Roof top unit - rigid body consideration

[Samwise Gamgee]

I have a heavy roof top unit which is supported on a steel frame which sits on HSS posts. The base connections of RTU to the steel frame can only transfer shear. As the unit is heavy and the resisting moment is greater than the overturning moment , can we idealize this as a rigid body and design the frame only for shear at the bottom of the unit as shown in the second image ?

 

[EngDM]

You will still have the downward force on point A due to overturning. Just because overturning is resisted from the weight, doesn't mean overturning forces don't still happen. The dead weight on the uplift side exceeds the overturning uplift, such that there is no net uplift (in reality your wind is unloading your dead weight of unit). But for the toe side you still have this downward overturning force.

 

[Samwise Gamgee]

You will still have the downward force on point A due to overturning. Just because overturning is resisted from the weight, doesn't mean overturning forces don't still happen. The dead weight on the uplift side exceeds the overturning uplift, such that there is no net uplift (in reality your wind is unloading your dead weight of unit). But for the toe side you still have this downward overturning force.

Yes, there will be an additional downward load at this location which is not a concern. I am more concerned about the connections at the base as they can only transfer shear. The way I comprehend, the wind load will push the unit and there will be a T/C couple = 11*6/10 = 6.6kips (Load on each side = 20/2 = 10kips). So net at point A = 10+6.6 = 16.6k and net at point B = 10-6.6 = 4.6 (no uplift). As long as I account for the extra downward force, its ok.

 

[EngDM]

Yes, there will be an additional downward load at this location which is not a concern. I am more concerned about the connections at the base as they can only transfer shear. The way I comprehend, the wind load will push the unit and there will be a T/C couple = 11*6/10 = 6.6kips (Load on each side = 20/2 = 10kips). So net at point A = 10+6.6 = 16.6k and net at point B = 10-6.6 = 4.6 (no uplift). As long as I account for the extra downward force, its ok.

That's how I would do it. The cross member you have from beam to beam acts to relieve the torsion (albeit very small) that comes up from applying the shear load at the top of beam.

 

[dik]

and in these environs there would be a substantial snow load accumulation affecting the roof for several feet around the unit.

 

[Harbringer80]

I don't really get the purpose of the question. Are you trying to avoid using the appropriate load combos?

Apply the loads (with code required load factors), get the reactions, and apply those reactions to the steel frame.

 

[BulbTheBuilder]

I don't really get the purpose of the question

OP is trying to design the RTU base connection as pinned (shear) not fixed (no moment), if I understand correctly.-->Axial forces on post together with lateral shear.

 

[Samwise Gamgee]

The unit is just sitting on the frame without any welds. So I am trying to analyze it as it is. If its not connected at the bottom , it cannot transfer any moment directly into the frame. So I wanted to resolve the moment as a T/C couple and check the beams for additional compressive load instead

 

[Samwise Gamgee]

and in these environs there would be a substantial snow load accumulation affecting the roof for several feet around the unit.

How would that impact the lateral load ? Are you referring to snow drift potentially swaying the frame laterally ?

 

[Harbringer80]

The unit is just sitting on the frame without any welds. So I am trying to analyze it as it is. If its not connected at the bottom , it cannot transfer any moment directly into the frame. So I wanted to resolve the moment as a T/C couple and check the beams for additional compressive load instead

What do you mean it's not connected at the bottom? Is it bolted to the beam?

Regardless, yes I would get a T/C for overturning. Still not clear what other way this would go? This is just a simple statics problem unless I'm missing something....

 

[Samwise Gamgee]

What do you mean it's not connected at the bottom? Is it bolted to the beam?

Regardless, yes I would get a T/C for overturning. Still not clear what other way this would go? This is just a simple statics problem unless I'm missing something....

If its bolted to the beam, it will only transfer shear, It cannot transfer moment. As of now, its not even connected, (so I am telling them to at least bolt it). The overturning force will try to topple the unit and it will add extra compression/tension on one end of the beam. On the tension side (uplift), the unit is so heavy that half of its weight offsets the uplift.

 

[EngDM]

How would that impact the lateral load ? Are you referring to snow drift potentially swaying the frame laterally ?

If the frame is up higher than the snow pack you actually get a windswept condition under the unit. I wouldn't put any additional load laterally due to snow on the side of the unit.

 

[dik]

How would that impact the lateral load ?

Not at all, it was just a 'heads up'. Snow accumulation can often exceed the weight of the RTU.