Wind loads for circular cross-section lattice structures?

[Vigardo]

Dear experts,

How should I compute the wind loads for a Shukhow tower, i.e. a circular cross-section steel lattice structure? The structural members should be rectangular or circular.

I´ve been checking EUROCODEs EN 1991-1-4 (2005) and EN 1993-3-1 (2006) without success. Is there any other code that considers this kind of structures? Would you help me?

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[jrmichler]

The codes that I have seen all specify a design wind velocity. It's then up to the design engineer to calculate the loads on the structure at that wind velocity.

A good, probably the best, source for aerodynamic drag information is Fluid Dynamic Drag, by S.F. Hoerner. He does assume that you have a solid foundation in fluid dynamics.

 

[Vigardo]

Hi jrmichler, thanks for the quick response and help!

I know the Hoerner's book about drag, it splits the contributions of each structural member but I think it is mainly intended for aeronautics. However, I need something more "construction code compliant", like EUROCODE.

I've found in EN 1991-1-4 how to obtain the velocities profile v(z) from 0 to 200 m height (z); it depends on roughness and orography. In this web the EN 1991-1-4 principles have been applied to obtain the force coefficients for isolated elements. I wanted to know if there is some code that considers the aerodynamic effects of arranging beams in a circular manner, like in Shukhow towers.

In EN 1993-3-1 (including code for steel towers and masts) they only talk about triangular and squared cross-sections for lattice structures, but I wanted something more general to compare among the different types of cross-sections (triangular, squared, circular, etc.) in a construction context.

Do you think I would just add all the forces computed for individual isolated elements to obtain the total action of wind on the tower?

 

[jrmichler]

Hoerner has a whole chapter about interference drag, where the total drag of two elements is greater than the drag of each individual element. He only covers streamline shapes, but the principle applies to other shapes.

I would attack this problem by calculating the drag of the isolated elements of a squared cross section lattice structure, and then compare that to the information for the entire assembly. The ratio of the difference is a combination of interference drag plus safety factor. Cross check with Hoerner on interference drag. Apply that ratio to the drag of the isolated elements of the circular structure, and then add an additional safety factor. Don't forget a safety factor for stuff mounted to the top of the tower because somebody someday will add just one more thing up there.

 

[Vigardo]

Hi jrmichler, thanks again! I'll check the chapter about interference drag you've said.

If I understood well, what you propose is to validate "our" procedure of calculating the wind action of a circular cross-section lattice structure by comparing, for a squared cross-section lattice structure, the results obtained from EN 1991-1-4 for the whole structure and the results obtained adding up all isolated elements drag from Hoerner's book. Is this correct?

 

[jrmichler]

Yes, and don't forget the extra safety factor.

 

[tech99]

Yes, and don't forget the extra safety factor.

This is my recollection from tower design many years ago.
Having found the forces in each member at the Design Wind Speed, the permissible load for any cross section can be found from a design code. Safety factors for structural members are built into codes, and may be about 2. It depends whether the load is tensile, compression or bending etc. The structure depicted is very difficult to analyse (and build) because it is rigid jointed, by the way. I do not recall Interference being taken into account, but we did not allow for shielding.
The structure might carry antennas at any height, so this must be specified at the design stage.
The Design Wind Speed needs to be chosen having regard to the risk of failure that can be tolerated in addition to geographical and topographical data.
A typical figure might be the 50 year 3 second gust with a given probability. In Southern UK around 100mph.
Foundations need to be designed for uplift, usually mass foundations with a FOS of 1.5.

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[Vigardo]

Hi tech99, thank you very much for your contribution!

Just to clarify, in your opinion, the effects of interference are not very important in such an open structure (low solidity factor). Am I right?

Then, do you think that just projecting the area of each member in the plane perpendicular to the wind direction and using the Eurocode formulas for the forces would be enough to estimate a good wind design load?

By the way, I've calculated from EN 1991-1-4 the design wind speed on the top of a 120 m height tower (including turbulence and gusts). You're right, it is around 165 km/h (100 mph) for an urban environment, and around 190 km/h (120 mph) for the countryside.

 

[tech99]

Just to clarify, in your opinion, the effects of interference are not very important in such an open structure (low solidity factor). Am I right?

Then, do you think that just projecting the area of each member in the plane perpendicular to the wind direction and using the Eurocode formulas for the forces would be enough to estimate a good wind design load?
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Hi Vigardo
Yes, in my time interference was not included.
I never did a circular tower, but I think you are correct to use the projected areas for front and back faces. In other words, measure the member lengths from the elevation drawing. Remember to apply the wind loading factor for circular members, maybe 0.8 or something like that from memory.
For the structure shown, you need to be careful how point loads (from dishes etc) are applied. There is a tendency with structures like this for people to want to add loads low down in later years, so it is wise to build in some spare capacity. The structure shown cannot be easily strengthened.
Don't forget wind load on the climbing ladder and platforms.

 

[Tom.G]

Don't forget wind load on the climbing ladder and platforms.

And the two to four people on it wrestling an antenna!

 

[tech99]

Hi Vigardo
Yes, in my time interference was not included.
I never did a circular tower, but I think you are correct to use the projected areas for front and back faces. In other words, measure the member lengths from the elevation drawing. Remember to apply the wind loading factor for circular members, maybe 0.8 or something like that from memory.
For the structure shown, you need to be careful how point loads (from dishes etc) are applied. There is a tendency with structures like this for people to want to add loads low down in later years, so it is wise to build in some spare capacity. The structure shown cannot be easily strengthened.
Don't forget wind load on the climbing ladder and platforms.

As I mentioned, I have never done a circular structure, and I can see that the wind acting tangentially on the "sides" of the tower will have reduced effect due to shielding. It is like the wind on the sides of a square tower, which we take as zero for the case when wind is blowing directly on to a face. For a square tower, we used to also consider a case of wind on the diagonal, so I suppose that taking the projected area of all members, as we discussed, will be reasonable.