View previous topic :: View next topic 
Author 
Message 
gsrinivasbabu SEFI Member
Joined: 26 Jan 2003 Posts: 8

Posted: Mon Dec 04, 2006 2:12 pm Post subject: Tower foundatin 


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
Posted via Email 

Back to top 


forvipul SEFI Member
Joined: 26 Jan 2003 Posts: 18

Posted: Tue Dec 05, 2006 4:31 am Post subject: Tower foundatin 


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
Posted via Email 

Back to top 


vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 2212

Posted: Tue Dec 05, 2006 5:43 am Post subject: Tower foundatin 


Tower foundations
i) we can permit 20 % upliftment of raft for foundations resting on rocky strata ( as per IRC 7 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
Posted via Email 

Back to top 


anand.shah SEFI Member
Joined: 26 Jan 2003 Posts: 8

Posted: Tue Dec 05, 2006 5:57 am Post subject: Tower foundatin 


Dear Vipul,
Is % permission in mentioned in IS? Because as per IRC78 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)
Posted via Email 

Back to top 


gsrinivasbabu SEFI Member
Joined: 26 Jan 2003 Posts: 8

Posted: Tue Dec 05, 2006 6:21 am Post subject: Tower foundatin 


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)

Posted via Email 

Back to top 


abhilashjoy SEFI Member
Joined: 26 Jan 2003 Posts: 3

Posted: Tue Dec 05, 2006 7:35 am Post subject: Tower foundatin 


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 7 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
(

Posted via Email 

Back to top 


vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 2212

Posted: Tue Dec 05, 2006 9:54 am Post subject: Tower foundatin 


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
Posted via Email 

Back to top 


gsrinivasbabu SEFI Member
Joined: 26 Jan 2003 Posts: 8

Posted: Tue Dec 05, 2006 1:56 pm Post subject: Tower foundatin 


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 7 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
(
(

Posted via Email 

Back to top 


gsrinivasbabu SEFI Member
Joined: 26 Jan 2003 Posts: 8

Posted: Tue Dec 05, 2006 2:00 pm Post subject: Tower foundatin 


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)
(

Posted via Email 

Back to top 


