Wind Loads: Dynamic or Static 2

[spaseur]

We are having a discussion in our office as to whether wind loads are considered static or dynamic. We have a geotechnical report that give 2 different allowable axial capacity. The report gives one value for static loads and a lower value for seismic loads. It does not mention wind loads. We may reach out to the geotech for clarification. But was wondering what some of you may think? Would you consider wind loads as static or dynamic?

 

[phamENG]

You have two different questions here:

1) Are wind loads static or dynamic? The answer to that...it depends. What is a static load? Me walking across a room is not a static load, but it's wrapped up static live loads. Wind loads, as defined by the codes, are static equivalents. So you would look at them as static loads of short duration.

2) Why are static and earthquake loads given different capacities? More than likely, you have some sort of liquefaction potential. So that's not a capacity for a dynamic load - it's a reduced capacity because when the soil starts to move in an earthquake it looses its ability to support the building above. Remember, the building doesn't exert a force on the soil in an earthquake - the tectonic plates move, which moves the soil, which moves the building. So it's actually the soil pushing on the foundations that cause it to move.

 

[Greenalleycat]

I completely agree with pham's view on this one
You'll have to ask the geotech for clarification as ultimately only they can answer it though

Sometimes here we get higher values for seismic as code allows less reduction in soil strength for seismic shaking compared to sustained gravity loading
However this assumes no other issues e.g. liquefaction

 

[spaseur]

I was thinking wind loads are considered by codes as static loads, but could not find that statement in the code or commentary. Do you know where in the code or commentary it says wind loads are considered static equivalent loads?

 

[Greenalleycat]

That will depend entirely on your code. Ours doesn't really make it specific, beyond having a few words around "equivalent static wind force" in some sections of our wind code
However, there is no real distinction made between 'static' and 'dynamic' in our codes
The consideration is given to the load origin itself, which is better described as sustained (dead, long term live), transient (live, wind, snow loads in most areas), or seismic
Wind receives no special treatment for 99.9% of structures

 

[spaseur]

We are under ASCE7-16. Could not find anything in the code saying one way are the other. Do you know if wind is considered a static load in ASCE7?

 

[ThomasH]

Wind loads are dynamic but can often be considered as static or rather static equivalent. It all depends on how the structure you are designing responds to the wind load or perhaps more correct, the air flow. There are a number of parameters in this but one of the more inportant is the structures natural frequency.

I don't know what type of structure you are discussing. But typically I would not expect a geotech to define if wind should be considered static or dynamic in a specific situation. I agree with phamENG, is is probably more a question of how the soil responds to static vs dynamic loads.

Thomas

 

[KootK]

Normally, your wind capacity will be higher than your seismic capacity because of its transient nature. If your site actually has liquifactiion potential, that's a pretty big deal and I would expect that exact phrase to show up somewhere in your soils report.

 

[spaseur]

We do have liquifaction at this site. It sits on the beach in a fairly high seismic area.

 

[KootK]

I would expect the lower values for seismic to be a result of the cyclic load reversals increasing the soil pore pressures and, thereby, temporarily reducing shear strength.

 

[KootK]

In that case, disregard my previous statement

 

[Greenalleycat]

Well your answer seems pretty obvious then
The distinction between capacities provided by the geotech seems to be for liquefied vs not
Is liquefaction likely to occur under wind? (no)

 

[KootK]

Remember, the building doesn't exert a force on the soil in an earthquake...

I was one of the original purveyors of that philosophy around these parts, perhaps even the OG (unfortunately). Regardless, I've come to view that line of thinking as false. In a dynamic scenario, I'm now quite confident that the building does, in fact, exert a force on the soil. Or, at the least, the "real-ness" of that force is every bit as truthy as the "real-ness" of the force that the soil exerts on the building. And this is marvelous news, really, since it means that general equilibrium is not violated in dynamic situations.

I've been meaning to issue a mea culpa on this with some more commentary. Soon... ish.

 

[driftLimiter]

@KootK you better hurry up on that mea culpa. I badly want to see the 'current' thinking on this, I wont say anything about it for the time being.

@spaseur As others have indicated I suspect that the reduced capacity provided is so that footings are large enough to endure seismic liquefaction. Better confirm with the geotech just to be safe.

Usually we see an allowable stress increase for short duration loads (wind / seismic). But it seems like here you have a different scenario.

From your original post it seems like the point is moot anyway....

The report gives one value for static loads and a lower value for seismic loads.

If that is the actual labels of each of the loads it seems to clearly indicate seismic loads as it's own category. Not dynamic, not transient, not wind.

If in fact it does say static vs dynamic loads then the geotech should clarify reg'd wind loading.

 

[KootK]

I badly want to see the 'current' thinking on this, I wont say anything about it for the time being.

Excellent, and thanks for the latitude. As a teaser, ask yourself by what means the ground causes the building to move? Or, more generally, what does it really mean to "push" something? You're going to default to something like "blah, blah...a push generates a force... blah, blah". Go deeper. What is a force and by what action does it cause things to move? I've come to see "force" as a purely operational definition rather than an absolute one with true physical meaning. Force = that which creates an accelerative tendency, positive or negative. The end.

Once you wrap you head around the reality of things at the below grade force that you can see -- or think you can see -- the situation above grade becomes a lot more palatable.

Don't get me wrong, none of this will matter one lick with respect to how real engineers prosecute real work. I just find it fascinating to try to tease out the truth of this stuff.

 

[KootK]

In the past, I've somewhat mockingly asked "what is it that would push upon the superstructure, the hand of God?". I'm atheist, for now. Turns out, it kind of is the hand of god in the sense that it's intimately connected to the very fabric of the universe and quite inaccessible to most folks not named Einstein. And even Einstein never quite got it all the way sorted.

 

[driftLimiter]

okay okay im already hooked. thanks for bringing this up to push our thinking , sometimes my mind tends to want to stay either at rest or in motion.

 

[phamENG]

Regardless, I've come to view that line of thinking as false. In a dynamic scenario, I'm now quite confident that the building does, in fact, exert a force on the soil.

If you mean it the same sense as a building exerts a force on the air when the wind blows and the floor exerts a force on my foot when I walk on it, I agree.

Of course, there are two sides, too...the side where the dirt pushes on the building to create positive acceleration and the side where the dirt pushes against the building to create negative acceleration. If that's what you mean, I'd not say it's false, but perhaps an incomplete picture.

Either way, I'm curious and look forward to hearing your thinking.

 

[Tomfh]

For general design purposes I’d consider a wind load a transient static force. I wouldn’t call it “dynamic” until we’re concerning ourselves with the dynamic response of the structure.

 

[phamENG]

And even Einstein never quite got it all the way sorted.

Are you about to tell us that buildings are quantumly paired such that shaking in my building will cause yours to shake?