RTU on Existing Bar Joists - Load Combination Question
RTU on Existing Bar Joists - Load Combination Question
(OP)
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.
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.






RE: RTU on Existing Bar Joists - Load Combination Question
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.
RE: RTU on Existing Bar Joists - Load Combination Question
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
RE: RTU on Existing Bar Joists - Load Combination Question
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.
RE: RTU on Existing Bar Joists - Load Combination Question
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.
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
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.
RE: RTU on Existing Bar Joists - Load Combination Question
H57, is that PDF either of these two: http://steeljoist.org/LiteratureRetrieve.aspx?ID=3... or http://steeljoist.org/LiteratureRetrieve.aspx?ID=3...
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.
RE: RTU on Existing Bar Joists - Load Combination Question
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.
RE: RTU on Existing Bar Joists - Load Combination Question
just wondering if the relatively recent vintage of the OPs structure, 2007ish, enters into your considerations regarding resorting to chapter 34's 5%?
RE: RTU on Existing Bar Joists - Load Combination Question
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?
RE: RTU on Existing Bar Joists - Load Combination Question
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.
RE: RTU on Existing Bar Joists - Load Combination Question
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.
RE: RTU on Existing Bar Joists - Load Combination Question
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.
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.
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
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.
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
I really appreciate everyone's assistance with this!
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
I think they can just burn through the deck. No different than a puddle weld do the TC of a joist.
Triangled,
The channel detail is provided in the Fisher book "Designing with Steel Joists, Joist Girders, Steel Deck".... and the weld you are talking about doesn't need to be much.
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
So you only use beams to support RTU's? Or do you use a different detail to support the RTU's? I would think no matter what detail you used, you would be performing some type of welding no?
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
Triangled, do you typically specify angles beneath the decking which are then welded to the top chords of the joists? Do you cope the channels and weld directly to the top chord, or do you have some sort of gusset welded to the top chord and then weld the regular channel to that?
RE: RTU on Existing Bar Joists - Load Combination Question
http://www.canam-steeljoists.ws/www/v4/epublica.ns...
http://www.canam-steeljoists.ws/www/v4/epublica.ns...
EIT
www.HowToEngineer.com
RE: RTU on Existing Bar Joists - Load Combination Question
Jbuening, for my apart of the question, I often develop a kind of hanger connection going over the top chord and a across the vertical legs of the joist top chord angles. I try not to use a gusset or similar, at least on relatively thinner joist chords as mentioned above, as those also generally involve transverse welds on the axially loaded truss member.
RE: RTU on Existing Bar Joists - Load Combination Question
Triangled, that involves cutting out portions of the steel decking to access the area between the deck and the top chords, correct? I've used details such as what you describe (coped-end angles going over the top chord joist angles), but these have always been new construction and are welded and placed prior to the deck being placed. With existing construction and lack of access to the top face of the top chord, I wasn't sure how this was handled outside of cutting the decking. Attached is a PDF detail used on RTUs for this building, with the coped-end angle to the joist shown in the middle. I really appreciate all of the help!
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
RE: RTU on Existing Bar Joists - Load Combination Question
good job engineering dudes... this horse is dead.
:p