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.

 

[jimstructures]

What building code applies???

If in the US of A ASCE 7-xx will likely apply and it discusses this specific issue.


Jim

 

[rowingengineer]

My guess would be eurocode, and I think that at this size it would be 0.9 x Cpe assuming no permeability.

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."

 

[HTURKAK]

BS EN 1991-1-4:2005

7.2.9 Internal pressure is applicable..

When the area of the openings at the dominant face is at least 3 times the area of the openings in the
remaining faces,
Eq. (7.2) is applicable .. cpi = 0,90* cpe

Or , conservatively i will suggest cpi = 1,0* cpe

Pls be careful for the signs , and check for both case, shown below..

 

[Mark.Martin]

Hi,

I'm aware of the clause 7.2. that says cpi=0.9xCpe (Internal pressure coefficients Cpi for buildings with dominant openings), but I'm not sure if I interpret this as I should.

For example:
I have a building like this (with external coefficients cpe):

When I calculate cpi=0.9xcpe for areas A,F and G I obtain:
Cpi(A)=0.9x1.20=1.08
cpi(F)=0.9x1.80=1.62
Cpi(G)=0.9x1.20=1.08

for all the remaining areas cpi becomes quite small in my opinion when wind is blowing inside the building directly from open front side.....for example area E following the same logic would be 0.9x0.4=0.36

Than, I found an article online with this scheme (very straightforward compared to clause 7.2):

Internal pressure is +0.80 when wind is blowing inside the building and -0.50 when it's blowing from 3 other closed directions.
When applying this logic to areas A, F and G wind force will be smaller compared to clause 7.2. (Cpi=0.9xCpe), but all other areas will receive more wind force.

Also, the old BS 6399-2:1997 has a similar Table 18 that looks like this:

What is the correct approach to this problem? Use Cpi provisions for "buildings with dominant openings" or Cpi for "open-sided buildings" that apparently are not mentioned in EC1991-1-4?

Kind regards!

 

[r13]

IMO, if not addressed directly by code, you shall conservatively use C_pe 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).

BTW, look up the definition of "buildings with dominant openings" in your code. It should have a guide in how to determine the degree of "dominance" that the provision is applicable.

 

[HTURKAK]

Will you pls confirm that the dimensions shown are in cm ?.. and the open side is front say at front elevation area shown D ?...
And the coefficentss are for wind direction from wall D to wall E ?..

Your conclusion ( for all the remaining areas cpi becomes quite small in my opinion when wind is blowing inside the building directly from open front side.....for example area E following the same logic would be 0.9x0.4=0.36 ) is not correct..

 

[Mark.Martin]

@HTURKAK
Hi,

Dimensions are in meters. The open side is area D and the direction for wind is from area D to area E.

Kind regards.

 

[HTURKAK]

Ach so..the dimensions of the bldg are, W= 3200 m, L= 3000 m, H =1200 m...so higher than Burj al Khalifa..Got it..

 

[r13]

It makes better sense if the units is in centimeter.

 

[Mark.Martin]

sorry...dimensions are in cm.

 

[Settingsun]

There's one single design internal pressure, based on the external pressure that would apply to your opening if there were a wall there. See the image in Hturkak's first post.

I think you're trying to apply different Cpi values all over the building based on the Cpe at those locations, instead of the Cpe at the opening. If the opening is at the windward wall, the building fills up with air and the internal pressure acts outwards. For opening in leeward wall, the air is sucked out and the internal pressure acts inwards.

 

[Tomfh]

I’d use Cpe

An opening that large will immediately pump up (or deflate) the entire building.

 

[Mark.Martin]

@steveh49

Hi,

Is it correct to assume that if wind is blowing normal to area D and cpe for area D is 0.72 that cpi for entire building (roof and walls) is 0.9*0.72=0.65?

 

[r13]

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?

 

[HTURKAK]

In your case h/d=12/30= 0.4 Table 7.1 is applicable .. Find the external pressure coeficient assuming the wall-D - is closed

For h/d=1.0 Cpe=0.8

For h/d =0.25 Cpe =0.7

With linear interpolation for h/d=0.4 Cpe =0.7+ 0.1*(0.4-0.25)/(0.75)=0.72

The internal pressure coeff. Cpi= 0,90XCpe = 0.65 This value will be applied to all internal surfaces..

I will remind the picture in my first post and ( Figure 5.1 Pressure on surfaces )..

Does this respond answers to your question?

 

[Mark.Martin]

@HTURKAK

Thank you!

This is internal pressure (cpi) when wind is blowing inside the building (angle 0).


What would be cpi when wind is at angle 90 and 180 degrees?

Kind regards!

 

[r13]

Below is ASCE7 definition of "open building", and suggested internal pressure coefficient. In your case, the internal surfaces suffer the same wind pressure as an external wall, 0.72Cpe, as Cpi = 0 when wind is blowing into the building (blow air into balloon). Note that the internal pressure will be zero for wind blows in any directions.

 

[rowingengineer]

R13, that is some really confusing advice, I would recommend anyone reading this thread ignore this advice. The building being discussed does not have 80 percent open walls requirement on all sides to meet the open building definition, this post by R13 is misleading at best.

It is a partially enclosed building and should only be discussed in this context for ASCE code requirements, however this is a eurocode discussion so ASCE doesn't have any standing and is mostly irrelevant.

However I am reluctant to provide a response as in-depth as HTURKAK, I would prefer Mark Martin have a go a re-doing the original diagram based on the information above.

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."

 

[hetgen]

What would be cpi when wind is at angle 90 and 180 degrees?

When the wind is blowing at 90' the internal pressure coefficient should be the range coefficient as shown in Table 18 you attached. For 180' wind the coefficient should be 0 to the negative suction figure shown in Table 18. or alternatively you can use Fig 7.13 below taken from EN 7.2.9-6

*Edit.. the above figure is only applicable if you have at least 1/2 the size of the main opening on the side of the building and roof eaves.