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support sinking
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veerababudodda
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PostPosted: Wed Nov 17, 2010 6:43 am    Post subject: support sinking Reply with quote

Dear All,

I have a query regarding support displacement loading for a steel structure. The costumer has asked to design the structure for 8mm differential settlement and not mentioned any details how to apply on the structure.

1)in what way we should apply this loading to columns when there are more columns?
2)how this loading can be combined with other loading ie whether  can i  apply this loading simultaneously with wind and seismic?

kindly suggest me.

regards
babu
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umeshrao
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PostPosted: Wed Nov 17, 2010 10:20 am    Post subject: Re: support sinking Reply with quote

[quote="veerababudodda"]Dear All,

Dear Mr. Veerababu,
     I think you need to describe your structure , atleast.
      If the structure is multiple bays and multi storeyed then the support shall be modeled as spring and check back for differential settlement. Then only forces in beam elements will be accounting for differential deflections between the support.
     Regards Umesh Rao
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vikram.jeet
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PostPosted: Wed Nov 17, 2010 11:08 am    Post subject: support sinking Reply with quote

Dear Er Bapu

8mm would be the differential settlement between adjacent supports.
Accordingly depending on No of supports , sinking support cases can
be added to the loading list.
This is Nodal loading under support displacement in STAAD.
This load case can be combined in load combination as others.
This loading shall also be combined with wind/EQ. But loading cases  
without sinking support s shall also be present for appreciating governing  
values in members.

best wishes  

vikramjeet

Dear All,
I have a query regarding support displacement loading for a steel structure. The costumer has asked to design the structure for 8mm differential settlement and not mentioned any details how to apply on the structure.
1)in what way we should apply this loading to columns when there are more columns?
2)how this loading can be combined with other loading ie whether can i apply this loading simultaneously with wind and seismic?
kindly suggest me.
regards
babu

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veerababudodda
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PostPosted: Wed Nov 17, 2010 3:28 pm    Post subject: Reply with quote

Dear Vikram,

my structure consists of 22 columns with 5 bays in longitudinal direction and 3bays in transverse direction. If i have to apply differential settlement how can i proceed with this type of configuration.

Is giving nodal displacement for each column (each load case) individually correct?

Is this type of loading comes under accidental loading? If yes,can we combine this with wind/seismic loading as the stresses will be very huge?

Regards,
babu
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knsheth123
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PostPosted: Wed Nov 17, 2010 4:05 pm    Post subject: support sinking Reply with quote

Dear Sefian

In Safe Bearing Capacity of Soil, Total Allowable Settlement is considered. For Isolated Footings on Sand, it is 50mm. Normally a differential settlement of the order of 2/3 of total settlement is assumed to occur.

Other Criteria prescribed in IS 1904 is Angular Distortion : Diif. Settlement / Span should not be more than 1/500.

In general most of the structures are capable of accommodating 25mm diff. settlement, and steel structure can accommodate still more. Hence for 8mm diff. settlement no more analysis is required.

K. N. Sheth

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vikram.jeet
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PostPosted: Thu Nov 18, 2010 5:01 am    Post subject: support sinking Reply with quote

Dear Er Babu

it would be sufficient to see response /result of one frame in each direction separately.
(if frame is symmetrical geometrically, only two columns in transverse and three columns
in long. direction could be seen for sinking)
and effect could be added at design stage ,if design stage is attempted manually/spread sheets.

But if design is also attempted through staad, the input and out put would definitely be voluminous

It all depends on proof consultant since such a loading is prescribed loading for your project.
In my opinion this shall not be combined with EQ/wind.

best regards

vikramjeet


Dear Vikram,
my structure consists of 22 columns with 5 bays in longitudinal direction and 3bays in transverse direction. If i have to apply differential settlement how can i proceed with this type of configuration.
Is giving nodal displacement for each column (each load case) individually correct?
Is this type of loading comes under accidental loading? If yes,can we combine this with wind/seismic loading as the stresses will be very huge?
Regards,
babu

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ibarua
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PostPosted: Thu Nov 18, 2010 8:47 am    Post subject: support sinking Reply with quote

18th Nov 2010

For computing responses due to differential settlement, please see attached file.

As this settlement may occur at any time, say, immediately after construction, the responses due to such settlement have to be accounted for along with responses due to earthquake or wind actions.

For a frame with more than 2 columns, it may be sufficient to account for such settlement in relation to any column and the column next in line to it, in isolation from the other columns. This is to say that differential settlements need be accounted for between any two columns at a time, and not for all the columns in a line having settled by 8mm in relation to a preceding column, which is very unlikely.

The effect of differential settlement may be accounted for in all the beam-column joints along the frame, because such diff.settlement could occur anywhere..

Indrajit Barua.


On Wed, 17 Nov 2010 13:32:26 +0530 "veerababudodda"  wrote
Quote:
     Dear All,

I have a query regarding support displacement loading for a steel structure. The costumer has asked to design the structure for 8mm differential settlement and not mentioned any details how to apply on the structure.

1)in what way we should apply this loading to columns when there are more columns?
2)how this loading can be combined with other loading ie whether can i apply this loading simultaneously with wind and seismic?

kindly suggest me.

regards
babu
     
Quote:





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lele_raj
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PostPosted: Fri Nov 19, 2010 7:37 am    Post subject: support sinking Reply with quote

Dear Vikram / and Babu,

If client has given you magnitude of sinking, he must give you the boundary conditions as well, where the sinking has taken place and how it has been distributed. If not, you can apply logic. Look at it this way. All 5 (X) and 3 (Y) columns can not have exactly the same displacement, otherwise, it's just a free body movement, and no stresses / strains would be developed. You can start by assuming that the displacements are linear from 0 at one end to maximum at the other, assuming soil of uniform properties. (For nonuniform soil, you'll need an expert's opinion.). It's highly unlikely that the displacement will be random, provided continuity of the structure and the support. This gives rise to 3 loading conditions: 1. displ 0 at one end of bay and max at other, for one direction, say X. 2. Ditto for Y. 3. Diagonal displacement, ie. diagonally, one column has zero displacement, diagonally opposite column would have max displacement.

Assuming same linear displacement, you can also develop another set of loading conditions, this time with maximum at the central column and diminishing to 0, either in X, or Y or in diagonal direction.

I've discounted the possibility of nonuniform displacement along one direction as that would suggest warping of the superstructure, which will be quite unusual. I've also assumed that the displacements are only in one direction, say downwards. This means that there are no columns experiencing uplift and there is no structure - foundation separation.

In my view, if you list all these assumptions, and add some more logical / rational ones, the way towards solution will become quite clear. At the end of the exercise, you'll need to validate these assumptions.

With due respect to Vikramj's tremendous knowledge and experience, I believe that this loading condition must be combined with other loading conditions as per the applicable standard. It's not unlikely that the max. wind will hit when the structure has experienced some sinking....!



Best regards,

Rajendra (Raj) Lele


P Please consider the environment before printing this e-mail or any attachment/s.



From: vikram.jeet <forum@sefindia.org>
To: general@sefindia.org
Sent: Thu, 18 November, 2010 7:58:16 PM
Subject: [SEFI] Re: support sinking

     Dear Er Babu

it would be sufficient to see response /result of one frame in each direction separately.
(if frame is symmetrical geometrically, only two columns in transverse and three columns
in long. direction could be seen for sinking)
and effect could be added at design stage ,if design stage is attempted manually/spread sheets.

But if design is also attempted through staad, the input and out put would definitely be voluminous

It all depends on proof consultant since such a loading is prescribed loading for your project.
In my opinion this shall not be combined with EQ/wind.

best regards

vikramjeet


Dear Vikram,
my structure consists of 22 columns with 5 bays in longitudinal direction and 3bays in transverse direction. If i have to apply differential settlement how can i proceed with this type of configuration.
Is giving nodal displacement for each column (each load case) individually correct?
Is this type of loading comes under accidental loading? If yes,can we combine this with wind/seismic loading as the stresses will be very huge?
Regards,
babu
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vikram.jeet
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PostPosted: Sat Nov 20, 2010 5:56 am    Post subject: support sinking Reply with quote

Dear Er Rajendra Lele ji

I agree with your posting.

Only small apprehension of not combining the same with EQ/Wind is that:

Wind /EQ resistance of only those joints which suffered extra  
rotations due to support sinking is affected whereas other joints  
have reserve MR due to provision for moments due to sinking
support at all joints.

However in a grid, if all supports sink ( EQ perpendicular to  
grid - -critical) will definitely require sinking support case to be
combined with EQ. But such probability is remote.

Considering a Four bay frame in both direction with 16 columns(4X4)

Total no of beam restraints of entire system
=24 in X direction (and 24 nos in Y direction)
( Each beam has two end restraints and therefore at continuous support two
sides of beam posses double)

For each beam restraint
M1= Moment of resistance for gravity loading
M2= MR for sinking support
M3= MR for EQ loading

Considering that one penultimate support sinks by settlement =y
This will induce moment of 6*EI y/L^2 in 4 beam restraints in X direction
and 4 beam restraints in Y direction in beams passing over it.

Induced moment due to support sinking in entire structural system in  
each direction = 4*M2

MR provided for support sinking in entire structural system in  
each direction is = 24*M2

Moment of resistance of structural system in each direction:
Gravity loading case:
Moment of Resistance provided =[ 24*M1 + 24* M2] in each direction

EQ case(in case EQ is not combined with sinking)
Moment of Resistance required =[(24*M1 + 24* M3 + 4*M2)] in each direction
(only one penutimate support sinks),  
Moment of Resistance available =[(24*M1+24*M2) + 24* M3- 4*M2] in each direction
which is equal to = (24*M1+24*M3) + 20* M2 in each direction
still higher than required to take seismic /wind

Further to say that load factor which is only 1.2 under EQ as against 1.5 under normal
will further add to reserve strength  

Case of 3supports in a grid sinking with EQ perpendicular to it.
MR required =24*M1+24*M3+12*M2

MR available=[(24*M1+24*M2) + 24* M3- 12*M2] in each direction

i.e Available MR is equal to required one OK

Similarily case of All supports in one grid sinking with
EQ perpendicular to it

MR required =24*M1+24*M3+16*M2

MR availablel =[(24*M1+24*M2) + 24* M3- 16*M2] in each direction

i.e Availabel MR is short by 8*M2

But such a possibility is remote

However for small buildings , combining this case will be a better and safe preposition

Sorry for the long mail and suggestion for better understanding of concept will be highly
appreciated.

with best regards

vikramjeet








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veerababudodda
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PostPosted: Mon Nov 22, 2010 4:15 pm    Post subject: Reply with quote

Dear Sirs,

Thank you for your valuable suggestions.

Regards,
babu
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