Seismic design takes advantage of larger damping owing to structure ductility to reduce the design load by R factor (of 3 to 8 value). Apparently, both seismic and wind governing loads are derived from similarly rare event at about 500 years return period, although some recent codes use 2/3 of MCE 2500yrs for seismic. But why only seismic design allows some structural yielding and acceptable damage, while wind design should remain elastic? Is there some concept that I misunderstand?
I think earthquake forces are impulsive in nature and they also act back and forth demanding ductility in the structure to prevent sudden collapse of structure. Whereas wind loads are build-up over time and not dynamic loads.
The main difference between the two loads lie in the prediction of their occurrence. While wind loads could be predicted with much accuracy(including the time of occurrence & intensity levels), unfortunately we have not yet developed any methodologies that enable prediction of time-specific or intensity specific occurrence of earthquakes at a site. Only probabilistic seismic hazard analyses (PSHA) are undertaken for better prediction of earthquake occurrence.
Secondly it is highly uneconomical and impractical to design a structure to remain elastic for a highly uncertain event such as earthquake. Therefore, the ductility of the structure is called for( by introducing R factor) while designing the structure for a much lower level of earthquake. Further it shall be noted that during earthquakes, structures are permitted to undergo different damage states depending on the return periods of the earthquake event.
Thank you for your answers sir Chhangani and Puttatt
I think Chhangani is correct that wind load is built-up over time and become dominantly static in windward direction. But I guess wind is also dynamic load going back and forth in leeward direction due to reynolds effect, which often noticably large.
So probably wind design should remain elastic to avoid cumulative/fatigue damage during this long-time dynamic wind load. As we know, strong wind strikes hours while earthquake hits less than 3 minutes. Materials that remain elastic can sustain millions of load cycles, while yielded materials can only resist few hundreds cycles top.
Maybe...
Yes, You have correctly pointed out the difference in time frame when wind load will build up and the very short time in which EQ will strike a structure back and forth, which I mentioned in my reply. Mr. Jayarajan Puttatt has also beautifully explained the reasons for taking R factor in design of earthquake resistant structures.
In any case you have raised very important point and there should be more comments from experts.
I agree with all of you. It is totally uneconomical to design an elastic structure to withstand a strong earthquake that may not happen in whole structure life, while in case of wind it happens all the time causing fatigue to the structure elements, so this structure must be designed as an elastic structure to sustain millions of cycles of loading. Thanks
General practice of design is to behave structures elastically for the applied loads. Earthquake load considering its magnitude, it is uneconomical to design the structures for the entire seismic load. Rather, the force is reduced by a factor known as Response reduction factor which accounts for over strength, material factor of safety and ductility. This approach is adopted by Codes. However, if demands, the entire load can be accounted without considering response reduction factor. However, wind load considering its lower magnitude, is taken fully into consideration for analysis and design
In the design of structures against earthquake we reduced by a certain factor maximum considered earthquake (MCE) to Design basis earthquake (DBE) to take into account the probability of its occurance and allowing certain damages without collapse of the structure. Whereas the response reduction factor is based on how structure is going to respond to the earthquake. If it is rigid and brittle like simple masonry then its value is less but if it is ductile and able to absorb energy its value is large.
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