×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Depth of wood floor trusses

Depth of wood floor trusses

Depth of wood floor trusses

(OP)
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.

RE: Depth of wood floor trusses

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'

RE: Depth of wood floor trusses

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

RE: Depth of wood floor trusses

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.

RE: Depth of wood floor trusses

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...

RE: Depth of wood floor trusses

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.

RE: Depth of wood floor trusses

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

RE: Depth of wood floor trusses

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.

RE: Depth of wood floor trusses

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

RE: Depth of wood floor trusses

(OP)
More walls, etc. on the second floor. Masonry perimeter walls.

RE: Depth of wood floor trusses

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.

RE: Depth of wood floor trusses

(OP)
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.  

RE: Depth of wood floor trusses

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

Dik

RE: Depth of wood floor trusses

(OP)
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.

RE: Depth of wood floor trusses

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

BA

RE: Depth of wood floor trusses

@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?

RE: Depth of wood floor trusses

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.

RE: Depth of wood floor trusses

(OP)
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.  

RE: Depth of wood floor trusses

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.
 

RE: Depth of wood floor trusses

(OP)
Thanks for all the input.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources