4x8 beam opinion

[beckhamk]

Hello all i wanted to get an opinion about using a 24'  4x8 beam for use as decrative vs functional.  I had seen dug's build in the owner build section where he took 4x8 and used them for rafter ties across his open area oposite of the loft. I really like the look and wanted to do the same thing.  But I want to place these on the top plate and our length would be 24'.  These beam will no be supporting anything, I might place some light weight lighing on them but nothing else.  Is there anything I need to consider when doing this?  I just wasnt sure if with that span if the beam would sag over time and cause more problems then its worth.

thoughts?

Thanks in advance!

[dug]

Actually the beams I used are 4 by 8's. They span about 18 ft. and in my case they are structural (rafter ties) and under tension. I'm not sure if a 4 by 6 spanning 24 ft. might sag a bit over time but I'm sure someone smarter than I will come to the rescue.

[Don_P]

Under their own weight I'm coming up with about 1/2" of sag, add anything and a bit of time and it'll increase pretty fast at that span. Add the weight of a man at midspan and the deflection jumps to about 1.25" and it goes into overload in bending.

I took some pics at work today. The bottom chord of these trusses, and these are structural, is a 6x10 spanning 20'.



In the second pic you can see how the kingpost tenon will insert down into that bottom chord. It gets a peg. If you take a look at the top of the kingpost it is pinched between the top chords of the truss. The kingpost is actually more like a cable dangling down from the peak and holding the middle of the bottom chord up. Anything hanging from the ridge and attached to the beam, be it a kingpost, a rod or chain will cut the span of the bottom chord, your beam, in half. The pics are more elaborate than this would need to be.

I had a brain fart moment on that first truss that's up. I had both a 1-1/8" and a 1" drill on the floor and grabbed the larger drill accidently while drilling the peg hole. My pegs are 1". The homeowner is turning me a replacement. Anyway, I inserted a 1" peg temporarily and raised the truss, jacking it the last bit as it took the roof on it's shoulders and inserted posts under the ends. I was walking out on the bottom chord later and it dropped a bit underfoot, settling down the 1/8" of slop in the temporary pin. So my weight on a 6x10 with a 20' span can make a noticeable deflection.

Dug, notice the coupling on the sink drain in the background.

This is how the tension is restrained in the heeljoint of these trusses;

[beckhamk]

wow - those look heavy!  Very sharp looking 

So don are you using some sort of online calculator tool for this? Or is the a best estimate base on your experience? 

[beckhamk]

I also wanted to ask is there any advice or examples for a noob owner builder on how to do something like what I am seeing here maybe even with metal tension rope or woord to help with the sag.  Nothing to complicated. 

Dug - have you seen any sag in your ties yet and or are you concerned about it.

I guess I am going to have to get my ownder build pics up. as we are coming along nicely.

[Don_P]

I did have my beam calcs online... someone else seemed to want my website more than me  . I have them all on the computer here. There are a few other calcs online, they all take a bit of know-how to use. I was plugging in moderate strength and stifness wood.

If you look around at buildings you'll probably find an example of a king rod suspened from a ridge down thru a tie beam. An angle attached to the ridge that the rod can go through and then a hole in the beam with nuts and washers at each end works.

[Ernest T. Bass]

I panicked when I found out your calcs were down a little while back, Don! I can't find any others that compare. Luckily I found a few of them still up at that forestry forum you're on.. They're not exactly easy to find, though..

[Don_P]

They do email, holler if you ever need one. 

[Ernest T. Bass]

Thanks, Don. They really come in handy often.. I might be musing about a header size or something and never feel like digging out my books and sitting down with a pencil and paper..