BRE 489 wind Loads equation

[CG_Sparks]

Hi. I'm having a bit of trouble trying to do the calculation in BRE 489 for solar pv wind loads. I understand the equation;

F = qs x Cp, net x Ca x Aref

But what i don't understand, is what to do with the number at the end.
Do i look in say a schuco mounting rail, hooks etc specification booklet and find out their maximum force capability?

I've paid for a structural engineer to do the static loading calcs on the roof, but i'm yet to find a company that will do the wind loading calcs as well.

Any help would be much appreciated

Cheers

 

[Worcester]

Your structural engineer can do this quite easily for you, take a look at the new 3rd Edition, whilst not yet a standard, it will help a lot, (http://www.microgenerationcertification.org/admin/documents/Guide v3.8 DFC.pdf)

Wind Loading comes under three standards

BRE 489
BS6399-2-1997 Wind loads
and Eurocode 1

And then don't forget BRE 495

Part of the MCS requirements is that if you don't have these, then you should have them listed in your QMS as to where you can find / borrow them from.

 

[Farley]

Hi

Is it Aref that your are relating to? if so this is the Area m2 of the entire array. this calculation looks quite daunting but if you follow step by step and look at the example it is not so bad.

As you say the difficulty is finding the n/m2 of the fixing, I wouls suggest the fixing bolt would be the require figure, try hilti fixings

In addtion to other guidance the National federation of roofers have just published an interesting document advising that any fixings to rafters should be pre drilled.

 

[CG_Sparks]

Hiya, thanks for that, the 3rd edition tells me that i should use BRE 489 for the pressure coefficient value, as it's a retro fit install.

What i meant by i don't understand is, at the end of the equation, you come up with a number for upward force and downward force measured in newtons.

it then says in a paragraph at the bottom that all modules, roof hooks and fixings should be able to take the upward and downward force as indicated and that for example, one roof hook should be able to take 1/4 of the force (if using 4 roof hooks)

How do i find out this information? - If it's too complex i'll have to find structural engineers other than Brewsters.

thanks for you help guys

 

[dansk]

Divide the figure by the Kn of the hook = number of hooks to use.

 

[Green]

The resistance force provided by the anchor is going to be largely dependent on the fixings you have used to secure the anchors to the rafters. Some manufacturers have this data available online - if yours don't then non product specific guidance can be found in the Eurocode docs. Hope that helps...

 

[Noddy67]

Dansk is right but another way to look at it is to divide the Newton number by 10 to get the Kg figure.

so if you end up with 20000Newtons, it's 2000kg. If you use schuco anchor which holds 100kg each you'll need 20 of them.

remember to distribute them in relation to the loading factor... ie more at the edges, even more at the corners (edges and corners being 10% of the longest roof lengh).

 

[Worcester]

And then you need a structural enginner to apply the uplift calculations to the roof timbers...

 

[toomanylogins]

We have just updated our sap pv calculator to include a wind load calculation which is compliant with BRE 489. It calculates the force per panel and the total for the system. I will have a demonstration version available tomorrow 12/11/11. if you Google for "MCS QMS SAP" you should find us.
Regards
Paul

 

[Worcester]

@toomanylogins

Is it compliant with and does it follow the steps as outlined in the Draft of the 3rd Edition?

Will you warrant the results?

 

[toomanylogins]

@ worcester

Its compliant with BRE 489. As the 3rd edition is a draft we will wait until its published and then update the software.

Software is never warranted because of the incompatibilities of installing on different systems. We are no different from anyone else in that respect.

 

[Worcester]

If it is not warrantied, then almost everyone issuing technical software of this nature will have it independently validated by in this case one of the well known Civil Enginnering Consulting Companies, are you planning to do this?
Else, how can we be sure of that the alogrithms have been correctly applied?

 

[toomanylogins]

This is a red herring the "Civil Enginnering Consulting Companies" you mention will use excel or similar to check it which iteslf is sold without warranty. For example the BRE use Excel to approve the results of our SAP software.

In a software context warranty is meaningless because of interdependency between software packages and operating systems. A much better measure of quality is customer feedback through forums such as this one.

 

[Andrew Bradbury]

hi i carnt find your software what is the name of it or could you email me at [email protected] please with a link or somthing so i can try it out.

 

[pwboston]

can you just confirm what you mean by hook etc.

If I have calculated that a 16 panel system has the following results

44.296KN Upwards acting
34.074KN downwards acting

how do i use this figure? is it against the clips/hooks holding the panel down, or is it the number of fixings holding the bracket to the wood, this is my only query, the windload calc is ok its knowing what to do with it im stuck with.

If its on the schuco fixings for instance, IE 28 mid and 8 end clips then its 44.29 divided by 36? what do you value your result against?