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Author Topic: Clear Span Roof Truss Calculator  (Read 4444 times)

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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #100 on: March 13, 2019, 11:31:54 PM »
Can do DF, but it would be green and the pine is dry. What concerns me if we tighten the side plates we are starting the same, but the DF will shrink more, where as the pine won't much.

I am thinking of using metal for the middle webs instead of any wood. I was having it come back down the ridge 6' on each side, plus in the middle.

The 1/2" metal plates extend over the top plate of the shorter studs, so the width would be 7". Can add in some OSB to make the difference to, and make it tight though.

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Re: Clear Span Roof Truss Calculator
« Reply #101 on: March 14, 2019, 07:54:19 AM »
The brits sometimes call the kingpost a kingrod which is a better description. It is a tension rod hanging from the peak, dangling down and holding the tie up from sagging. Then the web struts are posts in compression jacking up under the rafter chords supporting them from bending so think about their size in terms of keeping them from buckling as the rafters bend down onto them.

The DF at 6" thick won't shrink more than about 1/8" in thickness, tighten the bolts when dry, the heel stop is what is doing the work, the beetle kill has unknown degrade. Wood is pretty strong in tension so if you inspect it, it is probably fine in the bottom chord, bending in the rafter chord is real work and degrade is very unpredictable in terms of strength loss. In all of this remember a truss is a pretty high performance building element.
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Re: Clear Span Roof Truss Calculator
« Reply #102 on: March 15, 2019, 12:04:58 AM »
Thinking something on these lines.

1/2" metal. Each piece is laser cut and snaps together like a puzzle. Then once the 1" bolts are all tightened, the pieces of the web that sticks out from the flange will be welded onsite.

The web portions of the ibeams are 1/4" narrower than between the flanges. this will allow movement and compression before welding. Could even wait for a year, then weld, but will be tightening with a 1" impact, so will compress the wood some.  

Considering having bottom plates designed like the ibeam webs and they would be welded in after it's bolted tight. If I do that then the plates would extend 1.25" below the wood so there would be material under the piece that sticks through the side plates.

I'm also adding a decorative bottom plate of the bottom cord. Forgot to show it.





Hows the welding comin along?

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Re: Clear Span Roof Truss Calculator
« Reply #103 on: March 15, 2019, 07:48:19 AM »
Correctly a truss has pinned connections free to rotate at each panel point. What you have drawn is rigid. As the truss deflects it can put tearing stresses into the wood. For instance look at the web 2 pin upper connections and think about the stress on the wood as the truss deflects vs if one pin were used there. At the junction on the lower chord same kind of rigid connection. Ideally if you draw a line down the center of each member they should all line up concentrically about the pins to avoid eccentricity, that was obviously not done in one of the bar joist trusses you showed earlier, I didn't comment on it then but it was pretty obviously not well designed, that point was outside of the trusses which put eccentricity into the connections to the chords of the truss. Here if the webs were pinned to the top chord and pinned to the king which was then pinned to the bottom chord it would solve that in my mind. Anyway, this is technically a frame. All that said a modern lightweight metal connected wood truss is basically the same, really a frame rather than a true truss. Those are just general comments, that stuff is getting deep into an engineers playground, I can't say whether it will work or not just more info.
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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #104 on: March 15, 2019, 10:33:20 AM »
 ???

How can it rotate if bolted? I know you mentioned pinned, but that's how it would be made if all out of wood, but how does one use pins if using metal?

I've done several searches and I see them braced off of the king rod and in the web and some even down to the bottom cord, near the king rod. Some all wood and some with metal connectors.

If you were designing it, how would you lay it out?

Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #105 on: March 15, 2019, 12:09:13 PM »
Here's a new one. Not like you were talking really, but I think better that the earlier one. Still would like to see what you are thinking.




Online Don P

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Re: Clear Span Roof Truss Calculator
« Reply #106 on: March 15, 2019, 09:11:53 PM »
Sorry, I was geeking out, pinned is a non rigid connection that is free to rotate, it can be a bolt, nail, screw, steel pin, wooden peg. To keep the confusion rolling, those are all referred to as dowel type connectors. Ideally a truss would be pinned in that the connections are restrained in space but free to rotate. I think I'm probably deep in the land of navel staring there :D. My point was think of the prying action of the bolts on the wood as the truss is loaded, you don't want them to cause it to split down the grain.

Back in the real world. Here's a scan from "Design of Building Trusses" I think this short excerpt would be considered fair use in light of our discussion. If you hit ctrl and scroll your mouse wheel it should zoom and be readable, that's how I'm reading your pics. I'll post this one and dig in another manual for something I think I remember there that might be pertinent.

<br
Next page;
...steel members.
4. Maximum diameter of bolts <1/4 width of wood and <1/3 width of steel plates. (they mean total of all the bolts in that section of the member, in other words don't compromise more than 1/4 of the net section. Another caveat don't use a bolt larger than 1" dia in wood, chances are you are outside of the wood's strength)
5. A minimum of 2 bolts should be used in each member at a joint.
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Re: Clear Span Roof Truss Calculator
« Reply #107 on: March 15, 2019, 10:19:07 PM »
Now for a slightly different point of view, this is from "Wood Engineering and Construction Handbook" Dr Woeste at Va Tech suggested I get this and it is a good one to have on the bookshelf. There's a good chapter on trusses and in fact, there's about 11 pages detailing the engineering of a very similar truss to yours as a design example. Check interlibrary loan in your neighborhood (there's really lots there you should probably study) or PM me your email if you want a scan of those pages.

Anyway from earlier in that chapter just basic info;
Quote
Heavy Timber Truss Joints. The most common type of joint used in heavy timber trusses is a bolted connection using steel side plates. A number of pitfalls related to this type of joint can be avoided by remembering the principles of consistent deformation for all components and materials and for all conditions to which the connection may be subjected. The use of steel side plates, while common, economical, and practical, can lead to in service problems. The thermal characteristics and dimensional changes  of wood with moisture change are greatly dissimilar to to those of steel. Large steel side plates and multiple connector rows of fig 9.18a can lead to splitting of the truss member as cross grain tension is induced by shrinkage with moisture change. This can be avoided by the use of multiple straps as fig 6.18b. In all wood truss connections, it must be recognized that moisture content at the time of fabrication may be quite different from that in use.
...
A single large connector plate, as shown in 6.19a, while commonly used, should be avoided. A degree of fixity occurs, and the couple generated between the two web connectors produces cross grain tension which may split the web end. Also differential stiffness between the chord and side plates can induce unanticipated stress in the chord connectors. Fig 6.19b shows one possible arrangement to avoid these potential problems.

Both recommended connections at first seem more complicated than their single member counterparts. However, the multiple strap details typically use less steel and fabrication tolerances are less critical because of the lesser number of connectors in a given component.. Of course, steel side plates must be designed in accordance with appropriate standards, with checks made on bolt bearing, net section tension, end and edge distance, and stability in compression members.

The preceding discussions make reference to cross grain tension stresses. These are the lowest strength characteristics of wood and should be avoided whenever possible. The eccentric connection of fig 6.20 induces very complex tension and shear stresses and secondary bending moments and must be expected to perform very poorly unless the stresses are very low.

 
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Re: Clear Span Roof Truss Calculator
« Reply #108 on: March 16, 2019, 12:02:13 PM »
Sorry, I was geeking out, pinned is a non rigid connection that is free to rotate, it can be a bolt, nail, screw, steel pin, wooden peg. To keep the confusion rolling, those are all referred to as dowel type connectors. Ideally a truss would be pinned in that the connections are restrained in space but free to rotate. I think I'm probably deep in the land of navel staring there :D. My point was think of the prying action of the bolts on the wood as the truss is loaded, you don't want them to cause it to split down the grain.

Back in the real world. Here's a scan from "Design of Building Trusses" I think this short excerpt would be considered fair use in light of our discussion. If you hit ctrl and scroll your mouse wheel it should zoom and be readable, that's how I'm reading your pics. I'll post this one and dig in another manual for something I think I remember there that might be pertinent.

(Image hidden from quote, click to view.)<br
Next page;
...steel members.
4. Maximum diameter of bolts <1/4 width of wood and <1/3 width of steel plates. (they mean total of all the bolts in that section of the member, in other words don't compromise more than 1/4 of the net section. Another caveat don't use a bolt larger than 1" dia in wood, chances are you are outside of the wood's strength)
5. A minimum of 2 bolts should be used in each member at a joint.
On the bolts. Since 6" thick wood, then that would be 4, 3/4" bolts or 3, 1" ones, right? And how big is a 'section'?

On the steel plates. Don't follow  that one.

Do you prefer this style truss or one with a single king rod based on how we are using the steel for webbing?

Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #109 on: March 17, 2019, 01:52:11 AM »
Can't sleep, so i thought i'd draw up a quick one. I'm not showing any web detail, but I think I followed what I was able to pick up for this design except for the king rod. I just thought it added some look to the design..... Not sure i like the design yet though.




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Re: Clear Span Roof Truss Calculator
« Reply #110 on: March 17, 2019, 08:38:58 AM »
With the exception of the king rod you have drawn a Fink truss (Albert Fink first patented it). It is a good solution. As mentioned in one of those articles it is not a bad thing to reduce the number of members coming together at any one point. This brings 2 web members together in a place instead of 3 as in your kingpost design. I was actually thinking of moving the web struts up onto your kingpost in that design. Anyway, this is a good truss, normally it divides the bottom chord into thirds and the top chords into halves as you have done.

For more exploring another variant on the kingpost model is a Howe truss. With your kingrod in this model you are heading towards a Pratt truss. Keep exploring, looks like you are having fun working through the possibilities.
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Re: Clear Span Roof Truss Calculator
« Reply #111 on: March 17, 2019, 07:34:19 PM »
Don't really care for the fink truss. If it's a lot better or we need to use, sure, but would rather not if at all possible, but I can see the advantage of reducing the number of members coming together in 1 point with it.

Have a question. If I'm connecting everything by metal plates, what's the difference between the 2 designs really? I know the top cord web is further down the top cord on the second one and a few too many bolts, but besides that since it's all metal to metal.








Online Don P

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Re: Clear Span Roof Truss Calculator
« Reply #112 on: March 17, 2019, 10:43:23 PM »
In my opinion, and that's all it is, the upper drawing has the webs torqueing on the bottom chord under load and in the second drawing they are not, the web struts can rotate easier as the truss deflects. Is it enough to matter, above my pay grade. I can help rough you in and am happy to share what I know however I am a carpenter not an engineer. Hopefully after wading through those 11 pages of equations it becomes clearer, an engineer is a good idea and earns his pay.

This is the article on kingposts from the TF Guild and the NPS I was thinking of, good to read through, notice the truss on Mt Sinai, pretty cool they understood it that well back then.
https://www.ncptt.nps.gov/blog/timber-framing-no-72-june-2004-2004-12/

Another neat graphic on bridge trusses while I was googling the above;
https://www.nps.gov/hdp/samples/haer/truss%20poster.pdf
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Re: Clear Span Roof Truss Calculator
« Reply #113 on: March 18, 2019, 12:51:32 AM »
When you said that I looked at the truss again and I could see it plain as day, err I mean night......smiley_brick hits_hardhat  Thanks! smiley_beertoast  Even though it would be bolted and how, it would still move some overtime.

Ok, so now we are on the bottom truss design. In your opine, how far down from the ridge would you bring those webs? 1/3? 7' 8" is where it's at now roughly in that drawing. The length of the top cord is roughly 19' 7 1/2". 6' 6" is about 1/3.

In the old ways all that knowledge would get passed down from master to apprentice who then became a master and on and on with new knowledge added in. Pretty neat when you think about it.

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Re: Clear Span Roof Truss Calculator
« Reply #114 on: March 18, 2019, 07:30:14 AM »
Or as one contractor says "They don't build them like they used to, we have laws against that now :D" There is not a master/ apprentice program in the states. In the first half of the 19th century "rational design" began replacing empirical design, that is using science and mathematics to design rather than experience alone. The industrial age and more modern loadings along with increasing failures in empirically designed structures were driving forces in that change. I'm a bit nostalgic as well, I think we would do better with more mentoring such as the internship that engineers do have to go through for craftsmen as well but do understand where it all comes from and can't disagree.

I like to see the web struts at around a 45 or steeper but you can calculate the forces as the angle changes. That works well with a steeper roof as far as keeping the chord sections equal, less so with a shallower pitch and a kingpost design, that is when the Fink or one of the other trusses begins to shine.
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Re: Clear Span Roof Truss Calculator
« Reply #115 on: March 19, 2019, 01:03:05 AM »
Sadly that's about how it is. 

Yes, i agree, but we both know that just because ones an engineer on paper doesn't mean they know how to actually engineer.......

Do you have an engineering firm that you use for this types of things? I looked around here and couldn't find anyone today. All those that are here that I could find are for large commercial projects.

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Re: Clear Span Roof Truss Calculator
« Reply #116 on: March 19, 2019, 08:43:10 AM »
That is the truth of it, every person is an individual, so I've dealt with... am dealing with, all kinds. I'll obey Mama's rules and hush there :D. Engineers, like contractors, are licensed at the state level. The local guys I like here are only licensed in my state, well one goes down into Carolina as well, but they aren't licensed out there. Many folks here use Firetower for timberframe work, they do work nationwide or at least substantially so, I imagine they could take care of you or would know someone. I've spoken with those guys a few times and they are very knowledgeable, a fair amount of what I've been pontificating here comes from listening to them.

If you are pretty well settled on a basic design, then it is time to retain an engineer and have it detailed. At that point I do not drive the bus, I listen, maybe redirect, my design might end up not being the best way to go, it is still just an idea. The good ones give me the closest to what I want while satisfying the laws of nature. As coach Hilton said while getting ready to give us driver's ed,"Now boys, there's some laws that a man cannot break, two vehicles cannot occupy the same place at the same time" He later jammed his brake when I punched it at a yellow light, we slid to a halt a little abruptly and sorta sideways "We stop for those things :D" That's the kind of thing the engineer is checking for while giving me the hottest car he can.
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Re: Clear Span Roof Truss Calculator
« Reply #117 on: March 19, 2019, 11:02:29 PM »
Oh yeah, I hear ya. I dunno yet if it makes me mad or makes me laugh when people pay for professional help and then don't listen.... I guess it depends on how it effects me....

Agreed on the comment about the hottest car. Funny story also. :D

If they are just designing a truss, why's it matter where they be located? They aren't doing any plans besides it. No site inspection etc.

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Re: Clear Span Roof Truss Calculator
« Reply #118 on: Yesterday at 07:32:47 AM »
It is frustrating what falls under state control vs federal. States control licensing of professionals and all depending some professions and states have reciprocity and most of the time they do not respect one another's licensure. Back in the day I could travel and build fairly freely, that is a thing of the past now even though for instance when I went next door to work I went for contractor's testing and it was the same group that had tested me here. Oh well, render unto ::). But yes physics is the same each side of the border. Interestingly from welding class, that certification is recognized internationally and most of the structural welds are self certified by the welder, I pointed that out to our building official after he asked for a special inspector to be brought in, he still has power to trump that but AWS pulled some strings.
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Re: Clear Span Roof Truss Calculator
« Reply #119 on: Yesterday at 07:24:50 PM »
Yeah, makes no sense like most of what government does..... All I need is a design of a truss, not an actual building engineered. Sil no luck finding one here .

From what you wrote, you more partial for the fink truss since it's a 4/12 I take it?

On the 6x12's, they need to be FOHC?


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