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.

 

[Greenalleycat]

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.

Yea basically that - the house is two story but roof over the ground floor extends outwards like a skirt to create some sort of inspired architectural design/expensive engineering headache

I know what you mean about the $$ thing - it is so incredibly frustrating, especially in residential
You're damned if you do and damned if you don't hold them to a high standard

 

[phamENG]

US codes are silent on wind drift serviceability. Deflection of walls and beam/floors/roofs under wind loads are based roughly on a 10 year MRI.

For lateral drift, I usually do 10 year MRI with H/500 as my target, but willing to go down to H/400 in some circumstances. I also try to limit story drift to 3/8" as more than that and drywall damage is more likely.

 

[Greenalleycat]

Interesting, our Code racking limit is H/300 under a 1/25 year event - significantly more friendly than your limit
I think this is too relaxed personally but that's the world we live in eh

Our beam deflections are L/300 to L/500 depending on the particular case, but I don't think these work so well for inherently flexible / low redundancy systems like long roof cantilevers
The relevant metric IMO is absolute deflection but there's not a lot of guidance on that

 

[phamENG]

At least your code has a limit. Mine is self imposed - most of the houses I design are also at the beach and are owned by lawyers.

Here's a clip from the commentary from ASCE 7-16 (defines loading for structures):



Here's deflection limits from the IBC:

 

[Greenalleycat]

Ah yes, lawyers, the exact kind of client Ron was discussing...

Our code sadly does not give "limits" - it gives "recommended limits" which means you can still be sued even if you meet them
The plasterboard one is particularly baffling as it's based on H/300 testing data but, if you look at the hysteresis of plasterboard, there is a huge stiffness loss at >2mm deflection
So by the time you reach your 'serviceability' limit, the plasterboard is irretrievably rooted

 

[Ron247]

most of the houses I design are also at the beach and are owned by lawyers.

I love lawyers that show me how a golf ball rolls across a floor. I ask them what should it do? They say stay still. I say even if someone designed a floor so flat the ball would stay still, it will roll after they put furniture, belongings and themselves in the room. They don't generally understand there is such a thing as floor sag.

Also had one show me that all 4 corners of room were out of square by using a framing square. They put it in each corner and it would rock back and forth indicating more than 90 degrees. I explained a 4-sided box CANNOT have all 4 corners greater than 90 degrees. The built out drywall compound in the corner was causing the rocking. I told him to clip the corner off his square and see if it does better. He thought I was just making it up.

 

[Tomfh]

The difference between mandatory and recommended serviceability limits is an interesting one.

Australian codes usually offer “suggested” or “recommended” serviceability limits, which everyone tries to meet, and often exceed, because the minimums often aren’t enough to stop your phone from ringing.

In the cases where the recommended limits aren’t actually met, the standard fallback in my experience is for engineers to argue they’re only guidance, not hard requirements, and that the mandatory standards, like strength, have still been satisfied. In that sense, it’s a bit regrettable they’re not more strict, because the non-mandatory status often gets used to excuse serviceability problems.

However that being said I’m not sure I’m entirely “sad” about serviceability limits being non-mandatory. It’s one of the few areas where engineers can still apply judgment.

 

[human909]

It’s one of the few areas where engineers can still apply judgment.

It is a little sad, though also understandable, that so much of "engineering judgment" has been taken out of things....

Thankfully for myself many of the projects I am involved in, engineering judgment is alive a well! I'd like to believe that in "normal structures" my designs are efficient but also have healthy safety margins over and above the minimum required. But I also would enjoy to take "liberties" away from the straight jacket of codes. And the nuances of some of my work does allow that.

I have spend years working for a D&C firm with a large variety of projects. That flexibility has allowed me to enjoy "liberties" beyond the straight jacket of codes.

I have had two arms length mentors. One who is largely plays within all the rules. The other seems to try to break as many as he can get away with, but does so very much knowingly and with a fantastic engineer mind. It is a good balance.

 

[phamENG]

so much of "engineering judgment" has been taken out of things....

But...has it?

Is it engineering judgement that has been taken out, or is it that the uncertainty that comes from a lack of data and experience? Because let's face it - does any one engineer, no matter how long they've been practicing, have the breadth of knowledge or seen enough of what can happen to a structure to provide a consistent level of reliability to the built environment? I'd say no.

The current code environment is far from perfect. The codes are complex, and perhaps overly so. Particularly for engineers working on small to smaller medium sized structures (which is probably the majority of engineers). But do we want to leave the determination of how deep an anchor bolt should be embedded to an engineer's judgement, or do we want to leave it up to empirically derived procedures based on a large body of real world experience from many engineers and techs. Do we want to decide what strength of an earthquake a building should be able to decide based on what an engineer thinks it should be able to resist, or based on what geologists and seismologists have determined the local fault can do (looking at you, Seattle). Sure, there's some absurdity buried in there - such as the shear question in ACI 318-19 - but it's an opportunity to better understand the research process and figure out where it came from, why it is or isn't wrong, and see if there's a better way to do something. In other words, it forces us to understand something more deeply, increasing our ability to make a sound judgement (if in a roundabout and infuriating way).

To me, it's a mindset thing. The codes don't tie my hands. They establish a minimum standard. It's my job to use my judgement to determine if the things that happen in the real world meet that standard. No building goes together exactly the way I draw it, and a lot of buildings were built before these standards were set. Figuring out if they can be made to meet the standard, and how, requires judgement and creativity.

So, to me, the practice of engineering judgement has been improved by giving us better and more consistent standards to base that judgement on.