Coefficient of drag for Rectangular Hollow Sections 1

[sdz]

Does anyone know where I can find Coefficient of Drag for typical SHS and RHS sections. I have data for a square edge prism and a large rounded corner but not for corner radii typical of square and rectangular steel hollow sections.

 

[JedClampett]

Not worth refining down to that detail unless your doing some supersonic design. Just use a square section with square corners. If anything it will be very slightly conservative.

 

[Trenno]

From Aus Wind Code (AS1170.2 Appendix E). Check out that Appendix for a good run down on drag coefficients for different structural shapes.

 

[sdz]

Trenno, I saw that but for a 75x4 SHS the corner radius is about r/b=0.13. Sharp edge gives Cd=2.2. Interpolating I get Cd=1.8
I wonder if that is close and what about an RHS.

 

[Trenno]

I've never really delved into exposed structural members getting hit by wind...

But surely the difference between 1.2 and 2.2 is not in the big scheme of things?

 

[jrbaus]

Another extract from AS1170.2 (see attached), however this time for rectangular prisms.

For an aspect ratio of 2:1, I calculate a Cd of 2.78/1.6 depending on wind direction.

Regards Jake

 

[jreit]

AASHTO Standard Specifications for Luminaries provides drag coefficients for various shapes.

 

[btrueblood]

A good reference would be W. F. Hoerner's Fluid Dynamic Drag, if you can find a copy (likely have to look in a good university Engineering library).

An excerpt from same is shown here, and may answer your question:

http://www.boatdesign.net/forums/at...c-method-hoerner_3-13_fig23_cylinder_drag.png

Though the result is pretty dependent on Reynold's number based on this:

http://www.fgg.uni-lj.si/~/pmoze/ESDEP/master/wg13/l0100.htm

 

[sdz]

jreit, I don't have access to the AASHTO standard.

btrueblood, useful information. From what I have seen a useable approximation is
r/b

Cd​

0.0

2.2​

0.05

2.2​

0.2

1.2​

0.5

1.2​

This ignores reduction that can occur due to Reynold's number.
In my case for r/b=0.13 I get Cd=1.7 which is a useful saving over a sharp corner at Cd=2.2

 

[sdz]

I have found an answer in Eurocode 1 Actions on structures - Part 1-4 General actions - Wind actions, Clause 1 7.6 Structural elements with rectangular sections

https://law.resource.org/pub/eur/ibr/en.1991.1.4.2005/en.1991.1.4.2005_066_02.jpg

 

[Leftwow]

That drag coefficient is huge against structural members exposed to wind, that's all I deal with usually. We use ASCE 7-10, and the Code on wind loads on petrochemical facilities book. My boss wants to use 0.7 for Cf on Circular HSS beam, however I wouldn't mind going a little more into depth with this and finding out some kind of literature on this! If you can get this book - Wind Loads and Anchor Bolt Design for Petrochemical Facilities, it uses 0.7 for piping.

 

[bowlingdanish]

Wind loading on exposed members... Just use Cd = 2! Why waste your time.

If you really want to get technical, drag coefficients are basically characterized by:

1. Flow: Laminar or Turbulent
2. Form: Sharp edged or rounded
3. Longitudinal Length: Long or short
4. Shielding

The first two are intertwined. Sharp edged objects almost always have turbulent flow, rounded objects can be laminar at low fluid velocities (however for structual engineering ultimate wind will be turbulent). A circular (infinitely long) section will have Cd = 1.2 in laminar flow and Cd = 0.4 in turbulent flow when the boundary layer transitions. A long object will always have a high drag coefficient than a short one (out of plane to wind direction), as the fluid can't shed out of plane around the object. Then there's shielding; if the flow has become turbulent or somewhat disturbed before reaching the member in question, ascertaining a drag coefficient other than a conservative guess is futile. Basically it's too complex to waste your time with, and you should increase your beam size if you're concerned.

 

[Leftwow]

I feel like the structure should be as efficient as possible and going overly conservative is good, however I like to design as light as possible b/c everything exponentially gets costlier the more conservative you get. A wind load on a 100 ft structure with 50 lbs/ft on your beams will result in ridiculous overdesigns from the top to the bottom. I want to design every structure with as much complexity and time as possible, even if my boss gets mad b/c its taking to long!