Reinforcing of Existing Wood trusses for RTU

[mfstructural]

Hey everyone,
I'm working on a project where an RTU is being added to a roof framed with wood trusses. I'm placing the RTU over a steel beam down the center of the commercial space, at a column location. I have access to original design drawings which specify 15 psf top chord loading and 10 psf bottom chord loading. All we have at the building is the trusses, sheathing and layer of roofing.

The unit is 74 inches long parallel to the trusses, so curbs located 3'-1" off the center of the beam (see attached screenshot).

I evaluated the trusses by first creating moment and shear diagrams for the original span with 15 psf TC dead loading, and 20psf snow loading (snow drift does not affect this area of the truss). I then created a second moment and shear diagram set for the same truss span, but with 10 psf TC dead load and the point load of 173 pounds at 3'-1" from the end of the member. I reduced the dead load to 10 since the 5psf is considered miscellaneous and wanted to take advantage of it. When I do this, I overstressed in shear by 6% at the point of load. Everywhere else along the beam, and in the 3'-1" span between the point load and end of beam, the stress difference is about 3%.

I'm debating on whether to reinforce the truss for shear based on the 6%, which exceeds the 5% allowed by code.

My other question is, what are the preferred methods of reinforcing? I'm deciding between attaching 1/2" sheathing to each side of truss for first three feet, or just fastening an LVL to the top chord. I understand the shear will be transferred through the truss connector plate connections, but I think the LVL attached to top chord should be sufficient for this case. I wanted to get some thoughts on how people have done this in the past. see images attached

Thanks

 

[XR250]

I would go with a 2x12 or LVL as shown (my guess is that a 2x12 would work without the expense of LVL.)

 

[dik]

Depending on the size of the RTU, snow accumulation loads often are much greater than the actual RTU. Just a caution.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[jayrod12]

To build on what Dik said, where him and I regularly practice, the snow build-up around a unit that size would be in the range of 75 PSF right adjacent the unit and tapering down over about 10 feet to our base snow load of 36 PSF.

How far do the roof trusses span currently?

 

[CDLD]

If this is in the US, snowdrift can be neglected for roof equipment with plan dimensions less than 15 ft.

 

[mfstructural]

The trusses current span 22' on top side of page and 29' on bottom of page. I just ran a snow calc and with roof height difference of 2.75', length of upper roof = 7.2' (length of unit), and length of bottom roof = 19' (length of roof to back edge of roof), I get a drift width of 5.36' with snow drift of 23 psf. This will obviously increase the shear and flexure. In this case I think sheathing on both sides of truss would be a better reinforcing solution, depending on loading obviously. This is in the US.

 

[CDLD]

mfstructural, you can ignore snow drift.

Your RTU is less than 15 ft wide.

 

[mfstructural]

With smaller RTUs like this we haven't really taken it into account. I have another project with 10'x30' RTUs....for those drift will need to be considered obviously. So I will go with reinforcing with 2x12 of Top chord.

 

[CrabbyT]

You could get an identical truss and sister it to the existing one

 

[KootK]

1) As detailed, I don't see how a 2x12 helps with the shear capacity of the truss. Can you -- or anyone -- explain the mechanism of reinforcement there?

2) Were I to use the 2x12's, I'd break them at the beam rather than making them continuous. That, to avoid snapping them when they try to make the trusses continuous.

 

[dhengr]

Mfstructural:
The RTU curb should not be by others, you design it to work for you. It should canti. 3'-1" on either side of the stl. bm. carrying the wooden roof trusses. The three cont. long members should be 6'-2" long, the two end members are double the truss spacing in length, and there should be to two short cross members which are the truss spacing in length, over the stl. bm. line. Put nine bearing pls. under this frame, one at each truss out at the end members and one at each truss line over the stl. bm. line. Make the three middle brg. pls. 1/2" thick and the end brg. pls. 1/4" thick, or some such. Better yet, calc. the frame deflection, supported only over the stl. bm. and let that guide you on the brg. pl. thk., also consider some small balancing reaction under the other brg. pls. out at the frame ends. Then, install squash blocks, double 2x6's, standing vert., on each side of the trusses, under each of middle brg. pls. over the stl. bm. line, from the roof sheathing to the wooden top pl. on the stl. bm. and check C⊥ on that wooden pl.

 

[XR250]

As detailed, I don't see how a 2x12 helps with the shear capacity of the truss. Can you -- or anyone -- explain the mechanism of reinforcement there?

If the RTU is located properly, it could just balance on the 2x12 teeter totters and not need any help from the roof trusses.


Yeah, I know, there will be deflection and the truss is stiffer and it won't be balanced etc.

It would be interesting to try to model this.

 

[gte447f]

I agree with KootK about the LVL, and expecting the RTU frame to cantilever from the steel beam like a seesaw seems ridiculous to me. If you expect it to cantilever, you would need a moment connection to the steel beam. Mental gymnastics and overly complicated framing schemes to try to avoid loading the wood trusses are not the route to go on this. Keep it simple and reinforce the trusses if necessary. Reinforce as simply supported spans.

 

[XR250]

expecting the RTU frame to cantilever from the steel beam like a seesaw seems ridiculous to me

I don't think anyone thinks that. The 2x will take a portion of the load off the trusses but the trusses will provide some support.

 

[RontheRedneck]

Is there any reason you couldn't draw both details, and let the contractor choose which one to use?

 

[XR250]

1) As detailed, I don't see how a 2x12 helps with the shear capacity of the truss. Can you -- or anyone -- explain the mechanism of reinforcement there?

2) Were I to use the 2x12's, I'd break them at the beam rather than making them continuous. That, to avoid snapping them when they try to make the trusses continuous.

You could look at the 2x12 fix this way. Just attach it to one truss and let it cantilever. I modeled the 2x12 here with a 200 lb imbalance (i.e., the part under the compressor) This is just a guess. The cut sheet for the unit is need to figure out the actual CG.
Anyhow, the 2x12 only deflects 0.02" and has plenty of capacity. The 200lb uplift is resisted at the panel point of the attached truss - which should not be a big deal for a truss of this scale. I would attach it to both trusses and not worry about anything snapping. Use an LVL if this is a concern as they are pretty impossible to break.