Wind ASCE 7-16

[smokiibear]

We have a small worshop that has an arched roof that will slide to the side for observatory functions. We are attempting to develop wind loads, for an open building. I'm unclear how to derive the loads. Chapter 27 part 1 allows for open buildings, but Figure 3-3 is for arched roofs for enclosed or partially enclosed. I'd think that my only option is to use Figure 27.3-5.

From Chapter 28, part 1, the title says enclosed or partially enclosed. the note adds that chap. 28 is also for open buildings. 28.3 equations refer to Figure 28.3-1, which is for enclosed or partially enclosed, NOT open buildings. 28.3.5 is for "horizontal wind load on open or partially enclosed buildings," but does not account for vertical loads.

Any recommendations.

 

[phamENG]

I'm not sure you have an "open building." Review the definition of an open building as ASCE intends it. You'll still have full walls, right? Your internal pressure coefficient will change, but you're probably closer to a partially enclosed classification. I'd say a building with a kinetic roof isn't going to fit nicely into any category, so you'll have to use some serious judgement.

Another thing to consider - would the roof be open during a design wind event? Doubtful, but could be possible. You'd have to determine if that's a sufficiently probable scenario to consider it in your design. Seems more like a MWFRS design for it closed, and a C&C loading of all the parts and pieces in their various positions with a close eye on serviceability.

 

[smokiibear]

I think those are good points, but the roof in an open position will have 3 sides open, and it will be an open structure while the building uncovered may be partially enclosed. I'm still curious how a wind loads for a dome roof without walls would be derived. The code seems to circumnavigate this particular case.

 

[r13]

Wind load is not mathematically derived before testing. Maybe the umbrella designer can provide you the information you are seeking though.

 

[smokiibear]

Forgive me. Are you joking, Retired13?

 

[klaus.]

not every structure is covered by Codes
that's why wind tunnel guys make good money

 

[smokiibear]

I do understand, but the real point of my question is that the code seems to have some errors regarding open structures. If one follows what I've written, it appears Chapter 28 is a cluster regarding all open structures. There doesn't even appear to be a provision for an open structure in the vertical direction.

 

[r13]

smoklibear,

Not entirely. The structure of umbrella isn't an assemblage of rod and clothe, it is trade marked with spec and calculation to proof its strength and durability for consumer use. Engineers were involved, just like the development of many other consumer products. Anyway, the linked paper may help you to an extent, if not completely. Link

 

[phamENG]

smokiibear - an omission of a particular structural type isn't an error. It's simply that the particular type of building is rare and there has either been little physical research done on the specific of behavior or there has been insufficient interest in the industry for the code writers to wrap what research there is into the book. Remember - the ICC is a for profit company, so they're not going to spend a bunch of time filling out the book with every conceivable scenario as that would cost too much and cut into profits.

For this type of structure, you'll need to dig into some fundamental research on wind/structure interaction and make some judgement calls on just how you think it will behave. A good place to start may be considering the walls as freestanding - the code does have information on that. A walled enclosure with no roof is going to have positive pressure on the windward wall, negative pressure on the leeward wall, and the internal pressures will vary based on the size of the building. For a small one, I would guess negative on the interior all around. Look up wind separation - as the wind hits the wall, the flow "separates" and creates an eddy over the wall. If the leeward wall is at a distance close enough that the flow has not recombined into a relatively laminar flow, then you'll probably have a negative internal pressure. If it's far enough away that the air flow has smoothed out, you'll just have two freestanding walls. Designing as freestanding walls with wind in both directions is probably conservative.

 

[phamENG]

I'll add that for a unique structure a wind tunnel test is certainly prudent. However, you mention this is a "small workshop." To me that's about the size of a 2 or 3 car garage. Probably not worth a wind tunnel. If our idea of scale is significantly different, or if this is going to be used to frequently house public events, even low level wind tunnel testing to validate some assumptions may be worth considering.

 

[canwesteng]

I think retired has an extremely optimistic view of what kind of engineering goes into an umbrella...

 

[phamENG]

 

[r13]

Quite impressive editing. That's how we get fake news

 

[phamENG]

Wish I could take the credit. I think the best part is the fact that there are no vehicles on the road at grade (that would have to drive through the umbrella column). Well thought out by whoever did it.

 

[smokiibear]

I appreciate the insights. Frankly, I'm ok treating the walls as parapets (Chp 27, Part 1), and using the open roof gable coefficients for the roof when it slides off the building (aslo Chp 27, part 1).

What I didn't do well when I posed the initial question was break it down into two questions. One was and has been addressed above by some of your answers, (1) What would be a recommended approach to obtain wind loads for a 16'wide x 25'long x 8' high building with an arched roof that will slide off of it. The building will usually only house telescope equipment, and rarely after set up contain people. It is certainly not going to be conditioned space. The telescopes will be controlled via remote control.

2) has anyone actually gone through chapter 28 to confirm the chaotic way it is written with regard open structures per my original post. There doesn't actually appear to be a Chapter 28, part 1 path for evaluating open structures, except for horizontally. Chp 28, Part 2 is definitely not for open structures. That leaves the only two paths for evaluating open structures is Chapter 27, Part 1, so long as a figure exists in the chapter, or Chapter 31 (Wind Tunnel). Chapter 28 seems to make reference to open structures, but then makes reference to two figures which are not for open structure, except by provision of 28.3.5 for horizontal wind loads only.

 

[phamENG]

I don't have a copy of 7-16, but as I understand it this part didn't change (please correct me if I'm wrong):

Building, Open: A building having each wall at least 80% open. This condition is expressed for each wall by the equation A[sub]o[/sub]>=0.8A[sub]g[/sub] where...(defines A's). (Emphasis mine)

So no, there's no direct way for evaluating a structure with an open roof. All of the open provisions relate to walls, since an open building is defined by it's walls. Typically a structure without a roof isn't classified as a building, and a structure with a kinetic roof is outside the very narrow confines of this pretty limited code.

 

[smokiibear]

I'm refering to the specific wording in 7-16, chp 28.

Also, I have two structures that will combine to make 1. When the roof is off, I have walls with no roof, and I have a roof with no walls. The roof will slide to another structure without walls, just columns.

 

[phamENG]

I think I see what you're getting at. Not sure why the user note says that. Might be something to send to the ASCE committe in charge of the standard to have them fix it. My personal rule is to take user notes as advice, because that's usually all they are. The code says enclosed and partially enclosed - just because somebody added something extra to the user note doesn't change the fact that the code doesn't say you can do it. That seems moot, though, since you said it's an arched roof. None of the seminars/articles I've read mentioned adding arched roofs to the envelope procedure, so unless they did that chapter wouldn't be applicable to your roof shape anyway.

Sounds like the appropriate procedure would be looking at your building with the roof in place as an enclosed building with an arched roof using the directional procedure, then looking at the walls as either free standing or using your parapet approach with no roof and working out a method to look at the arched roof on columns as an open structure. Maybe start with looking at a gable roof open vs enclosed and then seeing what the arched enclosed might look like if compared to the arched open. Should get you close enough.

There may also be some white papers and research reports out there. The code does allow you to use "the literature" for design purposes - you just have to make sure it's well vetted. Here's one to start with: [URL unfurl="true"]http://scholarworks.uark.edu/cgi/viewcontent.cgi?article=3460&context=etd[/url]

 

[smokiibear]

phamENG, I appreciate your response. That's exactly the direction I'll take. Appreciate you taking the time to confirm the confusion!

 

[phamENG]

No problem. I enjoy studying wind loading. There's always more to learn so I enjoy an excuse to dig into it a little more.

Sounds like a pretty interesting project. Come back and post a picture when it's done, or a video of the roof sliding off.