Wind loading on low rise buildings with large openings at gable ends 2

[Motorspirit]

All

We are often asked to design industrial portal frame buildings for use as loading facilities for trucks etc. Typically these are long rectangular single story buildings with gable roofs or similar. They are fully clad along the side walls and roof but fully open at both ends to allow the trucks to drive in and out.

Due to the large openings we have typically treated both side walls as "windward" as a conservative approach to allow for wind that may hit the building slightly off perpendicular to the side walls (ie cross wind). Although there would certainly be some shielding from the first wall we often dont account for this.

I would like to improve on this design method and would be interested in hearing how others treat these type of structures for cross wind loading.

Appreciate any feedback/ advise

 

[msquared48]

Open Structure, definitely.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Surely you would allow for shielding and not apply double the windward wall pressure. Think of it this way...if the wind is at 45 degrees, the sum of windward pressures on the two surfaces would be 1.4 x the single wall pressure. For smaller angles, affecting the ends of the building, you could have larger pressures.

 

[Ron]

Well....actually it is a partially enclosed structure. In order to be an open structure, at least 80 percent of each wall must be open.

Fairly often such buildings are in relatively open areas and should be designed under Exposure Condition "C", not "B" as is often used, keeping in mind that if any upwind direction has 600 feet or more of unobstructed space, then "C" must be used. Obviously, design for the condition now, not what might be in the future.

Your approach is probably overly conservative. Do your analysis with the wind "face on" and oblique and you'll see the actual difference. You might be overdesigning by 30 or 40 percent.

 

[hokie66]

Some of the terminology is a problem here. Might help if the OP identified which country and code he is working to.

 

[Ron]

Hokie66...good point. We jumped the gun here again and assumed ASCE 7. The OP shows NZ, so closer to Oz than here.

Apologies to the OP for assuming!

 

[Motorspirit]

Thanks guys, some good points here. Ron is correct, we normally design structures in NZ and Aus. In parts of Aus winds are often cyclonic and hence we chose the conservitive approach. AS1170.2 has a couple of ways to deal with shielding. 1 is a multiplier to deal with near by buildings, the other is used for multiple open frames.In this case I could use the former and treat the windward wall as a neighbouring building. I will investigate more.

Thanks

 

[rowingengineer]

I like Hokie's way of thinking. I think applying the loading twice is very heavy handed. The way I see it, is at 0-22.5 deg you would develop a air bubble in the middle of the building thus you will get some varying internal pressures but no ability to generate internal pressures. at 77.5-90, the walls should be treated as free standing as per D2.1.

so this leaves the 22.5-77.5 which is up for grabs, I I would suggest you read- Holmes, J. D. (2000) ‘Wind loading of parallel free-standing walls on bridges, cliffs and embankments’, he also talks about it in his book, "wind loading of structures".

He shows that with closely space free standing walls you have a reduction in the Cpe. he also show that a elevated compared to a non-elevated wall there is an 80-90% difference in the loading, thus I think the roof would also reduce the Cpe from a free standing wall n ground. thus I normally use engineering Judgment based on the length of the building and spacing of the walls and a roof reduction, however normally give the walls are very closely spaced compared to height for the truck stop, I find the worst case is as defined by the code for cross wind however the main conservative step I take is I adopt a higher Kl than required by the code.

However to get some real advice as to the interpretation of the code send John H (or Cam Leitch/John Ginger from JCU) an email, they all are normally very helpful. Well I wouldn't mind knowing the answer as well so I might send him an email for kicks.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[rowingengineer]

sorry when I wrote wall on ground compared to bridge I meant to write hoarding compared to bridge.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[msquared48]

Ron:

Initially, I read too much into the OP's post here.

Nevertheless, check ASCE 7-05, 6.2 under "Building, Partially Enclosed", specifically the latter portion of note 2.

I do not know the dimensional particulars of the OP's building here, but the way I read the note is that if the sum of the two endwall areas is 20% or greater of the total wall area of the structure, then it defaults to an open structure. If not, then it is partially enclosed.

Please correct me if my interpretation is incorrect.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

Mike,
The default, if it doesn't meet the criteria for a partially enclosed building, is an enclosed building, not an open building (see commentary on 6.2). Further, the intent of defining an "opening" is one that only receives positive pressure.

Ron

 

[msquared48]

Thanks Ron for making that clear.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

The NBC has external and internal wind coefficients for low rise buildings with various roof slopes. The windward wall has a positive coefficient. The leeward wall has a negative coefficient indicating suction. The magnitude of these coefficients varies substantially with the slope of the roof.

Large openings on the end walls could contribute to internal pressure which might affect the design of cladding and roof structure but would not affect the lateral force on the portal frames.

BA