Apply Wind Code or CFD modelling?

[psyclones]

Hello,

An Engineer I know determined wind loads for a small but rather odd shaped structure assembled from various standard steel sections.

It's a frame attached to the topside of a stadium roof to suspend speakers.

 

We have AS/NZS 1170.2 (Australian) Wind Code, which gives detailed tables to calculate drag co-efficient’s for standard members.

But my problem is this;

Even if the sum of all areas and co-efficient’s were calculated, the wind loading (on any given face) would be incorrect, or very conservative!  Due to the standard only applying to individual members.

*Wouldn’t be easier to use a CFD package to model the wind loading, the wind velocities (per direction) can be easily found in Aust. Standard?

 

O/A approx. dimensions; 1300mm x 370mm x 670 mm,

Steel sections include;

• 100mm sq hollow section,
• 50x50x5 equal angles.
• 12mm and 16mm thick plates to mount speakers & structure existing rafters.

I’ve very hesitate to provide an image of model due to confidentially. Which I realise without it could make it difficult to get an idea of the structures geometry. 

Edited January 1, 2023 by psyclones
correction

[studiot]

What about a test model ?

 

[sethoflagos]

Couldn't the inherent conservatism of the design calculations be intentional?

Particularly given the potential for multiple fatalities if the structure failed, a safety factor of 10x would not be unusual (as with elevators etc.)

[psyclones]

On 1/1/2023 at 11:09 PM, studiot said:

What about a test model ?

 

As far as I know, there was no (physical) test model created. 

But, the code is very robust if applied properly.  Structural Engineer's for domestic buildings (I know) rely on the Wind code not CFD modelling. 

This maybe of little importance, but I know once all loads were determined the structural analysis would have been designed using (Siemens) FEMAP. I have the (3D) CAD model. 

On 1/1/2023 at 11:12 PM, sethoflagos said:

Couldn't the inherent conservatism of the design calculations be intentional?

Particularly given the potential for multiple fatalities if the structure failed, a safety factor of 10x would not be unusual (as with elevators etc.)

Yes. I reviewed the ASCE 7-16 for Wind loadings, and the American Standard (in some aspects) is very similar to the Australian.  

And Yes. "multiple fatalities if the structure failed" is correct. 

 

Maybe your right, due to where this structure will be mounted, Winds loads become a primary concern, so conservative loading would be ok, before applying SF.

 

But;

on second thought, if we take conservative loading as a given, could we/I just treat the whole structure as a Rectangular Prism (of which it would occupy <50% Vol). Then trying the calculate wind loads becomes much less cumbersome? 

 

Edited January 3, 2023 by psyclones

[sethoflagos]

1 hour ago, psyclones said:

But;

on second thought, if we take conservative loading as a given, could we/I just treat the whole structure as a Rectangular Prism (of which it would occupy <50% Vol). Then trying the calculate wind loads becomes much less cumbersome? 

I'd guess that method would give you a total stress loading on the structure as a whole, but how would it indicate how those stresses were distributed amongst the various sections? 

 

[studiot]

5 hours ago, psyclones said:

Yes. I reviewed the ASCE 7-16 for Wind loadings, and the American Standard (in some aspects) is very similar to the Australian.

 

5 hours ago, psyclones said:

As far as I know, there was no (physical) test model created. 

But, the code is very robust if applied properly.  Structural Engineer's for domestic buildings (I know) rely on the Wind code not CFD modelling. 

This maybe of little importance, but I know once all loads were determined the structural analysis would have been designed using (Siemens) FEMAP. I have the (3D) CAD model. 

 

It is very difficult to comment specifically on so little detail.

I do not know about US and Australian codes but the UK wind codes are designed for different purposes from what I understand your requirements to be.

The are designed for two principle purposes.

1.  The broad brush total forces acting on large surfaces such as wall, roofs, etc, ignoring most openings (which may in any case be shut) and applies an air stream which passes over, under around the structure.
   Structurally the distribution and transmission of these forces to the foundations should be considered.
2. The forces arising from pressure differences from trapped air, eg uplift on the underside of roofs, sway moments applied to structures etc.

What is not considered theoretically would be the effects of oscillations induced by, for instance fluctuating sway moments.
These would (should) normally be resolved by model testing. Testing is doubtless best doubly so in the case of small additional 3D members like your support brackets, which ae mounted is especially vulnerable locations.

[sethoflagos]

2 hours ago, studiot said:

I do not know about US and Australian codes but the UK wind codes are designed for different purposes from what I understand your requirements to be.

I think the OP may fall more into the orbit of BS 5950 than BS 6399.

[studiot]

7 minutes ago, sethoflagos said:

I think the OP may fall more into the orbit of BS 5950 than BS 6399.

Quite likely.

My knowledge of both BS5950 (Structural Steelwork)  and BS6399 (Loading for Building) is rather out of date, but a quick gander at the latest versions suggest they still follow the lines I outlined.

[psyclones]

7 hours ago, sethoflagos said:

I'd guess that method would give you a total stress loading on the structure as a whole, but how would it indicate how those stresses were distributed amongst the various sections? 

 

Good point. 

2 hours ago, studiot said:

 

 

It is very difficult to comment specifically on so little detail.

I do not know about US and Australian codes but the UK wind codes are designed for different purposes from what I understand your requirements to be.

The are designed for two principle purposes.

1.  The broad brush total forces acting on large surfaces such as wall, roofs, etc, ignoring most openings (which may in any case be shut) and applies an air stream which passes over, under around the structure.
   Structurally the distribution and transmission of these forces to the foundations should be considered.
2. The forces arising from pressure differences from trapped air, eg uplift on the underside of roofs, sway moments applied to structures etc.

What is not considered theoretically would be the effects of oscillations induced by, for instance fluctuating sway moments.
These would (should) normally be resolved by model testing. Testing is doubtless best doubly so in the case of small additional 3D members like your support brackets, which ae mounted is especially vulnerable locations.

My apologies, I can't give more info,

Yes. I agree the code(s) goes into great detail about how to deal with different wall/ roof pressures and local area pressures on corners and around eaves of buildings.

Sway becomes a significant problem for multi level buildings also.

25 minutes ago, psyclones said:

Good point. 

My apologies, I can't give more info,

Yes. I agree the code(s) goes into great detail about how to deal with different wall/ roof pressures and local area pressures on corners and around eaves of buildings.

Sway becomes a significant problem for multi level buildings also.

 

What information would you need? 

[studiot]

2 hours ago, psyclones said:

What information would you need? 

Well, short of actual drawings here are some guesses and thoughts.

 

1. I would imagine that the additional load imposed on the structure by your brackets (may I call them brackets ? - it's nice and short) are small compared to the main building frame loads.
2. I would however imagine that locally these brackets could impose quite high additional loads due to wind ( and perhaps other) forces, particularly at their mounting points.
3. As I have already noted the structural dynamics of these connections would therefore become important.
4. A further consideration might be maintenance access  -  gantryways etc.
5. Cabling, grounding and lightning protection are often an afterthoughts rather than part of the design and not always satisfactory as a result.
6. It is not clear what is meant by " a frame attached to the topside of a stadium to suspend speakers".
   Are the speakers up there as well ?
   How are they to be distributed over the stadium, canopy, roof or whatever ?
   Are these individual mountings or is there a single (central ?) frame or what ?

 

 

[psyclones]

On 1/4/2023 at 2:56 AM, studiot said:

Well, short of actual drawings here are some guesses and thoughts.

 

1. I would imagine that the additional load imposed on the structure by your brackets (may I call them brackets ? - it's nice and short) are small compared to the main building frame loads.
2. I would however imagine that locally these brackets could impose quite high additional loads due to wind ( and perhaps other) forces, particularly at their mounting points.
3. As I have already noted the structural dynamics of these connections would therefore become important.
4. A further consideration might be maintenance access  -  gantryways etc.
5. Cabling, grounding and lightning protection are often an afterthoughts rather than part of the design and not always satisfactory as a result.
6. It is not clear what is meant by " a frame attached to the topside of a stadium to suspend speakers".
   Are the speakers up there as well ?
   How are they to be distributed over the stadium, canopy, roof or whatever ?
   Are these individual mountings or is there a single (central ?) frame or what ?

 

 

Sure. It's an awkward shaped bracket. 

Concerns regarding cabling & access is not an issue. 

Thanks for your help. your right you can't do much without the drawings, and I can't provide them (again apologies).

 

 

CCF_000006.pdf

[Marifernanda12]

The decision to apply a wind code or CFD (Computational Fluid Dynamics) modeling will depend on the purpose and scope of the project in question.

If the objective is to perform wind analysis at the macroscopic level, i.e. to evaluate the effects of wind on large structures such as buildings, bridges or wind energy towers, then a wind code may be sufficient. Wind codes are simplified tools that allow the simulation of airflow around large, bulky structures and provide a general understanding of how wind affects the performance and safety of such structures.

On the other hand, if a more detailed and accurate analysis of airflow is required, such as in the design of an automobile, aircraft, or the analysis of the aerodynamics of a small object, then CFD modeling is recommended. CFD models are capable of simulating airflow around complex and detailed objects, allowing a much deeper understanding of the physical phenomena involved in airflow.

Edited April 24, 2023 by Phi for All
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