Depth of wood floor trusses

[ron9876]

I have a house with 24' spans with wood floor trusses. What is the recommended depth to limit vibrations?

Also at the ledger for the second floor trusses, where the walls are full height masonry, is there a good method to resist lateral load reactions without cross grain bending in the ledger and without casting something into the walls.

 

[MiketheEngineer]

Years ago - I think it was about 18:1. In your case about 16'' deep. I always was a bit wary about that. If you have wall running perpendicular on top - it should be OK. But if it is in a "great room" - you can expect some bounce.

Such ledgers are usually bolted to grouted cells. Most codes say 6' oc - I like 4'

 

[BAretired]

The depth of truss is not the only factor. The following link shows a table of open web joists spanning as much as 30' with 16" depth and meeting the vibration criteria of several building codes.

http://www.ufpi.com/literature/ojbroch-187.pdf

Vibration criteria are presented in the Canadian Wood Design Manual and probably other sources.

BA

 

[woodman88]

For MPC Wood Floor Trusses, the more depth the better deflection/vibration control. I would recommend a 24" depth, the typical maximum depth that a floor truss machine will make trusses.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[a2mfk]

Maybe I am being anal, aren't we mixing apples and oranges with deflection and vibration? Ron asked specifically about vibrations, which can be difficult to quantify and are related to mass, damping, harmonics, etc. Then there is good ol' deflection.

Since it is a house with a 24' span I will assume you will have some interior hallways with walls, so there is your damping most likely. Deflection is a separate issue...

 

[MiketheEngineer]

To BA -

It meets the deflection criteria of several codes. But I would NEVER spec a wooden floor truss at 30' only 16'' tall. It will be bouncy and almost impossible to "fix" or deaden.

 

[ron9876]

Most of the area below is open. Great room, etc. Any takers concerning the ledger.

 

[a2mfk]

I meant interior walls bearing on your trusses on the second floor, these will act as dampeners, as will furniture, etc.

1st floor walls are wood or CMU?

If wood, I'd avoid ledgers, bear on top of the wall if you can.

 

[BAretired]

MikeTheEngineer,

Neither would I. I was a little surprised to see that they showed this as an option in my earlier reference. I like the rule of L/18 for wood trusses and might be tempted to go to L/12 as suggested by woodman88.

BA

 

[ron9876]

More walls, etc. on the second floor. Masonry perimeter walls.

 

[MiketheEngineer]

Typically the wood sill is bolted into grouted cells. I like anchors at 4' oc but code allows for 6'-0'' in residential work.

Then the truss is nailed, bolted or "clipped" to the sill. Use a 2x6 sill if you want.

 

[ron9876]

Yeah but. I am referring to transferring the negative horizontal wind load thru the bolts to the floor truss and then to the diaphragm and thus bending the ledger cross grain.

That is the typical detail used locally for houses with masonry walls. NDS recommends no cross grain bending.

 

[dik]

I would typically be using 18" deep for OWSJ... and likely 24" deep for wood...

Dik

 

[azcats]

ron - you willing to use adhesive anchors? Perhaps a rod at needed spacing (depending on load) into the CMU and use it to attach a simpson holddown/tie to the truss top chord.

http://www.strongtie.com/ftp/catalogs/c-2011/C-2011-p053.pdf

 

[ron9876]

azcats--I won't make any snide comments about your basketball team out of respect but...

I know that will work but it is the type of thing that I am trying to avoid. Typically not done like that in the local market and you know how it goes when you try something different.

 

[BAretired]

Looks like a good detail to me. The local market will learn to love it.

BA

 

[azcats]

@ron - Don't get all mad now that you've seen the beginning of the next 25 years of Pac-10 (or 12 - doesn't matter) domination.

Regarding the detail, anything else I'm coming up with is even more exotic and would involve odd sized ledgers, plate washers or weird truss connections.

Hmmm....lag bolt a clip (of some sort) into the ledger at the same line as the ledger bolts? Still exotic though.

Steel ledger an option?

 

[dhengr]

There are all kinds of things the local market doesn’t like at first. Just because something has been done and hasn’t failed yet, doesn’t always make it right, or good design (we’ve always done it that way, and haven’t had any problems that could be or were blamed on us, but then its never seen the full design load either, so there). You didn’t write the current code you’re dealing with, at least no sane engineer would, you just have to comply with it.

If you’re dealing with a ledger for truss support, you’re going to need bolts a lot closer than 4' o/c., they’ll relate to the actual joist locations to take the vert. loads, and not be overloaded. Without seeing your actual detail it’s tough to see exactly what the possibilities are, or the magnitude of the loads you’re dealing with. There is probably a strap that you could wrap behind a ledger, bend horiz.; and nail to the T or B chord of the truss; locating these bolts properly w.r.t. the strap would have the bolts acting in pull-out/tension, with little cross grain bending. Maybe have the truss manuf’er. detail the truss end bearing so the last couple vert. chord members bear vertically on the ledger and allow the ledger to fit up in the truss depth. Then lag through the full depth vert. end chord members into the ledger if your loads are low enough for lag screw pull-out values.

Or just weld the damn thing onto the ledger and get on with it.

 

[ron9876]

dhenger thanks for the input. South Florida and horizontal reaction will likely require a bolt each side of each truss bearing with hardened washers. Blocking between truss bearings with Tapcons into wall to carry diaphragm loading parallel tio wall. The bent strap that you reference is exactly what the typical detail is. Just not sure they can/will get it in tight/straight enough to be effective so I was hoping someone had seen something better.

 

[dhengr]

The bolts to carry the vert. load should be in the lower half of the ledger so as not to overstress it in bearing and tension perpendicular to the grain. Bolts to take the strap load into the T or B chord of the truss should be nearer the strap location so as to rationalize away the bending across the grain. That sounds like a pretty clean detail to me, but maybe it needs a little special inspection or training at the outset to get the contractor going in the right direction. When they bend the strap, they should bend it down/up in line with the truss, not crank it to either side to mess up the bend. Hit it with a hammer and a block of hardwood to seat it to the truss chord, but not start to cracking it, a gentle bend. That’s what USP or SIMPSON worry most about w.r.t. field bending their hardware; over bending starting edge cracks or re-bending and un-bending work hardening the stl. and starting cracks. Then take a 12 or 16" long pry bar, with a 16d (or whatever nail size) tit on the end of it, .75" long; put this in the end most hole in the strap, at an angle, and use the pry bar to stretch the strap, and place a few nails.