Ideas to Get additional capacity out of Fink Truss

[AdamJSR]

Hello All,

I have analyzed a fink truss (2x4 TC, 2x4 BC, 1x6 Webs (nailed) & plywood Gusset) that has had some substantial foam encased HVAC duct work hung from it. I am analyzing it to see if there might be a way that I can get some additional capacity out of it, but it appears that with my conservative values for the snow load, wind load, and HVAC loads the TC and BC are above 1.0 stress index!

I'm thinking that the BC is fine because there is a living space below with walls bracing it, but the TC is my issue!

Other than sistering the TC with new lumber, any other tricks you folks have found. I'm looking to strengthen it to get another 5-6psf out of it.

Thanks all

 

[msquared48]

Brother the top chord.

Mike McCann
MMC Engineering

 

[msquared48]

... or you could sheath one entire side in plywood, and around the ducting.

Mike McCann
MMC Engineering

 

[XR250]

If there are interior walls bracing the bottom chord (I assume you mean support - intentional or un-intentional), then the top chord axial load is lessened as well.

 

[AdamJSR]

Sorry. Supported, yes. Long day yesterday!

 

[Archie264]

mquared48,

A quick pedantic question if I may. I'm familiar with the term "sistering" and I and use it when describing how to strengthen a wood joist, i.e., "sister" a new member to the existing member. Is "brothering" another way to say the same thing, or does it mean something different? Thanks.

 

[XR250]

That is when it is sistered, but not too well

 

[charliealphabravo]

Yeah, I'm pretty sure that "brothering" was an attempt at humor

Either that or I'm about to learn something new...

 

[charliealphabravo]

What is the span and the roof pitch? Is the bottom chord fully supported or is it open? I'm guessing the 2x6 webs are in order to reduce lateral bracing requirements on the webs.

I think you might see some benefit from a more rigorous analysis. Your conservative estimates might be what's killing you. For example, none of the load cases require 100% snow + 100% wind or 100% live + 100% snow/wind. Attic live loads are not required to be concurrent with other live loads. Are you considering snow load and live load reductions for roof pitch and roof area? Can you discount attic live loads for trusses without a ceiling or pull down stairs? An analysis will tell you what is the weak link. Perhaps it is the connections. Perhaps it is the lateral braced length of the webs or the bottom chord. The solutions to these problems are different.

 

[msquared48]

You sister on the right side, and brother on the left. But that too depends on how you look at it.

Mike McCann
MMC Engineering

 

[AdamJSR]

Charlie...

I am actually calculating the actual snow load (based upon ground snow load in our area, and then reducing it for pitch), adding in the DL of sheathing, etc, adding the DL from the HVAC installation and then determining web stresses.

Both the bottom chord and top chord are overstressed, webs seem to be fine. Bottom chord is supported by walls below, so I'm thinking I am ok there. Top chord is my concern, as well as the gusset plate (it is wood) at the heels.

http://www.eng-tips.com/

I was considering sistering a new TC member (2x4) glues and screwed to existing member, and then adding an additional heel gusset plate.

Thanks again for the help.

 

[XR250]

Is the TC failing mainly in compression or bending?

 

[AdamJSR]

Bending. Buckling seems to be ok.

 

[XR250]

is the ductwork hanging from the top chord?

 

[AdamJSR]

YEs, hanging from the top chord, but not even tied to every truss member, about every other or every three.

There is also four laterals running on each side that is fairly rigid and only supported from the mail trunk and at the drywall ceiling as it enters the ceiling below.

 

[XR250]

sounds like sistering may be the best option