RTU on Existing Bar Joists - Load Combination Question 2

[jbuening]

We have a client that wants to add a generator to a roof of an industrial building that has metal decking and 32LH07 bar joists feeding to an interior 40G 6N 12k girder joist. The generator is a 2600lb unit of 42"x94" base (92psf). My question for those that may do these more often than us, when analyzing the existing joists (we have erection drawings) how do you typically handle the load combinations with an RTU? It should fall under the roof live load criteria, but do you include the design roof live load on the existing plans (20psf) over the entire area and also add the concentrated load in the area of the RTU? In our area the snow load is the same as the roof live load, 20psf.

Any concerns with placing the generator centered over the interior 40G girder joist near the support? My thought is the load of the generator would have four contact points, one on each 32LH joist that connects to the 40G, as well as two contact points on the 40G. This would minimize the concentrated loads on the top webs compared to centering it over a single 32LH joist. This assumes the joist can handle the shear loads as well as the bearing on the precast tilt up wall panels, which we would check. Another option is centering it between two 32LH joists and add angles to transfer the loads to the joists (joists @ 6'-8"cts and generator is 3'-4"). Attached is a roof framing plan in case it helps. The client wants to keep it away from the 2nd floor area (to the right in the plan), but the electrical boards are all located under the 2nd floor area at ground floor. One option was to put the generator towards the wall at the far left, but then you have a long run of electrical wire to the boards. Thoughts?

The Lennox weighs 2570lb, the Carrier weighs 755lb, and the Cook vent fan is unknown at this point but definitely less than 500lb.

 

[H57]

My recommendation would be to buy technical digest 12 "Evaluation and modification of open-web steel joists and joist girders" from the steel joist institute. You can purchase this as a pdf.

It outlines how you can use bar joist load tables to determine adequacy for additional loads and how to transfer loads to additional bar joists if needed. In a worst case scenario it can be used to design modifications to the bar joists to support the additional load.

 

[msquared48]

Don't forget snow drifting in the area of the generator. Could locally mount up to 60 t0 80 psf.

I would consider, for example, if these are Vulcraft joists and joist girders, to let Vulcraft analyze them for you with the additional load and design any strengthening required. This way any existing manufacturer's warranty of the joists and girders would not be in jeopardy.

Mike McCann
MMC Engineering

 

[JAE]

As far as load combinations, I would do this:
1. Include roof dead load as actually occurs over entire area
2. Include snow loading over the entire area (i.e. on the "roof" of the unit)
3. Determine drift loads around the unit (if the unit is larger than 15 feet across per ASCE 7)
4. Include roof live load only around the unit (not on top of it).
5. Include lateral wind loads (equipment on roof per ASCE 7) that create vertical force couples - two load cases - X wind and Z wind.
6. Include vertical and lateral seismic loads.

Then with the above DL, RLL, SL, WLX, WLZ, ELX, ELZ, combine these per ASCE 7 and check the joists for each load combination.
Even if your snow loading is small, the drifting might be more than the RLL - maybe not - depends on the area.
The wind might cause a higher downward force (from the down force couple loads) on an individual joist.
Same for seismic if it is significant enough.

 

[wannabeSE]

JAE provided a nice list. By the way, the IBC and ASCE 7 include fixed service equipment in the definition for dead load. Therefore, I would consider the generator to be a dead load.

Because I have had problems in the past with MEs increasing the weight of equipment a couple days before plan check, I often increase the equipment dead load by 20 - 40 % (depending on the ME) during preliminary design. Also, don't forget to consider the weight of the curb. Sometimes, the curb's weight can be significant.

Deciding where the unit are located is part of the design process. It needs to be coordinated with the other disciplines. It may be that only one location on the roof will work without significant structural modifications. Or, it may be that any location will work without changing the structural construction costs.

 

[Triangled]

Where is it located geographically?

 

[Triangled]

Regarding loading conditions, I have no problem with deducting the roof live load from beneath the footprint of the HVAC in most cases. I don't think the concept is written down anywhere except in forums such as this, but it is completely reasonable to me. You may get wind uplift, especially close to the roof edges, depending on your locality, and if the building is older, it quite possibly wasn't designed for it. Asce 7-10 has a new and fun item in there for RTUs on roofs, Gc uplift = 1.5.

Regarding locations, generally speaking, closer to girders, and closer to columns sees to make for happier joists and girders. Watch for stress reversals in joist webs, ie webs originally designed for tension only may now experience compression depending on the placement of the new equipment. Watch for curb loads not coincident with joist panel points, ie create a new panel point.

 

[jbuening]

Wow, excellent feedback!

H57, is that PDF either of these two:

http://steeljoist.org/LiteratureRetrieve.aspx?ID=37354

or

http://steeljoist.org/LiteratureRetrieve.aspx?ID=37367

msquared, I will contact the manufacturer to get their input. I definitely don't want to affect any warranty.

JAE, thanks for the excellent rundown of loads. And I had a duh moment, the generator is definitely a dead load.

Triangled, it is located in Illinois. Thanks for the information as well as the stress reversals. The building is fairly new, 2007 I believe.

Getting feedback on those who do this more often is priceless and thanks for the tips. Regarding the curbs, what have others done regarding curbs for generators? I know of details for raised curbs to mount HVAC units on, in which they have holes through the roof for the ductwork. This won't have that and will only have a simple conduit through the roof. The roof consists of metal decking with insulation atop that and then the membrane. I'm considering just using pressure treated timber to replace the insulation in the location where the generator would sit, and then cover the timber with the membrane.

 

[SteelPE]

There are many different ways you can look at this. You have the existing joist size which may be to your advantage.

When adding new loads to an existing member I usually consult chapter 34 of the building code. They allow you to change the loads (or stress I forget which one) by up to 5% w/o making modifications to the member in question. If you can keep you loads less than 5% you should be OK (just add the appropriate web reinforcing). You may be able to have the generator placed over a girder or a column which would help you with the 5% limit. The 5% limit is sometimes helpful if the snow loads have changed.

If not then you need to check the the roof members in question for the loading the code would apply today. You have the joist size and there may be a little extra capacity which you can use. If not, I would lean towards adding spliced joists (vs reinforcing the joist) if allowed, and reinforce the girder as necessary. With the 5% limit, girders tend to be more accepting of load than the joists

Loading conditions for the 5% limit are already discussed above.

 

[Triangled]

SteelPE,
just wondering if the relatively recent vintage of the OPs structure, 2007ish, enters into your considerations regarding resorting to chapter 34's 5%?

 

[SteelPE]

Triangled,

I thought the title of chapter 34 was Existing Structures? So if the structure is existing why wouldn't this apply?

In my jurisdiction, they recently increased the snow loads (about 4 years ago). In some areas, the increase is 30% based upon some line the jurisdiction thought was a good idea. So if I had a structure that was built in 2007 and I went to add a new RTU I am running the risk of having the joist, joist girder, column and footings not work because of this increase. So are you suggesting that all items in the previous sentence be reinforced accordingly?

 

[Triangled]

SteelPE,
I'm not really suggesting anything, and your hypothetical is really quite interesting. I was just wondering if the relatively recent vintage of the OPs structure, 2007ish, would enter into your considerations regarding the utilization of chapter 34. You are, of course, correct that it is entitled "Existing Structures", and therefore the OPs resort to this is within the letter of the law. The topic is simply one in which I am curious as to how other professionals look at things.

 

[SteelPE]

Triangled,

Here is my flawed opinion. If the building is complete (building permit closed out) then it is an existing structure.

I have always looked at this provision as allowing the owner of a building to make some minor modifications here and there with out having to bring his building up to the current code. The limit of 5% for gravity member is really not that much. The deeper you get into the structure (joist, joist girder, column/footing) the more and more load you can apply. The probability of your joist failing the additional load from a RTU/Generator is much higher than a joist girder.

If the 5% provision wasn't in the code then I imagine we would all be much busier with a lot of angry clients.

 

[Triangled]

SteelPE

This is the heart of the matter and the opinion I was seeking: "I have always looked at this provision as allowing the owner of a building to make some minor modifications here and there with out having to bring his building up to the current code." Thank you.

You may be perfectly correct, I really don't know. I am pretty sure that none of us are thinking the building becomes instantly 5% stronger the moment the Cert of Occupancy is issued. Building Officials are not that awesome.

With some structural elements, such as steel joists which are engineered by the manufacturers, I am inclined to make a distinction between a 5 year old existing steel joist and a 50 year old existing steel joist. With the sophisticated software today calculating KL/rs to 4 digits and joist manufacturers utilizing angle stock varying by 32nds of inches, I think there is a reasonable probability that the 5 year old joist was designed to 99.9% of the EOR specified and code permitted allowables. When it was slide rules and triangles, I think there is a reasonable probability they were designed to 95%. But for other elements, say a 50 year old wood truss, considering that the allowable tension values in wood dropped around 50% since that time, obviously none of us would say a 5% increase per chapter 34 is ok.

 

[jbuening]

I have another question. I'm familiar with the typical HVAC units where you place angles between joists and angles tying those angles together (around the duct), but not so familiar with units that don't have ductwork. If you were to place the unit centered/directly over a joist and assuming the stresses all check out in the joists, is it as simple as providing the typical vertical angle strut at the concentrated load point on the joist connected to the upper and lower chords? Or do you put angles in tying the adjacent joists together where the curb overhangs the joist to support the curb? The sled that the generator is mounted on is rigid and probably safe to assume the load would be transfered as a point load at the contact points on the joist, but I'm not so sure the curbs are as rigid and could therefore transfer forces from the weight of the generator to the steel decking between joists.

 

[jayrod12]

When we do roof top units we provide angles (or channels depending on the load) below the whole unit curb and then angles around the opening to be cut in the roof. So in this case we would be providing something below the whole unit curb. We usually then try to line up one edge of the unit with the joist so we only have to put angles in one or two joist spaces.

 

[Triangled]

jbuening
I agree with your hesitation. I wouldn't assume anything they provide above the deck will properly transfer loads unless they provide to you some kind of design indicating so. The HVAC contractors I work around say that they can obtain a "structural" curb for more money that would span between joists loads, but that is not what is normally provided. Jayrod12 describes what is similar for us... structural support for the curb which also receives the curb holddowns and also framing for the deck openings as needed.

 

[SteelPE]

If I were to put a new unit on a roof and everything was OK, and I knew the existing deck was a 1-1/2" B deck, I would use a C6x8.2 with the flanges turned down into the flutes of the deck running from joist to joist. Weld the channels through the deck to the joist below. I would place the curb for the RTU on these channels. Pretty simple detail

 

[jbuening]

SteelPE, do you weld the ends of the channels together? That is a very simple detail.

I really appreciate everyone's assistance with this!

 

[jbuening]

Also, by welding the channels through the deck to the joist, is this a full length weld or just a couple small welds to keep it from moving? Do you have them remove a small strip of the deck to get good penetration into the joist or just have them weld directly to the decking and it'll blow through to the joist?