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Tower foundatin

 
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gsrinivasbabu
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Joined: 26 Jan 2003
Posts: 8

PostPosted: Mon Dec 04, 2006 2:12 pm    Post subject: Tower foundatin Reply with quote

Design of raft for a tower can be done as usual normal raft foundation,But
is to be checked for Ceck for 1) Over turning 2)Down thrust 3)Punching
shear Check
against overturning plays key role in the design of raft foundation for a
tower. Tocheck for OT   First calculate the overturning moment whch can be
obtained by multiplying the base width of the tower with twice the down
thrust. Then calculate the Stabilizing moment Stabilizing moment= W*B/2 Where
w=weight of soil above raft+weght of concrete including self weight of raft B=
base width of foundation   FOS =Stabizing moment 7 Overturning moment and
must be greaster then 1.5 Checking don thrust can be done by calculating
P/A*M/Z of the raft and this value must be less then SBC depth of pad can be
calculated from Punching shear calculations as usal. Generally we get
minimum steel in in pad in both the direction at top and bottom. Columns can
be designed with help of SP16 charts as biaxial members(Tension+ bending)  I
think it is sufficient for you.
Any clarifications pls dont hesitatae to ask me
G.Srinivas Babu
Senior designer
Towers wing
SVPCPL
Hyderabad

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forvipul
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Joined: 26 Jan 2003
Posts: 18

PostPosted: Tue Dec 05, 2006 4:31 am    Post subject: Tower foundatin Reply with quote

Dear Sir,
I have few quries

1. While calculating P/A + M/Z can we permit 80% of contact area in raft design.

2. Can we put some extra weight in form of PCC on one side to account for overturning moment

Regards

Vipul Dobhada
wrote:

Design of raft for a tower can be done as usual normal raft foundation,But
is to be checked for Ceck for 1) Over turning 2)Down thrust 3)Punching
shear Check
against overturning plays key role in the design of raft foundation for a
tower. Tocheck for OT First calculate the overturning moment whch can be
obtained by multiplying the base width of the tower with twice the down
thrust. Then calculate the Stabilizing moment Stabilizing moment= W*B/2 Where
w=weight of soil above raft+weght of concrete including self weight of raft B=
base width of foundation FOS =Stabizing moment 7 Overturning moment and
must be greaster then 1.5 Checking don thrust can be done by calculating
P/A*M/Z of the raft and this value must be less then SBC depth of pad can be
calculated from Punching shear calculations as usal. Generally we get
minimum steel in in pad in both the direction at top and bottom. Columns can
be designed with help of SP16 charts as biaxial members(Tension+ bending) I
think it is sufficient for you.
Any clarifications pls dont hesitatae to ask me
G.Srinivas Babu
Senior designer
Towers wing
SVPCPL
Hyderabad

Structural Engineers Forum of India

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vikram.jeet
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Joined: 26 Jan 2003
Posts: 2212

PostPosted: Tue Dec 05, 2006 5:43 am    Post subject: Tower foundatin Reply with quote

Tower foundations

i) we can permit 20 % upliftment  of raft for foundations
resting on rocky strata ( as per IRC -7Cool
However for footings resting on soils this not permitted

But in case of Tower foundations where overturning
moment is primarily due to wind, we can permit
the redistribution provided that after redstbution,
max base pressure does not exceed the allowable BC.

ii) since in tower foundation,moment is primarily due to
wind action which is reversible or can take any direction
, providing PCC mass on one side will be further unsafe
and  same could be provided concentrically to footing
/ raft to have more W/A.

with best wishes

vikramjeet

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anand.shah
SEFI Member
SEFI Member


Joined: 26 Jan 2003
Posts: 8

PostPosted: Tue Dec 05, 2006 5:57 am    Post subject: Tower foundatin Reply with quote

Dear Vipul,

Is  % permission in mentioned in IS?
Because as per IRC-78  it is only permitted if below starta is rock.

Regards,
Anand

forvipul[AT]ind...
es.com
To
12/05/2006 10:55          anand.shah[AT]pmc...
AM                                                         cc

Subject
Please respond to         Tower foundatin

org

Dear Sir,
I have few quries

1. While calculating P/A + M/Z can we permit 80% of contact area in raft
design.

2. Can we put some extra weight in form of PCC on one side to account for
overturning moment

Regards

Vipul Dobhada
wrote:

Design of raft for a tower can be done as usual normal raft foundation,But
is to be checked for Ceck for 1) Over turning 2)Down thrust 3)Punching
shear Check
against overturning plays key role in the design of raft foundation for a
tower. Tocheck for OT First calculate the overturning moment whch can be
obtained by multiplying the base width of the tower with twice the down
thrust. Then calculate the Stabilizing moment Stabilizing moment= W*B/2
Where
w=weight of soil above raft+weght of concrete including self weight of raft
B=
base width of foundation FOS =Stabizing moment 7 Overturning moment and
must be greaster then 1.5 Checking don thrust can be done by calculating
P/A*M/Z of the raft and this value must be less then SBC depth of pad can
be
calculated from Punching shear calculations as usal. Generally we get
minimum steel in in pad in both the direction at top and bottom. Columns
can
be designed with help of SP16 charts as biaxial members(Tension+ bending) I
think it is sufficient for you.
Any clarifications pls dont hesitatae to ask me
G.Srinivas Babu
Senior designer
Towers wing
SVPCPL
Hyderabad

Structural Engineers Forum of India (
http://www.sefindia.org)

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gsrinivasbabu
SEFI Member
SEFI Member


Joined: 26 Jan 2003
Posts: 8

PostPosted: Tue Dec 05, 2006 6:21 am    Post subject: Tower foundatin Reply with quote

Dear Mr.Vipul,
1) We must take 80% to 85% of conatct area.
2) We should not take PCC weight in accounting for Overturning Moment.

Thanks and regards
G.Srinivas babu
9440583864

On Tue Dec 5 10:01:10 2006, forvipul[AT]ind... <forvipul[AT]ind...
Quote:
wrote:

Dear Sir,
I have few quries

1. While calculating P/A + M/Z can we permit 80% of contact area in raft
design.

2. Can we put some extra weight in form of PCC on one side to account for
overturning moment

Regards

Vipul Dobhada
wrote:

Design of raft for a tower can be done as usual normal raft foundation,But
is to be checked for Ceck for 1) Over turning 2)Down thrust 3)Punching
shear Check
against overturning plays key role in the design of raft foundation for a
tower. Tocheck for OT First calculate the overturning moment whch can be
obtained by multiplying the base width of the tower with twice the down
thrust. Then calculate the Stabilizing moment Stabilizing moment= W*B/2
Where
w=weight of soil above raft+weght of concrete including self weight of
raft B=
base width of foundation FOS =Stabizing moment 7 Overturning moment and
must be greaster then 1.5 Checking don thrust can be done by calculating
P/A*M/Z of the raft and this value must be less then SBC depth of pad can
be
calculated from Punching shear calculations as usal. Generally we get
minimum steel in in pad in both the direction at top and bottom. Columns
can
be designed with help of SP16 charts as biaxial members(Tension+ bending)
I
think it is sufficient for you.
Any clarifications pls dont hesitatae to ask me
G.Srinivas Babu
Senior designer
Towers wing
SVPCPL
Hyderabad

Structural Engineers Forum of India (
http://www.sefindia.org)


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abhilashjoy
SEFI Member
SEFI Member


Joined: 26 Jan 2003
Posts: 3

PostPosted: Tue Dec 05, 2006 7:35 am    Post subject: Tower foundatin Reply with quote

Dear sir,

I am a structural engineer from Cochin

In a tower structure predominant force acting is wind, while we analyse the
structure for wind, we apply load on the whole surface of tower
profile rather than the area of steel section. Since it is a hollow
structure and we analyse as a solid structure, are the foundation forces we
get too high. Then the question of drag on the tower due to wind. How drag
affects tall towers. I am raising this primary question due the observation
that, most of the towers section fails and collapses, rather than
foundation. Are we doing a wrong analysis

Regards

Abhilash joy

On Tue Dec 5 11:13:10 2006, vikram.jeet[AT]jal... <
vikram.jeet[AT]jal...> wrote:
Quote:

Tower foundations

i) we can permit 20 % upliftment  of raft for foundations
resting on rocky strata ( as per IRC -7Cool
However for footings resting on soils this not permitted

But in case of Tower foundations where overturning
moment is primarily due to wind, we can permit
the redistribution provided that after redstbution,
max base pressure does not exceed the allowable BC.

ii) since in tower foundation,moment is primarily due to
wind action which is reversible or can take any direction
, providing PCC mass on one side will be further unsafe
and  same could be provided concentrically to footing
/ raft to have more W/A.

with best wishes

vikramjeet

(


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vikram.jeet
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Joined: 26 Jan 2003
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PostPosted: Tue Dec 05, 2006 9:54 am    Post subject: Tower foundatin Reply with quote

Dear Abhilash,

I think that for tower design, the legs in compression
are more critical as permitted stress in comp.(based on L/R)
is less compared to tensile. Check on tension leg on net area
basis is also essential.( all legs can be in comp/tension)

Also it is always better to check stability
ie overturning / sliding with wind blowing
along  XX direction  or  YY direction.

However for design of raft, wind blowing
perpendicular to Diagonal  proves to be
more critical due to more area of tower
members exposed to wind. This may be
also critical for bracing members

Thus ,Tower members and its foundations
shall be analysed for crital of following cases
i) wind along XX
ii0 wind along YY
iii) Wind along Diagonal ie XY direction
and Two orthogonal components
acting simultaneously shall be
computed

I expect other sefians who are involved exclusively on
tower design to throw more light for benefit of us all.
regards
vikramjeet

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gsrinivasbabu
SEFI Member
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Joined: 26 Jan 2003
Posts: 8

PostPosted: Tue Dec 05, 2006 1:56 pm    Post subject: Tower foundatin Reply with quote

Dear Mr. Abhilash,
We should not apply wind load on the whole surface.

Regards
GSBABU

On Tue Dec 5 13:05:46 2006, abhilashjoy[AT]gma... <abhilashjoy[AT]gma...>
wrote:
Quote:

Dear sir,

I am a structural engineer from Cochin

In a tower structure predominant force acting is wind, while we analyse
the
structure for wind, we apply load on the whole surface of tower
profile rather than the area of steel section. Since it is a hollow
structure and we analyse as a solid structure, are the foundation forces
we
get too high. Then the question of drag on the tower due to wind. How drag
affects tall towers. I am raising this primary question due the
observation
that, most of the towers section fails and collapses, rather than
foundation. Are we doing a wrong analysis

Regards

Abhilash joy

On Tue Dec 5 11:13:10 2006, vikram.jeet[AT]jal... <
vikram.jeet[AT]jal...> wrote:

Tower foundations

i) we can permit 20 % upliftment  of raft for foundations
resting on rocky strata ( as per IRC -7Cool
However for footings resting on soils this not permitted

But in case of Tower foundations where overturning
moment is primarily due to wind, we can permit
the redistribution provided that after redstbution,
max base pressure does not exceed the allowable BC.

ii) since in tower foundation,moment is primarily due to
wind action which is reversible or can take any direction
, providing PCC mass on one side will be further unsafe
and  same could be provided concentrically to footing
/ raft to have more W/A.

with best wishes

vikramjeet

(

(


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gsrinivasbabu
SEFI Member
SEFI Member


Joined: 26 Jan 2003
Posts: 8

PostPosted: Tue Dec 05, 2006 2:00 pm    Post subject: Tower foundatin Reply with quote

If it is a communication tower it is OK, But for transmission line towers we
should take wind in only one direction upto 220kV as per IS 802.

Regards
G.Srinivas babu

On Tue Dec 5 15:24:15 2006, vikram.jeet[AT]jal...
<vikram.jeet[AT]jal...
Quote:
wrote:

Dear Abhilash,

I think that for tower design, the legs in compression
are more critical as permitted stress in comp.(based on L/R)
is less compared to tensile. Check on tension leg on net area
basis is also essential.( all legs can be in comp/tension)

Also it is always better to check stability
ie overturning / sliding with wind blowing
along  XX direction  or  YY direction.

However for design of raft, wind blowing
perpendicular to Diagonal  proves to be
more critical due to more area of tower
members exposed to wind. This may be
also critical for bracing members

Thus ,Tower members and its foundations
shall be analysed for crital of following cases
i) wind along XX
ii0 wind along YY
iii) Wind along Diagonal ie XY direction
and Two orthogonal components
acting simultaneously shall be
computed

I expect other sefians who are involved exclusively on
tower design to throw more light for benefit of us all.
regards
vikramjeet

( http://www.sefindia.org)

(


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