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.

Post tensioned slabs
Goto page 1, 2, 3, 4, 5  Next
 
This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies. Thank Post    www.sefindia.org Forum Index -> E-Conferences-2008 [ Flat Slab Design issues ]
View previous topic :: View next topic  
Author Message
kajal.chopra
...
...


Joined: 26 Jan 2003
Posts: 102

PostPosted: Mon Dec 22, 2008 8:34 am    Post subject: Post tensioned slabs Reply with quote

Hi All,

This is a question which is bothering me from a long time and I need some explanation for the same.

In all the post tensioned slabs I have come across, I find that the post tensioned tendons do not pass through the column; that is; whenever a column is encountered the tendons are either curved (in plan) so that they do not intercept the column(s).

I want to know whether my reasoning for the above (that is the reason for the tendons being curved (or) not directly passing through the column is correct:

1. The tendons are highly stressed to the order of 1600 MPa.

2. If the tendon pass through the column, we could imagine the free body of the column: it woild be : at the location where the tendon passes through the column there would exist an enormous point force acing on the column which indeed would cause the bending of the column which could be detrimental.

3. I want to know if my interpretation/intuition of the above scenario is correct?

Can anyone please comment on my thinking above?

Please help!
Back to top
View user's profile Send private message
Thankful People
2 user(s) is/are thankful for this post.
sahi
SEFI Member
SEFI Member


Joined: 02 Oct 2008
Posts: 4

PostPosted: Mon Dec 22, 2008 8:58 am    Post subject: Post tensioned slabs Reply with quote

For Interior col. take them straight. For end column if not
practicable U may curve then. Col. + Slab act as monolith concrete
unit. No concenteration of forces due to cables.

On 12/22/08, kajal.chopra <forum@sefindia.org> wrote:
Quote:
Hi All,

This is a question which is bothering me from a long time and I need some
explanation for the same.

In all the post tensioned slabs I have come across, I find that the post
tensioned tendons do not pass through the column; that is; whenever a column
is encountered the tendons are either curved (in plan) so that they do not
intercept the column(s).

I want to know whether my reasoning for the above (that is the reason for
the tendons being curved (or) not directly passing through the column is
correct:

1. The tendons are highly stressed to the order of 1600 MPa.

2. If the tendon pass through the column, we could imagine the free body of
the column: it woild be : at the location where the tendon passes through
the column there would exist an enormous point force acing on the column
which indeed would cause the bending of the column which could be
detrimental.

3. I want to know if my interpretation/intuition of the above scenario is
correct?

Can anyone please comment on my thinking above?

Please help!









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


Joined: 26 Jan 2003
Posts: 230

PostPosted: Mon Dec 22, 2008 9:48 am    Post subject: Post subject: Post tensioned slabs Reply with quote

In the Indian scenario we usually use wide ducts comprising 3, 4 or 5 tendons whose total width is approx 70 mm. Usually in a ductile detailed column there is not enough space to accommodate this width. I have been trying to tell manufacturers to provide single tendon ducts so at least a few can be accommodated.

The stress in steel is high but not so in concrete (approximately 2 to 3 MPA).

Bending of column & its interaction with the slab is accounted for in design.

Hope that answers your questions

Regards

Vipul Ahuja

**************

Hi All,

This is a question which is bothering me from a long time and I need some explanation for the same.

In all the post tensioned slabs I have come across, I find that the post tensioned tendons do not pass through the column; that is; whenever a column is encountered the tendons are either curved (in plan) so that they do not intercept the column(s).

I want to know whether my reasoning for the above (that is the reason for the tendons being curved (or) not directly passing through the column is correct:

1. The tendons are highly stressed to the order of 1600 MPa.

2. If the tendon pass through the column, we could imagine the free body of the column: it woild be : at the location where the tendon passes through the column there would exist an enormous point force acing on the column which indeed would cause the bending of the column which could be detrimental.

3. I want to know if my interpretation/intuition of the above scenario is correct?

Can anyone please comment on my thinking above?

Please help!
Back to top
View user's profile Send private message
kajal.chopra
...
...


Joined: 26 Jan 2003
Posts: 102

PostPosted: Mon Dec 22, 2008 10:33 am    Post subject: Reply with quote

Thanks for the reply.

You said , in a ductile detailed column, there is not enough space to accomodate the width of (say) 70 mm.

Let us there is a provision to accomodate that width of 70mm(supposing),
then if these tendons do pass through the column would not that high force exerted on that location of the column by the tendon cause column to bend heavily?

Would not this be detrimental for the column?Would not this effect be avoided by not passing the tendon through the column (assuming there is sufficient space for the duct to pass)?
Back to top
View user's profile Send private message
kajal.chopra
...
...


Joined: 26 Jan 2003
Posts: 102

PostPosted: Mon Dec 22, 2008 11:13 am    Post subject: Reply with quote

What I mean to say is that in order to avoid the bending of the column is it not better to avoid passing the tendon through the column?
Back to top
View user's profile Send private message
cancer_kk
SEFI Member
SEFI Member


Joined: 03 Oct 2008
Posts: 1

PostPosted: Mon Dec 22, 2008 12:32 pm    Post subject: Post tensioned slabs Reply with quote

Dear kajal,
I hope that passing of tendons through should not cause you worry about column bending,because what I feel that cosine part of the postensioning force in tendon actually compensate for shear as well bending.
further if I am not wrong, Even if one avoid tendon to pass through column,passing of moments to columns from tendons through distribution cant be avoided.

On Mon, 22 Dec 2008 kajal.chopra wrote :
Quote:
What I mean to say is that in order to avoid the bending of the column is it not better to avoid passing the tendon through the column?








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


Joined: 26 Jan 2003
Posts: 230

PostPosted: Mon Dec 22, 2008 6:45 pm    Post subject: Post tensioned slabs Reply with quote

It is not only desirable but actually required by some codes to run tendons thru the column cage (for ductility of the column-slab connection). Further additional bottom steel is also required to prevent progressive collapse.

The concern for column bending is academic. This is accounted for in design. However very stiff columns or shear walls must be isolated from the PT operation-- say by casting in 2nd stage after the PT operation.

Regards

Vipul Ahuja

***********************
What I mean to say is that in order to avoid the bending of the column is it not better to avoid passing the tendon through the column?
Back to top
View user's profile Send private message
kajal.chopra
...
...


Joined: 26 Jan 2003
Posts: 102

PostPosted: Mon Dec 22, 2008 7:19 pm    Post subject: Reply with quote

You mean the bending moment induced in column due to tendons running through the column is accounted in design of the column?Through which clause of IS 456 or IS 1343?
Back to top
View user's profile Send private message
kajal.chopra
...
...


Joined: 26 Jan 2003
Posts: 102

PostPosted: Wed Dec 24, 2008 4:19 am    Post subject: Reply with quote

My only concern is:

Let us say a tendon passes through the olumn.

Now imagine the free body of the column.

In its free body, there would be a huge concentrated force acting at the location where the tendon passes, right?

Thus, this single concentrated force can cause enormous bending in the column irrespective if the stiffness of column is high/low)

Please anyone - throw some light on this issue.
Back to top
View user's profile Send private message
Kedar
SEFI Regulars
SEFI Regulars


Joined: 23 Jul 2008
Posts: 32
Location: Mumbai

PostPosted: Wed Dec 24, 2008 6:02 am    Post subject: Reply with quote

Hi Kajal


Can you please specify


1.eccentricity of tendons wrt column centre
in horizontal or vertical plane?

Lets calculate roughly the force applied by
tendon on column


1.Let's assume only tendons(say 4 nos 13mm dia stressed to 1000MPa)
are passing through column (say 900x900)with eccentricy if 400 mm (max
we can take ! )from centre of column.


2.Let's assume tendons are bent in horizontal plane by 6 dgrees for
all practical purpose wrt the direction of cable from its origin.


3.Let's assume tendons are bent in vertical plane by 4 dgrees for all
practical purpose wrt the horizontal plane. for slabs this is a bit
conservative.


  
4.Force in cables=no of cable x area x stress
=4 x 133 x 1000=532000 N = 532 kN ~ 53 Ton

5.Lets calculate forces due vertical dip by resolving forces

5a.Vertical component = Force in tendons x sin(theta)
=532 x sin(4)~ 37.5 kN
5b.Horizontal component = Force in tendons x cos(theta)
=532 x cos(4)~531 kN

6.The force transfer mechanism due profile in vertical plane

6a.Vertical components

Moment induced in column = vertical component in tendon x eccentricity
in column section

= 37.5 x 0.4 = 15.0 kN.m for all practical purpose this a small
if you want you can account for it with other LCs.

Please note that this the worst case we assumed that cables are
concentrated at one point/line but in actuals the are seperated by
some distance hence actual moments induced due to cable eccentricity
is low.

6b.Horizontal Component

The force is passed from column to slab due to diphragm action(since
column slab are integrally casted)the inplane rigidity of slabs we can
safely assume infinite.Hence the more number of columns this force is
shared less by every column.In most cases/practical the forces are
cancelled out by considering other tendons in the system.

7.Lets calculate forces due horizontal turning of tendons.

Here whatever forces/components are generated due to bend all are
transferred in diaphragm action in slab.We can safely apply same logic
of point No 6b.


I hope possibly this would solve your querry regarding moments induced
in column due to bend in tendons.In single word in most of the case
they are trivial for all practical purpose? but needs to be assess on
case-to-case basis.


Please correct me if I am wrong.


Regards
Kedar
Back to top
View user's profile Send private message
Display posts from previous:   
This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies. Thank Post    www.sefindia.org Forum Index -> E-Conferences-2008 [ Flat Slab Design issues ] All times are GMT
Goto page 1, 2, 3, 4, 5  Next
Page 1 of 5

 

 
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 can 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