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.

 

[steellion]

I just wanted to add that per ASCE 7 it is not necessary to consider snow drift for sides of the mechanical unit with length < 15 feet. I consider permanent mechanical units where the unit dimensions and weight are known to be dead load, but I add live load on top of the unit in snowy regions (snow can build up on top of the unit).

 

[Triangled]

i like the channel on top of the deck detail for wide flange beam framing but usually avoid it on joist framing because of the challenge of field welders providing transverse welds on small cross section joist chords.

 

[SteelPE]

jbuening,

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.

 

[Triangled]

Yes, I'm familiar with and have several of Fisher's books. I am just aware that a lot of these joists have rather thin top chord angles. The concern about a possible 1/16" undercut across the horizontal leg(s) of, say, a 3/16" thick angle(s) by an inexperienced or distracted field welder, reducing the cross section by 8% if just one leg undercut and 16% if both legs undercut, is enough cause me to shy away from that detail. Even more so if there is significant wind load on the RTU, both lateral and uplift, increasing the demand and therefore size of that weld. I am not opposing the use of the detail, I do use it and will use it for most wide flange framed roofs and may occasionally for steel joists if the chords are of substantial size or understressed by calculation. Perhaps I am too untrusting of field welders or perhaps I am mistaken regarding the ease of undercutting, but, that's my 2 cents anyways.

 

[SteelPE]

Triangled,

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?

 

[Triangled]

We use welding of course. It is the transverse welds on the predominantly axiallly loaded members of premanufactured steel joists that we studiously try to avoid.

 

[jbuening]

Good info guys. Lets say we utilize the upside down channel like in Fig 5.4.2 in the Vulcraft manual to span between joists. What is used in the opposite direction (parallel with the joists)? With an existing structure, its not like we can provide blocking on the underside of the decking like shown in Fig 5.3.1. Assuming we have even distribution around the perimeter of the structure, there will be 115 lb/ft (not including curb weight) of force running the length of the girder. I don't think that is enough to crush the decking, but thought I would ask. Having a channel directly atop the joist (welded) would strengthen the joist as well, but one would have to cut into the decking ribs which would weaken the decking's ability to resist negative moments at the joists.

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?

 

[RFreund]

Although I would like to hear the responses to jbbuening's question here is some information on 'crushing':

http://www.canam-steeljoists.ws/

http://www/v4/epublica.nsf/va_doc/853084824DA4FD218525796B004D927C/$File/Avoid1.pdf

http://www.canam-steeljoists.ws/

http://www/v4/epublica.nsf/va_doc/94D3B59D30B5C0C98525796B004D927D/$File/Avoid2.pdf

EIT

http://www.HowToEngineer.com

 

[Triangled]

RFreund, useful info, thanks.

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.

 

[jbuening]

Thanks for the info RFreund!

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!

 

[Triangled]

I'd personally revise (for steel joists) the frame to top chord weld from "all around", to, just the line parallel to the chord. For existing construction they can perform a similar weld from the underside, parallel to chord.

 

[jbuening]

Those details are from the existing plans. So do they cut the decking or is it located somehow between puddle weld and slid between the joist and decking? Sorry for all the questions.

 

[jayrod12]

We do a detail similar to the one you posted however we continue the angle complete over the joist/beam to the other side and weld a vertical steel plate to the angle creating a saddle over the joist/beam. That way no welding is required to the joist unless uplift resistance is required. we are yet to have a problem with the contractor sliding the steel angle between the decking and the joist.

 

[jbuening]

One last question for the masses. I've decided to use the inverted channel atop the steel deck since this generator needs isolators at each corner of the skid and the spacings on the isolators is within a couple inches of the joist spacing, so the channel just made sense. My question is, when dealing with something that vibrates such as this would it be a good idea to weld the inverted channel to the steel deck? My thought is that it will prevent the channel from moving on the deck. There will be ISO insulation all around it but that isn't likely enough to hold it in place from vibrations. I don't want this thing moving around and tearing the EPDM roofing. Just curious if anyone typically does this or avoids it for a reason I may not be thinking of. Thanks!

 

[TDIengineer]

One heck of a thread for unit on a steel joist roof!!

good job engineering dudes... this horse is dead.