Do you ever consider a '1 in 1 year return period' wind

[Greenalleycat]

This topic coming at you from some office musings

In the residential world it's pretty common to get lovely things such as a house on the side of a hill facing an ocean
These sites get the lovely combo of both higher wind loads due to the ocean/terrain combined with regular/reliable winds i.e. a higher frequency of windy days than inland sites

These types of houses also inevtiably have lovely architectural features such as 4m cantilever floating roofs (definitely not basing this on a real job...)
For such features, the cantilever tip deflection is a critical consideration - the client doesn't want to see a permanent sag, and they don't want to sit inside and watch it flop around in a wind
So we design to a serviceability condition that is based on a 1/25 year wind - say L/150 or L/300 maybe
For our 4m cantilever that still equates to a tip deflection of 15-25mm which I think would be a bit displeasing to watch - but hey, it's only 1/25 years, right?

Enter the 1/1 year wind - under our code, the 1/1 wind is 63% of the loading of the 1/25 year wind
So you'll be getting ~10-20mm of tip deflection occurring once a year (and you can keep reducing that fraction for more frequent winds again)
To me, even 10mm of deflection occuring ever year feels like it could be enough to get a displeased owner ringing you up for an explanation - they do say it's serviceability issues get you sued, not ultimate issues, after all...

Is a 1/25 year wind actually the right metric to be considering? Maybe, as I'm not aware of issues in reality?

 

[ANE91]

Up to the client and their wallet.


For a house in Tennessee: we showed our fairly sophisticated client the different statistical windspeeds broken down by return period to include/exclude thunderstorms and compared that to ASCE’s curve (right column). Then we showed them how that turned into deflections.

A 4m cantilever might just be flexible enough for relatively low winds to induce a bit of dynamic excitation. I’d prepare them for a bit of wobble no matter what.

 

[human909]

IMO the risk/return doesn't seem make sense for an engineer to be playing those games. With longer spans and cantilevers it is pretty much all about serviceability in the residential world.

These types of houses also inevtiably have lovely architectural features such as 4m cantilever floating roofs (definitely not basing this on a real job...)

I remember seeing one of these type of designs. Except there was an ugly great column in the corner. Definitely looked beautiful, long and thin with no ugly column in the corner. Then along came the engineer and introduced them to reality.

I agree with ANE91, I'd also be worry about dynamic excitation. I got burnt by slender SHS fluttering in even moderate winds. I believe it was vortex shedding. Either way, I was the one who first noticed it and being D&C we just fixed it. But it was a lesson learnt. All the normal load checks were quite conservative, but I didn't considered vortex shedding induce vibration!

 

[Ron247]

These types of houses also inevitably have lovely architectural features such as 4m cantilever floating roofs

When you say "floating roof", what are you describing? Is it the fact a cantilever sags/rises at the free end?

I would think serviceability issues are difficult to explain after the fact rather than before the project is built. The Owner does not want to deal with them and after it is built, it is someone else's fault to them. If you explain before, then you are not only appealing to their judgement, you are also talking to their wallet.

I remember a contractor who everytime he asked his client a Time-Cost-Quality question, they answer was "whatever is the cheapest" without even thinking about it. By the 10th time of asking T-C-Q questions, the contractor decided he knew what the answer would be, so he just did "whatever was cheapest". For every serviceability issue like we are discussing here that the Client was not asked about, the Client was suddenly Diamond Jim Brady, money was no object. And therefore, thought the contractor should make all those corrections at his own expense.

 

[Tomfh]

For every serviceability issue like we are discussing here that the Client was not asked about, the Client was suddenly Diamond Jim Brady, money was no object. And therefore, thought the contractor should make all those corrections at his own expense.

Sounds familar lol. There is wisdom in this. For possibly contentious matters, get it in writing whether the client wants the code minimum, or something better.

 

[human909]

As a builder pointed out to me recently. He has to give a 6year warranty on the home based on local laws. It doesn't matter what the CURRENT owner agrees to regarding serviceability. If the owner in 4 years time takes issue with the outcome he will have problems.

We were discussing the architects desire to shift the location of the sink and toilet in a bathroom when the building was 75% completed which would have required local excavation of the concrete slab for the replumbing.