I’ve thought about doing this a time or 2 but didn’t know how much interest it would generate. Some of you might like it. Others might think it’s as boring as watching paint dry.
I put this in the general forum, as it COULD be considered educational. If the moderators disagree they can move it to the tavern.
The following is a picture of a small girder truss. It’s not from a real job, but is similar to many that I see. And it has what I consider a common mistake in it.
Anybody wanna take a shot at what’s wrong?
We have enough youth, how about a fountain of smart?
Replies
O.K., so amybe I DIDN'T manage to get it posted in the general forum.
Or maybe the moderators are just REAL fast at moving stuff.
(-:
The one that learns by reading.
The few who learn by observation.
The rest of them have to pee on the electric fence for themselves.
I'll take a shot...
the two non vertical webs should go from the top of the king post to third points on the bottom chord??
MrT, SamT, DanT,RonT,Ms.T…
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It needs vertical pieces right below where it says 2x3 down to the bottom chord?
I,m gotta go with Mike Smith and the others that the diagonal braces are wrong, I feel that the load will tranfer to the center point in a concentrated load right were you don't want it,or then again maybe I should stick with casing , base and doors, but somethings gotta hold it up.Mike,Any word about the trip to Isle Royale?" I reject your reality and substitute my own"
Adam Savage---Mythbusters
Its a truss for a shed that will have a John Deere parked in it?
A 9' span can be spanned cheaper and better by stickbuilding?
The truss plates will dull your saw blade when you cut for the skylight?
The overhangs are to small?
Its not made out of the right wood?
MrT, SamT, DanT,RonT,Ms.T…<!----><!---->
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Whatever…<!---->
<!---->
Welcome to Breaktime!!!<!---->
Where…<!---->
The free advice is worth every Penny!!!<!---->
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You fergot the 1/4" luaun and liquidnails and Piffen screws?
Spheramid Enterprises Architectural Woodworks
"Sell your cleverness, Purchase Bewilderment"...Rumi
How about the corrugated fasteners and staples?
The pitch vs.the rise measurement don't add up.uhhh, what was the question?
The slope doesn't start until 4-3/16" up from the bottom of the truss. Subtract that from the 2'7-3/16" overall rise and you get a rise of 2'3" over a run of 4'6".
Well, dang - I thought it would be more obvious than it apparently was. The plates are fine. The bottom chord detail at the bearings is quite normal. The 2' 7" 3 is feet, inches, and sixteenths. The danged program dimensions everything like that. It's annoying if you're not used to thinking in sixteenths..The problem with this truss is that the 2 diagonal webs are unnecessary - It should just have the vertical kingpost under the peak. Girders are typically loaded along the bottom chord. Since this truss is short, the top chords don't have much stress on them. So all you really need is the vertical at the center of the truss. Take out those 2 unnecessary webs, and the price of this truss drops from about $90 to about $80. So it's not HUGE deal. But why spend the money if it won't do you any good?.So - Was this interesting? I can come up with another one at work tomorrow. Didn't think it would generate this much of a response.
Q: Why are men immune to Mad Cow Disease?
A: Cause they're all pigs.
That was interesting.
Value engineering....the love of my life.
Save the owls!
blueJust because you can, doesn't mean you should!
Warning! Be cautious when taking any framing advice from me. There are some in here who think I'm a hackmeister...they might be right! Of course, they might be wrong too!
Looks like the game was over before I go there so yepper, let's do it again
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
I thought it was interesting--good to have a "brain teaser"--makes you think about how and why things are done.
> The problem with this truss is that the 2 diagonal webs are unnecessary
Actually, they do have a function. They make it impossible for a guy my size to crawl around in the attic and mess with plumbing, electricial, etc. ;-)
-- J.S.
"Any word about the trip to Isle Royale?"
There's a meeting Monday night, and I should learn more then.
I know I'm narrow minded. But I can afford to be - I'm right.
I hope it goes well and I will keep you in my thoughts. i am confident that the trip will be worth all the work--Besides you have a pit stop all ready set up in Minneapolis.Mike" I reject your reality and substitute my own"
Adam Savage---Mythbusters
Rats! Did'nt have my reading bi-focals on. lolAhh, what was the question?
"Besides you have a pit stop all ready set up in Minneapolis."
Thanks, but I doubt we'll be coming through there. And I have relatives in the twin cities too.
Ever hear of an orthopedic doc by the name of Paul Cedarburg?
But if we get near there and time allows, I'll look you up.
Actual size *doesn't* matter. It's how big you can convince her it is that matters.
You and who ever your traveling with will be welcome---I have been fortunate not to need to services of an orthopedic doctor ---Yet----but if I ever do I will be sure to look him up.What city does he practic in? I grew up in Chicago and travel back reg. I have a spot in my heart for IL. and it always drives me nuts when the name is mispronounced.Then again when I was young long before my move to Mn. I thought the Twin Cities were Minneapolis and Indianapolis. Hows that for ignorant?Mike" I reject your reality and substitute my own"
Adam Savage---Mythbusters
boss.. to me ... the top chord is ok.. but there should be verticals dropped from those points to the bottom chord..
View Image
or.... more commonly it would be a "W" truss... with 3 panel connectors on the top chord and three panel connectors on the bottom chord
Seem like the 2x3 gusset plates are whimpy......?
I can only guess that the plates aren't big enough.
How simple.
"Poor detail" at the Bearing Points along the top plate
Bottom splices prone to shear?
My guess is that the angle cuts on the ends of the bottom chord (that rest on the top plates) are too deep. They should not have taken away so much material. That's just an un-educated guess tho.
Boss,
Looks to me most of the weight of the structure, roofing, snow etc.
is being transferred to the center of the bottom chord (if that is the right term?). Which would not be desirable since isn't the point of a truss is to construct a strong structure out of lightweight parts and have those parts working together to disperse load among them?
But hey I was never meant to be an engineer!
Tom
OK, without looking at anybody else's answers, there are a couple things I don't like about the drawing, but I don't know enough about trusses to criticize the design.
1) It took too long to figure out that there are no dimensions for the lumber, except the length of the bottom chord, and that the <N> X <N> dimensions all referred to the plates.
2) I don't understand the notation for fractional inches. 2' 7" 3 means two feet, seven inches, and three what?
From a purely stick-figure, engineer's model point of view it's a valid truss -- all triangles, and arranged in a semi-logical fashion. So the devil must be in the details.
The first thing I'd suspect is the plates bonding the tops of the diagonals. This joint is subject to a lot of sideways force (due to the angled cut on the diagonals) if subjected to a lot of load from the top, and those plates aren't very good with sideways forces. Either the joints should be beefed up or the diagonals should be made to meet the rafters at a more perpendicular angle.
I'm with Blue Thumb, the top chords should butt the bottom chord, 'steada vicee versee.
SamT
Girder trusses should have their bottom chords have better bearing than this one shows.
First it is a 'king-post' design. The weakest of the lot. Often adding a king post, from ridge to the center of the bottom cord, actually weakens a truss design.
Further the center of the rafters, the weakest point under load because it is the center of their span, is being reinforced by the center of the bottom cord. Also the weakest point.
Likely the web members only benefit in this general design are only reinforcing the geometry. Not the actual strength of the truss. In other words the triangulation is god but it is more likely to crack at the middle of the bottom cord, the walls spread and the truss fall into the building.
Of course given the spans represented a simple triangular structure, a truss closer to a stick built structure, would actually be more structurally efficient, cheaper and easier to build. Run 2by4s as rafter and bottom cord, include some plywood, glued and nailed, to link the pieces into a unified structure if desired and it should hold up to just about anything.
I would increase the height of the roof, same slope, and make it a 'dropped cord' truss so insulation is easier and more effective. Keep the top plates where the plywood reinforcements are, insert short web members as needed to adapt the design, to maintain strength.
On so short a span I don't't think I would worry too much about what was done unless you have extreme snow, or other, loads.
"...adding a king post, from ridge to the center of the bottom cord, actually weakens a truss design."
Actually that's not at all true. Kingpost designs are quite normal on smaller trusses - Both girders and commons.
Life is full of uncertainties...or I could be wrong about that?
Even after your answer, IMHO the comments of #10, 12, and a couple others about the intersection at the ends are a valid critisism of that particular truss, putting the tie plate in all shear vs. mostly compression. But like Mr T said, there cant be much force there for a 9 ft span.
"IMHO the comments of #10, 12, and a couple others about the intersection at the ends are a valid critisism of that particular truss, putting the tie plate in all shear vs. mostly compression."
A couple of points here -
Truss plates are tested extensively in shear, tension, and tooth holding in all sorts of configurations and angles. So the plate in this case has to meet the published and code accepted standards AFTER those tests.
Trusses have been through many full-scale tests to determine acepted join configurations, strenghts, and design methodology since the 1960s. So there's nothing wrong with the heel joint as it's shown.
When the bosses talk about improving productivity, they are never about themselves.
Boss,
>>So there's nothing wrong with the heel joint as it's shown.
Yeah, there is. For the exact same amount of material, cuts, plates, and labor it could be stronger.
Whether it is strong enough doesn't matter. It's wrong to design obvious weakness in without a balancing gain somewhere else.
It's bad philosophy, grasshopper.
SamT
Edited 3/31/2005 7:43 am ET by SamT
>> For the exact same amount of materialAh, but you don't pour wood out of a vat. Does <n> cubic inches of 2x8 cost more, less, or the same as <n> cubic inches of 2x4. If the 2x8 costs more, and the design is adequately strong as shown, then it makes sense to make those cubic inches part of the top chord rather than the bottom chord.
"Does <!----> cubic inches of 2x8 cost more, less, or the same as <!----> cubic inches of 2x4."<!----><!---->
That's not a consideration in this case. Lumber is bought in (mostly) 2' increments. So you'd use a 10' piece for the bottom chord either way.
Life is like golf. If you keep in the fairway, you never have to ask for a ruling.
>> "Does cubic inches of 2x8 cost more, less, or the same as cubic
>> inches of 2x4."> That's not a consideration in this case. Lumber is bought in (mostly)
> 2' increments. So you'd use a 10' piece for the bottom chord either
> way.It makes a heck of a difference in a well-run truss shop. A big part of your profit margin is dependent on getting all the squeal out of the pig, so cut lists are made up to waste as little as possible.
"A big part of your profit margin is dependent on getting all the squeal out of the pig, so cut lists are made up to waste as little as possible."
But regardless of how you do the heel joint, you still need a 10' board for the bottom chord.
It's what you cut it out of that matters, not how many square inches you use.
If one less person had put out one less percent we would have lost. [Tommy Prothro]
Yeah, but if you buy a 12' board (or 16'), you may have a waste piece that's exactly what's needed elsewhere. And the angle of the cut may be a factor.
"...but if you buy a 12' board (or 16'), you may have a waste piece that's exactly what's needed elsewhere."
I'd say that's wrong for 3 reasons.
First - Longer lumber is almost without exception more expensive PER FOOT. So you may not actually save money by using longer lumber.
Second would be that it just isn't practical to save a bunch of cutoffs. You'd have short pieces of lumber stacked everywhere. We cut 1,000 to 1,500 pieces per saw per day on the average. There just isn't room for a big pile of cutoff lumber.
Third - Truss parts are cut on specialized conveyor fed saws. They aren't designed to cut 9' boards out of 12' stuff. The extra 3' would at best fall into the waste conveyor. At worst, it would hit something on the saw, jam the conveyor, or cause it to automatically shut itself down. (Depending on the type of saw)
Which is harder -- to make education entertaining or entertainment educational?
Quite the contrary, there's an outfit near here that makes automated saws used for truss and factory home manufacture. The system includes cut list software that figures out how to squeeze the most out of a piece of wood, and, last I heard (about two years ago), they were working on a printer to print a part number on each piece as it was cut, to help keep things straight.
After 20 years in this business, I think I know quite a bit about the saws that are available. The only one I can think of that even comes CLOSE to what you're describing is an automated chop saw. It's made for square cut pieces only (Last time I saw one) and is mainly used for parts for panelized walls. The printers are nothing new. Although the ones I've worked with are tempermental, and the printing isn't particularly clear.
The squeaking wheel gets annoying.
This saw is articulated and numerically controlled, so it can make any angle of cut you want, including compound. Saw a video of it at work. Quite impressive, though you don't want to get within ten feet of the thing when it's running.
You're probably thinking about one of the newer style single blade saws. there are 2 on the market that I know of. We have a Hundegger brand. Alpine makes one that's similar, but it's hand fed. Theoreticaly, you can do what you suggested - Cut a bunch of small parts out of long lumber. But as I said before, that doesn't really work well. Like the fact I mentioned earlier about longer lumber being more expensive. We buy lumber as short as 3'long in truckload quantities. Anything much shorter than that can't easily be cut by an automated saw unless it's cut out of a longer and more expensive piece. There's also the problem of lumber grades. If you try to cut a bunch of small 2X4 pieces, you have to pull lumber of the highest grade in the series. If you have to use higher grade lumber than you need to, you've again wasted money.
If you drink don't park - Accidents cause people.
"For the exact same amount of material, cuts, plates, and labor it could be stronger."
Not necessarily. If you're talking about runnoing the bottom chord through, that wouldn't necesarily be any stronger. The joint is all about connections. Do the heel like I assume you're suggesting, and the plate isn't any bigger. So overall, the truss isn't any stronger.
"Whether it is strong enough doesn't matter. It's wrong to design obvious weakness in without a balancing gain somewhere else."
I don't agree that it's a weakness.
Truss designs have to have a 3X safety factor. And they rarely see their full design load. So if the joint design is a proven one I can't imagine any reason not to use it.
Every time - all the time - I'm a perfectionist. I feel I should never lose. [Chris Evert]
View Image
Edited 3/31/2005 8:44 am ET by SamT
Your drawing would make sense if the bottom chord was the only thing there. But you're completely ignoring the top chord, and the fact that the joint is held together with metal plates.
I have as much authority as the Pope, I just don't have as many people who believe it. [George Carlin]
Did your mind go blank?You post was.(-:
As long as a person doesn't admit he is defeated, he is not defeated . . . he's just a little behind and isn't through fighting. [Darrell Royal]
I'll take a wild guess.
A superior way to make the joint at the bearing ends would be to run the bottom chord all the way out and create a level joint where the top chord intersects.
blue
Just because you can, doesn't mean you should!
Warning! Be cautious when taking any framing advice from me. There are some in here who think I'm a hackmeister...they might be right! Of course, they might be wrong too!
That's easy!
For that span, they could have stick framed it in the time it took to figure out how to draw this and what angles to cut the pieces. While they are still trying to nail on gusset plates, I am already laying sheathing
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
Thought of something just before I cheated to look at your answer--it would be hard to ventilate the roof here--guess you could use bullet vents in the fascia. But then your ceiling insulation would be kind of messed up--have to keep it back and leave edges less insulated. Okay, now I'm going to look at answer!
swap the top chord for the bottom and i'm happy.
The brace should be at 90° to the rafter at mid rafter span. The gussets at the wall plate should be turned 90° for optimal strength.
mike
Well, dang - Nobody came close again. I thought this one would be easier. Basically, there are too many webs in the truss. Once I re-webbed the truss, it would look more like the attached truss picture.I'll post another one tonight when I get home.Edit: Sorry - Got confused and posted this pic in the in the wrong thread.
Edited 3/31/2005 6:58 pm ET by Boss Hog
Pssstt Boss...I think you're in the wrong thread.
http://forums.taunton.com/tp-breaktime/messages?msg=56379.1
Jon Blakemore
OK, in cathedral2.gif, what if you were to replace B2 and W2 with one continuous piece? Would it be stronger? Cheaper or easier to make?
-- J.S.
I thought we were BOTH being fairly civil myself.
I think 2 reasonable people can disagree. And we obviously don't. Not a big deal.
But please remember that I've spent 20 years doing this. I've dealt with all types and brands of manual and automated saws. Including the one that sits 50 feet from my desk. So I know the problems we face on a daily basis and the limitations of the saws and software all too well.
So when you come in trying to tell em I'm wrong, I take it kinda personal. This what I DO - Not my hobby.