Reinforcing Composite Open Web Trusses for RTU

[CANeng11]

A client is placing a new RTU on two existing composite trusses. The trusses are in the style of the Redbuilt "Red-L" trusses, with wood top and bottom chords and steel webs. The building is from the 50's or 60's, so I believe the trusses were likely manufactured by Weyerhauser before they sold off this product to Redbuilt. Speaking with a rep from Redbuilt, these trusses were likely designed to be fully utilized, and adding any additional weight would overload them.

My question is whether reinforcing these trusses should be something we consider, or if we should add new support to take the RTU load separately from the existing structural elements? How would you reinforce this style of truss? Our initial thought was to stack an additional ply below the bottom chord, and add LVL's to either side of the top chord, but the connection may be an issue and this doesn't reinforce the webs, if that is required.

 

[jayrod12]

New support. Reinforcing those trusses would be time consuming and wouldn't give me a whole ton of confidence.

Generally, location dependent, my typical issue when adding a roof top unit to an existing roof isn't the weight of the unit itself, but rather the additional snow build-up around the unit causing reinforcing being required to not only the members supporting the unit, but the members surrounding the unit as well.

Often, when the existing roof structure is difficult to reinforce, or too full of mechanical and electrical that adding new supports would be difficult, I've talked clients into putting the new mechanical unit on the ground and ducting it back up to wherever it needs to service. Sometimes that results in a more expensive unit. But the costs of the slab-on-grade plus the premium for the larger unit and ducting are usually far outweighed by the costs of reinforcing once you account for removal of existing mechanical and electrical, roofing, etc.

 

[dik]

I think you're SOL, and the best option is to provide independent support for the RTU... Like most proprietary items, they are normally tightly designed and have little additional load capacity, if any... any snow accumulation around the unit? Just had a recent project where they were adding a large AHU... the weight was 14K... and the snow accumulation was 68K...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[CANeng11]

This is a fairly small unit, the weight is only ~1000 lbs. The longest horizontal dimension of the unit is also only 2.3 meters, so according to 4.1.6.7 (3) of the NBCC, drift surcharge need not be considered.

 

[XR250]

Maybe check the chords and sheath both faces to transform into a plywood boxed beam. Add additional support at each wall as needed.

 

[jayrod12]

Must be nice that your local AHJ has adopted the 2015 NBCC. Unfortunately ours is still stuck on 2010, and then wonder why we don't trust them.

 

[dik]

Thanks... the one I referenced was about 10m x 3m x 1.5m high... Although unit is small, there is likely little reserve... trusses may possibly be reinforced by plywood glued (PL Premium) and screwed to the bottom chord if load is small, and you could tack a couple of pieces to the top chord on the sides...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

I was recently taken to task by a local Plan Examiner who objected to me referencing the latest CSA S16 for steel design rather than referencing the one stipulated in the code... A dilemma... engineers should be using the latest information... solved it by referencing both standards... our taxpayer's money hard at work...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[jayrod12]

I was recently taken to task by a local Plan Examiner who objected to me referencing the latest CSA S16 for steel design rather than referencing the one stipulated in the code... A dilemma... engineers should be using the latest information... solved it by referencing both standards... our taxpayer's money hard at work...

Likely the same AHJ. It's ridiculous to me that they make us update our general notes to reference old versions of the standards. The argument that the superceded standards are no longer valid falls on deaf ears.

 

[dik]

Pat in Winnipeg? My project notes stipulate that referenced standards are the ones currently referenced in the applicable code... and I don't update the 'year'... only thing is to remove those standards that have been replaced. From my Project Notes:

REFERENCE OR COMPLIANCE WITH A STANDARD SHALL MEAN REFERENCE TO THE BUILDING CODE APPLICABLE EDITION OF THAT STANDARD AS WELL AS SUPPLEMENTS AND ALL DOCUMENTS REFERENCED THEREIN UNLESS REFERENCE TO A SPECIFIC DOCUMENT EDITION IS MADE


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[jayrod12]

Correct, or Mr. Khan. I'm pretty sure they have a checkbox that has to be filled regarding referencing the standards in the adopted code.

 

[dik]

Small world...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

For small load increase, I like XR's idea. Just be careful with fireproofing requirements - you may need to fill the web with insulation bats or something to eliminate the concealed space you'd be creating.

 

[KootK]

1) What's the span of the trusses?

2) Where about on the span will the unit go?

3) What's the spacing and depth of the trusses?

4) Keep in mind that the week link in these things, and the trickiest part to evaluate, is the bearing seats.

5) Distributing the load among four or so trusses with a stiff, perpendicular strong back might be a useful strategy.

6) This product may have a uniform shear capacity, similar to a KCS steel joist. That might be exploited to advantage.

 

[CANeng11]

The plywood box beam is a good idea, but how would you deal with all the existing mechanical through the trusses?

 

[EdStainless]

Have you though about reinforcing 3 or 4 and putting the unit on a raft? I hate the thought of loading 2 of these.
Many years ago I helped reinforce some of these, we added to the lower cord and ply sheathed both sides. We used a $hitload of fasteners to assure load transfer.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[phamENG]

Existing mechanical can be a problem, but it depends on where it is. Can you leave those sections without reinforcement, or cut the reinforcement around the mechanical equipment and treat it like a web opening? You'd likely have to strap the bottom chord, but that may be needed at reinforcement joints anyway.

You may just have to do the analysis to see if some web openings are viable, and then figure out how to accomplish them.

 

[KootK]

I very much like some aspects of the box beam solution as well. Keep in mind the tricky bits however:

1) If you're going to sheath some of the truss, you'll probably have to sheath all of the truss. This won't be a setup where you can pass your vertical shear in and out of the original webbing at will.

2) If you'll sheath all of the truss, you'll need to design stiff and rotationally stable, external splices where the sheathing breaks are.

3) At the sheathing to chord joint, you'll need to design the fastening to move your vertical loads out of the chord and into the sheathing. And this goes for your point loads as well.

4) You will almost certainly need to abandon the existing seat bearing connection and, instead, introduce some kind of clip or seat to he wall below to transfer your end shear.

 

[CANeng11]

There is one large duct running through the trusses near the support (the support opposite the masonry wall is a steel beam). The only other obstructions to contend with are the continuous bottom chord bracing that runs inside the trusses. When you say you should sheath all the truss, would you completely avoid any openings for mechanical and the bracing?

 

[KootK]

When you say you should sheath all the truss, would you completely avoid any openings for mechanical and the bracing?

As I see it:

1) There would be two, parallel shear mechanisms in play: the truss webs and the sheathing.

2) There will be no practical way to shift the shear load path out of one system and into the other.

3) Because of #2, whatever shear goes into the sheathing needs to stay in the sheathing until it is deposited into the supports at the end of the truss.

I wouldn't sweat the bottom chord bracing. You can yank that off and replace it when the reinforcing is complete easily enough.