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Anchorage of roof deck furniture and accessories on an assembly space type roof deck.
4

Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

(OP)
What is the best way to calculate suction forces on rooftop furniture? For example, 18th floor of a high rise building (250 feet high), they want to put a roof paver system and outside deck furniture consisting of aluminum chairs and tables, lounge style sofas and a bar. If you use the ASCE7 components and cladding forces, these items would literally fly off the roof (I am being dramatic intentionally!).

Are there other design procedures, beyond wind tunnel testing, that are available that reduce these wind pressures? Or am I on the right track in the assumption that roof deck furniture, such as that described above, simply need to be anchored down (or ballasted?) to the roof structure to resist the ASCE7 design wind pressures? The issue clients would have is that now the furniture cannot be moved, which they find as a significant drawback.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

1. ASCE 7 wind is a design level wind force. With 90 mph speeds (or even 50 mph speeds) not sure anyone would want to be outside during that kind of event.
2. For their furniture, no anchorage is needed if a) wind is slow/moderate or b) the furniture is cheap or c) they are disciplined to bring the furniture in or tie it down every time there is a big wind.
3. Even if you used a wind tunnel test, the wind speed going from 90 mph to 65 mph would still require furniture tie-downs.
4. As a structural engineer I'm not sure why I'm even thinking about furniture.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

This is an issue during extreme wind events I imagine? Is the pressure still an issue with 10 year recurrence interval wind loading? Perhaps its a sign posted near the terrace that furniture must be stored / tied down during storms? It seems like a tricky thing to manage / control as a design professional.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

I'd think that one of that main concerns would be things flying off the roof and landing on pedestrians, as with construction materials. Any chance chains or cables might be pallatable? Probably just trip hazards more than anything.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

The first question I would ask if I were in your shoes is: is rooftop furniture even in the scope of what you are doing? In a 25+ year career, I've never issued drawings once that said anything about furniture (or their anchorage).

If it is part of your scope, I'd have some kind of "catch" system set up where it couldn't happen. (Similar to something to stop jumpers.) Having it tied off to cables or something would probably be too cumbersome.

As far as wind loads go.....the values for components and cladding in ASCE 7 would probably be sufficient. (As they tend to be high.)

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

Miami Dade County has requirements for elements that are part of the building enclosure which go beyond the base requirements of the Building Code. These assemblies must be tested to certain wind pressures, and are provided with a NOA. Identifying a paver system with an applicable NOA which meets the wind pressures for the building is a common requirement of design professionals in South Florida, especially for the type of construction you have mentioned. I have not had to deal with any furniture, it is assumed that during a wind event these items will have to be sheltered. Elements such as trellises and light poles on amenity decks are designed for wind pressures as appropriate.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

The self weight of the furniture must be able to counter the lift force, overturning should be fine, though not desirable. Otherwise fasten the furniture or weight them down.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

People install ballasted solar arrays on roofs all the time.

Consider using ASCE 7 to develop maximum roof suction loads (use rooftop equipment section) then bolt some concrete to the furniture.

It's no concern if the furniture slides around since there will be guardrails around the edges of the habitable rooftop area, and if anyone is up there during the design wind event, god help them.

Then, so long as the dead load is at least 1.5 times the ASD level wind uplift, you know the furniture won't be leaving the floor.

This will likely give you some hilariously heavy furniture, but it won't be going anywhere.

=========================
David Reber www.kadent-structures.com

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

Wrought iron furniture usually works.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

Anchoring to a paver system is not a particularly good idea. While the paver might have some level of building code approval (such as the Notice of Acceptance (NOA) mentioned by EZBuilding) it is not tested with anything anchored to it. You might consider alternate anchorage for the furniture. Treat the furniture as you would any rooftop appurtenance under ASCE 7

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

(OP)
In this particular case, the landlord was directly asking the tenant, who also happens to be a law firm, to submit signed and sealed structural calculations stating the furniture would not "fly off the roof" and was adequately anchored to the roof. As a landlord, their primary concern is an instance where something does fall off the roof and injures a pedestrian below (JAE). I provided a detail, in the end, to anchor the furniture down and comply with code, but it was far from ideal and far from what an architect would deem "aesthetically pleasing." I did suggest posting a sign stating similar to what structSU10 mentioned, but neither landlord or tenant wanted to assume that risk themselves. I thank you all for your shared input, it is very helpful and valuable. If there are any other insights, please continue adding to this thread! Thank you.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

The only solution which I have seen to work for rooftop furniture is to store it when not in use. This requires a storage structure at that level, and vigilance on the part of the users to put the furniture away.

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

While it's not going to satisfy anyone's liability shedding desires, I had the same thought as retired13 regarding the cast iron. I live in an extraordinarily windy city and am too lazy to ever bring my stuff in. So we got a cast iron chair and table set. These things are just how you'd imagine them to be: a bit heavy and at least 2/3 open to allow the wind to pass through. The chairs aren't going anywhere, even on top of the Burj Dubai. The table might catch flight in a hurricane if the wind grabbed the glass top eating surface.

Lawyering up aside, I feel that a very practical solution would be cast iron chairs and tethered tables. And sure, for sport, tell users to bring the chairs inside during a hurricane.

The one I always seem to get saddled with is tying down treas on high-rise green roofs. You know, smart stuff like a blue spruce in a little box between balconies at the corner of the 30th floor of a building downtown with 18" soil depth...

RE: Anchorage of roof deck furniture and accessories on an assembly space type roof deck.

Maybe you could run a continuous aircraft cable along the base of the parapet wall, similar to a safety/fall protection line, anchored every 8ft or so and then tether the furniture by the leg to the cable, with other smaller cables. That way the furniture would still be kindof movable, but it can't fly away (ideally)? That avoids anchoring to the pavers, and is slightly more aesthetically pleasing than screwing conduit clips around the leg of the pool chair to the paver.

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