Can I support a pergola on roof without anchorage? 1

[milkshakelake]

I've been tasked to design this pergola to be on a 12th story rooftop. The wind loads are not bad; it's 98 MPH per ASCE 7-05 and 117 MPH per ASCE 7-10+. Negligible seismic loads. The owners want to do it without anchorage, which means using weights for uplift + sliding.

Is it permissible to do this without anchorage?

ASCE 7-10 says that anchorage is needed. Using 13.4 of ASCE 7-10 for guidance, it says that

Nonstructural components and their supports shall be attached (or anchored) to the structure in accordance with the requirements of this section and the attachment shall satisfy the requirements for the parent material as set forth elsewhere in this standard. Component attachments shall be bolted, welded, or otherwise positively fastened without consideration of frictional resistance produced by the effects of gravity.

This is taken from seismic part of the code, but it makes sense for wind.

However, I'm inclined to disagree with the code. I'm wondering if this is any different from a rooftop table or umbrella. I've seen trees in unanchored planters go on roofs. I'm not sure at what point something I consider to be merely a live load becomes a nonstructural component. I'm thinking that this is a matter of engineering judgment, but if there's code guidance, it would be useful.

 

[gte447f]

I agree completely with BAretired on this. Some of the comments to the effect of saying, "something like this is not likely to lift off and will probably just slide if it moves at all", is unrealistic to say the least. Things like this go airborne all the time at ground level, much less on a 12 story roof. I say anchor it or have nothing to do with it. Give the client sound advice. If they don't agree, let them go their own way without your blessing.

 

[CrabbyT]

I'm in agreement with BAretired and gte447f. Sometimes we have to try and save the client from themselves.

 

[Alistair_Heaton]

What the view on the solar lot and their ballast mounted installations?

There are some utterly colossal racks going in with some serious surface area to catch the wind. And the go to solution is 200kg of ballast every 1.5 meters, 100kg each side. I would have wanted more on the high side.

I have seen some absolutely crazy ballast mounted system in solar social media groups. I usually comment please go and get it checked by a local Structural engineer that knows your local building codes. Have got a few messages back saying thanks, that was 500$ well spent.

 

[AaronMcD]

I tend to agree with "anchor it"! I wouldn't even consider not anchoring something that wind can get under.

Pagodas on grade always end up with a bigger foundation than anyone thinks is reasonable.

Is it going to be obstructed? Could receive much higher loads if there's a wall on one side.

I recall a couple years ago I was going to the gym. It was during COVID and so they put up giant shades outside so people could workout under them. These things were attached at ~16 ft intervals with a tiny little masonry screw. The other end was attached to a ~12 ft tall 4x4 post at ~16 ft intervals. Looked super sketchy, I know a 4x4 barely passes for a 42" guardrail. Sure enough, first storm comes through and breaks down almost every post. Somehow the masonry anchors didn't pull out. Must have been a small storm.

http://www.hellodwell.design

 

[milkshakelake]

With the horror stories mentioned, I'm kind of reconsidering if I want to take this on. No anchorage is a strict requirement by the client. I haven't decided yet, but I think adding some secondary cables to hold it down in case the ballast is insufficient alleviates some of the concerns (though it opens another can of worms with waterproofing). Thanks for the comments!

 

[TylerM94]

The pergola should definitely be anchored. I recently investigated a pergola that blew off a 6 story building into the parking lot below... Give the client sound advice that these types of rooftop structures should always be anchored to the base building structure.