Serviceability limit state under wind load (Motion Perception) 1

[KenStructural]

Hi Guys,
May I have your thoughts is regards to serviceability check of building motion for wind loads particularly motion perception (vibration, acceleration). Tall buildings are usually exposed to wind and in this regards acceleration should be taken into account (motion perception). Common example includes human discomfort caused by wind if resonance occurs.

 

[bookowski]

In my experience the wind accelerations of the building are calculated by the wind tunnel consultant - you provide them building dynamic properties and geometry (mode shapes, mass etc). There are standard criteria for various levels of acceptance based on use and percent that in theory will perceive motion. I have some old reports somewhere that I could dig out. Similar to vibration wind acceleration can be difficult to control as you need both mass and stiffness.

 

[KenStructural]

Hi bookowski,
I’ve found some reference Serviceability limit states under wind load (Engineering Journal) by Lawrence G. Griffis which I think it’s a good reference, however I have a hard time dealing with the grafts, what is shown there are grafts the most common one is Root-Mean-Square (RMS) versus Peal Acceleration and from there you can actually determine the comfort limit.

I think you’re right, the acceleration of the building due to wind should be calculated by wind tunnel analysis specially for tall building as well as iregular building.

Would it be possible if you could send some reference about this. It would be a great help.

 

[MrHershey]

The traditional method has been to keep deflections to L/500. I've also seen L/400.

Though this isn't a hard limit, it's not codified anywhere. And if you actually dig into some of the articles promoting it, this limit is based on a 10-20 year return on the wind (I've seen articles say 10, commentary to ASCE 7-05 Appendix C says 20; commentary isn't included in most printings, you'll need to search for errata). Most use the limit in conjunction with service-level wind (i.e. without the 1.6 factor), which is wildly conservative. ASCE 7-05 Appendix C commentary recommends using a 0.7 factor on wind to get to 20 year return.

I would only use this limit for schematic stages or projects without a wind tunnel consultant. Just something to keep your designs in check during the early stages before wind tunnel consultants get involved (which should be much earlier than it typically is, but that's a separate discussion). Humans don't 'feel' displacement, only acceleration. So this limit has zero bearing on human comfort. You'd need to look at maximum accelerations to address the human comfort issue. You'll likely need to get a wind tunnel consultant involved for this aspect. Once a wind tunnel consultant gets involved, I'd throw the 1/500 limit out the wind and let them do the heavy lifting on this part. They know more than you. Just make sure that you let them know of any major changes in building geometry or stiffness. Major changes in building geometry may require a new analysis if they're major enough. Major changes in stiffness they should be able to address without as much rework, it's mostly just plugging numbers in but shouldn't need any retesting for the typical level of analysis.

 

[bookowski]

I've attached a page/figure from a typical wind tunnel report, this one is from RWDI but we've used others and they are similar. I've removed the project name. Working with the wind tunnel is a bit of a black box situation - you will provide them with geometry and dynamic information (modes and mass) and they give you results. In the past they have given us a spreadsheet to play with to see the affect of more mass and/or stiffness but they say it's not 100% accurate, you need to give them the final properties and they redo the analysis (not sure if it's truly that complicated or they just want to stay in business).

A good resource is CTBUH. I don't think I can upload any of their papers here as they are not free - but you may find some around your office or online to purchase.

 

[bookowski]

This may be of use for you

 

[bookowski]

and this