[SEFI] Drift Limits

[nykochak]

I have following querry regarding drift control.

As per IS 456 page 33 clause 20.5 total drift limit for wind loading is H/500.

Where as

As per IS 1893 no such criteria for total drift exists. Inter storey drift not to exceed 0.004 x storey height, which works out to L/250. If we multiply by R factor, in case of two adjacent units to avoid ponding effect, as 2.5 it works out to L/100.

Overstrength is same in both the cases.

I am not able to understand and convince vast difference between two drift limits.

N. Y. Kochak

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[drnsmani]

Dear Mr. Kochak,

There is no confusion. In IS 456:2000, Clause it is clearly stated that under wind load, sway at top should not exceed H/500 and under for earthquake load, refer IS 1893. If the building is not located in EQ Zone, it is enough if you check for H/500 at top. In EQ zones, the deflection requirements are more stringent, as indicated by you the Inter storey drift itself should not to exceed L/250, which is twice that of the total defln. specified for wind!

Reg. pounding, the gap between two adj. structures should be R factor x allowable drift.

Best wishes
Subramanian

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Fri, 8/1/08, nykochak wrote:

Quote:

From: nykochak Subject: [SEFI] Drift Limits To: gen...@sefindia.org Date: Friday, August 1, 2008, 3:09 PM

http://sefindia.org/forum/viewtopic.php?p=7923#7923

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[sdec.in]

In addition, if the floor levels of adjacent Blocks are same, the R factor can be modified to R/2;i.e. if the two blocks have a total deflection of let's say 25 and 33 mm,;the minimum gap of R/2 x 58 mm is recommended for such Blocks.
regards
Sangeeta Wij
[quote] ----- Original Message -----
From: drnsmani (fo...@sefindia.org)
To: gen...@sefindia.org (gen...@sefindia.org)
Sent: Friday, August 01, 2008 11:12 PM
Subject: [SEFI] Re: Drift Limits

Dear Mr. Kochak,

There is no confusion. In IS 456:2000, Clause it is clearly stated that under wind load, sway at top should not exceed H/500 and under for earthquake load, refer IS 1893. If the building is not located in EQ Zone, it is enough if you check for H/500 at top. In EQ zones, the deflection requirements are more stringent, as indicated by you the Inter storey drift itself should not to exceed L/250, which is twice that of the total defln. specified for wind!

Reg. pounding, the gap between two adj. structures should be R factor x allowable drift.

Best wishes
Subramanian

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Fri, 8/1/08, nykochak wrote:

[b]--auto removed--

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[nykochak]

Dear Sir,

Thanks for your reply.

I want to stress again that

H/500 is stringent criteria (For wind loading)

and

not L/250

Under EQ load we are allowing sway as (L/250) x R

If R = 2.5

When you apply R factor it means you are permitting that much deflection.

or Actual earth quake forces are scaled down by R factor.

Permissible inter storey sway is L/100

If floor height is 3000 then permissible sway = 30 mm for EQ loading

If floor height is 3000 then permissible sway = 6 mm for wind loading

If you consider single storey building.

For multistorey you may convert (extrapolate) inter storey drift limits in to total drift for EQ loading

(L/250) x R = (H/250) x R

Even if you donot consider R factor, for lateral forces EQ and Wind there are two separate limits.

With regards

N. Y. Lochak

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[abdulbasit]

Dear Mr. N. Y. Kochak,

As per IS 456 page 33 clause 20.5, it is not total drift, but it is lateral sway at top. A comparison can not be made between two different quantities and also in one case it is total height of the building H and in the other it is the storey height.

Quote:

----- Original Message ----- From: nykochak (fo...@sefindia.org) To: gen...@sefindia.org (gen...@sefindia.org)

http://sefindia.org/forum/viewtopic.php?p=7975#7975

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[nykochak]

Dear Sir,

I fully agree with you.

Please do not get carried away by stipulations.

Please imagine.

Please read my second e-mail also.

If two cannot be compared does it mean difference between the two can be anything ?

Also two are lateral forces only.

In fact drift limit due to earth quake should be more stringent since it is storey based.

With regards.

N. Y. Kochak
[quote] ----- Original Message -----
From: abdulbasit (fo...@sefindia.org)
To: gen...@sefindia.org (gen...@sefindia.org)
Sent: Monday, August 04, 2008 10:42 AM

Subject: [SEFI] Re: Drift Limits

Dear Mr. N. Y. Kochak,

As per IS 456 page 33 clause 20.5, it is not total drift, but it is lateral sway at top. A comparison can not be made between two different quantities and also in one case it is total height of the building H and in the other it is the storey height.

[b]--auto removed--

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[nykochak]

Respected Sir,

Thanks for reply.
My original question remains.
If you consider a single storey only then
Drift limit is
H/500 is For wind loading
&
L/250 is for EQ loading

Under EQ load we are allowing sway as (L/250) x R
If R = 2.5

Permissible inter storey sway is L/100
If floor height is 3000 then permissible sway = 30 mm for EQ loading

considering R factor.
If floor height is 3000 then permissible sway = 12 mm for EQ loading
without considering R factor.
If floor height is 3000 then permissible sway = 6 mm for wind loading.

For multistorey one may convert (extrapolate) inter storey drift limits in
to total drift for EQ loading since total drift limit is not specified.

I feel interstorey drift should have stringent limit as compared to total
drift

(L/250) x R = (H/250) x R

Even if R factor is not considered, for lateral forces viz. EQ and Wind

there are two separate limits.

Why ?

With best regards,

N. Y. Kochak

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[rahul.leslie]

Sir,
I find it cofusing to undertsnd what you have actually meant. But as I can understand,

1) The limit of sway is L/250, with partial load factor equal to 1.0 (Cl.7.11.1)

2) In seismic Zones IV and V, structural components shouldn't lose their vertical load-carrying capacity from sway equal to R times sway calculated as per 7.11.1. (Cl.7.11.2)

3)For adjacent units, separation joint in between be equal to R times sum of the sways as per 7.11.1 of each of them. When floor levels of two units are at the same elevation levels, R may be replaced by R/2. (Cl.7.11.3)

So permissible sway is different from limit to which structure should be intact, which is again different from width of construction joint. It is not that (L/250)*R is the permissible sway

Regards,
Rahul Leslie

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[nykochak]

Dear Sir,

Thanks for your reply.
I want to stress again that

H/500 is drift criteria (For Wind loading)
and
L/250 is drift criteria (For EQ loading)

Under EQ load we are anticipate sway as (L/250) x R

When you apply R factor it means you are permitting that much deflection.
or Actual earth quake forces are scaled down by R factor.

If R = 2.5
Permissible inter storey sway is L/100

If floor height is 3000 then actual sway = 30 mm for EQ loading (L/100)

If floor height is 3000 then permissible sway = 6 mm for wind loading (H/500)

If you consider single storey building.

For multistorey you may convert (extrapolate) inter storey drift limits in to total drift for EQ loading

(L/250) x R = (H/250) x R

Even if you donot consider R factor, for lateral forces EQ and Wind there are two separate limits.

Why ?
When analysis and design parameters are same.

With regards

N. Y. Lochak

Quote:

----- Original Message ----- From: rahul.leslie (fo...@sefindia.org) To: gen...@sefindia.org (gen...@sefindia.org) Sent: Tuesday, August 05, 2008 7:43 AM Subject: [SEFI] Re: Drift Limits

Sir, I find it cofusing to undertsnd what you have actually meant. But as I can understand, 1) The limit of sway is L/250, with partial load factor equal to 1.0 (Cl.7.11.1) 2) In seismic Zones IV and V, structural components shouldn't lose their vertical load-carrying capacity from sway equal to R times sway calculated as per 7.11.1. (Cl.7.11.2) 3)For adjacent units, separation joint in between be equal to R times sum of the sways as per 7.11.1 of each of them. When floor levels of two units are at the same elevation levels, R may be replaced by R/2. (Cl.7.11.3) So permissible sway is different from limit to which structure should be intact, which is again different from width of construction joint. It is not that (L/250)*R is the permissible sway Regards, Rahul Leslie

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[rupen]

Dear Mr. Kochak:

Design for earthquake load and wind load are totally different.

Earthquake resistant design relies heavily on inelastic actions - ductility. Thus, use R, the response reduction factor. Here, it is expected that there will be large lateral deformations (with suitable detailing to prevent collapse).

One the other hand, one does not design structures to go into inelastic action under regular wind load. Structures are required to remain elastic under wind loads, and therefore, cannot permit large deformations. Thus, the limiting lateral drift limit (whole or inter-storey, whichever way you look at it) is small. As a result, it looks very stringent compared to the one for earthquake loading.

Please do not compare these two!

With warm regards...
Rupen Goswami


________________________________

From: nykochak [mailto:fo...@sefindia.org]
Sent: Wed 06-Aug-08 01:19 PM
To: gen...@sefindia.org
Subject: [SEFI] Re: Drift Limits

Dear Sir,

Thanks for your reply.
I want to stress again that
H/500 is drift criteria (For Wind loading)
and
L/250 is drift criteria (For EQ loading)

Under EQ load we are anticipate sway as (L/250) x R
When you apply R factor it means you are permitting that much deflection.
or Actual earth quake forces are scaled down by R factor.
If R = 2.5
Permissible inter storey sway is L/100

If floor height is 3000 then actual sway = 30 mm for EQ loading (L/100)

If floor height is 3000 then permissible sway = 6 mm for wind loading (H/500)

If you consider single storey building.

For multistorey you may convert (extrapolate) inter storey drift limits in to total drift for EQ loading

(L/250) x R = (H/250) x R

Even if you donot consider R factor, for lateral forces EQ and Wind there are two separate limits.

Why ?
When analysis and design parameters are same.

With regards

N. Y. Lochak

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[nykochak]

I agree with you.
But EQ or Wind (regular ?) are forces induced in the structure once in life time.
I suppose probability is same.
Elastic or inelastic action is the behavior of structure and does not depend on our wish.
As such non linear behavior is always there.
In Eurocode experiments are carried on structure with drift limit as 8%. (For EQ)
Drift limits vary from code to code (0.001h to 0.005h)
All the parameters (analysis viz stiffness, design viz load combinations, formation of plastic hinges, moment curvature relations) are same except drift ?
Then ductility / capacity design is not required in Wind design ?
Even if we do not apply R factor still there is a difference of two times.
How we can say that in EQ inelastic actions are considered and not in Wind loading ?

With regards

Narayan Kochak

:

----- Original Message ----- From: rupen (fo...@sefindia.org) To: gen...@sefindia.org (gen...@sefindia.org)

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[alpa_sheth]

Mr. Lochak:

This issue has been already raised at the Earthquake Code Committee level. In most international codes the permissible drift in earthquake loads is also L/500.
I hope the next code revision accounts for the anomaly and limits seismic drift to L/500.

regards,
Alpa Sheth

:

----- Original Message ----- From: nykochak (fo...@sefindia.org) To: gen...@sefindia.org (gen...@sefindia.org) Sent: Wednesday, August 06, 2008 1:19 PM Subject: [SEFI] Re: Drift Limits

Dear Sir, Thanks for your reply. I want to stress again that H/500 is drift criteria (For Wind loading) and L/250 is drift criteria (For EQ loading) Under EQ load we are anticipate sway as (L/250) x R When you apply R factor it means you are permitting that much deflection. or Actual earth quake forces are scaled down by R factor. If R = 2.5 Permissible inter storey sway is L/100 If floor height is 3000 then actual sway = 30 mm for EQ loading (L/100) If floor height is 3000 then permissible sway = 6 mm for wind loading (H/500) If you consider single storey building. For multistorey you may convert (extrapolate) inter storey drift limits in to total drift for EQ loading (L/250) x R = (H/250) x R Even if you donot consider R factor, for lateral forces EQ and Wind there are two separate limits. Why ? When analysis and design parameters are same. With regards N. Y. Lochak

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[avinash_alaska]

Dear Mr Kochak,

Almost all the recent seismic codal provisions are based on performance based design.. In this, we set some performance requirements for the structure.. Say for example, minor damage under moderate earthquakes and collapse prevention under large earthquakes.. In order to achieve the performance objectives in cost effective way, we highly depend on the inelastic behaviour of RCC elements in the structure.. Thus a large deformation is acceptable under the severe seismic event, however without collapse of the structure.. The ductlity of RCC is used in this context..

Whereas in the case of wind loading, which is more frequent than seismic loads, we expect no severe damage to the structure.. Even the damage of partitions/external glazing are not acceptable.. Thus the drift limits set for the wind loads are more stringent as compared to drift limits for seismic loads..

Senior members, please correct me if I am wrong in my conclusions..

Avinash S.
Structural Engineer
Dubai, UAE


--- On Thu, 8/7/08, nykochak wrote:

:

From: nykochak

Subject: [SEFI] Re: Drift Limits

To: gen...@sefindia.org Date: Thursday, August 7, 2008, 10:45 AM

:

I agree with you. But EQ or Wind (regular ?) are forces induced in the structure once in life time. I suppose probability is same. Elastic or inelastic action is the behavior of structure and does not depend on our wish. As such non linear behavior is always there. In Eurocode experiments are carried on structure with drift limit as 8%. (For EQ) Drift limits vary from code to code (0.001h to 0.005h) All the parameters (analysis viz stiffness, design viz load combinations, formation of plastic hinges, moment curvature relations) are same except drift ? Then ductility / capacity design is not required in Wind design ? Even if we do not apply R factor still there is a difference of two times. How we can say that in EQ inelastic actions are considered and not in Wind loading ? With regards

Narayan Kochak

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[scpatel_74]

Dear Sefians...

Yes I do agree with Mr. Kochak.
For structures designed using concept of Response reduction factor for earthquakes, we expect the structure to under go large deformation beyond so called elastic yield limit with some structural damage but without collapse. The deformation
In the inelastic range helps to dissipate energy.

Whereas for design wind loads we do not want any damage to finishes and structural members hence more stringent drift criteria for wind loading as compared to earthquake loading..


Regards,

-Sanjay Patel
Maire Engineering S.p.A.
Turin, Italy

From: avinash_alaska [mailto:fo...@sefindia.org]
Sent: Friday, August 08, 2008 11:45 AM
To: gen...@sefindia.org
Subject: [SEFI] Re: Drift Limits


Dear Mr Kochak,

Almost all the recent seismic codal provisions are based on performance based design.. In this, we set some performance requirements for the structure.. Say for example, minor damage under moderate earthquakes and collapse prevention under large earthquakes.. In order to achieve the performance objectives in cost effective way, we highly depend on the inelastic behaviour of RCC elements in the structure.. Thus a large deformation is acceptable under the severe seismic event, however without collapse of the structure.. The ductlity of RCC is used in this context..

Whereas in the case of wind loading, which is more frequent than seismic loads, we expect no severe damage to the structure.. Even the damage of partitions/external glazing are not acceptable.. Thus the drift limits set for the wind loads are more stringent as compared to drift limits for seismic loads..

Senior members, please correct me if I am wrong in my conclusions..

Avinash S.
Structural Engineer
Dubai, UAE


--- On Thu, 8/7/08, nykochak wrote:
:
From: nykochak
Subject: [SEFI] Re: Drift Limits

To: gen...@sefindia.org (gen...@sefindia.org)

Date: Thursday, August 7, 2008, 10:45 AM


:
I agree with you.
But EQ or Wind (regular ?) are forces induced in the
structure once in life time.
I suppose probability is same.
Elastic or inelastic action is the behavior of structure
and does not depend on our wish.
As such non linear behavior is always there.
In Eurocode experiments are carried on structure with drift
limit as 8%. (For EQ)
Drift limits vary from code to code (0.001h to 0.005h)
All the parameters (analysis viz stiffness, design viz load
combinations, formation of plastic hinges, moment curvature
relations) are same except drift ?
Then ductility / capacity design is not required in Wind
design ?
Even if we do not apply R factor still there is a
difference of two times.
How we can say that in EQ inelastic actions are considered
and not in Wind loading ?

With regards

Narayan Kochak

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[jiwajidesai]

The very concept of performance and safety are essentially different in case of wind and earthquakes. While the structures subjected to wind are expected to be designed for safety under serviceability criteria like resistance undue vibrations, fatigue-resistance, and deflections significant enough to affect and damage glass, partitions, finishes etc all under normal operational conditions, earthquake effects are sudden, short-lived collapse-prone and infrequent. For the latter, one has to allow for large deformations - ductile behaviour - (except in case of some important structures), and hence conventional serviceability criteria are nowhere significant compared to survival under life-threatening situation. In other words, the structural collapse has to be delayed by dissipating destructive seismic energy thru inelastic deformations, as large as needed.

To this thinking, having limits on deformations in medium to large-intensity earthquakes does not seem reasonable, where saving lives is far more important.

Regards

Jiwaji Desai
Sr. GM (C & S[b])
WSP Engineering Services Ltd.[/b]

To: gen...@sefindia.org (gen...@sefindia.org) (gen...@sefindia.org (gen...@sefindia.org))

Date: Thursday, August 7, 2008, 10:45 AM


:
I agree with you.
But EQ or Wind (regular ?) are forces induced in the
structure once in life time.
I suppose probability is same.
Elastic or inelastic action is the behavior of structure
and does not depend on our wish.
As such non linear behavior is always there.
In Eurocode experiments are carried on structure with drift
limit as 8%. (For EQ)
Drift limits vary from code to code (0.001h to 0.005h)
All the parameters (analysis viz stiffness, design viz load
combinations, formation of plastic hinges, moment curvature
relations) are same except drift ?
Then ductility / capacity design is not required in Wind
design ?
Even if we do not apply R factor still there is a
difference of two times.
How we can say that in EQ inelastic actions are considered
and not in Wind loading ?

With regards

Narayan Kochak

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[Parth.Vyas]

Dear Mr. Narayan,
I totally agree with Mr. Rupen as earthqk and wind is totally different. Earthqk is considered as a transient load which comes with a very less probability (once in a life span of structure) and it is for a very short duration (that is why it is called as transient load). In other word you can say that application of which is much faster than structure behave. (I mean to say forcing function time pr is much short than fundamental time period of structure.)
2nd Design of earth qk is totally based on performance based approach. It means if structure performance shall be ductile and reliable (concept of earthqk resistance is in zone v or under savior earthqk building shall not be collapsed.)
That means we are allowing structure to under go large inelastic expected deformation. The limitation on elastic drift is to ensure the stiffness of structure. (I mean to say there must be some limit on flexibility of structure.)


Now let me talked about wind. As we want to talk about wind is also a transient load if you consider 3 sec. gust reference wind pressure and that is too once in 50 years. That also mean once in life time of structure.
But If you refer Canadian NBC (national building code) it is clearly written that for serviceability check of structure for deflection you should consider wind which is generally expected (I mean more probability) during life period of a structure. Hence for deflection check as per NBC code it is clearly written that “ reference velocity pressure, q for the design of structural members for deflection and vibration shall be based on a probability of being exceeded in any one year of 1 in 10.”


Now let me compare to ASCE code It is clearly written as per C.6.5.5 Commentary that “For applications of serviceability, design using maximum likely events, or other application, it may be desired to use wind speeds associated with mean recurrence intervals other than 50 years. To accomplish this, the 50-year speeds of figure 6-1 are multiplied by the factors listed in Table C6-2.”
That mean the 3 sec gust wind once in 50 year you can reduced by multiplying that factor listed in that table to check for serviceability condition.

So it is very clear that serviceability shall not be check for the transient condition but for service load condition. The loads, which are expected during life span of the structure.


I hope I am clear enough. Friends please correct if I am wrong. I am just sharing experience of different code on which we are working.



Thanks and Regards,



Parth Vyas
Jacobs Canada Inc.
205 Quarry Park Boulevard SE
Calgary, Alberta, T2C 3E7, Canada
(403) 385 3001

-----Original Message-----
From: nykochak [mailto:fo...@sefindia.org]
Sent: Wednesday, August 06, 2008 11:15 PM
To: gen...@sefindia.org
Subject: [SEFI] Re: Drift Limits

I agree with you.
But EQ or Wind (regular ?) are forces induced in the structure once in life time.
I suppose probability is same.
Elastic or inelastic action is the behavior of structure and does not depend on our wish.
As such non linear behavior is always there.
In Eurocode experiments are carried on structure with drift limit as 8%. (For EQ)
Drift limits vary from code to code (0.001h to 0.005h)
All the parameters (analysis viz stiffness, design viz load combinations, formation of plastic hinges, moment curvature relations) are same except drift ?
Then ductility / capacity design is not required in Wind design ?
Even if we do not apply R factor still there is a difference of two times.
How we can say that in EQ inelastic actions are considered and not in Wind loading ?

With regards

Narayan Kochak
:

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[nykochak]

Respected Madam,

Sorry for delayed reply.
Thanks for your reply.
You must have read some replies to my querry.
I cannot understand, from the replies I got from the engineers in SEFI, how H/500 ensures that building is with in elastic limit ? Then if permissible drift crieteria varies from country to country then whether elastic / inelastic criteria changes from country to country ? Safe crietria for non structural elements varies from country to country ?
I request you to carefully recommend dtrift crieteria in EQ and Wind loading.

With regards.

N. Y. Kochak

Quote:

----- Original Message ----- From: alpa_sheth (fo...@sefindia.org) To: gen...@sefindia.org (gen...@sefindia.org)

Sent: Thursday, August 07, 2008 3:57 PM Subject: [SEFI] Re: Drift Limits

Mr. Lochak: This issue has been already raised at the Earthquake Code Committee level. In most international codes the permissible drift in earthquake loads is also L/500. I hope the next code revision accounts for the anomaly and limits seismic drift to L/500. regards, Alpa Sheth : ----- Original Message -----

From: nykochak (fo...@sefindia.org (fo...@sefindia.org))

To: gen...@sefindia.org (gen...@sefindia.org) (gen...@sefindia.org (gen...@sefindia.org))

Sent: Wednesday, August 06, 2008 1:19 PM

Subject: [SEFI] Re: Drift Limits

Dear Sir, Thanks for your reply. I want to stress again that H/500 is drift criteria (For Wind loading) and L/250 is drift criteria (For EQ loading) Under EQ load we are anticipate sway as (L/250) x R When you apply R factor it means you are permitting that much deflection. or Actual earth quake forces are scaled down by R factor. If R = 2.5 Permissible inter storey sway is L/100 If floor height is 3000 then actual sway = 30 mm for EQ loading (L/100) If floor height is 3000 then permissible sway = 6 mm for wind loading (H/500) If you consider single storey building. For multistorey you may convert (extrapolate) inter storey drift limits in to total drift for EQ loading (L/250) x R = (H/250) x R Even if you donot consider R factor, for lateral forces EQ and Wind there are two separate limits. Why ? When analysis and design parameters are same. With regards N. Y. Lochak

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[nykochak]

Dear Parth,

Sorry for delayed response.
I agree with you.

I would like to bring to your notice that IS 456 - 2000 clause no. 20.5 page 33 mention that
Under transient wind load the lateral sway at the top should not exceed H/500, where H is the total height of the building. For seismic loading, reference should be made to IS 1893.

I want to stress again that

Is interpretation of word transient wind loading is different from code to code ?

I think now you will agree with me that earth quake and wind both are transient loads as far as Indian Code is concerned. There is lot of variation in permissible Drift limits if you compare codes of various countries. (From 0.001xh to 0.005xh).

Does that mean if I design the building in China is safe and the same will be unsafe in Poland ?

With regards.

N. Y. Kochak

Quote:

----- Original Message ----- From: Parth.Vyas (fo...@sefindia.org) To: gen...@sefindia.org (gen...@sefindia.org) Sent: Wednesday, August 13, 2008 3:31 AM Subject: [SEFI] Re: Drift Limits

Dear Mr. Narayan, I totally agree with Mr. Rupen as earthqk and wind is totally different. Earthqk is considered as a transient load which comes with a very less probability (once in a life span of structure) and it is for a very short duration (that is why it is called as transient load). In other word you can say that application of which is much faster than structure behave. (I mean to say forcing function time pr is much short than fundamental time period of structure.) 2nd Design of earth qk is totally based on performance based approach. It means if structure performance shall be ductile and reliable (concept of earthqk resistance is in zone v or under savior earthqk building shall not be collapsed.) That means we are allowing structure to under go large inelastic expected deformation. The limitation on elastic drift is to ensure the stiffness of structure. (I mean to say there must be some limit on flexibility of structure.) Now let me talked about wind. As we want to talk about wind is also a transient load if you consider 3 sec. gust reference wind pressure and that is too once in 50 years. That also mean once in life time of structure. But If you refer Canadian NBC (national building code) it is clearly written that for serviceability check of structure for deflection you should consider wind which is generally expected (I mean more probability) during life period of a structure. Hence for deflection check as per NBC code it is clearly written that “ reference velocity pressure, q for the design of structural members for deflection and vibration shall be based on a probability of being exceeded in any one year of 1 in 10.” Now let me compare to ASCE code It is clearly written as per C.6.5.5 Commentary that “For applications of serviceability, design using maximum likely events, or other application, it may be desired to use wind speeds associated with mean recurrence intervals other than 50 years. To accomplish this, the 50-year speeds of figure 6-1 are multiplied by the factors listed in Table C6-2.” That mean the 3 sec gust wind once in 50 year you can reduced by multiplying that factor listed in that table to check for serviceability condition. So it is very clear that serviceability shall not be check for the transient condition but for service load condition. The loads, which are expected during life span of the structure. I hope I am clear enough. Friends please correct if I am wrong. I am just sharing experience of different code on which we are working. Thanks and Regards, Parth Vyas Jacobs Canada Inc. 205 Quarry Park Boulevard SE Calgary, Alberta, T2C 3E7, Canada (403) 385 3001 -----Original Message----- From: nykochak [mailto:fo...@sefindia.org] Sent: Wednesday, August 06, 2008 11:15 PM

To: gen...@sefindia.org (gen...@sefindia.org) Subject: [SEFI] Re: Drift Limits

I agree with you. But EQ or Wind (regular ?) are forces induced in the structure once in life time. I suppose probability is same. Elastic or inelastic action is the behavior of structure and does not depend on our wish. As such non linear behavior is always there. In Eurocode experiments are carried on structure with drift limit as 8%. (For EQ) Drift limits vary from code to code (0.001h to 0.005h) All the parameters (analysis viz stiffness, design viz load combinations, formation of plastic hinges, moment curvature relations) are same except drift ? Then ductility / capacity design is not required in Wind design ? Even if we do not apply R factor still there is a difference of two times. How we can say that in EQ inelastic actions are considered and not in Wind loading ? With regards

Narayan Kochak :

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[drnsmani]

Dear Mr. Narayan Kochak,

In IS 456:2000, Clause it is clearly stated that under wind load, sway at top should not exceed H/500 and for earthquake load, refer IS 1893. If the building is not located in EQ Zone, it is enough if you check for H/500 at top.

As per IS 1893, in EQ zones, as indicated by you the Inter storey drift should not to exceed L/250.

I agree with you that if you compare the values for a single storey building, drift limit for wind load is more stringent. But wind loads will be governing only for buildings which are slender (as per Is 875(part3) clause 7.1, dynamic effects of wind have to be considered only if the height to min. lateral dimension ratio is more than 5.0 and/or when the natural frequency in the first mode is less than 1.0 Hz). Hence dynamic effect of wind need to be considered only for tall buildings, with narrow base width. For shallow buildings wind may be considered as static load. Hence for tall buildings earthquake criteria may become stringent.

The drift is checked as serviceability limit state with a load factor of 1.0 only.

In earthquake mass is the governing factor. If you have more mass at the top, you have to design it carefully. Hence for tall buildings, for which drift is the main consideration, the inter-storey drift for earthquake may become critical than the total drift specified for wind loads.
It may be of interest to note the following max. drift limits given in performance based earthquake codes.


Fully operational 0.2% (H/500)
Operational 0.5% (H/200)
Life-safe 1.5% (H/66.67)
Near collapse 2.5% (H/40).

As per ASCE 7-05, whose provisions are adopted by codes of several other countries, the design storey drift is calculated as per the following.


Step1: Calculate lateral deflection Dxe at each floor level as per elastic analysis.


Step2: Amplify it by Cd (this is similar to R factor-the code gives several values of Cd for various categories) and divide it with Importance factor to give Dx.


Step 3: calculate the design storey drift based on steps 1 and 2 and check it with the allowable limits(which is 2%, 1.5% and 1% depending on occupancy category.



The footnote in table 12.12.1 says drift limit need not be checked for single storey buildings! (I am attaching the table- which may opened with MS Paint)

Regards
Subramanian

Respected Madam,

Sorry for delayed reply.
Thanks for your reply.
You must have read some replies to my querry.
I cannot understand, from the replies I got from the engineers in SEFI, how H/500 ensures that building is with in elastic limit ? Then if permissible drift crieteria varies from country to country then whether elastic / inelastic criteria changes from country to country ? Safe crietria for non structural elements varies from country to country ?
I request you to carefully recommend dtrift crieteria in EQ and Wind loading.

With regards.

N. Y. Kochak

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Computer Design Consultants: www.cdcstruct.com
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559

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Attachments:
Drift_as_per_ASCE-07-2005.tif

[Parth.Vyas]

Hi some times ago the discussion was going on about some confusion about lateral drift criteria in the case of transient wind. I also have discussed with one of my friend working with me in Canada and I am coating his reply as mentioned below. We are working in Canadian code and in this code this criteria is very much clear.


--- Please read the discussion below for understanding the subject and then read my comments.

Abhijit Parkhi

My comments :-

Hi,

Let's first understand that, there is a difference in the concept of strength and serviceability limit states.
The drift / deflection limits imposed constitutes serviceability. Hence there is no question of safe and unsafe
in drift / deflection limits. The mail mentions that due to difference in drift limits the structure designed in one
country is unsafe while in other country is safe. This is not correct. The limits on drift are the part of serviceability
which corresponds to discomfirt to users or cracking of partition walls or cladding. This is nothing to do with
safety. Evenif we are not following drift limits, nothing is going to happen to structure, if it is safe in strength limit
state. Serviceability need to be checked always for service loads and not factored loads. Service loads are the load values
of common occurence. For those loads, your structure shall be serviceable. That's all.

The limits on drift can not be imposed by concrete/steel code which mainly deals with the strength limit states.
Drift/deflection limits are set depending upon the type of building. Those can not be generalized. For example,
for cladded building the limit of drift can be H/500 or H/600 but for steel piperack, that limit can be H/200 or H/150.
Since cladding is too sensitive to drift and can be cracked under higher lateral drift but the pipes are flexible and
can accommodate the large lateral drifts. In short, the type of building is very important in setting the criteria for
the drift. Therefore you will find there is large variations in those values. It totally depends upon the functionality
of the structure(depends upon intended use of building as serviceability should be read as discomforts to occupants or a functional use of that particular building).

It is worth to mention that, the wind drift need to be checked not only for service loads but also for reduced wind
pressure. (typically use 1 in 10 year return or 1 in 15 year return period). So this will reduce the wind considerably.
This is because, generally speaking, structure shall be governed mainly by strength limit state and not serviceability
limit state. (Since, the serviceability limits are just the thumb rules and based on experience and can not be taken
precisely.)

If you refer to US codes, Steel code as well as concrete code, no drift limits are given and those codes are referring to the
building codes for serviceability limits. This is how it should be done.

In Canadian steel and concrete codes, no limits on wind drift or deflection is given in main code, and the limits are
suggested in the appendix which is not the part of the code.

Lot of other international codes are following the similar practice. Evenif some are giving the limits, those can altered
depending upon your particular structure and it's use.

In Indian code, the lateral sway limit is mentioned under stability of structure. Please note that stability of the
structure is not the part of serviceability. Stability contitutes the strength limit state. If the structure is not
stable under lateral loads means it will fail. There is a question of safety involved in that. Hence we can not
use the clauses mentioned under stability, as serviceability limits. Now the last thing is mention of lateral
sway limits. If the intent of the code is to have limit on lateral drift, this is not the place where the limit shall be
written. This limit can not be a single value and shall vary from structure to structure. What I am thinking is
this limit is given not for checking serviceability but to restrict the first order deflection. Because stability criteria
should give the limit of lateral deflection to minimize the second order effects (P-Delta effects). This can
be the intention of the code. The only problem is that, in IS 456 there is no mention of P-Delta analysis (Analysis
involving second order effects) anywhere. So if I want to use this provision I will read the statement as
"Under transient wind load, the lateral sway limit at the top may not exceed H/500". I will read as a suggestion
and not mandatory clause. If we can not follow that limit, then we have to do second order analysis to design the
structure. (please note that the code says lateral sway and not lateral drift. The word sway indicates that the code
is talking about the stability and not serviceability)

I hope this clarifies the confusion.

Regards,

Abhijit Parkhi P.Eng
Civil Structural
Suncor Voyageur Project
Jacobs Canada Inc.
Location : 3E206
205 Quarry Park Blvd SE,
Calgary, AB - T2C 3E7
Ph: 001-403-640-8062

Parth Vyas
Jacobs Canada Inc.
205 Quarry Park Boulevard SE
Calgary, Alberta, T2C 3E7, Canada
(403) 385 3001


-----Original Message-----
From: nykochak [mailto:fo...@sefindia.org]
Sent: Thursday, August 14, 2008 2:35 AM
To: gen...@sefindia.org
Subject: [SEFI] Re: Drift Limits

Dear Parth,

Sorry for delayed response.
I agree with you.

I would like to bring to your notice that IS 456 - 2000 clause no. 20.5 page 33 mention that
Under transient wind load the lateral sway at the top should not exceed H/500, where H is the total height of the building. For seismic loading, reference should be made to IS 1893.

I want to stress again that

Is interpretation of word transient wind loading is different from code to code ?

I think now you will agree with me that earth quake and wind both are transient loads as far as Indian Code is concerned. There is lot of variation in permissible Drift limits if you compare codes of various countries. (From 0.001xh to 0.005xh).

Does that mean if I design the building in China is safe and the same will be unsafe in Poland ?

With regards.

N. Y. Kochak

[b]--auto removed--

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