www.sefindia.org

STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

 Forum SubscriptionsSubscriptions DigestDigest Preferences   FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups  RegisterRegister FAQSecurity Tips FAQDonate
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log in to websiteLog in to websiteLog in to websiteLog in to forum 
Warning: Make sure you scan the downloaded attachment with updated antivirus tools  before opening them. They may contain viruses.
Use online scanners
here and here to upload downloaded attachment to check for safety.

Top and bottom reinforcement in isolated footing
Goto page 1, 2, 3  Next
 
Post new topicReply to topic Thank Post    www.sefindia.org Forum Index -> SEFI General Discussion
View previous topic :: View next topic  
Author Message
suresh_sharma
...
...


Joined: 23 Mar 2011
Posts: 783

PostPosted: Tue Apr 05, 2011 5:29 pm    Post subject: Top and bottom reinforcement in isolated footing Reply with quote

To all Sefians

From the design consideration bottom reinforcement is worked out by
the designer for RCC isolated footing. I have observed from the design of
some designers and I too am practising to provide some reinforcement
in the top layer of the isolated footing say 0.12% of the section
distribted both ways. In almost all the design books the top layer of
the footing is shown as inclined. But I do not favour this approach as
it becomes difficult to vibrate the concrete in inclined state.
Further the RMC having higher slump will flow away in course of
vibration from the inclined surface.My senior is asking me to convince
him about the top layer of reinforcement. My feeling is that it is
very difficult to predict the exact behavior of footing, hence it is
advisable to provide minimum reinforcement in the top layer too so
that entire footing behaves as one unit of concrete. I seek your help
in this regard to enable me to convince my senior with adequate
justification.
Back to top
View user's profile Send private message
Thankful People
1 user(s) is/are thankful for this post.
thirumalaichettiar
Silver Sponsor
Silver Sponsor


Joined: 26 Jan 2003
Posts: 3459

PostPosted: Tue Apr 05, 2011 5:56 pm    Post subject: Reply with quote

Dear Er.Sureshpsharma,

Usually the Top reinforcement is provided against :

1. Seismic forces but the usual reinforcement is .12%. This is in the event of any vertical forces arising fro EQ. Also some engineers provide steel rein. as nominal one.
2. In retrofitting of footing also the top reinforcement will be of great help.
3. Some software like SCADDS gives detail with reinforcement for pedestal.
4. Also the footing- underground buried members once buried can not be easily inspected to check their durability and deterioration. As a precaution providing some extra steel will be of beneficial only.  you can arrange the reinforcements in such a way to have access for the vibrators or use very small dia needle.

Using a sloped footing is having the following practical and technical problems:
1. casting of concrete has to be carried out with very stiff0 low slump concrete.
2. Vibration is not possible only hand tampering is possible which will result loose and honey combed concrete.
3. Curing is to be done not by ponding but with hessian.

Due to poor compaction the strength designed may be achieved. Hence better to design footing as stepped one which may consume little more concrete.  Refer Concrete handbook by Mark Fintel and Design of concrete structure by Dr.Ramachandran ( the title may be different).

Hope that above may be of some help.

T.RangaRajan.
Back to top
View user's profile Send private message
vikram.jeet
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 2212

PostPosted: Wed Apr 06, 2011 9:22 am    Post subject: Top and bottom reinforcement in isolated footing Reply with quote

This topic has been discussed earlier also.

Top reinf in column footings of buildings (RCC construction) is not needed unless
there is possibility of tension at top. It may be a wasteful expenditure. SP-34 sketch
on footings for RCC column ,also indicate only bottom mesh.

Top mesh ,in addition to bottom mesh may be definitely needed in following
cases:

Light weight steel sheds on RCC pedestal and footings. Wind uplift  
will be countered through footing &soil weight over footing necessitating
top reinf.

Independent RCC columns(not having connection to building behind)
supporting Cantilevers like entrance porticos , bus shelters etc derive their
stability from footing and soil weight over it. The rcc footings must have  
top mesh reinf designed for top tension from bending.

And all other cases requiring reinf at top from BM considerations

EQ effect:The effect of EQ Moment on footing is generally reduced due to  
provision of plinth beams/and (or) beams provided at footing level itself
and designed for EQ moments.
Plinth beams are generally provided in almost all cases of structures in  
severe EQ zones
However ,in absence of plinth beams, the effect of EQ moments on footing
need to be seen for deciding whether top reinf.is needed or not.

best regds

vikramjeet


To all Sefians

From the design consideration bottom reinforcement is worked out by
the designer for RCC isolated footing. I have observed from the design of
some designers and I too am practising to provide some reinforcement
in the top layer of the isolated footing say 0.12% of the section
distribted both ways. In almost all the design books the top layer of
the footing is shown as inclined. But I do not favour this approach as
it becomes difficult to vibrate the concrete in inclined state.
Further the RMC having higher slump will flow away in course of
vibration from the inclined surface.My senior is asking me to convince
him about the top layer of reinforcement. My feeling is that it is
very difficult to predict the exact behavior of footing, hence it is
advisable to provide minimum reinforcement in the top layer too so
that entire footing behaves as one unit of concrete. I seek your help
in this regard to enable me to convince my senior with adequate
justification.

--

Posted via Email
Back to top
View user's profile Send private message
gpsarathyy
...
...


Joined: 28 Jun 2010
Posts: 483
Location: chennai

PostPosted: Thu Apr 07, 2011 4:39 am    Post subject: Reply with quote

Dear Sefian,

It is better to provide the min. top rebar. If your site is in high seismic zone, where the stresses can be reversed due to the seismic/wind effect.
Otherwise it shall be provided only where the tension occurs as stated by Vikram Jeet.

Regards,
G.PARTHASARATHY
CHENNAI
GPSARATHYY@GMAIL.COM
Back to top
View user's profile Send private message Visit poster's website
padamkr
SEFI Member
SEFI Member


Joined: 26 Jan 2003
Posts: 4

PostPosted: Thu Apr 07, 2011 3:32 pm    Post subject: Top and bottom reinforcement in isolated footing Reply with quote

Dear Er vikramjeet,

plz go through the clause 34.5.2   (IS--456:2000) regarding top reinf
in footings,

which states that for concrete footing of thickness more than 1.0 mtr ,

360  sqmm per mtr length in each direction on each face  should be provided .

correct me if I am wrong.

Thanks and regards

padam kumar


On 06/04/2011, vikram.jeet <forum@sefindia.org> wrote:
Quote:
This topic has been discussed earlier also.

Top reinf in column footings of buildings (RCC construction) is not needed
unless
there is possibility of tension at top. It may be a wasteful expenditure.
SP-34 sketch
on footings for RCC column ,also indicate only bottom mesh.

Top mesh ,in addition to bottom mesh may be definitely needed in following
cases:

Light weight steel sheds on RCC pedestal and footings. Wind uplift
will be countered through footing &soil weight over footing necessitating
top reinf.

Independent RCC columns(not having connection to building behind)
supporting Cantilevers like entrance porticos , bus shelters etc derive
their
stability from footing and soil weight over it. The rcc footings must have
top mesh reinf designed for top tension from bending.

And all other cases requiring reinf at top from BM considerations

EQ effect:The effect of EQ Moment on footing is generally reduced due to
provision of plinth beams/and (or) beams provided at footing level itself
and designed for EQ moments.
Plinth beams are generally provided in almost all cases of structures in
severe EQ zones
However ,in absence of plinth beams, the effect of EQ moments on footing
need to be seen for deciding whether top reinf.is needed or not.

best regds

vikramjeet


To all Sefians

From the design consideration bottom reinforcement is worked out by
the designer for RCC isolated footing. I have observed from the design of
some designers and I too am practising to provide some reinforcement
in the top layer of the isolated footing say 0.12% of the section
distribted both ways. In almost all the design books the top layer of
the footing is shown as inclined. But I do not favour this approach as
it becomes difficult to vibrate the concrete in inclined state.
Further the RMC having higher slump will flow away in course of
vibration from the inclined surface.My senior is asking me to convince
him about the top layer of reinforcement. My feeling is that it is
very difficult to predict the exact behavior of footing, hence it is
advisable to provide minimum reinforcement in the top layer too so
that entire footing behaves as one unit of concrete. I seek your help
in this regard to enable me to convince my senior with adequate
justification.
-- 限









Posted via Email
Back to top
View user's profile Send private message
suresh_sharma
...
...


Joined: 23 Mar 2011
Posts: 783

PostPosted: Fri Apr 08, 2011 3:13 am    Post subject: Reply with quote

Dear Padam Kumar,

I was exactly looking for a reply that you have posted. I had some vague memory that top reinforcement will be guided by the thickness of footing.

Thanks
Back to top
View user's profile Send private message
thirumalaichettiar
Silver Sponsor
Silver Sponsor


Joined: 26 Jan 2003
Posts: 3459

PostPosted: Sat Apr 09, 2011 6:57 pm    Post subject: Reply with quote

Pl. find attached a sketch showing the top reinforcement provision for a wall footing as per page 433/ structural concrete by Nadeem Hussain.

T.RangaRajan.



Warning: Make sure you scan the downloaded attachment with updated antivirus tools  before opening them. They may contain viruses.
Use online scanners
here and here to upload downloaded attachment to check for safety.
footing details.pdf
 Description:

Download
 Filename:  footing details.pdf
 Filesize:  95.85 KB
 Downloaded:  2360 Time(s)

Back to top
View user's profile Send private message
vikram.jeet
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 2212

PostPosted: Mon Apr 11, 2011 5:11 am    Post subject: Top and bottom reinforcement in isolated footing Reply with quote

Dear Er Padam Kumar ji

You are very right in enligntening about clause 34.5 on Nominal
reinf.
34.5.1 Minimum reinf and spacing shall be as per the requirements of SOLID SLAB
34.5.2 - - in spirit of this clause  
For concrete sections of thickness greater than 1.0m
The reinf of 3.6cm2 per m(in each direction) is nominal reinf to be provided on each face
Thus ,this may also include side faces besides top face &bottom face

This is same as the treatment for mass concrete where surface reinf
is recommended on each face of concrete.

Many thanks for timely information and regards

vikramjeet


Dear Er vikramjeet,
plz go through the clause 34.5.2 (IS--456:2000) regarding top reinf
in footings,
which states that for concrete footing of thickness more than 1.0 mtr ,
360 sqmm per mtr length in each direction on each face should be provided .
correct me if I am wrong.
Thanks and regards
padam kumar

--

Posted via Email
Back to top
View user's profile Send private message
muralidhar.sivaraju
Progressive Member
Progressive Member


Joined: 26 Jan 2003
Posts: 40

PostPosted: Mon Apr 11, 2011 5:54 am    Post subject: Top and bottom reinforcement in isolated footing Reply with quote

Dear Sir,

please guide me on the middle reinforcement in the thicker(more than 1m) foundations, any codal references?

regds
muralidhar

On Mon, Apr 11, 2011 at 11:06 AM, vikram.jeet <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Dear Er Padam Kumar ji

You are very right in enligntening about clause 34.5 on Nominal
reinf.
34.5.1 Minimum reinf and spacing shall be as per the requirements of SOLID SLAB
34.5.2 - - in spirit of this clause
For concrete sections of thickness greater than 1.0m
The reinf of 3.6cm2 per m(in each direction) is nominal reinf to be provided on each face
Thus ,this may also include side faces besides top face &bottom face

This is same as the treatment for mass concrete where surface reinf
is recommended on each face of concrete.

Many thanks for timely information and regards

vikramjeet


Dear Er vikramjeet,
plz go through the clause 34.5.2 (IS--456:2000) regarding top reinf
in footings,
which states that for concrete footing of thickness more than 1.0 mtr ,
360 sqmm per mtr length in each direction on each face should be provided .
correct me if I am wrong.
Thanks and regards
padam kumar
--








Posted via Email
Back to top
View user's profile Send private message
vikram.jeet
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 2212

PostPosted: Wed Apr 13, 2011 4:54 am    Post subject: Top and bottom reinforcement in isolated footing Reply with quote

Mass concrete /massive foundations
I think the reinforcement provision is from the consideration of shrinkage and  
temperature on the surface and therefore reinforcement is recommended at  
surface only. Concrete mass within may not need such reinf.
e.g. Mass concrete abutments for bridges, solid gravity retaining walls

However in case of massive foundations having Anchor bolts for towers/machines
etc, the concrete mass within needs to be reinforced to distribute the localized
tensile force from Bolts to the concrete mass and internal reinf layers may be  
provided to avoid local failure of concrete around bolt locations

best wishes and regds

vikramjeet



Dear Sir,
please guide me on the middle reinforcement in the thicker(more than 1m) foundations, any codal references?
regds

muralidhar
--

Posted via Email
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topicReply to topic Thank Post    www.sefindia.org Forum Index -> SEFI General Discussion All times are GMT
Goto page 1, 2, 3  Next
Page 1 of 3

 

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


© 2003, 2008 SEFINDIA, Indian Domain Registration
Publishing or acceptance of an advertisement is neither a guarantee nor endorsement of the advertiser's product or service. advertisement policy