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Wind Loads
2

Wind Loads

Wind Loads

(OP)
I am working on an optical sensor that will be mounted on a tower located out at sea. I have just been asked for a report/calculation on wind loading. I am not a civil engineer. I am not a PEng (PE). I have no access to a civil PEng to look over my shoulder. Failure could be catastrophic. I am hoping that there are qualified people at the customer's end who can review what I do.

I see that ASCI has a manual Minimum Design Loads for Buildings and Other Structures. How complicated is wind loading analysis? Should a mechie be able to the wind loading section and produce notes, calculations and a design that will satisfy a qualified civil engineer, or this is a job for a consulting engineer?

--
JHG

RE: Wind Loads

The wind section of ASCE7-10 consists of 6 chapters and 117 pages of code to determine wind loading (not counting the Appendixes)
It is also based on ultimate loading vs. ASD loading, so appropriate load cases need to be applied (from a different part of the ASCE7-10)

I would recommend hiring a structural engineer

RE: Wind Loads

If you're only determining the wind load on the sensor itself, and not the tower, you may be able to come up with something reasonable and conservative using the provisions for "other structures".

Some areas to be careful about -- if your tower is flexible (as defined by ASCE) you can get higher loads due to the Gust Effect Factor. Also, if your tower is latticed or sufficiently slender, you may get some odd vortice or aeolian vibration effects.

If either of these are the case, and you can't afford to just throw steel at the problem, you might still need a consultant.

RE: Wind Loads

How far out at sea are we talking? A big part of the ASCE code is wind speed, which is based off of local measurements & statistical analysis of wind speeds on land, not over the water.

RE: Wind Loads

If you're designing the sensor doohickey itself and not the tower, the better approach would be to ask the owner/tower designer what wind loadings are applicable to the project and go from there. Loading may vary with elevation on the tower.

If the stability of the whole tower with the added doohickey is in question, get the weight, general geometry, and projected area of he equipment to the tower designer and let them work on that end of it.

ASCE 7 is intended for stationary buildings on land in the USA, so it would be of limited use for that. Antennas, wind turbine towers, offshore platforms, ships, etc., would be covered by other standards (presumably) and you wouldn't use ASCE 7 for them.

RE: Wind Loads

(OP)
Thanks everybody.

I am responsible for the sensor only. I don't know how rigid the tower is. If our customer wants useful output, it will have to be rigid. I believe the winds are rated at 60m/s.

--
JHG

RE: Wind Loads

The basic equation for wind pressure in US customary units is Pressure = 0.00256 x Velocity^2 where Pressure is in psf and Velocity in mph. So for example, 120 mph = 37 psf.

I imagine out a sea during a hurricane velocity is even higher.

I remember years ago (1970's_before all the complications in ASCE-7, I used to design highway sign structures in the New York area for 110 mph, with just this simple equation as per NYC/NYS DOT procedures, no K factors, etc. None failed!

RE: Wind Loads

Will the attachment of the equipment to the tower be your responsibility?

RE: Wind Loads

(OP)
DETstru,

The mount flanges are mine. The floor and everything underneath is somebody else's. The customer will do the actual installation.

--
JHG

RE: Wind Loads

You are not a registered or licensed engineer, and are designing a structure and/or deriving loads for a structure for which you have stated failure could be catastrophic. Your area of expertise is mechanical engineering. In tandem with this, you are hoping that the customer has qualified engineers who will catch anything you miss.How good is your liability insurance?

Hire someone to do this for you. If for no other reason than the fact that whoever is designing the tower probably won't accept any wind load numbers you give them unless they are sealed.

RE: Wind Loads

Here's the equation for wind pressure: qz=0.00256KzKztKdV2 (psf)

qz = velocity pressure you're going to use
Kz = velocity pressure coefficient, which will depend on the height. If you're dealing with an ocean that's exposure D, and the higher the height, the higher the coefficient. If it was 50 ft, for example, Kz = 1.27. 100 ft, Kz = 1.43. Table T30.3-1 contains the values in the ASCE 7-10.
Kzt is usually one, and will be if there's no windspeed increase due to elevation change.
Kd = directionality facor, which I would take as 0.85 (grouped with signs).
V = you'll need to obtain the basic windspeed from either the drawings or the tower structural engineer of record.

If you want more information, you can find the equation and information in Chapter 26 in the ASCE7-10 (Or ASCE7-15). Included are simple conversions if you're dealing with metric, also.

Once you have all these and finally have your qz, I would apply that to the cross section of your sensor assembly and design the attachment for the resulting shear + moment.

If you want to do the engineering on this that's on you, just know you're assuming the liability for it.

RE: Wind Loads

(OP)
gwynn,

Right now, it appears that the customer understands this stuff, and we don't. I have some estimates of wind load, and I am doing structural analysis. My bolts are getting bigger, and I am hardening some flanges. When I am done, we will submit drawings and calculations to the customer. We build instruments. They know how to mount stuff on elevated platforms at sea. They will have to decide if they want to approve this or not.

Most of my stuff gets mounted in aircraft, so I have experienced the attitude. This is a new set of problems and hardware.

Thanks.

--
JHG

RE: Wind Loads

Go to see ASCE, and there are some steps to tell you how to calculate wind load, good luck

RE: Wind Loads

(OP)
I have submitted my calculations to our customer. This is as opposed to us sending them a report claiming that we have analyzed the thing and found it to be safe. The second section of the report is as follows...

Quote (report)


Warning

I am a mechanical engineering technologist specializing in mechanical design, not a structural/civil engineer. This must be reviewed by a qualified engineer.

They are reviewing it.

Thanks for the help everyone.

--
JHG

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