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Axial loads on Trusses

Axial loads on Trusses

Axial loads on Trusses

(OP)
Is there a limit on the axial loading I can get out of a truss, or multiple trusses? I need to transfer 6.75 kips to a shear wall via a fabricated wood truss acting as a drag strut.

RE: Axial loads on Trusses

Consider adding a beam to transfer the loads.

RE: Axial loads on Trusses

That seems like a pretty large load for a truss or multiple trusses to transfer, but why not contact your local truss plant and see if it is possible?

RE: Axial loads on Trusses

For my designs I just add a glue Lam beam in the roof system.

RE: Axial loads on Trusses

It can be handled with perhaps a 3-ply truss, if you can find a suitable Simpson connector. Drag loads in single truss can be  about 2500 pounds, routinely.

RE: Axial loads on Trusses

(OP)
Thank you all for your replies. I've decided to grid the entire roof support system with glulam's and use knee bracing. I'm looking at 130 mph winds on the bad side of the Coastal Construction Line and I've got bays open to the oceanside and on both ends.

Thanks again.

RE: Axial loads on Trusses

Multi truss's require care in the field to ensure proper nailing size and spacing.  All the time I see gun nails being substuited for common nails and 1/2 of the required nails.  The truss sheets will specify the required nailing.

I have installed the glue lams up in the trusses so we did not have to furdown, breaking the ceiling lines.

RE: Axial loads on Trusses

Gun nails are much better than commons because they are rated at about 85% the capaciy of commons (which we take into account anyway), and they have MUCH less a tendency to split wood when spaced closely, as opposed to commons.
As far as 1/2 the nails being installed, that's the Builder's mistake, and that can happen irregardless of truss vs. glulam anyway.

RE: Axial loads on Trusses

I agree the type and quantity of fasteners used to install are critical to truss, wall and beam performance.  This is one the areas I find significant installation errors in the field.  If the truss package requires common nails, I ensure commons are used.  The 16d gun wire nail (Senco) is 0.131" in diameter.  However, a 16d common nail is 0.162" in diameter (a 16d box or sinker is 0.135" in diameter).  Which is a big reduction in area.  The reduction in fastener strength is a function of bearing area, wood density and fastener strength (you indicated you use 15% reduction for gun nails, I have also seen a 25% increase used).

I wish the codes and NDS would change to add the wire gun nails in there design values.  I have used the following link for comparing nails.

http://www.hughesmfg.com/nails.shtml#optionalnails

RE: Axial loads on Trusses

JackTrades,
 6.75 Kips into a truss? What does that translate into 6,750 lbs of horizontal force give or take?
  I see those loads from time to time and depending on span / pitch / method of connection and location of connection it can be done. Can you give me more info?
 

RE: Axial loads on Trusses

(OP)
Hi jptruss,

The building is a 5 bay, 70'x21', rectangle with the long direction parallel to the ocean. I was attempting to tranfer wind loads from the roof of two open end bays (14' x 21') via a ceiling level diaphragm to an interior shear wall. This cantilevered diaphragm was a bad idea or at least one I don't have the expertise to feel comfortable with and I'm not sure it's even allowed by the code, which seemed a bit ambigious on that point. I decided instead to use a combination system of X and knee-bracing at the ends bays and diaphragm/shear walls at the interior ones. There may be some issues with the dissimilar stiffnesses of the two systems but I'm hoping that with bumping up my required lateral loads and neglecting the roof diaphragmatic action, it may be ok. The architect has forced be out of my comfort zone so perhaps the old "belt and suspenders" idea will work.

RE: Axial loads on Trusses

Jack youll have to excuse me I dont, understand all of your terminology as Im am not a engineer by trade but a Truss Designer for 12+ years.
 I ran a sample truss 21-0 long, 24" tails , "D" exposure an a 7K drag load through one truss and connected on the full length of the bottom chord.
 What specific information can I give you ?

JP
Truss Designer

RE: Axial loads on Trusses

(OP)
jptruss,

I have a 21'-0" span, 5'-0" overhang on one end, 12" overhang on the other, a 5/12 pitch, with the truss seated along its full bottom chord length on a glulam or a glulam/cmu wall combination.

Load case 1. A longitudinal axial load of 5405 lbs in conjunction with 28.6 psf on the windward slope and 2.6 psf on the leeward slope. (both are positive pressure normal to and in the direction of the top chord of the truss.)

Load case 2. A longitudinal axial load of 4881 lbs in conjunction with -20.8 psf on the windward slope and -44.3 psf on the leeward slope.(both are negative pressure normal to and away from the direction of the top chord of the truss.)

The trusses are at 24" on center.

Would this require two (or more) trusses screwed together? What would be a good way of attaching the bottom chord to the glulam or masonry nailer? What information particular to this should I put on the roof framing plan for use by the truss designer?

Thanks much for your assistance.

-Jack

RE: Axial loads on Trusses

Jack,

 A 5'-0" overhand will be an issue. Even with 2x6 for a top chord there are deflection problems. 2x8 top chords will work without support. A rather pricey option.
 Is there some sort of walkway at the front of the building where the tails/overhangs bear on a beam? That would be ideal. If this is the case and there is support for the tails the a 2x4 top chord would work.
 With the loads described above I can run a single ply truss. This is a small truss and I have seen larger loads then this belive it or not.
 
 For connection , have you looked into the Simpson DSC Drag Strut Connectors?
Have a look here, perhaps this may suit your project.

 http://www.strongtie.com/products/connectors/dsc_pt.html

JP
Truss Designer

RE: Axial loads on Trusses

(OP)
jptruss,

Good information. Thanks.

-Jack

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