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IS 800:2007 - Section 12 application
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abhishek.theone
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PostPosted: Sun Jan 30, 2011 2:30 pm    Post subject: IS 800:2007 - Section 12 application Reply with quote

Dear All,

I hope that people reading this topic are best in their health.

Background:
Design of a very light structure say a 13m high warehouse shed of size 50m x 30m. The place of construction is in seismic zone IV & wind velocity in range of 55m/s.

Since the structure is light the governing load combination is one having wind load. Since the structure is shed, the slender column is coming up of very high size.

Now, when section 12 of IS 800: 2007 comes into picture. Clause no. 12.12 of the IS 800 say the base plate is to be designed withstanding 1.2 times the plastic moment of column size. Due to which, the base plate sizes coming are abnormal when we apply this code.

My interpretation goes like:

If governing load case is other than seismic loads, the design shall be done as per actual design actions and the ductility shall be checked for earthquake loads only as per section 12 of IS 800:2007.

Hence, Base plate shall be designed withstanding the actual design actions with adequate factor of safety.

I will like to know the interpretations of the expert readers.

regards,

Abhishek
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Yogesh.Pisal
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PostPosted: Sun Jan 30, 2011 4:36 pm    Post subject: Reply with quote

Dear Abhishek,

Actual seismic forces coming on the structure are much higher than the forces specified in the code. Here, we are designing for the reduced forces by considering factors like ductility, redundancy, overstrength etc. Hence, it is not justified to compare the design forces and design for the wind loads if they are governing.

When we are designing the base plate for 1.2 times plastic moment capacity, we are ensuring that base plate will not fail before the failure of the member.

Hence, in my opinion we have to design and detail the structure as per Chapter 12, IS 800:2007 even if it is governed by the wind load.

Regards,
Yogesh Pisal
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amarjeetsingh
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PostPosted: Sun Jan 30, 2011 6:02 pm    Post subject: IS 800:2007 - Section 12 application Reply with quote

dear sir,

i agree with the contention of yogesh pisal that even if the seismic loading is not teh governing case we have to apply the provisions of the Chapter 12.

But there are divergent views on the applicability of the chapter 12 as a whole for the light weight warehouse structures .

I will advise that U should check and confirm with ur seniors in ur office and if theproof consultant is on board advisable to take his view. i will mail u the exemptions which were allowed on other project fopr warehouse type of building.

cheers

amarjeet singh


On Sun, Jan 30, 2011 at 10:06 PM, Yogesh.Pisal <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Dear Abhishek,

Actual seismic forces coming on the structure are much higher than the forces specified in the code. Here, we are designing for the reduced forces by considering factors like ductility, redundancy, overstrength etc. Hence, it is not justified to compare the design forces and design for the wind loads if they are governing.

When we are designing the base plate for 1.2 times plastic moment capacity, we are ensuring that base plate will not fail before the failure of the member.

Hence, in my opinion we have to design and detail the structure as per Chapter 12, IS 800:2007 even if it is governed by the wind load.

Regards,
Yogesh Pisal







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abhishek.theone
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PostPosted: Sun Jan 30, 2011 6:53 pm    Post subject: Reply with quote

Dear Mr. Yogesh,

First of all thanx for the response, i really appreciate it.

So, Basically even if governing load combination is other than seismic loads, structure will be designed as per section 12 with considering governing load combination that is other than seismic load combination case.

Dont you that it will be an economical design and from construction point of view not viable. This is because design was done was done for following different load combination (in my case):

i) Critical load combination with seismic loads
ii) Overall governing load combination

The member size for the second load combination is much higher than those obtained considering 1st load combination. So, now if we consider load combination II and design as per section 12, it will further be much higher.

I dont know but i still request you to review. thanx for the time.

Regards,

Abhishek Kumar
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anshugoel
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PostPosted: Mon Jan 31, 2011 2:38 am    Post subject: Re: IS 800:2007 - Section 12 application Reply with quote

abhishek.theone wrote:
Dear All,

I hope that people reading this topic are best in their health.

Background:
Design of a very light structure say a 13m high warehouse shed of size 50m x 30m. The place of construction is in seismic zone IV & wind velocity in range of 55m/s.

Since the structure is light the governing load combination is one having wind load. Since the structure is shed, the slender column is coming up of very high size.

Now, when section 12 of IS 800: 2007 comes into picture. Clause no. 12.12 of the IS 800 say the base plate is to be designed withstanding 1.2 times the plastic moment of column size. Due to which, the base plate sizes coming are abnormal when we apply this code.

My interpretation goes like:

If governing load case is other than seismic loads, the design shall be done as per actual design actions and the ductility shall be checked for earthquake loads only as per section 12 of IS 800:2007.

Hence, Base plate shall be designed withstanding the actual design actions with adequate factor of safety.

I will like to know the interpretations of the expert readers.

regards,

Abhishek


Hi..

If you don't mind, can you please upload the particular section where this requirement is stated.

It looks to be an unreasonable requirement from the face of it. This issue has come up before also in the forum and some how we have not been able to have an intense discussion about it. At that time also, i had requested some one to please upload this particular paragraph.
    We should all be doing design per the loads - not the plastic moment capacities - in order to force a particular outcome - at least as per me.
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dipakdgaikwad
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PostPosted: Mon Jan 31, 2011 4:08 am    Post subject: IS 800:2007 - Section 12 application Reply with quote

Dear Yogesh
The factor takes account of ductility, redundancy, overstrength etc is Response reduction factor(R).Suppose I take R=1 and then also seismic is not governing load case; will it be wise to design structure or any of its element (baseplate in present case) for ductility??
I think if seismic (with appropriate 'R' factor) load case is not governing one should reanalyse the structure with R=1 and if then also seismic load is not governing there is no need to design baseplate for 1.2 times plastic moment capacity of column.
Experts please correct me if wrong .
Thanks and Regards
Dipak D Gaikwad

-----Original Message-----
From: Yogesh.Pisal <forum@sefindia.org>
Sent: Sunday, 30 January 2011 10:06 PM
To: general@sefindia.org
Subject: [SEFI] Re: IS 800:2007 - Section 12 application

Dear Abhishek,

Actual seismic forces coming on the structure are much higher than the forces specified in the code. Here, we are designing for the reduced forces by considering factors like ductility, redundancy, overstrength etc. Hence, it is not justified to compare the design forces and design for the wind loads if they are governing.

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nwalavalkar
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PostPosted: Mon Jan 31, 2011 4:37 am    Post subject: Reply with quote

Dear Yogesh
Your interpretation of code is perfect. However
Base plate is a unique element where Axial force, shear and moment acts simultaneously. Hence P-V-M interaction has to be there in place. Probably that may be the reason AISC 341 / 05 has not adopted this type of clause but preferred to caution engineers about more practical approach.
Regards
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bharatthej
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PostPosted: Mon Jan 31, 2011 7:07 am    Post subject: Reply with quote

Dear Abhisek,

You can verify the demand to capacity ratio for D +L, D+W combo and compare it with lateral load. From the abnormalities you figured for baseplate, i expect that the column is vey tall (more than 8m unsupported probably). Hence forth due to the single curvature bending of that tall span, it can be more critical for lateral laods or gravity loads.(one of these loads are having high impact on the slenderness which you can find from D/C ratio). If this is the case you can tru reducing the effective length of column by bracing (preferably K bracing at girts location). Hope this could solve your problem by bringing the column dimensions down and hence the design capacity.

PS: ALWAYS THE BASE PLATE NEED TO BE DESIGNED FOR THE CAPACITY OF THE CONNECTING MEMBER (COLUMN) OR EVEN HIGHER (AS STATED IN IS800)

THANKS,

BHARAT THEJ

STRUCTURAL ENGINEER

DAR AL HANDASAH
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bharatthej
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PostPosted: Mon Jan 31, 2011 7:28 am    Post subject: Reply with quote

Forgot to add,

did you consider plastic moment capacity of the column also??

the type of connection is also critical. i hope you have used a pinned connection.

best way to do that is to form that sort of connection between column to baseplate in which case for the design strength of column you can only take the compressive/tensile capacity and need not consider moment capacity
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gpsarathyy
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PostPosted: Mon Jan 31, 2011 7:34 am    Post subject: Reply with quote

As stated by Er. Yogesh, it should be designed for the column capacity and not for the forces. (If possible reduce the column size by provided tie beams and bracings.)

It means that during seismic enormous amount of force will be realised by the structure at that time the structural members shall yield and deflect, but it shall not collapse catastrophically due to failure of bolts, baseplates or foundations.

Regards,
G.Parthasarathy
Chennai
GPSARATHYY@GMAIL.COM
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