Beam Help?

EddieWalker · Aug 11, 2006

Mudcat,

Thanks for the picture. My understanding on sissor truss design is that the bottom cords have to tie straight into the top cords. As in a straight line to have it's maximum strength. In your picture, I can see that it was done differently. The company that built them for you did a standard pattern inside the cords, but didn't maintain that straight line on the bottoms that I thought was necessary.

There are allot of things that I do just becuase that's how it's done, and has always been done. Not that I fully understand or know why in every instance.

Kind of like putting studs on 16 inch centers. I do it because that's code and the way it's always been done. I have no idea of the math or engineering for it, and obviously the same applies to your truss design.

Thanks,
Eddie

schmism · Aug 11, 2006

sissors are annalized with a roller on one end so they are forced to be self supporting with no horzontal "tie".

as a concequence you can see "significant" horzontal deflections under full design load.

for instance if i ballparked his trusses right. (60', 2x8 over 2x8, 4 over 2) with 25,5,1 @ 2' OC (about as light as your gonna get) your looking at something like 3/4" horzontal deflection at full load and almost 1.5" vertical deflection. (meets L/240L, L/180T, ie fairly "squishy" truss)

the other food for thought the top beam carring that truss has to support nearly 1000 PLF gravity load. Addtionaly based on 90mph wind, and an "open" building design each truss would need ~654# uplift connection. Else youll find your shed in the next feild over should you get a down burst out of a thunderstorm.

EddieWalker · Aug 11, 2006

schmism,

Any chance of translating that to english???

Sorry, you kind of went over my head there. Are you saying that sissor truss's will move 3/4 of an inch, 1 1/2 inches or just plain keep moving around?

I'm planning on using them in a future project and have allot of interest in what works and what I can do to make it work better.

The lack of horizontal tie, translates into not having a straight bottom chord. Is that right? So the design of the truss allows the walls to move out and in as the truss flexes, or am I making too many assumptions?

My understanding of it revolves around the bottom chord holding the side walls together. This stops the walls from bowing out. It's also why you can use smaller material on the bottom chords than the top chords.

The top chord is tied to the ends of the bottom chords and held in place, creating the angle of the roofline. When the top ends of the top chord come together, you have a complet triangle and the majority of your strength.

Then the bridging is there to eliminate deflection of the chords and tie it all together.

With a sissor truss, I thought the bottom Chord has to run all the way through to the top chord to hold it together. It was my understanding that this is what stops the truss from opening up and allowing the walls to bow under load, or just over time. If the bottom chords are not anchored to the top chords, then there is nothing to stop the truss from opening up.

Thanks,
Eddie

mudcat · Aug 11, 2006

schmism said:
sissors are annalized with a roller on one end so they are forced to be self supporting with no horzontal "tie".

as a concequence you can see "significant" horzontal deflections under full design load.

for instance if i ballparked his trusses right. (60', 2x8 over 2x8, 4 over 2) with 25,5,1 @ 2' OC (about as light as your gonna get) your looking at something like 3/4" horzontal deflection at full load and almost 1.5" vertical deflection. (meets L/240L, L/180T, ie fairly "squishy" truss)

the other food for thought the top beam carring that truss has to support nearly 1000 PLF gravity load. Addtionaly based on 90mph wind, and an "open" building design each truss would need ~654# uplift connection. Else youll find your shed in the next feild over should you get a down burst out of a thunderstorm.

Close, 40' 2x6 tc & bc, .36" horizantal due to live load, .11" due to dead load.
I have 1200# uplift connections.

Timber · Aug 11, 2006

Nice to see we have the right people on the job

mudcat · Aug 11, 2006

Amen, brother.

bugstruck · Aug 12, 2006

randy41 said:
the right way to do this is to compute the actual load and then to refer to a span table.

Exactly, I sure didn't mean to infer anything other than that. That's how we've done it for quite some time now.

Paddy seemed to take some offense at my comment that a standard engineering disection of beam loading would miss some contributing factors to why those 2 x 12 beams aren't failing or deflecting past L/360. No offense was meant. Just my observations and those of many others. I do understand his comments on Celotex sheathing and the like. Not much to it structurally and the observations on windows and doors not working is dead on. I simply agree with him on all that. Materials make a difference. Add to the fact that most of those headers from decades past are fir or hem-fir and that's another part of the equation.

So follow the tables and everyone will be happy. Until you point load a beam of course and the tables are out the window. Then it is engineering time. Frankly, I want to quantify what that point load value is though and make sure the engineer and I are near the same values. Wood structures aren't the strongest of most structural or civil engineers domains. The ones who work it though, are true experts. Sharp guys. Just aren't as many as people might think that delve hard into the wood side and it has some peculiar aspects. We have a couple of Engineer contributors here though that are rather well versed in wood among other materials. Just haven't seen them in this mix.

I've seriously re-engineered some designs the structural guys, who didn't spend much time in wood, were struggling with. They were collaborations and we both pitched in to get a solid cost-effective result. I needed them at least as much as they needed me. They just took the wrong approach to the design solution at the outset. Engineers know all about that and work hard to avoid it, but you need some experience. Engineers specializing in failure analysis are a very competent group in thier respective fields. They've seen most of the dumb stuff.

bugstruck · Aug 12, 2006

Sorry Schmism. I hadn't read that far down the thread. Your one of the wood-smart Engineers I was referring to. Good job.

ovrszd · Aug 13, 2006

schmism said:
you guys crack me up!

so who actually has access to beam softwere?

(Schmism raises his hand)

ok guys check it out. (i work for ALpine engineered products and provide engineering services to the truss induistry)

so on a structual gable end (non weight bearing) then the header has to hold pu the sheathing above it. (all roof load is carred by the last stuctual truss)

so at 5psf * ~8' tall = 40PLF on the top of the header.

punch that in the puter and it says a 2ply SP#2 2x8 will work fine.

Technically speaking loaded to 62% capacity with a .66" deflection from dead load at mid pannel and .72" total load (with addtional live load) or L/328 dead L/295 total which exceeds the 480L/240T default defection crytieria

see http://www.snjschmidt.com/pics/header1.jpg for a pic of the actuall computer output

so at min i would use a 2ply SP#2 2x8 beam. Any addtional size will make the beam stiffer and still work.

PS although i dont want to get bombarded with 8million requests i can run any shape, size, loading etc beam or truss configureation you can come up with with any kind of material including TGI's for beams and LVL for both trusses and beams.

(yes i have access to fun toys)

WOW!!! An Engineer answer that doesn't require a second mortgage!!!! AWESOME!! Thank you very much for this input Schmism!!!!

MUDCAT, awesome trusses!!! Send more pics as you move along with this project we are all interested!!!