Wind Pressure Return Period 1

[KAR FUNG]

Dear Sir,
Could you explain what is the reason for the wind pressure on building the design return period must be 50 years? Why cannot be 100years or 20 years?

Thanks.

 

[Lomarandil]

Unless a specific building lifespan (and design return period) is required by the local building codes -- you are right, it's an arbitrary convention that society has agreed is appropriate.

As such, it would be OK to design a structure for some other lifespan/return period. But because you would be changing the "standard of care" commonly applied in engineering, it would be important to have an in-depth discussion with the building owner* about the consequences of such a decision. And I'd want to make it obvious on any design documents (in some cases, even a posted sign might be appropriate) so that any future owners/engineers/etc are aware.

* If the building is financed, that might also come into play -- I'm not sure.

----
just call me Lo.

 

[driftLimiter]

Like Lomarandil says, society agreed this appropriate. Here society means professional engineering societies, insurance agency's, public officials, financeers, contractors, etc have attempted to balance risk and economy. Using statistical analysis they 'agreed' upon a standard convention for return intervals for the various transient loadings. Also worth noting that design methods set forth by the building code (LRFD, ASCE 7) for example, use adjustments on the design side to scale the wind forces to different levels of return period for different risk category buildings. The levels of wind pressure (for example) that the code supplies are statistically balanced to match those pre-determined return intervals.

Also extending on Lomarandil's point, it is important for owner's to understand the code level design especially for seismic. Most people think if a building is engineered then it is earthquake proof, I have seen some engineers require their client sign an acknowledgement that they understand that the code level of seismic design implies damage at the design level earthquake, maybe so much damage that the building is torn down.

Just digging into a bit more, we can think of the consequences of altering the return interval up or down a bit.

-If you design for a 20 yr return interval level of wind, it would be a lower value than the 50 year. And the risk that the design wind is exceeding during the life of the building is increased.
-On the flip side you design for 100 yr return wind, there is less chance that that level wind event would occur during the lifespan of the building, this means perhaps a lot of the value of structure is being wasted over the life of the building and 'society' has determined that the additional cost is not worth the extra reduction in risk.

the ASCE 7 commentary, and any reading on LRFD design methods and risk factors would shed further light on this topic.

 

[dik]

...just as a caution, just because you design the structure for a return period of 10 years, doesn't mean that it can't experience two hundred year storms, next year... it's just unlikely.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[RPMG]

The return period is associated with the risk category, and the return period is nonsense. Quoting from the commentary, "Similarly, the 25-year return period wind speed associated with Risk Category I buildings equates to a 300-year return period wind speed with a wind load factor of 1.0." What they're trying to say is that Risk I, II, and III have probabilities 1/300, 1/700, and 1/1700 of having the wind load being exceeded in any given year. I believe that those numbers are based on statistics from collecting 50 years of hurricane wind speed data, so they normalized to that.

 

[Lomarandil]

Good point RPMG -- I don't use return period in discussion if at all possible, I talk about probability of exceedence (either on a year-to-year basis as above, or over the agreed design lifespan). Your average person doesn't understand the difference, and return period tends to be misleading.

----
just call me Lo.