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[VETRI]

Dear ALL,
While providing the foundation system for our steel structures like Conveyor gallery supporting Trestles & Junction towers, we are following R.C Framed foundation system consist of columns, Tie beams and footings. More cases the governing load case for super structure design (Trestles & Junction Towers – Steel structure) is Wind load. Reaction from super structure is applied as load to the foundation system.
My doubt is as follows,
1.   Whether ductile detailing is required for this foundation system?
2.   If so what is the ductile detailing provisions for underground structures like Tunnel, Pit etc.?
3.   For R.C buildings, whether the ductile detailing is required for below ground level (Plinth level)?
4.   One of our client referring Cl.6.1.3 (last Paragraph) of IS 1893- 2002- Part-1, and insisting us to provide ductile detailing for Conveyor Trestle & Junction tower foundations, even though it is not governed/designed for Seismic loads.

Also please note that the Structure is in Zone - III and the junction tower loading is more or less uniformly distributed than masses of concentration like floors.

Kindly give the clarifications with reference to any Standards / Books. So that I can able to convince the Client

Thanks & Regards,
V.Vetrivelan

[Dr. N. Subramanian]

Dear Er Vetrivelan,

Even though the structure you are designing is not governed by earthquake load, note that as per the code you are taking the actual EQ load but the design basis EQ divided by a factor R. If the R value is 5 then you are dividing the actual EQ load by a factor of 10! How do we say that the structure will be able to resist the actual load. It is by means of over strength and ductility.

How do we provide ductility? It is only through seismic detailing. Hence provide close spacing of stirrups in the footing column connection unto a length of 300 mm as given in the code. Though the foundation will also be affected, we normally assume that the footing is supported by soil on all the sides and hence will not be affected. Of course, check whether the soil is not having liquefaction potential. Also turn the column rods in the footing toward the column and not outside the column as shown in the text books.


Best wishes,
NS

[sakumar79]

Dear Sir,

1. The clause you have mentioned does not talk about ductile detailing, but only design to earthquake load even if it may not appear to be predominant.

2. IS13920 however has a clause 1.1 which insists that Ductile Detailing should be adopted for all RC structures in Zones III, IV and V.

3. The ambiguity rises in whether we should consider the foundation component of steel structures (sheds, silos, etc) as RC structures or if the term represents only those buildings where the superstructure is also of RCC. In my humble opinion, even foundation component of steel structures should be considered for ductile detailing. Exceptions may be sheds without intermediate/mezzanine floors and cranes where the dead load is small and live load is only at roof level (which is neglected in calculation of seismic weight). There has already been debate on this particular ambiguity over in SEFI where some senior sefians have recommended ductile detailing for foundation of such sheds also.

Yours sincerely,
Arunkumar

[SANGEETA WIJ]

Dear Mr Arun Kumar

Referring to the third point in your email, I feel we should adopt ductility even in the RCC part of Industrial /Steel structures as well, although not specifically covered by our 1893 or 13920.

Regards
Sangeeta Wij

From: sakumar79 [mailto:forum@sefindia.org]
Sent: 27 April 2013 08:04
To: general@sefindia.org
Subject: [SEFI] Re: Clarification required in Ductile detailing (IS 13920)



Dear Sir,

1. The clause you have mentioned does not talk about ductile detailing, but only design to earthquake load even if it may not appear to be predominant.

2. IS13920 however has a clause 1.1 which insists that Ductile Detailing should be adopted for all RC structures in Zones III, IV and V.

3. The ambiguity rises in whether we should consider the foundation component of steel structures (sheds, silos, etc) as RC structures or if the term represents only those buildings where the superstructure is also of RCC. In my humble opinion, even foundation component of steel structures should be considered for ductile detailing. Exceptions may be sheds without intermediate/mezzanine floors and cranes where the dead load is small and live load is only at roof level (which is neglected in calculation of seismic weight). There has already been debate on this particular ambiguity over in SEFI where some senior sefians have recommended ductile detailing for foundation of such sheds also.

Yours sincerely,
Arunkumar

Posted via Email

[prakashsiyani]

Dear All

In IS 13920:1993 Cl 7.4.8, Confining reinforcement shall not be less than

Ash = 0.18 S h fck/fy (Ag/Ak-1)......... (Equation is derived from experiment equation of richart fcc = fcp+4.1ft)

But it's derivation only for 2 legged stirrups of column means only 1set stirrups (ACI Code)

What is about for column having 8 legged stirrups ??

example like,

300x900 column having 18 number of 20dia bar and 3 set+3link stirrups,
where fck =20 Mpa, Fy =415 Mpa & clear cover equal to 40mm


what is dia of stirrups used and at which spacing ?


Answer 1 like


Ash = 0.18*100*220*20/415 *((300x900)/(220x820)-1)
       = 94.78 mm2
so spacing for 12 dia bar = 113/94.78 *100 = 119mm c/c or spacing for 10 dia bar = 78.5/94.78*100 = 82 mm c/c


Means provided 10 dia at 82 mm c/c 3SET + 3link


Answer 2 like


Ash = 0.18*100*220*20/415*((300x300)/(220x220)-1)
       = 164 mm2
This area is only for 1SET of 2 legged stirrups we provided 3set +3link means equivalent to 4 SET
so 164/4 =41 mm2


So provided 8 dia = 50.26/41 *100 = 122mm


Means provided 8 dia at 122mm c/c 3SET+ 3link


Which is correct answer for IS CODE EQUATION ?? Think Twice ??


See attached figure

[sknsrinivasan]

Dear Prakashsiyani

   Generally we have to consider Ak value Hoops outer to outer dimension. How 220X820 you have considered I don't know  

   If 2 leg Hoops is enough as per shear design,  but Code is restricted the distance between outer face of hoops should not exceed 300mm so we have to provide internal hoops in form of overlapping hoops or cross tie in both direction. If we provide like this in the formula the value of h will be very less and required spacing will be more other advantage will be to avoid buckling of vertical bars If we provide 2 leg hoops that will prevent the buckling of corner 4 bars

   We have to assume dia of hoops and find spacing of hoops in this one the spacing not less than 75mm and grater than 100mm

   I think your Answer 1 is wright  and the hoops should have good ductility


Regards
N.Srinivasan

[prakashsiyani]

Dear N.Srinivasan

Thanks for your valuable reply, But In practical filed, I have seen that very less structural consultant provided stirrups details as 10dia @ 100mm or 75mm, Most of consultants in ahmedabad, Bangalore, Pune, Mumbai provided details like 8dia @ 100mm at support Hc/6 and center 8dia @150 c/c

In ahmedabad city, I have seen, all consultants used 8 dia as stirrups, if anybody would like to provided 10dia stirrups than they loss project from client. this scenario we have seen from last 5 year so what we are doing?? very serious!!

Prakash Siyani

[Abhishek_Singh]

Hi Er. Arun Kumar,

Against the popular opinion here, I would say that there is no need to do ductile detailing in foundations (there is no method to do it also ). Only the portion of the columns going into the footing will be detailed for ductility.

As Dr. NS Sir is saying and if I am understanding it correctly, the EQ coming into the footing will be absorbed by the surrounding soil through winkler spring energy dissipation. Since in the superstructure does not have any surrounding elastic material, we have no choice but to tell the building to absorb it internally through ductility.

This is just a judicious academic guess....not very sure about it.

Best,

Abhishek

[anviksha]

[anviksha]

dear Mr. Prakash
I humbly disagree with your statement specific to Ahmedabad consultants
mentioning " all Ahmedabad consultants use 8# stirrups " and " if they provide 10# stirrups, they will lose the project"
it is far from truth/reality.
structural detailing is being done purely on merits of calculations backed up by sound engineering judgements.
your observations might be true in some specific cases but not applicable to all.....

best regards
vatsal mahendra Shukla
principal structural engineer
anviksha::structural design studio
ahmedabad