Enclosed or Partially Enclosed: That is the question 1

[SmithJ]

Hello All,

I am in the process of designing an airplane hangar for a municipal airport. The building is designed as an enclosed structure per IBC 2000. The architect has raised concerns about the condition where the hangar door is open and if the building should be designed as a partially enclosed building.

Does anyone have any insight on this matter?. I have always been under the impression that structures are designed as enclosed structures unles they have a "permanent" opening. Have I been mistaken? Is there an actual need to design this hangar as partially enclosed?

Thanks in advance.

JS.

 

[ERV]

SmithJ,
In relation to wind loads this structure should be designed as an enclosed structure, unless there is no means of closing up the structure. Interior pressure may help load considerations.

 

[CTSeng]

The question is: If a serious wind event did occur would the hanger doors be open or closed? It should be obvious that whomever owns/maintains the structure would make damn sure the doors are closed.

I used to question whether to design as enclosed or partially enc. until I moved to Florida. In hurricane wind borne debris regions if windows and doors are not impact resistant or have hurricane shutters then the structure is designed as partially enclosed. In almost all other cases I would design the structure as enclosed unless permanent openings warranted otherwise.

Along the same lines I have recently been questioned whether ventilated roofs should be designed as partially enclosed because they allow free circulation of air thru them. Does anyone consider that to be true?

 

[ERV]

CTSeng,
A building designed as a closed structure should be analyzed as a closed structure due to the internal/external pressure differences. In lower wind design areas these differences may be insignificant, howerver in high wind areas this could be substantial and of course, depends on building design and level of infiltration.
All buildings have a certain amount of infiltration, or we could reason that in the case of a tornado, due to suction the windows would break out making this an open structure. If this is done than it would be wise to analyze the time it would take for the pressure drop to take affect due to infiltration. It is the pressure difference internal/external that determines uplift characteristics, loads on windows, etc. Once the pressure is balanced loading decreases.
In high wind areas large errors could result if an enclosed structure is mistakingly analyzed as open or partially enclosed.

 

[ishvaaag]

In our code the condition for constructions with openings greater than 33% is to select the specified (bigger) loads (corresponding to an open condition), or so I think most of my colleagues would sustain (if not pressed by commission or need to such thing assert)is the correct interpretation of the wording in the code. That is, most would think that the ability to keep closed the doors of the hangar under severe storm is not as warranted as to choose the less demanding solicitations.

 

[ERV]

Definitions as per ASCE-7:

Building, Enclosed. A buildign that does not comply with the requirements for open or partially enclosed building.

Building, Open. A building having each wall at least 80% open.

Building, Partially Enclosed. A building that complies with both of the following conditions:
1. the total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building envelope (walls and roof) by more than 10%, and
2. the total area of openings in a wall that receives positive external pressure exceeds 4 ft2 (0.37 m2) or 1% of the area of that wall, whichever is smaller, and the percentage of openings in the balance of the building envelope does not exceed 20%.

Openings. Apertures or holes in the building envelope that allow air to flow through the building envelope and that are designed as "open" during design winds as defined by these provisions.

 

[SmithJ]

Thanks for all the comments. Per the ASCE-7 Definition, there is no direct reference to "permanent" openings. It just refers to "the total area of openings in a wall ..." My thinking is that if a hangar door is stuck in an open position due to malfunction or any other reason and we have a 120mph or greater wind event, there could be damage to a structure that was designed as enclosed. In this case, is the Structural Engineer responsible for not anticipating this condition?

CTSeng, I think I agree with your comments that the Hangar doors should be closed during a high wind event but how common is common sense? And as Engineers, can we be held responsible for not designing against poor human judgement or mechanical failure?

JS.

 

[structuresguy]

Well, here is another thought. During Hurricane Andrew, Homestead AFB in Miami was devastated. Particularly the airplane hangars. Basically what happened is that the doors failed, creating a sudden pressure change inside the building. This caused the hangar to "pop" like a balloon. The hangars were destroyed. Now, this sudden pressure would probably be higher than if the doors were just left open. However, designing as partially open would likely have saved the structure.

I recently designed a 100,00 SF hangar in Orlando International. Even though we assumed the hangar doors would be closed and remain intact, I designed the hangar structure as partial enclosed. Also, the door manufacturer designed the door structure for 125+ mph. This way, if the doors are open and say a freak wind storm rolls thru (as they do often in Florida) then the pressures generated by say 60 mph wind in partially enclosed condition would not pose a problem. The design wind speed was 105 mph. [Int P(60) = 0.55 * 60^2] / [Int P(105) = 0.18 * 105^2] ~= 1 All other things being equal, the ratio from 60 mph partial to 105 mph enclosed is about 1. SO this is definitely a design loading case which should be considered.

I would definitely check out what kind of winds could be expected during a bad storm, but short of a predictable design wind event. You may find that this wind condition combined with open doors may result in higher internal pressures than full design wind with enclosed building. Also, many hangars have glazed walls for natural light. These should be checked for impact resistance to determine if they are design openings. They may result in very large ratios of the wall (or roof) areas.

 

[CTSeng]

Is the owner aware that you plan to make this conservative assumption? I wish I had the luxury of always assuming the worst case senario but the market in which I work owners are aware when the structure is 20% or more over designed than previous structures and will take their business elsewhere if not satisfied.

That seems to be my biggest problem with these forums and engineering in general. There is little unity between engineers everyone is so quick to point out the flaws of another engineers design. There's always more than one way to skin a cat but of course mine is the 'right' way.

 

[dik]

I've been involved with a few hangars (including 747 size) and the general design is to design it as closed and to insure that the user 'closes' the door... The resulting increases in load as a result of the greatly increased pressures from having the door open can result in a major increase in cost for design and construction. Make sure the owner is 'on the hook'!

 

[VoyageofDiscovery]

Hi CTSeng,

Obviously the codes do not address what to do. Perhaps they should address this "hole" (no pun intended). Then at least we will all be consistent.

Regards

VOD

 

[SmithJ]

Structuresguy,

What was the basis of your 60mph for the partially enclosed condition? If you were really designing as a partially enclosed structure, shouldn't you use the design wind load i.e Int P(105) = 0.55 * 105^2???

CTSeng has a good point. I would guess that an owner could be made aware of the cost implications with enclosed vs. partially enclosed and, if willing to assume the responsibility, can then make an informed decision.

Thanks.
JS.

 

[structuresguy]

Personally, I think that if you assume the doors will be closed and state that clearly on your design documents that structure is only valid for wind loads when doors are closed, you will have covered yourself. Of course, like everything else in the world, you can either get a Yugo or a Chevy or a Cadillac. However, if you check doors open with commonly occuring medium velocity winds, you will find that this load case may be equal to full wind velocity load case.

In my case the owner was aware of this design requirement. I mean, lets say a big storm rolls thru and knocks out your power. And the hangar doors are power operated. I guess they would have a manual backup, but now you are relying on backups and manual intervention, and the time required to manually close 6-8 heavy large doors.

When you consider that the planes in the hangar probably each cost more than the entire facility, and the hangar holds 8-10 of them at max capacity, then the cost of a little extra structure really seems inconsequential. Also, with these requirements known to the owner, he can receive a reduced insurance rate for his hangar to cover the cost of the planes inside.

And oh by the way, the bids all came in under budget by about 20%, enough to allow the owner to add another 23,000 SF hangar to the job for the same budget. After all, internal pressure really only affects the roof during uplift. Lateral load resistance system is unaffected since the internal pressure is negated overall, as it acts oppositely on opposite walls. And the footings grow a little for the increased uplift reactions, but not by very much. As for cladding, well that gets stronger as well. But the difference again was not that much, maybe a extra row or two of girts over the height of the building.

I am not trying to say that my way is the right way for everyone or every job. If just happened to be the right way on this job. Obviously, a lot of hangars are designed as cheaply as possible, hence the considerable use of pre-engineered hangars. Of course, if I am the general in charge at Homestead AFB, I think I would have rather payed an extra 5% and still had hangars standing after the hurricane. The difference between a Yugo and a Cadillac is literally cents per square foot.

 

[dik]

VOD
In Canada, the NBC has 3 categories for building wind pressures over and above the general pressure tables.

Cat 1, buildings without large or significant openings, but can have small uniformly distributed openings.

Cat 2, buildings with significant openings which can be relied on to be closed in the event of a significant windstorm.

Cat 3, buildings with large or significant openings through which gusts are transmitted to the interior.

 

[VoyageofDiscovery]

Hi dik,

The question is what to use for airport hangars, as those doors may not be closed or may be inoperable during the event.

Regards

VOD

 

[dik]

VOD
Generally Cat 2, or even Cat 1 with caution... stipulating that maximum wind is 25 mph (for Cat 2). With larger hangars, it's been my experience that the doors are nearly always closed... To appreciate the increase for Cg for Cat 3, it is twice that of Cat 2.

 

[VoyageofDiscovery]

Hi dik,

This is my point, your experience tells you, however others may treat this as a partially enclosed hangar as structuresguy states he did at Orlando International. Thus it appears the codes (US and Canadian) seem to not address loadings for hangars.

Regards

VOD