Internal pressure - open front buildings 5

[Mark.Martin]

Hi,

What internal pressure coefficient value cpi should be considered when calculating wind action on open front building according to eurocode?

Kind regards.

 

[r13]

First to say is I have no intention tp "mislead" anybody on this issue, but to offer my understanding, which admittedly can be an unintentional mistake due to misunderstanding. Secondly, I do recognize this is an Euro Code issue, so I've asked the OP to verify the definition of "open building by his code in the very beginning. ASCE is only provided for information on the US practice. Below is a discussion and example applications of internal pressure.

Internal pressure (i.e., building pressurization/depressurization). Wind striking a building can cause either an increase in the pressure within the building (i.e., positive pressure), or it can cause a decrease in the pressure (i.e., negative pressure). Internal pressure changes occur because of the porosity of the building envelope. Porosity is caused by openings around doors and window frames, and by air infiltration through walls that are not absolutely airtight. A door or window left in the open position also contributes to porosity.

Wind striking an exterior wall exerts a positive pressure on the wall, which forces air through openings and into the interior of the building (this is analogous to blowing up a balloon). At the same time the windward wall is receiving positive pressure, the side and rear walls are receiving negative (suction) pressure; therefore, air within the building is being pulled out at openings in these other walls. As a result, if the porosity of the windward wall is greater than the combined porosity of the side and rear walls, the interior of the building is pressurized. But if the porosity of the windward wall is less than the combined porosity of the side and rear walls, the interior of the building is depressurized (this is analogous to letting air out of a balloon).

When a building is pressurized, the internal pressure pushes up on the roof. This push from below the roof is combined with the suction above the roof, resulting in an increased wind load on the roof. The internal pressure also pushes on the side and rear walls. This outward push is combined with the suction on the exterior side of these walls. Therefore, a pressurized building increases the wind load on the side and rear walls (see Figure 4) as well as on the roof.

 

[Settingsun]

12m/30m = 0.4, not 0.3 as used by Hturkak so the final solution changes slightly because of that, but not enough to worry about.

For the 90 degree wind, with the open wall on the side (parallel to the wind), I'd take Cpi about -1.0. This is based on zones A and B from Fig 7.5 and Table 7.1 in the eurocode. Some judgment needed because the opening spans several zones.

For 180 degree (opening downwind), Cpi = 0.9*-0.34 = -0.31.

(I don't use eurocode so please check for yourself - ie read the fine print. The 90 degree case is more than my local code and the British code you posted earlier.)

 

[r13]

Besides the code, a review on internal pressure due to fluid medium on enclosed space may help.

 

[rowingengineer]

R13 what does that even mean? Are you suggesting a cfd analysis is required?

If you are suggesting such on a standard building then I think a sledgehammer on a nail comes to mind.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[r13]

Physics is something we experiences daily. For this matter, blow air into a balloon and image the pressure on the enclosure is the most simple example at your finger tip without the need for a hammer. Sure, you can use one, if you want to complicate it with air dynamics, drags..., which could be implied by the code by coefficients.

 

[rowingengineer]

I don't think a practicing engineer is going to get great results on the CPI coefficient from visualisation of air flows from a balloon. All I get is that the CPI should be equal to cpe to create a static dynamic, however I don't think that houses behaviour is similar. The code that he is using handles the problem as stated, it isn't difficult, no need to be using some abstract concept that is clear as mud to solve.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[r13]

If you are certain the code provision apply to your case without any question, then the argument is closed. My simplification is meant to serve as a reminder of the worst case scenario, that could only lead to an err on the conservative side. I wonder why it is so hard for you to swallow.

 

[Tomfh]

This is as standard a case as it gets. The simple code rules are appropriate. Cpi should be approximately equal to Cpe at the position of opening

 

[r13]

Cpi should be approximately equal to Cpe at the position of opening

.

Agreed.

 

[rowingengineer]

R13, i don't generally swallow stuff that I cannot understand what the intended point of the statement is ment to imply. If you were just trying to say what tomfh was stating a few times in the thread, you went about it in a most strange fashion.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Ron]

@rowingengineer....YES! Star for you!

 

[r13]

I think I've pointed it out on previous occasions as well, but people like you do not read.

r13 (Civil/Environmental)3 Jan 21 23:31
I don't know how you have determined the coefficient is 0.72 at D, but yes, the pressure calculated at the entrance should be applied all around and pointing to the internal surfaces. Now the interesting point is how the wind load is distributed on the external surfaces of the roof and wall E. Is this an air field hangar?

r13 (Civil/Environmental)3 Jan 21 13:02
IMO, if not addressed directly by code, you shall conservatively use Cpe for both internal and external. I don't think internal pressure exist, or effect little, when wind blows in direction normal to the opening (like blow air into the balloon).

r13 (Civil/Environmental)3 Jan 21 01:25
Without refer to any code, I think it is an open building, isn't it?

 

[BridgeSmith]

I'm not familiar at all with the eurocode, and only vaguely familiar with the ASCE. I may be wading in over my head, but based on what's been presented here, it seems that Cpe ranges from +0.8 to -0.4, and the Cpi ranges from about +0.85 to -0.6, depending on the direction of the wind, which is typically assumed to possibly come from any angle. The appropriate values of the 2 get added together for a given wind direction.

Again, from what I see presented in this thread, for the building pictured, the max total pressure coefficient (Cpe - Cpi) on the back wall would be about -0.4 - 0.85 = -1.25, and the max pressure coefficient for the side walls would be 0.80 - -0.60 = 1.4. Seems odd that the side walls would be higher, but Table 18 above would seem to indicate that there can be a greater suction on the inside of the building with the wind blowing approximately parallel to the opening than suction on outside of leeward end of the building - possibly a venturi effect?

I could be way off base, but that's how it seems to make sense to me. AASHTO really only addresses flat or solid structures (signs & traffic signals) and round shapes (tubes & poles), and dumbs it down to a single factor, Cd, which varies between 1.1 and 1.2 for most of those flat and solid shapes.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[rowingengineer]

R13, people like me don't read or can't read? Or is it an entirely different situation, where we care about nuances and hope that a person can come to this site and leave a better engineer.

First of all let's get a few items corrected for the above posts.

1. r13 (Civil/Environmental)3 Jan 21 23:31
I don't know how you have determined the coefficient is 0.72 at D, but yes, the pressure calculated at the entrance should be applied all around and pointing to the internal surfaces. Now the interesting point is how the wind load is distributed on the external surfaces of the roof and wall E. Is this an air field hangar?

Cpe and Cpi are coefficients in the code we are referring too, as the pressure external and internal are not equal as stated in your post. nothing could be further from the truth, this is due to average along the opening that occurs in a building like we are referring to, as the opening gets large the less the impact of local peaks, if we really want to hit this home we need to talk about frequency, but I don't think air slugs and frequency is required for today. This is why in some wind codes cladding had a pressure double to the main frame. The cladding can be subject to the peak pressure. They will also have modification factors that will vary between internal and external pressures often. If you had a header sheet like this building does the two design pressures will be different.

2. r13 (Civil/Environmental)3 Jan 21 13:02
IMO, if not addressed directly by code, you shall conservatively use Cpe for both internal and external. I don't think internal pressure exist, or effect little, when wind blows in direction normal to the opening (like blow air into the balloon).

Again this has an error or two, the internal pressure can govern in this type of building when blowing from other directions depending on the governing code for live and snow loads, if you code requirements have a combination requirement then it can govern. The header sheet must be designed for these pressures as it will govern the header sheet design.

I am still at sea with this Ballon theory.

3. r13 (Civil/Environmental)3 Jan 21 01:25
Without refer to any code, I think it is an open building, isn't it?

This is wrong, refer above, the building isn't open it is a partially enclosed building in your code asce16.

The only part I agree that is correct is when you copied and pasted tomfh post, that is reasonable.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[r13]

rowingengineer,

Your last point has let me know why I deserve the criticism because the mistake I made in the very beginning. Thank you very much. I shall keep my big mouth shut, but watch carefully on the sideline on further development of this thread as a leaner.

 

[HTURKAK]

Pls find below internal pressure (cpi) for angles 90 and 180 degrees;

for angle 90 degr. assuming clockwise rotation, ( on your sketch , west to east ),


elevation e ≤ d (e=30 m, d=32 m, h=12 m)

the widths of zone A,B, and C= (24/5),( 4*24/5),( 32-24)=4.8,19.2,8 meters..

External pressure coefficients Cpe for zone A=-1,2 , B= -0,8 and C= -0,5

and weighted average for WALL D assuming closed wall, CpeD= ( 1.2*4.8+0.8*19.2+0.5*8)/(32)=-0.6725

and Cpi= 0,90XCpe = -0.6


for angle 180 degr. assuming clockwise rotation, ( on your sketch , North to South),


h/d=12/30= 0.4 ,
for h/d = 1 +0,8 , -0,5
for h/d ≤ 0,25 +0,7 , -0,3



For wall D = 0.3 + 0.2*(0.4-0.25)/0.75=-0.34

and internal vacuum pressure when wall D is fully open, Cpi=0.9*0.34=-0.31

I hope this respond answers to your question . Pls post the total final figures for each wall , roof etc acc to wind direction.



Your approach is not correct. The use of Fig 7.13 (Internal pressure coefficients for uniformly distributed openings ) is explained at (6) For buildings without a dominant face, the internal pressure coefficient cpi should be determined from Figure 7.13, and is a function of the ratio of the height and the depth of the building, h/d, and the opening ratio µ for each wind direction θ, which should be determined from Expression (7.3).





Thank you for your correction.. Typo mistake but changes... slightly or not.. I have corrected the figures as per your warning.

 

[hetgen]

@HTURKAK I did state that the fig7.13 can only be used if the building has at least 1/2 the size of the main opening on the side of the building and roof eaves.

I think the building pic does show a long eave opening on the left side, if that is the case and if the communicative area is ~1/2 x the front opening then Fig 7.13 is valid and the calculation you present above is conservative.

 

[HTURKAK]

Dear HETGEN,
The picture at original post depicts IMO, a typical case for front face open structure. The sketch posted by OP , has flat roof with 3 sides closed. The OP should clarify the situation. However, if there are windows etc. at roof and other walls, still the design shall be based on for closed windows case..

The use of Fig 7.13 (Internal pressure coefficients for uniformly distributed openings ) is applicable in cases for , without a dominant face.. If there is an external opening, such as a door or a window, ( in this case there is a dominant face, the front face is full open ) would be dominant.

 

[hetgen]

@HTURKAK

Agreed, if there are no openings beside the front face then your calculation or Table 18 provided above by the OP should be used; however, if there are openings those high internal force coefficients will not materialize, for example, 0.9 x Cpe.

In addition to that, according to Eurocode the case you noted (i.e. all windows, etc, closed and the front openings open) is taken as an accidental loading case, an accidental limit state material factor and a load factor of 1.0 instead of 1.35 should be used. So it may not be the most critical load case.

 

[Mark.Martin]

Hi,

many thanks to all of you reading and writing something about this thread.

I carefully read every single post...

What I found lately is part (chapter) of a German publication with some additional explanations and guidelines for may case (building completely open on one side and totally closed on three sides).
The author pointes out that assumed internal pressure cpi can be as mentioned in "old" DIN 1055-4:2005-03...print screen below: