Roof Top Condensing Unit Fastening to Structure 1

[jandlo]

In the midwest, we typically do this for small roof top mechanical equipment. Anchoring through the roof results in penetrations that are prone to leaks and require more frequent maintenance. I've never heard of one moving problematically let alone blowing off the roof or tipping over. Yet an overturning calc using the wind loads provisions of ASCE 7-10 (Rooftop Structures And Equipment For Buildings...29.5.1) suggest this thing should tip over in a typical heavy wind. A 400 lb wind load on the face of a 200lb unit+stand in this configuration shouldn't work.

Is the code ridiculously conservative? Am I overlooking something?

I don't really want to get into theory, but I've had discussions about rooftop pavers and why they don't fly away even though the uplift calcs show they should. Could it be that these forces are for gusts of wind, if attached rigidly, might transfer to the structure, but because it's not, the force is dissipated by infinitesimal rotation of the object? Grasping at straws here. It's frustrating because I always get questioned about my designs and have the code required calcs to back it up, but clients and contractors point to hundreds of real life examples contradicting it. Hard to argue.

 

[jayrod12]

I've seen one almost blow off the top of a 10 storey building. A much heavier, and larger unit. The only thing that kept it on the roof was the electrical wiring. It had tipped over and was resting half on the roof, half out over the edge. Trust the numbers.

 

[jandlo]

I've seen one almost blow off the top of a 10 storey building. A much heavier, and larger unit. The only thing that kept it on the roof was the electrical wiring. It had tipped over and was resting half on the roof, half out over the edge. Trust the numbers.

I appreciate the example but what about the other thousands of them that have been in place for years without a problem?

 

[HTURKAK]

Anchoring through the roof in penetrations is not a good idea .In past i preferred the use of concrete pad over the roof layer to resist OT and sliding.

 

[bones206]

Probably has something to do with the recurrence interval of the design wind load. People only have anecdotal evidence if they’ve been somewhere when a design-level wind event happens. People may not necessarily experience one in their lifetime.

 

[human909]

I don't really want to get into theory, but I've had discussions about rooftop pavers and why they don't fly away even though the uplift calcs show they should.

I would suggest you reconsider how you are calculating uplift on the unit here. I would suggest that uplift here is minimal and if anything downwards pressure is more likely.

Regarding lateral loads and overturning. Yes, it looks like it is at risk and should be accounted for.

 

[ANE91]

You need to stress to your stakeholders how wind works, fundamentally.

Your calcs are for a dead-on wind, broadside onto the unit. You cannot predict when or how this happens. In real life, we have some that don’t tip over because the angle of incident wind is not 90 degrees.

Further, wind irl is considered over a 3-second running average, which is what airports and other ASOS report. This doesn’t mean that you had a design wind event applied for 3-seconds. Wind is extremely turbulent. If an anemometer samples the wind speeds 100 times per second, then there are 300 records over any 3-second interval, and the reported gust is the average of those 300 records. Going back to the fundamentals of dynamics, a fraction of a gust beyond your design wind likely does not have time to mobilize any meaningful response from your structure (RTU) vs. a gust beyond your design wind for a full 3 seconds.

There is also much to be said about spatial correlations. Smaller areas tend to have less variance in the wind load. This is why we calc exterior building elements to C&C pressures. Your calcs approximate this, only.

The bottom line is that your stakeholders aren’t aware of the many RTUs that do indeed blow off of roofs. Contractors build something and move on. They don’t check in 40 years later to see how the building performed. Ask a forensic engineer and they’ll tell you of the horror that is late-stage building performance absent maintenance.

 

[dold]

Florida actually has increased factors for RTU (and other roof top structures) wind loads compared to vanilla IBC/ASCE7. The did this after (Andrew?) hurricanes blew tons of RTUs off of roofs. I've had to spec simpson hold downs for several Florida projects. Its real.

 

[jayrod12]

I appreciate the example but what about the other thousands of them that have been in place for years without a problem?

This isn't a great philosophy to adopt. Just because something bad hasn't happened in many cases, doesn't mean it won't. Like I said, I've personally seen one. Prior to that, I had similar feelings to you, I no longer have those opinions. Anchor it down some way.