Shape of a structure to reduce wind loads on AS1170.2

[nathan7]

Hi all

I'm analysing two tall structure, one is rectangular and one is circular. The drag coeffiecient for a rectangular structure is 2.2, for a circular structure is 1.2.

Does the 1170.2 allow you to reduce the wind loads based on the shape of the structure? Other international codes allow to reduce by 20-30% the action on a circular structure compare to a square structure.

I don't think that I am allow to do it with AS1170.2

Look forward to hear some comments

Cheers

 

[rowingengineer]

does the structure not meet the requirements of appendix C5?

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."

 

[apsix]

The drag coefficient is affected by the Reynolds number, which the AS modifies by using bV as shown in Table E3, so Cd less than 1.2 & 2.2 may be appropriate, depending on size.
I don't have any information on the transition of a square edge rectangle at 2.2 to a rectangular building at (0.7 + 0.5)x0.9 (from Table 5.2 and a combination factor of 0.9)

 

[nathan7]

I don’t believe that appendix E should be used to calculate the effect of wind on a high rise structure as it is to be used only for exposed structural members, frame ad lattice towers.

To calculate the effect of the wind on building you should use chapter 5 and use the following formulas:

Cfigi = Cpi Kci
Cfige = Cpe Ka Kce Kl Kp

None of the two formulas take into account the shape of the structure

My understanding is that AS1170.2 doesn’t allow a reduction of wind force if my structure is more aerodynamic compare to a box structure.

Any suggestion on codes that allow the shape on the wind force calculation?

Thoughts?

 

[asixth]

Nope. Need to use the windward and leeward co-efficients and apply the 0.8 factor to allow for combinations and area reductions. A large highrise where the construction costs are several hundred million AUD's will have a wind-tunnel test done and you often find that this will recommend lower wind actions. Chamfered corners will reduce wind pressures but you won't find it written in AS1170.

One of the appendices in AS1170 has calculations for cross-wind excitation for flexible structures which is more used for side to side sway and accelerations.

 

[nathan7]

asixth

I know all of this thanks. My point is different. Right at the start of the project during pure feasibility stage when wind tunnel is not in the equation yet, the engineer should be able to check the building for strength and service with the code and should be able to calculate reduce wind loads based on code.

American code allow for reduction based on shape.

 

[asixth]

I would definitely put the idea on the table but wouldn't guarantee any material quantity reductions until testing is completed. The blokes in my office who have designed the odd highrise along the way talk about a 30% reduction when testing is done compared to AS1170 wind actions. This is just a general observation on testing and not really considering the shape of the building.

I have never done it myself and reducing the wind actions on the basis of a Cfig=1.2 compared to 2.2 is a big drop in force but this might be what a wind test reports.

If your a member of Engineers Australia there should be a webcast called "Structural Technical Presentation - Design Challenges of the Soleil Tower", the engineer quickly talks about the reduction in wind forces with chamfered corners but doesn't go too far into the topic.

 

[Trenno]

Maybe look into something like the Autodesk Robot CFD Module? Perhaps you will be able to get some savings using it?

Would love to know how different it is opposed to code based pressures.

Link

 

[nathan7]

Wind tunnel test give you a reduction only if you don't have building around the structure. If you have tall buildings around the structure the wind might accelerate due to Venturi effects.

cfd is a way to do it but it's another black box software and you must be good at using it before believing the results.

The real point would be to try to use a code which allow you to reduce loads based on shape of structures

 

[Trenno]

Turns out it's pretty good at predicting pressures...

Australian study provided here

 

[asixth]

I'm sure the example that software developer presented in their marketing doc's shows that it's great at estimating wind pressures.

 

[Trenno]

I wonder how involved Windtech was in that case...

 

[apsix]

I would use any reputable code (US, EU, UK etc) or textbook to obtain a more refined Cd if available for the shape of your building, especially if it's the feasibility stage and testing will follow. Of course the possibility of funnelling has to be considered.
However, the originally quoted 1.2/2.2 will never be appropriate for buildings; they are only used structural section sized elements.

 

[nathan7]

I probably wasn't clear at the start of the thread. I didn't mean to adopt 1.2/2.2 for the design.

All i meant was that there is a different on bending moment between a similar size square structure and circular structure. The problem is that the code doesn't allow a reduction as such until you run wind tunnel tests you will not know the reduction so the schematic design became more conservative (which is frustrating).

Using CFD is ok but you should model the building around it to have true response. In Robot this is difficult to do.