Wind Loading on Temporary Mobile Structures

[Steve Douce]

Hi All,

Just looking for some opinions around wind loading on Temporary Mobile Structures. An example would be a mobile access platform on wheels or a skid etc. 

The ACBC Standard for Temporary Structures specifies guidance around reduced wind speeds, depending on the wind region and approved installation duration. 

Unfortunately the standard wasn't written for these types of structures, accordingly there is nothing in the reduced wind speeds to account for the time taken to dismantle the equipment, relocate to a protected area, or secure in accordance with a tie down arrangement. 

Say for a mobile structure, which may be used year round (for which no wind speed reduction is applicable), would it be appropriate to apply the 1 week reference period to reduce the wind speed? 

I'm interested to hear some opinions and experiences in this area. Thanks 

[Shane Groom.2]

Like all things, the answer might depend on context. And it can vary state to state.

While it is risky to deviate from the code, if you can prove the same reliability, then you meet the safety standard. The ABCB is based on a published article, so you could base your design on the article.

If it is going through the VBA, you will need a performance based solution and a wind management plan. As you suggest, tie it down if it gets too windy.

It's reasonable to look at wind forecasts say 48 maybe 72 hours maximum. If you are doing a specific activity,  then you can be sure it will be safe if it only takes 1-2 days.

If it is in use for 1 year, but it can be tied down in a few hours with no notice, then it may be OK with a monitoring and action plan.

If the worksite is closed for 48 hours or more, you should be tying it down, one would expect.

If none of the above can be done, you may want to follow the code.

 

All advice is general and you should consult an engineer. No liability will be accepted for anyone relying upon the above.

[Shane Groom]

Hello,

I tried to reply to this on my way to work, but for whatever reason I think the guest log in didn't work.  

The ABCB tables you are referring to; I am pretty sure it came from the paper C H Wang & L Pham (2011) Design wind speeds for temporary structures, Australian Journal of Structural Engineering, 12:2, 173-177.

If you want to reference a research paper, as long as you achieve the same level of reliability as the minimum in Australian Standards, then you have satisfied the requirements.

If something is certified via the VBA, it can be a Performance Based solution.  Typically this may need a wind monitoring and action plan.  E.g. if strong winds are predicted or measured, the structure is tied down.

Even if not certified through VBA, you may be able to take a similar approach.  If you can tie down or stow plant or equipment in a few hours only, you can have an in-service wind gust which is quite low, and out-of-service is as per AS1170.2.

Wind gust predictions are likely only reliable for say 48 hours ahead.  If site is closed for a long weekend, for example, everything would need to be tied down.

You would like need to prepare and document a wind management and action plan to satisfy the Contractor or their Engineers.

Also, requirements can vary state to state.

 

All advice above is general in nature and does not consider your circumstances.  Always consult an Engineer prior to proceeding.  No liability will be accepted for anyone relying upon the advice above.

[Steve Douce]

Hi Shane, thanks for your insight, much appreciated. 

[Barnabas Ilko]

@Steve Douce, Shane's response is spot on. You have a few options to choose from and I would recommend looking up the research paper by C H Wang and L Pham as that is the basis of the tables in the ABCB Standard. That particular standard does not cater for all temporary works and you might want to take a different approach, or as suggested, a performance based approach. 

I have no experience with mobile structures, but I would think the same rules apply as for static structures. If you are using an equipment by a supplier, they should have specification for certain use. If it is custom built, I would use the above advice. I would note one item though. You should assess the risk of the equipment toppling over if the wind picks up before you can fix it down. You should assess what is in the surrounding and if failure of the equipment can lead to other H&S hazards, I would recommend using a higher FoS for overturing.