Opinions Wanted - Lateral Loads Types - Which is more accurate?

[Trenno]

Would you agree that, based purely on a codified approach, earthquake loading can be quantified more accurately/realistically compared to wind loading?

Would love to hear opinions.

 

[KootK]

I'm gonna roll the other way and say that wind load predictions are more accurate.

- we don't even try very hard to get EQ loads right. Instead, we go straight for force reduction modifiers and capacity design to avoid the need to be right.

- wind depends a little on the structure (stiffness/damping) and mostly only for taller things. Earthquake depends massively on the properties of the structure for structures of all height.

- I'm assuming that code EQ means equivalent lateral force method rather than modal, time history, or performance based. ELF apprioximates buildings as single degrees of freedom deforming primarily in shear. Pretty crude.

- for wind, you're keeping things closer to the elastic range. I feel that response us more accurately determined in that range.

I showed you mine now you show me yours damn it. Why do you think that EQ loads are more accurate?



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MrHershey]

Agree with KootK. Both are approximations and it's always important to remember that, but EQ as of right now is much more of a crapshoot. Building performance and even loadings are heavily dependent on building properties (which are highly dependent on modeling assumptions that may or may not be real). Could argue wind is similar, but with wind you're not neglecting a bunch of the load because you're assuming your building will behave a certain way. In earthquakes you are (assuming not doing performance-based design), even for buildings with no special detailing requirements you can use R=3 and eliminate over half your force.

I think the actual input (ground motion) may have an argument for being more accurate but even there you're not comparing apples to apples for code-level inputs because the wind return periods tend to be shorter than seismic.

 

[KootK]

In North America at least, you've also got that 2/3 "whatever factor" on the input ground motion. Not super confidence inspiring.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

A bit 'old' but nonetheless a good read on EQ forces and design:

Priestley, Nigel, “Myths and Fallacies in Earthquake Engineering - Conflicts Between Design and Reality,” Recent Developments in Lateral Force Transfer in Buildings: Thomas Paulay Symposium. ACI SP-157. 1993.

 

[msquared48]

Regarding seismic: We don't even know where all the active faults are, or the long term history the many that are known, let alone the trip mechanisms with certainty, and probably never will. In reality, this is really still a very young field.

In my opinion, engineering for seismic forces will always be a crapshoot. There are really just way too many variables.

Mike McCann, PE, SE (WA)