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[Saurabh Raj]

Dear All,

Hope you are doing well.

Clarification needed for IS 1893-2016. Cl.7.12.3 for seismic effect on compound wall/Boundary wall. In my case there is boundary wall with retaining wall(With 2.5m soil retaining)
IS-1893 says that 'Ah' shall be considered for 1.25Z with I=1 and R=1 which is 3times of IRC-06 (Design of retaining wall for seismic effect) whereas, in IRC-6-2017 Cl. 214.1.2.1, Ah shall be considered Z/2 and even in IIT commentary (IITK GSDMA EQ05-V4.0, GSDMA EQ15-V3.0) it is recommended that Ah=Z/2 Cl 7.14.3.
Now, 1. what shall I consider in my design.
        2. what is the reference of IS-1893 cl 7.12.3.
        3. Is compound wall and boundary wall same.
Your response will be much helpful.

Thanks & Regards,
Saurabh Raj

[vikram.jeet]

The design horizontal seismic coefficient  Ah = (Z/2)*(Sa/g)*(I/R)

As per this clause boundary walls / compound walls to be designed
using I =1.0   ; R=1.0  ; Sa/g = 2.5

Putting these values  Ah  works out = 1.25 Z

Compound walls  are free standing vertical cantilever structures .


For free vertical  projections above roof tops , the Ah value is five times . (  also for horizontal cantilever  projections at any floor Av*5 to be taken).

Yes , the 1.25 Z value amounts to a eq static force  = 0.3g in Zone 4

In case of    roof top vertical projections  on a  building with  I=1.0 ,  R= 5.0 (ductile detailing) ,  it works out to ,

Ah = 5[( Z/2) *(1/5)*2.5] = 1.25*Z

But for Load bearing masonry Buildings ,   the value of Ah for free standing projections  is still more since R =2.5  (masonry  with rcc  bands and corner bars) I.e
Ah =  5[Z/2  * (1.0/2.5) *2.5) = 2.5 *Z

(To be continued)

[vikram.jeet]

As regards query:

Compound wall is boundary wall .

However  retaining wall is retaining structure which needs to be designed under  seismic condition using higher Earth pressure coefficient  than normal non seismic case.

In yr case  Compound wall is retaining wall (retains earth) in lower 2.5 m  but  a boundary wall exist on it , above .

It is better to adopt  :

Ah= 1.25 Z for portion of wall acting as free standing boundary wall portion

Ah = (Z/2)*(1.0/2.5)* 2.5 = 0.5 Z for retaining wall portion
( R Value for  rcc abutment wall is taken 2.5)

[Saurabh Raj]

Sir your justification well explained is very clear to me.

Further, can I get the reference of ( R Value for  rcc abutment wall is taken 2.5)

Thanks for your support

[vikram.jeet]

May pl refer IRC 6 where for R value is given in table

Masonry/ pcc abutment  R= 1.0

RCC abutment  R = 2.5  ( without Ductile Detailing)
And R = 3.0  ( with DD)