Drift in Reinforced Concrete Structures - 2nd Time

[mkhuri]

Hay U All again Drifty People,

Thank U for your replies, but again look at this.

As Far as Drift Limits,

UBC97 and IBC2006 codes allow h/100 and h/50 in some cases.
Japanese Code allows h/120.
French code about h/1500.
Fintel h/500.

How did they decide that these are correct limits, and on what Basis?

Thanks Thanks and Thanks.

 

[hokie66]

Looks like they pulled rabbits out of hats. That is the problem with codes. They are no substitute for the institutional judgment of long established consulting firms.

Seriously, I doubt that the drift parameters you listed are all for the same type structure.

 

[kslee1000]

Not a hi-rise person, to me the veriation on drift limit is largely due to different perception on human confort. Also, reginal construction practices and philosophy on sustainable building system (flexible/rigid) during high wind/earthquake are likely factors in determining the drift limit.

I don't know what was the drift limit imposed on the design of Sears Tower in Chicago, however I do know nobody wants the upper floor spaces due to high drift - people claimed sea-sickness and constant fears. Also, my office was in 10th floor, during windy days, the doors will swing close by itself, the swing of picture frame was clearily visible, not to mention the loud, noisy steel joint was making.

 

[ErnestoD]

the h/500 for wind serviceability is to minimize damage on non-structural components like claddings and partitions. Same case for seismic, to minimize injury due to falling objects... ceiling loads, cabinets, etc.

Regarding perception on human comfort, it's not really the drift... it's more on the acceleration or vibration induced by wind.


Ernesto

 

[kslee1000]

ernestoD:

Would like to agree with your comment, but can you please provide the logic explanation for setting drift limits?

 

[ErnestoD]

These limits were provided by the Code committee experts based on their experiences. As I said, these limits are used to minimize damages to non-structural components.

This issue is explained further on some building codes. I've read it before on some british code (i forget the number) or refer to AS/NZS 1170.0:2002 Table C1.

Cheers,

Ernesto

 

[WillisV]

Though I agree that wind drift limits are primarily for limiting damage to non-structural components, I strongly disagree that the same holds true for seismic drift limits. Seismic drift limits are more of an overall stability check under ultimate loading conditions (an earthquake) as opposed to the 10yr type return period typically used for wind drift check.

 

[ErnestoD]

I agree with you Willis on the ultimate loading condition for seismic, but here in NZ we also consider the 10-20yr return period for earthquake (serviceability limit criteria).

 

[kslee1000]

"Modern tall buildings are expected to meet certain wind-induced drift (serviceability) and acceleration (occupant comfort) criteria to preserve the integrity of their building cladding system, interior finishes, and operation of elevator system and to ensure occupant comfort due to building motions."

Excerpt from paper presented in link below.

http://www.succeed.ufl.edu/asce09/Program/Abstracts/1466_Bashor_Kareem_Occupant_Comfort_(1).doc

 

[haynewp]

H/400 or H/500 for service level wind for occupant satisfaction, the H/50 range is for a one time expected max seismic event.

 

[kslee1000]

Conclusion to myself:

Wind induced drift, swaying, vibration are mainly service criteria. And, the bottom line, serviceability is directly tied to human desire/comfort/perception, which shall be seriously considered in the planning/design stages. The limits on such is highly based on locality, engineering judgement and past experience (studies).

 

[StructGen]

ASCE-7 does not limit the overall drift of the building in a wind event, instead it limits the story drifts of building to probably H/360 (I am not sure of exact number)as mentioned by other posters this is to ensure serviceablity during a 10 year storm. The load case that is used to check this criteria is D+0.5L+0.7W.

In seismic the deflection criteria is different if you want to compare it with wind multiply the number with your ductility demand factor. H/50 when multiplied by Rw = 8 will give you H/400 which is same as of wind design.
The way ASCE works is that they introduce this Cd factor which is multiplied by elastic forces to obtain in-elastic deformations and then these in-elastic deformations are compared with H/50 limit

 

[bazflexure]

As far as seismic design is concerned drift is limited to [0.025/(Rx0.7)] for structures with time period less than 0.5seconds fot T> 0.5 sec it is [0.02/(T^2/3)(Rx0.7)].