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?

 

[Greenalleycat]

It seems pretty clear that the building must exert a force on the soil in at least some way.

Imagine a case of a building over soil over bedrock. There is some static, assumed equal, compression stress on the soil resulting from the building weight
The EQ shakes the bedrock which transfers force to the soil via friction which transfers force to the building via friction on the underside of the foundation
That force is horizontal only as the shaking, in this case, is assumed to be horizontal only

Question: does the magnitude of the compression force under the building change at all?
Obviously it does, due to overturning caused by the lateral force distributing up the building leading to larger compression one side and reduced compression the other
How does the soil experience a vertical force when it is only applying a horizontal force to the building?
Given that the soil is only going left-right we have to conclude that the building is somehow applying vertical forces onto the foundation
Therefore buildings most definitely apply forces to soil and not just the other way round

QED thanks for coming to my TED talk

 

[Tomfh]

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

It doesn’t? How does the building move then?

 

[phamENG]

I get it. My initial statement, which was aimed at identifying the primary load path, was technically incorrect. Newton's Third Law and all of that.

My point was that the forces from an earthquake are, at least around here, most commonly "equivalent lateral forces" because there is no external force being applied to the building at those elevations. The ground pushes on the building (yes, the building pushes back) and moves. That causes acceleration of the mass in the building equal to the force applied by the ground to the building. So those forces are being applied from within - the dirt pushes on the foundation, the foundation pushes on the column base, the column base pushes on the beam/vertical brace, etc. And then, when the ground heads back in the other direction, it pushes from the other side.

 

[driftLimiter]

Pulls out a SDOF oscillator physical model. Stares at it blankly... puts it away until Koot has an appropriate thread for discussion.

 

[Tomfh]

My point was that the forces from an earthquake are, at least around here, most commonly "equivalent lateral forces"

Oh you meant our force arrows on the building (which combine into base shear) don't just appear from thin air and start pushing on the building?

 

[KootK]

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

Prefect, that's what I was going for. Best to leave the details for the mea culpa. It would be astonishingly impressive if anybody were able to guess where I'm headed with this. It took me 20+ years of reading pop-sci books every Christmas to arrive where I'm at and my understanding is still ridiculously shaky. I've not peered behind any curtains per se but, rather, feel that I've glimpsed enough of the shadows moving around behind them to be able to say a couple of things that will be worth discussing.

You'll recall that, earlier in the year, I was questioning the existence of "mass". This is very much related to that. I've got mass sorted now +/- and it was nothing more than me not paying attention in 11th grade physics. Mass = the subcategory of energy that we typically associate with stationary things in a particular frame of reference. That's all. I was thrown off by E=mc^2 seeming to convert mass to energy. That's somewhat of a read herring in that mass was always energy all along. The conversion is from the energy of stationary matter to the energy of matter travelling at the speed of light.

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

I wish. My quantum game is non-existent. I'm on track to not even have figured out general relativity before I croak.

 

[phamENG]

Oh you meant our force arrows on the building (which combine into base shear) don't just appear from thin air and start pushing on the building?

That's about the gist of it.

KootK - I await it with bated breath.

 

[KootK]

Pulls out a SDOF oscillator physical model. Stares at it blankly... puts it away until Koot has an appropriate thread for discussion.

Ha! I'll stamp something, walk the dog, and then put pen to paper on this. Clearly, it's time.

I think that part of the confusion on this is that most of us learn about inertial forces in the context of D'Alambert's principle. He called the dynamic inertia forces sets "fictitious forces" simply because his work preceded Einstein's and he didn't know what else to do with them. Still "spooky action at a distance" and all that jazz. So most of us tend to have latched on to inertial forces being fictitious as well. It turns out that "fictitious" inertial forces can do all of the same things that other, "real" forces do: accelerate things, generate heat, keep the universe from falling apart... The only thing "fictitious" about inertial forces is that typical human experience give us no great intuition for their origins.

 

[WARose]

Only time I've ever considered wind loading as anything other than static is when I am thinking about some type of vortex shedding (or other amplification). But that normally only happens with fairly flexible structures.

You may want to ask the Geotech if the seismic short term allowable is also applicable to wind. But my bet would be "no". (Not sure if I've ever heard of liquefaction limit under wind load.)

 

[MotorCity]

I think everyone can agree that in reality, wind and seismic are dynamic loads. For most of us everyday practicing engineers, they are more often than not treated as equivalent static loads (ASCE 7 blatantly refers to, arguably, the most popular method as the Equivalent Static Procedure for seismic design). This is to bridge the gap between the PHd's in the research world who create incredibly complex models and equations to predict these loads and the engineers working in the trenches who don't have the time or motivation to solve a differential equation only to find out that the wind loads is 15 psf.