Placing RTU on the roof of Prefab. Metal Building 1

[1asdfghjk]

Hello Engineering Community,

A mechanical engineer on our team asked me about the possibility of placing a 600 Ib RTU on the roof of a prefab metal building. I'm not sure what the reinforcement of the Z-purlins would look like, but given the structural inadequacy of the Z's, I'm hesitant to put such a unit on the roof. If any of you have had an opportunity to encounter such an engineering problem and come up with a solution, I would appreciation if you could share. Thanks!

 

[LittleInch]

PEMBs are usually built with zero allowance for any additional force.

But why don't you just assess the structure which only you can see.

But reinforcement is almost certainly required for such a load. If nothing else I suspect the roof will just act like a amplifier once the compressor kicks in...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[271828]

600 lb isn't much, even for zee purlins. It's like three normal guys standing on the roof; I don't think anybody would be concerned about that.

That said, if you're not into cold-formed steel design, the calcs might take some time.

Also, if the roof is standing seam, you can't count on it to brace the zee top flange. You could provide something else to brace them, such as bridging made of sheet metal straps or light angles.

Instead of using the zees, you might consider adding some light framing between the zees and count only that for the support of the RTU. The additional members could be CFS channels or W-shapes.

I'd be hesitant to punt to "it can't be done." They'll find someone who will make it happen. Years ago, we won a new client that way. An engineer was refusing to sign off on placing a 500 lb piece of equipment on existing precast planks because he supposedly didn't have any way to calculate that it would work. We took a quick look at it and made it happen for the client. It didn't take much to show that it would work.

 

[LittleInch]

What's the span between the rafters and where is the load?
How many Z purlins and spacing?
Definitely need to brace the purlins or add a spreader plate, but local reinforcement of the purlins looks like it is needed.
you might need to add a couple of purlins in between the existing ones to add some load capacity.

I wouldn't put three men in the same location on a roof like that.

A drawing . sketch or photo helps hugely to understand what you are looking at.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[phamENG]

Around here, it's rarely the weight of the unit that's the problem. It's the overturning reaction from wind load. I've never been able to prove even a lightweight RTU can sit on a PEMB without significant reinforcement. Distributed loads are a little easier (if they're small), but point loads are always problematic.

This is a good primer on some approaches. But don't attempt it if you don't know what you're doing. These buildings can be troublesome.

 

[XR250]

600 lb isn't much, even for zee purlins. It's like three normal guys standing on the roof; I don't think anybody would be concerned about that.

I would be very concerned about that.

 

[271828]

LittleInch and XR250, if you and two of your friends were on site, you wouldn't all three stand on a metal building roof and talk?

Granted, it's not like these are concrete slabs, but they're not that weak.

 

[LittleInch]

Does your Roof Top Unit need to be put in something horizontal?

Is your roof flat or pitched?

This is why a sketch / drawing is so useful...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JLNJ]

Put it over a girder. Use one of those flashing boots made for standing seam roofs. Tell the Owner the roof will probably leak anyway.

 

[XR250]

LittleInch and XR250, if you and two of your friends were on site, you wouldn't all three stand on a metal building roof and talk?

1) I weigh 150
2) Not with a snow load on the roof.

There is very little fat in these buildings. It needs to be analyzed properly regardless.

 

[CANPRO]

Around here, it's rarely the weight of the unit that's the problem. It's the overturning reaction from wind load.

Similarly, if the RTU is large enough to trigger snow drift considerations that often requires some local reinforcing. Even if you place the RTU over a girder as suggested by JLNJ, the snow drift can extend far enough out over the purlins that it requires reinforcing.

To minimize reinforcing I've always tried to place the RTU near or over a girder, and orientate the "snow-drift" side of the RTU (if applicable, and only one side causing drift) to cause the drift to run parallel with the girder. Unfortunately, you don't always have flexibility on the RTU location.

 

[LittleInch]

Nope, I wouldn't.

Far too much point load.

We haven't seen anything like a picture of what we're looking at yet, but most of the PEMBS can barely resist rain water.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JedClampett]

I wouldn't do it. Unless you get the original manufacturer to buy in (and you won't), you're going to own the building for posterity. Anything that goes wrong (a leaky door, excessive deflection) will be your fault for adding that equipment up there. How much money is your fee for taking that on?
Once you dig a little, you're going to find that the original design was sketchy in some way (snow, wind, something), and then what do you do? Tell the owner their building never worked?

 

[Aesur]

A mechanical engineer on our team asked me about the possibility of placing a 600 Ib RTU on the roof of a prefab metal building. I'm not sure what the reinforcement of the Z-purlins would look like, but given the structural inadequacy of the Z's, I'm hesitant to put such a unit on the roof. If any of you have had an opportunity to encounter such an engineering problem and come up with a solution, I would appreciation if you could share. Thanks!

Can you walk me through exactly what your concern is and why? What is your concern with the Z? How would you approach strengthening versus just adding new members? Do you believe the portal frame/girders are adequate, how would you go about checking these? Please walk through the steps and I believe you may find your answer very quickly.



That said, it's quite common to add mechanical units to a PEMB roof, although not always easy, but also not all that hard. Your profile says you are in the UK I believe - so I'm not familiar with the EU codes as they relate to existing buildings, so I can only speak to the US codes. Typically, the simplest method is adding new framing for the unit that spans girders. I assume you would need this regardless for unit support between purlins, it is highly doubtful it will fit perfectly on the existing purlins and you will need to frame the openings correct? As for the girders, many times these are checked with the existing building code allowable stress increases, normally girders are of such large trib area that when even using live load reduction it still works.

 

[driftLimiter]

I've had luck in the past just adding two WF beams up in the purlin space. The Main frames are generally controlled by lateral loading (or snow loading in my area).

 

[dik]

Snow accumulation is often a problem, even with normal steel buildings. Your PEMB likely has no capacity for overload. I consider them 'throw away buildings' after about 20 years. I spec additional design loads for them when I use them to accommodate this type of application.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[1asdfghjk]

Thank you very much for your expert comments, considering all the above concerns, we have come to the conclusion that placing the RTU on the roof is not feasible at this time due to the time constraints where the project is in its final phase. The unit will be placed on the ground.

 

[dik]

Good choice...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik