Rooftop Curb Structural Connection

[SunnyEng]

Hi All,

I am designing a rooftop HVAC system and we have a curb mounting system. I need a bit of help with the connection for the curb.

I have seen different designs for the structural system and I was curious as to what the benefits are? Its a carrier unit (50TCD14) and the manufacturer shows the base rail connecting to the curb without much detail (see link). I also have found similar designs but the rail connecting to a z-bracket (see link).

The z-bracket design makes sense. I did the wind loads on the unit. And now I am trying to determine the loads through the bracket and bolt. I have determined the moment at the bracket. I have created a free body of the bolt itself with 2 forces coming down from the Base Rail and reaction forces from the z-bracket. But I am having difficulty placing the moment to produce the correct reaction forces. My thought is to put the moment in between the 2 downward forces on the bolt, does this make sense?

Also with this setup, how do I go about calculating the overturning moment? I am used to concrete pads and since the pads self weight is more than the wind load it passes. In this case we are connecting to the building structure so it will obviously have enough force to prevent overturning, but how is this conveyed in a calculation? Or it can be assumed with a note?

And last question, is designing the z-bracket itself. I could not find a standard chart listing these z-brackets with the appropriate section modulus to determine the maximum bending or moment that it can handle. Is there a resource out there for this?

If there are other connection designs that you all suggest I am open to it.

Thank you for your help!

 

[SoFloJoe]

From my experience following the manufacturer's recommendations is the way to go.

I am not too sure about the Z-bracket, but seems like there should be some specification on this. I see you mentioned that you did some research on this. Maybe someone else here may have an idea.

It sounds like you are on track with the free body diagram of the bolt.

Hope I was of some help. Good luck!

 

[JedClampett]

I think you might be overcomplicating this. Consider the curb as building with four walls, supporting the HVAC equipment. The HVAC manufacturer has hopefully taken into account the environmental conditions (wind and/or seismic) to provide an attachment to the curb. You can do a calculation treating the steel curb as a shear wall and find out that even though it is gage metal, it's pretty strong. Then you get your overturning and lateral forces, resisted by the equipment weight and apply it to the top of the curb. Once again, the gage metal is stronger than you think. If the curb is firmly attached to the roof (sometimes through wood nailers), you're getting there. these usually work through strength in numbers, with a lot of little screws or nails resisting the forces and moments. Lastly, if you're patient, you can check the roof deck or members. But the forces are getting spread out over more and more structure, so the forces are dissipating.

 

[SunnyEng]

Thank you for your responses.

Yes I agree, it felt like I was over complicating it. Yes the overturning on the curb would be sufficient, this was fairly obvious to me. That is why I am contemplating if is it even necessary to do a calc for overturning. But as suggested, I can do the shear wall calculation.

Any ideas on the Z-bracket?

Thanks again!