Splicing floor trusses via top and bottom 2x4 additions. 2

[mansome]

Gentlemen; Though I am a mechanical engineer and I have 15 yrs experience I am wise enough to ask professionals that have intimate experience in this area. I have 160 floor trusses ranging from 15' to 19' in length and 14" in depth. My needs require floor truss lengths of 20'-7". My idea is to cut up some of these floor trusses to to provide extensions to the remainder of these trusses. My intention is to then use 10' lengths of .050 "U" shaped galvanized metal to slice the two pieces together. This 2x4 shaped metal will be nailed to the truss plates with 8d nails via a pneumatic nailer at 6". I intend to also nail the vertical metal flanges into the 1.5" side of the truss plates with 8d nails at 6" centers. I then intend to add southern white pine 2x4s, minimal length of 10'centered over the above metal splice and nail these 2x4s using 10d nails at 4"-5" staggered centers. This method will result in floor trusses 17" in depth with double top and double bottom 2x4 plates. These trusses will be installed at 16" centers along side existing 2x10 floor members. I need to have a minimum of 100 lbs/sq ft load capacity at 19'-0". I will be building 2x4 walls with double 2x4 studs placed directly under each extended floor truss. Will this setup give me 100#/sq ft loading capacity? I believe it should give me up to 150#/sq ft but am not certain. Any help on this concern of mine will be greatly appreciated.

mansome

 

[Trussdoc]

Some suggestions and thoughts...

By extending the trusses in the way you describe, it seems you will be losing the triangulation thet the trusses require, at least in the spliced area. You could use plywood/OSB gussets to create a box beam there.

The double 2x4 stud wall...will this be at the ends of the trusses or a center support? If so,the added support will reduce the span, but may create stress reversals that will cause the connector plates to be understrength.

It may also be a good idea to include a spacer block inside the U shaped metal splice piece, in order to (a) prevent buckling of the sheet metal under compression, and (b) you can reduce compressive stresses in the connection by 50% if you have good wood-to-wood contact.

I also wonder about the economics of this; it seems quite labor intensive. Compare the cost of doing this work with buying new wood I joist material.

 

[mansome]

Thank you for the speedy response.
1. The triangulation of the trusses will be preserved by cutting out truss extensions that will exactly continue the longer trusses triangulation pattern.
2. Thus there will thus be wood to wood contact at the splice point, top and bottom.
3. The double 2x4 stud wall will be at each end of the truss and centered directly under each truss end.
4. Because I will have wood to wood contact at the truss splice, I believe the spacer block will not be needed, correct?

Sincerely

Mansome

 

[RockEngineer]

I haven't done the calculations on your truss but I have some thoughts. Don't splice your top and bottom boards you are adding near the middle. This is where your bending stress is highest. Have a full length 2x4 in the center and splice on both ends of necessary. Make sure the splice in the truss and the splice in the top and bottom plate aren't close to each other. Strap along the top and bottom plate at the splice.

 

[RDK]

I would be very careful with this. The floor loading of 100 psf suggests high hazard occupancy. Splicing trusses and building this floor system is a job for someone with a lot of experience in this type of design and full familiarity with the application and site conditions. Unless there is a compelling reason I would suggest you look at alternative solutions to your problem.

The success of this will also be highly dependent on the workmanship of the carpenters doing the splicing.

I would get a local P.Eng (or PE depending where you are) to look at this and not proceed unless someone’s stamp is on the line. Pay for detailed inspection during the construction as well.



Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com