British wind loading codes

[HankVRSI]

Anyone have any information on where I can get some information on the British wind loading specifications or where I can download the specification?

 

[pba]

Uk codes are available from British Standards Institution
You will need BS6399 Loading for Buildings - Part 2: Code of practice for wind loads - 1997. The current version incorporates Amendment No1, Corrigendum No 1 and is Corrected and Reprinted June 2002. - All this is on the front cover!

BSI website is http:/

http://www.bsi-global.com

and telephone is +44 (0)20 8996 9001. There are PDF versions around...

I'm not familiar with the ASCE-7 code but I can confirm that BS6399-2 is NOT easy to understand. Use the standard method in preference to the directional method. If you need further information post your queries here.

NOW - I don't know what type of building you are looking at but several UK cladding manufacturers have produced free design software which assists in the calculation of wind forces. One such company is Ward Building Components -

http://www.wards.co.uk

Also

http://www.structural-engineering.fsnet.co.uk

used to have a spreadsheet for calculating wind forces to the code. I've never used it so I can't comment on how good it is.

Good Luck

 

[HankVRSI]

My company designs parabolic antennas. We have reliable wind tunnel test that provide drag coefficients for the wind at different angles to the antenna. I can use the basic wind pressure calculated from ASCE 7, along with these drag coefficients to determine the loads on the antenna. My question is, if we advertise our antennas as meeting a 125 mph wind, what basic pressure would a customer using the British code think a 125 mph wind would be?

I should have stated this clearly from the beginning.

 

[pba]

You will need the code. The short answer is the UK code is based on a 3 second gust and 1 in 50 year return period. There is a map with ground level wind speeds for the whole of the UK. the wind force is calculated (after applying may coefficients which can be greater that 2) using the equation wind force qs = 0.613V^2, where V is the wind speed. The wind force is then multipled by a cooefficient due to the shape - (for instance - free standing sign boards have a factor of 1.8). The appendices give information on the requirements of wind tunnel testing and on how to derive extreme wind events.

BS6399-2 is only applicable to static and mildly dynamic structures. If your antenna is classed as dynamic the code does not apply.

125mph is 55.8m/s which would be reasonable for most areas of the UK and for buildings of heights up to 100m. Some very high and exposed areas may have winds stronger than 125mph, but you are likely to have good coverage.

Obviously we have different codes for the design of materials as well...

 

[HankVRSI]

The 3 second gust and 50 year storm are the same as ASCE 7. It sounds like the basic calculation is similar.

Does the British code allow for any kind of stress increase due to wind and dead weight only forces, or a decrease in the load factor?

Is the classification of a dynamic structure based on the natural frequency?

 

[pba]

There is a Building type factor (remember this is a building code) This factor is 8 for a Welded steel unclad frame (I guess this is closest to your antenna). You then look at a graph of Dynamic Augmentation Factor against Building height and read off the curve line marked 8 For the structure to be sufficiently undynamic to comply with the code the maximum height is limited to about 20m. The Dynamic augmentation factor is 0.25 which adds about 25% to the wind speed.

BS6399-2 is for determining the wind force only. How you use the wind force is covered by other codes. Our steel work code (BS5950) is ultimate limit state and used partial safety factors of 1.0, 1.2 and 1.4 on wind forces.

It's difficult to be more precise without doing the actual calculations. If you make the antenna the material design is down to you. If you plan on using it in the UK it is sensible to confirm that the applied wind load is not higher and this is being done. It might also be worth considering snow loading (BS6399-3). We do not usually suffer from ice formation in the UK due to our lovely warm weather! Once you are satisfied that the applied loadings are not more that your design can take I think you are done. The materials are not going to get weaker by being in the UK.

This last point can be verified by contacting the local building control authorities or other parties who would ultimately be responsible for approving the design.

 

[HankVRSI]

Interesting as ASCE 7 has a safety factor of 1.6 for wind.

Okay, I checked out the spreadsheet you referenced and I might be able to use it. I'll also check the snow loading and appreciate the reminder.

Thanks for all the valuable information.

Regards
Hank

 

[HankVRSI]

I got the BS 6399-2 document you informed me about. The 1.4 factor you mentioned was something I decided to check into. We happened to have a 1990 version of BS 5950 Part 1, and when checking into getting the latest version, I discovered that it was quite expensive. In the version we have, table 2 seems to be the pertinent factors for our purposes. I hate to ask my company to spend that much money for just this little bit of information. Could you tell me if that table is still the same?

dead load 1.4
dead load acting with wind and imposed load 1.2
imposed load 1.6
imposed load acting with wind 1.2
wind load 1.4

Regards
Hank

 

[pba]

HA!

I have BS5950 Part 1 2000 on my desk and extacts from the 1985 (original) version. I don't have the 1990 version around.

Table 2 is much bigger in the 2000 version but contains basically the same information as the 1985 version - I infer that the 1990 version would look like either the 1985 version or the 2000 version. I can't remember - it is not the part of the code the gets committed to memory. If it WAS different I would know! Extracting and paraphrasing from the 2000 version, the following partial safety factors seem relevant:

Dead Load (alone and with imposed loads) 1.4
Dead Load with imposed and wind 1.2
Then there are factors for cranes...
Dead load resisting uplift 1.0

Imposed load 1.6
Imposed load with wind load 1.2

Wind load 1.4
Wind load with imposed load 1.2

There is then half a page of factors for tanks, earth pressures, temperature effects and various crane forces acting in combination with dead, imposed and wind...None of which probably have any effect on your case(?)

I read the table (both 1985 and 2000) to mean the following load combinations:
1.4DL + 1.6IL
1.0DL + 1.4WL
1.4DL + 1.4WL
1.2DL + 1.2IL + 1.2WL

Imposed load (IL) is live load and/or snow load.

For uplift conditions, convention in UK practice is to use the load factor of 1.0 on dead loads that are deemed to always be there. If there is a component of dead load that is removable this is usually assumed to be removed when its presence could be beneficial. A nebulous written justification often accompanies the establishment of a partial safety factor of 0.9 or thereabouts.

Serviceability calculations are based on partial safety factors of 1.0 for dead, imposed and wind conditions but clause 2.5.1 states: '...In the case of combined imposed load and wind load only 80% of the full specified values need be considered when checking serviceability...'

I assume that the practice is similar in the US?

We are now potentially talking about actual loads on a physical structure. Please remember that any comments by me in this forum are based on my experience of UK practice but carry no warranty as to their validity. You must confirm to your own satisfaction the applicability of my statements to your problem.

Having said all that; Do please post your conclusions, if you are able.

Good Luck

 

[HankVRSI]

I am concluding that since our structures are predominately loaded with wind, the ASCE 7 load factor of 1.6 for wind and 1.2 for dead load will effectively insure that our antenna’s will meet the British design code. However we will provide this basic information to our customers so that a determination can be made about further verification they may feel is necessary.

I understand your concern regarding comments you have made and want to assure you that they will not be used any further than our discussion here on this forum.

Thank you again for the information and your valuable time.

Regards

Hank