces). if trusses are used, they could simply be nailed down on top of these posts. The posts could be linked together in the longitudinal direction by attaching a 2x6 on the inner side of the posts. Because of the weaker wind load in the longitudinal direction and because of the combined effect from the multiple posts, it should be plenty strong enough.

The other idea I had was to combine this with trying to get rid of the trusses, as proposed. I came across this plan when searching for timber framing bends with roof overhangs: https://timberframehq.com/16x24-timber-frame-plan/

I like the idea of a similar structure, except for the fact that I have that venting "chimney" in my coop that goes up the middle of the roof to a cupola, which would interfere with the plate supporting the top of the rafters.
Also this design uses massive pieces of wood for the plates and the tie beams, because almost the whole weight of the roof rests on the king posts and and tie beams. It would be nice if I could find a design where the bends use some trussing action so I can stick with lumber. I have yet to find such a bend design with roof overhangs, as the ones I have seen so far have the end of the rafters connected to the posts. Is anyone familiar with one?

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #11 on: June 08, 2019, 07:01:52 PM »
I guess a design similar to this could be the solution: https://timberframehq.com/19x22-timbered-pavilion/ . It uses king post trusses, so maybe it could be made out of 4x4 for my narrower building... What do you think?

Edit: Maybe a possible issue with this design is that the tie beam joint is in tension and both roof load and wind load will put pull on it. Maybe it is not an issue though if glue and metal plates are used for that particular joint (unlike the posts that will be exposed to more moisture and maybe the knee braces I don't mind if the tie beam cannot be easily replaced)
« Last Edit: June 09, 2019, 05:18:32 AM by kreyszig »

Offline Don_P

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Re: Need advice, ideas, for roof design
« Reply #12 on: June 09, 2019, 06:40:21 AM »
Any of those roof types will work but they all have the same need for a brace of sufficient strength between the post and the roof in the plane of the bent. The 16x24 design would be called a crown post, the crown post is in compression as you said, it predates the kingpost truss where the kingpost is in tension.

But, I was wondering last night about solar up there. Chicken poop, even the fumes from it, is highly corrosive. Is this the right location for the panels, if not the design can change.

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #13 on: June 09, 2019, 07:07:51 AM »
Any of those roof types will work but they all have the same need for a brace of sufficient strength between the post and the roof in the plane of the bent. The 16x24 design would be called a crown post, the crown post is in compression as you said, it predates the kingpost truss where the kingpost is in tension.

But, I was wondering last night about solar up there. Chicken poop, even the fumes from it, is highly corrosive. Is this the right location for the panels, if not the design can change.

Thanks. Do you think the fumes could be an issue at roof level even with the open walls of the run? Or are you referring to the chimney for the coop? My house and garage have their roof rafters oriented in the east-west direction, so the idea I had was to orient the coop perpendicular to the other buildings and use an optimal 12/12 pitch for the roof to maximize the efficiency of the panels. I knew the poop is very corrosive and I am designing the building to avoid metal at ground level (I have yet to find a solution to fix the sill plate of the building (not pictured) to concrete sonotubes), but I did not expect it to be an issue at roof level...

Offline Don_P

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Re: Need advice, ideas, for roof design
« Reply #14 on: June 09, 2019, 01:21:05 PM »
I haven't got a clue, it is basically an ammonia pit, just thinking that is some expensive gear to potentially find out the answer with.

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #15 on: June 09, 2019, 05:27:03 PM »
I haven't got a clue, it is basically an ammonia pit, just thinking that is some expensive gear to potentially find out the answer with.

Ok, I will keep that in mind. Anyway I did not want to install solar panels right away. Currently at my location it would take 30 years to recoup the material costs through electricity savings even if I did everything myself. I will wait a bit.

Offline Don_P

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Re: Need advice, ideas, for roof design
« Reply #16 on: June 09, 2019, 05:47:46 PM »
If you're proceeding with that design maybe think about either sheathing the door wall as JRR suggested to provide bracing at that end, figuring out some form of good bracing in that end wall, the coop can provide bracing at the other end, or welding up a rigid frame there or at each post to truss. If you use each end as the bracing there needs to be a stout pair of plates connecting them. Basically you have a lot of hat up there and are trying to remove the bracing, that needs to be replaced somehow or it is very likely that when there is a good head of snow up top it won't take much for it to lay over. All of the roof designs discussed are fine, the weak point as I'm seeing it is the bracing between the posts and the roof.

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #17 on: June 09, 2019, 07:02:50 PM »
If you're proceeding with that design maybe think about either sheathing the door wall as JRR suggested to provide bracing at that end, figuring out some form of good bracing in that end wall, the coop can provide bracing at the other end, or welding up a rigid frame there or at each post to truss. If you use each end as the bracing there needs to be a stout pair of plates connecting them. Basically you have a lot of hat up there and are trying to remove the bracing, that needs to be replaced somehow or it is very likely that when there is a good head of snow up top it won't take much for it to lay over. All of the roof designs discussed are fine, the weak point as I'm seeing it is the bracing between the posts and the roof.

Thank you

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #18 on: June 09, 2019, 07:07:57 PM »
Do you know a truly free tool to generate and solve the system of linear equations for the loads and momenta on a structure such as a truss? I found https://skyciv.com/free-truss-calculator/ , but I can't even do calculations for a basic king post truss without having to pay... I am looking for a tool flexible enough to handle the asymmetric load generated by solar panels over one side of the roof. It does not need to be 3D.

Offline JRR

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Re: Need advice, ideas, for roof design
« Reply #19 on: June 10, 2019, 06:36:38 AM »
If you are thinking solar panels, and since the rafter length is only 10 feet (if I understand correctly) why not use a simple single-plane sloping shed roof?  It would be the most efficient for solar panels, it can't be simpler and even with snow the rafters could be simple 2x beams.  Even if the rafter spacing had to get as close as one foot, (I haven't done the math) i suspect the cost would be the lowest possible.  The anti racking could be included only in the walls as mentioned before, either by inlaid braces or sheathing.  (I enjoy spitting out frivolous ideas and letting others do the leg work!)

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #20 on: June 10, 2019, 06:25:40 PM »
If you are thinking solar panels, and since the rafter length is only 10 feet (if I understand correctly) why not use a simple single-plane sloping shed roof?  It would be the most efficient for solar panels, it can't be simpler and even with snow the rafters could be simple 2x beams.  Even if the rafter spacing had to get as close as one foot, (I haven't done the math) i suspect the cost would be the lowest possible.  The anti racking could be included only in the walls as mentioned before, either by inlaid braces or sheathing.  (I enjoy spitting out frivolous ideas and letting others do the leg work!)

I prefer not doing that because although the truss span + overhangs is 10 feet, it means the building would end up being 17 feet tall if I was using a single pane roof angled at 45 degrees and I kept the same width for the building. It would create more side load and bending stress on the building and require more material. It would be optimal for the solar panels if the building was 4 feet wide with a single pane roof, but I would end up with a building 38 feet long for the chickens to have enough room in the run... I aim more to have a chicken coop designed with solar panels in mind than the other way around. Also aesthetically I find that most buildings with a single pane roof look better when the pitch of the roof is not too aggressive.

Offline Don_P

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Re: Need advice, ideas, for roof design
« Reply #21 on: June 11, 2019, 02:54:58 AM »
This is a very basic truss program, you can move the load direction around and it seems to be easier than the skyciv to input.
http://pages.jh.edu/~virtlab/bridge/truss.htm

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #22 on: June 11, 2019, 03:53:16 PM »
This is a very basic truss program, you can move the load direction around and it seems to be easier than the skyciv to input.
http://pages.jh.edu/~virtlab/bridge/truss.htm

Thanks. Yeah their "M + 3 = 2*N" restriction makes it not very usable for roof trusses I think. skyciv seems to work great, I just find that $70 is a bit much for the convenience. I will just solve the system of linear equations by hand or using Python...

Offline Don_P

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Re: Need advice, ideas, for roof design
« Reply #23 on: June 11, 2019, 05:48:03 PM »
It actually works quite well for most roof trusses, a queenpost is one exception, the kingpost does fine within the limitations of the method of joints. The issue though is not the truss its the bracing of the bent frame, bracing the legs under the truss.  Just think it through whatever method you choose. An old engineer told me one time "remember at the end of all this that the building actually knows the load path"

Offline kreyszig

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Re: Need advice, ideas, for roof design
« Reply #24 on: June 11, 2019, 07:45:04 PM »
It actually works quite well for most roof trusses, a queenpost is one exception, the kingpost does fine within the limitations of the method of joints. The issue though is not the truss its the bracing of the bent frame, bracing the legs under the truss.  Just think it through whatever method you choose. An old engineer told me one time "remember at the end of all this that the building actually knows the load path"

Hmm i might miss something, but my simple king post truss with overhangs I have 7 members from the point of view of that method (2 rafters, the king post and the tie beam broken in 4 segments) and 6 nodes. If I add the posts I have 9 members and 8 nodes. Neither work with their constraint. Also the only way to add a load in the middle of a rafter with the tool is to add an extra node (and member), but this also messes up with their M+3=2*N constraint. I want to calculate the loads at the joints when I have angled and asymmetrical loads on the rafters...

I just wrote down the system of equations for my trusses. I have 20 unknowns (loads along both axes for all the joints, including the joints between the posts and the ground, plus the transfers of torque to the tie beam from the posts) and 20 equations using the sum of forces and moments. I could specify that the posts are able to transfer torque to the tie beam. I could also have considered the tie beam and posts as a single member and compute the loads for the inner joints as a second step. In order to have a unique solution though, I expect to have to throw in the assumption that only one of the posts can transmit side load. This can be  equivalent to saying that I rely on the brace in compression to prevent raking, which is a good conservative assumption I guess...
« Last Edit: June 12, 2019, 10:55:43 AM by kreyszig »

 

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