www.sefindia.org

[Yogesh.Pisal]

As per Cl7.5.1; IS1893 Part 4 : 2005
Structures in Category I shall be designed for MCE

As per Cl7.3.3; IS1893 Part 4 : 2005
STructure in Category I, need to be designed under MCE and checked under DBE. Load factors for combination with MCE are unity

I am designing one process column on elevated structures (As per Tb 5 it is under Category I)

I have following doubts in this regard

1. When structural analysis is as per IS800:1984 (WSM)
    There is no difference between load combinations for DBE & MCE
    We are getting only benefit of increased damping for MCE
    Benefit due to increased damping is not significant
    Then whether it is really required to check the structure for DBE

2. When structural analysis is as per IS800:2007 (LSM)
    We are getting benefit from unit load factors for MCE
    There is also benefit from increased damping

3. Vessel design code still follows WSM
    If we are designing the structure and vessel for MCE,
    Then there will be huge difference between the capacities of structures and vessel.
    Whether there are some provisions to reduce the effects of MCE when using WSM (For structures as well as for vessel design)

[sukanta.adhikari]

Dear Shri Yogesh Pisal,
 
I think u have read IS 1893:2005 (Part 4) very carefully…and the question what u have raised is very interesting..I think what code has mentioned to design the structure in category I was keeping Limit state method in mind…and so they have asked to design for MCE(without load factor) and DBE(for load factor)…
 
They have not kept in mind WSM and its reduce stress concept…and still we many structures need not be steel structure even some RCC structure are to be designed by WSM..as for instance T.G foundation…. .so naturally if u design for MCE and WSM….the sections will be very heavy...and I have also read the code very thoroughly…but I could not find any answer to ur question about the reduction factor for MCE and WSM….
 
But one thing I feel that while designing by WSM we should go for DBE….Experts can give their views.
 
And regarding the emphirical equations for rerular or irregular structure..i am working on it..and I will get back to u…
 
 
Regards,
S.Adhikari
09431568830
suku82in@yahoo.com (suku82in@yahoo.com)
 
 

On 11/6/09, alltimepisal <forum@sefindia.org (forum@sefindia.org)> wrote:

[sukanta.adhikari]

Dear Shri Yogesh,

A very old discussion of ours...The emphirical equation for time period given in IS 1893:2002 is for regular structure only....Actually it has not been seen by me as it is in the introduction part of IS 1893....But we wrongly use it for all type of structure and even our software while performing seismic anaysis by static method uses this for irregular structure....But still we r in a state of conservative design only in case whn base shear by static method is more than response spectrum method.because in that case we design for the former

Regards,
S.Adhikari
suku82in@yahoo.com
08051069555

[Yogesh.Pisal]

Dear Sukanta Adhikari Sir,

No doubt we are on conservative side. But, I will like to share my experience here.

- Pipe rack will come under RC frame building and empirical formula for fundamental time period is 0.075 h ^ 0.75.
- Pipe rack has the dimension of 8m width x 50m long. Above formula will result in equal time period in both the directions which is wrong.

There are so many other cases where we will be ending up with wrong value of base shear and scaling up.
Difference in base shear in some of the cases is upto 80% (In some of the structures I worked on)

Hence I feel that if scaling is at all to be done it should be as per appropriate empirical formula (ASCE manual for seismic design can be referred for the appropriate formulas) otherwise it is better not to do the base shear enhancement.

Regards,
Yogesh Pisal

[sukanta.adhikari]

Dear Shri Yogesh,

Can u please tell us what are ASCE provision for scaling base shear what u have mentioned below.It will be useful

Regards,
S.Adhikari

[Yogesh.Pisal]

Dear Sukanta Adhikar Sir,

Attached here is the provision of ASCE manual about scaling of results for structures in petrochemical facilities.

Thanks & Regards,
Yogesh Pisal

[sukanta.adhikari]

Dear Yogesh,

Thanks a lot for the attachment...Have u ever come accross emphirical time period formula for irregular structure as per other international standard...

Regards,
S.Adshikari

[Yogesh.Pisal]

Dear Sukanta Adhikari Sir,

I did not get any reference for fundamental time period of irregular structures

Regards,
Yogesh Pisal