AS wind load and overturning failure 1

[bratty]

It is realistic to consider 3 sec gust wind load in Aust Standards to check the overturning failure of a tower crane? Does it means a wind exceed the speed acting for 3 sec or a few second longer will topple a tall structure? Whereas the Eurocode is using 10 min wind speed to topple a s tall structure. For a force acting 3 sec as per AS and 600 secs in EC on a structure is a lot of difference to fail a structure. Which is more logical?

 

[Settingsun]

I think the Eurocode uses 10 minutes as a reference velocity then uses a turbulence factor to bring that up to the peak velocity. The Aus code uses the gust velocity as the reference velocity so saves one calculation stage with similar result.

Eurocode also uses a load factor to scale up the 50 year recurrence wind whereas Aus starts with the design recurrence interval.

 

[Agent666]

I've designed a few crane foundations, and you just use the standard AS/NZS1170 loads. There is no additional scaling required. Typically the overturning loads are given by the crane manufacturer.

As for which is more logical, as steveh49 eluded, they are attempting to give you similar approaches, but through a different path.

From my own perspective/opinion, using the peak velocity and hence peak pressure sounds more logical, but then that's all I know really having been trained in New Zealand.

Someone from the other side of the world might find the Eurocode approach more logical. Horses for courses really depending on how you were educated to think about things I guess. The end result is structures designed to either standard are intended to have sufficient margin against collapse as they are all intended to be benchmarked to the same level of safety index/reliability.

 

[IDS]

I'd be interested to see the final design pressures from the two codes, if anyone has done the numbers.

Whatever the results, I'd not recommend fine tuning wind loads by cherry picking values from different codes, especially for checking overturning loads on cranes.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Settingsun]

Re-reading the question: a wind gust lasting longer than three seconds shouldn't cause the structure to fail if the structure can handle the three-second load. There's a general assumption that the capacity isn't significantly reduced by load durations over which a wind load can act. So, if the structure can resist 100 kN (for example) for three seconds, it can also resist 100 kN for ten minutes.

Some materials do have lower strength for sustained loads, eg timber and some foundations (soils), but the ten-minute average wind load is less than half of the peak load. The capacity of the usual structural materials and foundations doesn't halve over a ten-minute period so the longer duration wind load isn't critical.

 

[Agent666]

I'm not so sure either that the peaks are based on 3 second gusts anymore either. It used to be the case, but it changed in the 2011 version of the standard.

The footnote to table 3.1 suggests 'the peak gust has an equivalent moving average time of approximately 0.2 seconds'

 

[Settingsun]

Agent, do you use a low return period for the crane foundation, like a temporary structure? I wouldn't myself as the crane is transportable rather than temporary, but wondering what the usual practice is.

 

[Agent666]

The suppliers usually have a data sheet that gives you the maximum overturning and slewing loads. I think they are actually based on a shorter design life on the assumption that the crane is only there for the construction period (I'd have to go searching to find one in my records to confirm). But even if it didn't I think you'd factor it down to something reasonable like less than 6 months or 5 years. It is intended to be the design installation life in accordance with AS1418 series of standards.

In terms of the foundation for a tower crane, it is considered temporary in that location if you like. The crane itself would of course need to be designed for the full design life that it will be in service.

The information they give you is very cryptic, often you are not even entirely sure what loads are or are not included, and sometimes a different configuration in terms of jib length or height. They are also envelopes so you cannot even back calculate out just the wind or seismic loads. Basically they make it really hard to determine what basis the loads were determined and they are not presented in terms of any of the tower crane load-cases from AS1418 for foundation design (refer to AS1418.4 table 2.4.3 for support structures). So you need to use a lot of judgement.

Half the time because the cranes have been on the market for a decade or two, a particular model might only have wind loads for older versions of standards (like NZS4203, the NZ loading standard before we adopted AS/NZS1170 joint standard).

So often we used to just take a huge load factor like 1.5 on the basis that the largest load factor in AS1418.4 is 1.6 for actions that contribute to the overturning and apply it just to be sure we weren't going to be the subject of the next YouTube crane collapse montage video... it was a fairly crude approach, but if something is likely to see it's design loading it will be a crane. Given the difficulty of sourcing information in the first place on loads and the frequency in which cranes fall over I'm surprised the suppliers and manufacturers don't have more complete information available to make a better assessment. I guess they sell more cranes if they fall down?!

They have in the past offered to provide more information or a specific assessment by going back to the original manufacturer and for ~1500 Euros provide some updated information. Never really took them up on that though.

The last few I did a few years ago and the crane is still up, as its part of a project that has seen it's fair share of troubles in the construction. 1.5 year total build time turned into 5 years now and still going (read 3/4 finished now)! So like you say taking a smaller design life has it's downsides.

 

[Settingsun]

Sounds like cranes have a lower safety expectation even if they will be in service as long as many 'permanent' structures.

 

[blihpandgeorge]

i was curious about the 0.2 second noted, and from the AWES Wind loading handbook it explains that the previous 3-second gust reference in earlier standards was 'incorrect' and the new standard uses a equivalent moving average time of approximately 0.2 seconds as noted by Agent666 above

 

[bratty]

Steven49
For 3 sec gust, the wind velocity is higher than 10 min wind velocity in Eurocode by a factor of 1.5 to 3 depending on the region. I do not know much about turbulence being added as it will dig into the environment condition. But I think they added a partial load factor to the wind for ultimate limit state whereas in Aust Standard it is directly an Ultimate Load. Problem is that, we are using European Manufactured tower crane and the manual is based on European Version and I doubt they co-relate to Aust Std.
3 sec gust gives a higher wind load which is appropriate to small and light weight structure. But it is relevant to large heavy structure where it's response inertia takes much longer?

 

[BJI]

The change to an equivalent moving average of 0.2 seconds was to reflect the current measurement practices in Australia (cup anemometers vs the Dines anemometers previously used). You should not need to adjust any historical data based on these differences, it is of more relevance to anyone looking to take wind readings. There are some minor differences between the international codes but overall effect is relatively negligible, for most applications I would just use the locally published data. There is a good publication by Holmes and Ginger (The gust wind speed duration in AS/NZS 1170.2) that is worth a read if you want to get into the details

http://www.acsev.org.au/images/document/47_1.pdf

They also provide some comparison between international codes that should answer your question.

 

[Settingsun]

I would say the reduction due to inertia is not much, maybe around 5-10% as an order of magnitude. The response time is still fairly short, measured in single-digit seconds I'd imagine.

 

[human909]

ISO 4353:2009 has some good references to comparison between standards. Page 10 is helpful.

https://www.slideshare.net/aronmichowski/iso-4354-2009-ethiopian