Joist/girder evaluation for RTU

[mfstructural]

When doing an evaluation of the existing girders and joists when adding an RTU unit on a roof, what method do you use to check the existing joists? Do you use the point loads on the joists from the corner of the curb? Or do you convert the unit weight into a distributed load and add that to the joists? Regardless I will reinforce the joist webs at the corners of the curb. Another question regarding two curb corners that don't coincide with a joist...do you run angles between the joists to carry the curb load to the joists?
Thanks

 

[kylesito]

I have converted it to a distributed load. However, I have not found one that has worked yet without significant reinforcing. We have had a couple of projects where we have had to build a frame above the roof level to suppor the RTU supported on posts located directly above the building columns.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[IsaacStructural]

I typically convert the load to point loads, but the difference between doing this and using an equal distributed load shouldn't be too much. I also account for some unbalance/movement etc, and thus, if the unit is carried between two joists, I design each joist to carry 75% of the unit weight.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[grantstructure]

Yes, you'll want an angle frame under the deck to take the curb loads. I generally then treat the load to be taken completely by the angle frame, which delivers point loads to your joists.

Without the angle frame, a concentrated load on the deck could cause it to deflect and pull the waterproofing apart between the deck and the curb (since the curb is stiffer, and because it doesn't see the loading on the deck and therefore isn't moving.)

 

[mfstructural]

Thanks all. Yea, I was planning on using angles to carry the curb loads. How about supporting the curb on the roof? Do you use an inverted channel (or something else) or just place the curb directly on the roof? I would think that you need some support and not place it directly on the roof. Also how do you connect the curb, curb support to the framing members? Is there a good source for typical details?

 

[grantstructure]

The curbs sit directly on the roof, and you don't need to provide anything for them above the surface of the deck.

 

[dik]

The biggest issue in our environs is the snow accumulation associated with the RTU's as well as the changes in snow live loading... we have gone from a 1/30 year return to a 1/50 year return...

Dik

 

[efsinc]

If your in a wind area, watch your anchorages. ASCE 7-05 (IBC 2006 & 2009) requires lateral GCf = up to 1.9 with uplift not specified but noted in commentary "one should design for uplift". ASCE 7-10 (IBC 2012) requires lateral GCf = up to 1.9 and contains a specific uplift design requirement using GCf = 1.5. Current Florida Building code requires lateral GCf = 3.1 and contains a specific uplift design requirement using GCf = 1.5.

 

[a2mfk]

Good point efsinc - Most of my work experience is in the sunshine state. The first company I worked for had designed a building in the Virgin Islands or somewhere in the Carribean, and the RTUs were lifted off the building and blown into the ocean by a hurricane, I believe in the late 80s or early 90s. This was a new building, not a retrofit. But at this time there was not a lot of thought given to the anchorage of the RTUs.

Not that it is a lot different, but I would convert the approximate point load into each joist into an equivalent MOMENT, and then add that to the code LL and in-situ DL MOMENT. There isn't usually any wiggle room in the LL but maybe they over-designed a bit for the actual DL.

I have used inverted channels, but usually they cut a hole in the deck and install angles and the curb, and then reflash. Or you can use small pipes or tubes and build a small rack. Depends on what is going on in the roof and what the owner will let you do.

 

[jike]

I develop the new moment and shear diagrams for the joist with the added RTU weight and compare it to the actual joist shear and moment capacities. Check the spec on the joist as to what should be used for the minimum design shear capcity at the joist midspan.

I find it does not make much difference whether you use conc. loads or uniform loads. If you do NOT have corner loads from the RTU vendor, I would suggest that you the RTU weight since it is generally heavier at one end than the other.

Don't forget to add snow on top of the unit and account for drifting around it. Anchorage for wind uplift is also very important.