[SEFI] Seismic force for vessel supporting structure
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alltimepisal
Aug 7, 2008, 12:13:08 AM8/7/08
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Seismic force for vessel supporting structure
When we design vessel supporting structure, seismic forces have two difference sources. First source is seismic force due to vessel and second source is seismic force due to structure itself
We use to generate seismic force due to structure by response spectrum analysis with appropriate factor. Seismic forces due to vessel will be applied directly on structure at appropriate location as static load. (These forces will be added with seismic forces due to structure)
Here, seismic forces due to vessel are provided by vessel engineers and are based on Response reduction factor R = 2. We use to take R = 3 / 4 / 5 for the civil structure based on the material and building configuration.
While considering seismic forces due to vessel for designing civil strcture, whether we should reduce these forces in proportion to the response reduction factor or apply these forces as it is?
Regards,
Yogesh Pisal
When we design vessel supporting structure, seismic forces have two difference sources. First source is seismic force due to vessel and second source is seismic force due to structure itself
We use to generate seismic force due to structure by response spectrum analysis with appropriate factor. Seismic forces due to vessel will be applied directly on structure at appropriate location as static load. (These forces will be added with seismic forces due to structure)
Here, seismic forces due to vessel are provided by vessel engineers and are based on Response reduction factor R = 2. We use to take R = 3 / 4 / 5 for the civil structure based on the material and building configuration.
While considering seismic forces due to vessel for designing civil strcture, whether we should reduce these forces in proportion to the response reduction factor or apply these forces as it is?
Regards,
Yogesh Pisal
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sangramiitk
Aug 8, 2008, 10:38:48 PM8/8/08
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Dear Mr. Yogesh,
What ever the structure may be while designing for seismic force you can not ignore the weight of vessle that participate in seismic behaviour in response spectrum method.
First of all forget about R value what vessle ppl using.
If you are using IS code refer IS 1893 part 1, for R value, depending un which structure you are using steel or concrete , if steel then moment connected or braced, if braced eccentrically braced or concetric braced etc and mostly vessle supporting structure is industrial one. Hence refer IS 1893 part 4.
Lump the vessle mass at appropriate place in structure , and take assumptions what ever required, and design the whole structure with response spectrum method.
With best regards
Sangram
On Thu, Aug 7, 2008 at 12:13 PM, alltimepisal fo...@sefindia.org)> wrote:
What ever the structure may be while designing for seismic force you can not ignore the weight of vessle that participate in seismic behaviour in response spectrum method.
First of all forget about R value what vessle ppl using.
If you are using IS code refer IS 1893 part 1, for R value, depending un which structure you are using steel or concrete , if steel then moment connected or braced, if braced eccentrically braced or concetric braced etc and mostly vessle supporting structure is industrial one. Hence refer IS 1893 part 4.
Lump the vessle mass at appropriate place in structure , and take assumptions what ever required, and design the whole structure with response spectrum method.
With best regards
Sangram
On Thu, Aug 7, 2008 at 12:13 PM, alltimepisal fo...@sefindia.org)> wrote:
| Quote: |
aapatil
Aug 9, 2008, 1:04:36 AM8/9/08
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Dear Yogesh,
Many times the vendor gives the vessel seismic loads in an isolated condition i.e vessel resting on ground as he is unaware of the structural configuration. While designing the supporting structure, it is always better to ignore the seismic forces given by the vendor and lump the operating weight of the vessel at the C.G of the vessel by modelling a rigid element in STAAD so that the seismic moment is captured properly along with that of the structure. Then you can use the response spectrum method in STAAD for designing the structure.
AKSHAY A. PATIL
"sangramiitk"
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[SEFI] Re: Seismic force for vessel supporting structure
Many times the vendor gives the vessel seismic loads in an isolated condition i.e vessel resting on ground as he is unaware of the structural configuration. While designing the supporting structure, it is always better to ignore the seismic forces given by the vendor and lump the operating weight of the vessel at the C.G of the vessel by modelling a rigid element in STAAD so that the seismic moment is captured properly along with that of the structure. Then you can use the response spectrum method in STAAD for designing the structure.
AKSHAY A. PATIL
"sangramiitk"
- "sangramiitk"
08/09/2008 08:08 AM Please respond to
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[SEFI] Re: Seismic force for vessel supporting structure
Dear Mr. Yogesh,
What ever the structure may be while designing for seismic force you can not ignore the weight of vessle that participate in seismic behaviour in response spectrum method.
First of all forget about R value what vessle ppl using.
If you are using IS code refer IS 1893 part 1, for R value, depending un which structure you are using steel or concrete , if steel then moment connected or braced, if braced eccentrically braced or concetric braced etc and mostly vessle supporting structure is industrial one. Hence refer IS 1893 part 4.
Lump the vessle mass at appropriate place in structure , and take assumptions what ever required, and design the whole structure with response spectrum method.
With best regards
Sangram
What ever the structure may be while designing for seismic force you can not ignore the weight of vessle that participate in seismic behaviour in response spectrum method.
First of all forget about R value what vessle ppl using.
If you are using IS code refer IS 1893 part 1, for R value, depending un which structure you are using steel or concrete , if steel then moment connected or braced, if braced eccentrically braced or concetric braced etc and mostly vessle supporting structure is industrial one. Hence refer IS 1893 part 4.
Lump the vessle mass at appropriate place in structure , and take assumptions what ever required, and design the whole structure with response spectrum method.
With best regards
Sangram
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alltimepisal
Aug 12, 2008, 1:52:30 AM8/12/08
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Akshay,
Whatever you are suggesting is absolutely right for the structure in which weight of vessel is less than 25% seismic weight of structure. But the structure i am considering is having different configuration.
About structural configuration -
It has 16m x 16m table top (Slab thickness - 3m) supported on 12 columns. Height of the table top is 8.3m
It supports 13m dia vessel (height of vessel is 40m) which has 25000 kN (2500 ton) weight. But weight of the table top is 15000 kN only.
Our interpretation -
Due to this structural configuration and loading mode shapes will be governed by vessel only. This is very similar to vessel supported on rigid support / ground (Table top will be very rigid and will have very less time period)
In this case mass will be concentrated on table top and vessel will be vibrating as cantilever fixed at table top. Hence lumping of mass at CG will not be justified. At the same time, if we lump the mass at the table top, then it will lead to wrong time period estimation.
Hence we have found out time period of combined system and fixed the seismic force percentage and followed the procedure given in my query on 7th August.
Query -
Based on this, can you suggest me whether i should go with the seismic force given by vessel group or i should go with lumping the mass at CG / table top. (In the first case, again question arises about whether we should use their forces as it is or we should factor them by appropriate factor to take care of R)
Whatever you are suggesting is absolutely right for the structure in which weight of vessel is less than 25% seismic weight of structure. But the structure i am considering is having different configuration.
About structural configuration -
It has 16m x 16m table top (Slab thickness - 3m) supported on 12 columns. Height of the table top is 8.3m
It supports 13m dia vessel (height of vessel is 40m) which has 25000 kN (2500 ton) weight. But weight of the table top is 15000 kN only.
Our interpretation -
Due to this structural configuration and loading mode shapes will be governed by vessel only. This is very similar to vessel supported on rigid support / ground (Table top will be very rigid and will have very less time period)
In this case mass will be concentrated on table top and vessel will be vibrating as cantilever fixed at table top. Hence lumping of mass at CG will not be justified. At the same time, if we lump the mass at the table top, then it will lead to wrong time period estimation.
Hence we have found out time period of combined system and fixed the seismic force percentage and followed the procedure given in my query on 7th August.
Query -
Based on this, can you suggest me whether i should go with the seismic force given by vessel group or i should go with lumping the mass at CG / table top. (In the first case, again question arises about whether we should use their forces as it is or we should factor them by appropriate factor to take care of R)
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aapatil
Aug 12, 2008, 4:49:34 AM8/12/08
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Dear alltimepisal,
If you calculate the weight of table top it comes to around 1920 T (16mx16mx3m) excluding column weight. If we consider a column of 0.6mx0.6mx(8.3-3)mx12 = 58T, the total combined wt comes to about 2000T which somewhat near to the vessel weight. If you analyse the structure only in STAAD it will not show the characteristic of a rigid structure as the dynamic model will have a mass of 1920T lumped at a height of 8.3m with column lateral stiffness equal to 12*12EI/L^3.
Hence i still feel that lumping the mass at the CG of the vessel will give precise results as the structure is not rigid and will still capture the integrated time period.
As we lump the mass of the vessel at the CG the value of R considered for the structural configuration will suffice.
However more suggestions are welcome from other members of SEFI which can give more idea on such analysis.
AKSHAY A. PATIL
"alltimepisal"
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[SEFI] Re: Seismic force for vessel supporting structure
If you calculate the weight of table top it comes to around 1920 T (16mx16mx3m) excluding column weight. If we consider a column of 0.6mx0.6mx(8.3-3)mx12 = 58T, the total combined wt comes to about 2000T which somewhat near to the vessel weight. If you analyse the structure only in STAAD it will not show the characteristic of a rigid structure as the dynamic model will have a mass of 1920T lumped at a height of 8.3m with column lateral stiffness equal to 12*12EI/L^3.
Hence i still feel that lumping the mass at the CG of the vessel will give precise results as the structure is not rigid and will still capture the integrated time period.
As we lump the mass of the vessel at the CG the value of R considered for the structural configuration will suffice.
However more suggestions are welcome from other members of SEFI which can give more idea on such analysis.
AKSHAY A. PATIL
"alltimepisal"
- "alltimepisal"
08/12/2008 11:22 AM Please respond to
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[SEFI] Re: Seismic force for vessel supporting structure
Akshay,
Whatever you are suggesting is absolutely right for the structure in which weight of vessel is less than 25% seismic weight of structure. But the structure i am considering is having different configuration.
About structural configuration -
It has 16m x 16m table top (Slab thickness - 3m) supported on 12 columns. Height of the table top is 8.3m
It supports 13m dia vessel (height of vessel is 40m) which has 25000 kN (2500 ton) weight. But weight of the table top is 15000 kN only.
Our interpretation -
Due to this structural configuration and loading mode shapes will be governed by vessel only. This is very similar to vessel supported on rigid support / ground (Table top will be very rigid and will have very less time period)
In this case mass will be concentrated on table top and vessel will be vibrating as cantilever fixed at table top. Hence lumping of mass at CG will not be justified. At the same time, if we lump the mass at the table top, then it will lead to wrong time period estimation.
Hence we have found out time period of combined system and fixed the seismic force percentage and followed the procedure given in my query on 7th August.
Query -
Based on this, can you suggest me whether i should go with the seismic force given by vessel group or i should go with lumping the mass at CG / table top. (In the first case, again question arises about whether we should use their forces as it is or we should factor them by appropriate factor to take care of R)
Whatever you are suggesting is absolutely right for the structure in which weight of vessel is less than 25% seismic weight of structure. But the structure i am considering is having different configuration.
About structural configuration -
It has 16m x 16m table top (Slab thickness - 3m) supported on 12 columns. Height of the table top is 8.3m
It supports 13m dia vessel (height of vessel is 40m) which has 25000 kN (2500 ton) weight. But weight of the table top is 15000 kN only.
Our interpretation -
Due to this structural configuration and loading mode shapes will be governed by vessel only. This is very similar to vessel supported on rigid support / ground (Table top will be very rigid and will have very less time period)
In this case mass will be concentrated on table top and vessel will be vibrating as cantilever fixed at table top. Hence lumping of mass at CG will not be justified. At the same time, if we lump the mass at the table top, then it will lead to wrong time period estimation.
Hence we have found out time period of combined system and fixed the seismic force percentage and followed the procedure given in my query on 7th August.
Query -
Based on this, can you suggest me whether i should go with the seismic force given by vessel group or i should go with lumping the mass at CG / table top. (In the first case, again question arises about whether we should use their forces as it is or we should factor them by appropriate factor to take care of R)
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ajeetkokil
Aug 13, 2008, 9:20:59 PM8/13/08
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Dear All,
One thing we have to bear in mind that vessel is an external attachment to the structure, and not the structural part.
Mode shapes, time periods are evaluated for the structure which is primarily designed to resist interial / climatic forces.
In this case, the lumped mass is the vessel weight which will ultimately trasferred to table top. Hence, lumping mass @ the centre of the vessel is not appropriate. Lumping of the vessel mass should be done @ the CG of table top.
We can imagine this vessel as an external attachement to the structure and not the structure (which is meant to resist impending forces / loads ). So, we need not account vessel / vessel CG while calculating time period of the structure.
However, vessel connections to the table top should be properly designed.
So, while preparing mathematical model, this system can be idealized as cantilever supporting forces (Inertial)@ free ends.
Hope this helps
Thanks & Regards
Ajit
On Tue, 12 Aug 2008 alltimepisal wrote :
One thing we have to bear in mind that vessel is an external attachment to the structure, and not the structural part.
Mode shapes, time periods are evaluated for the structure which is primarily designed to resist interial / climatic forces.
In this case, the lumped mass is the vessel weight which will ultimately trasferred to table top. Hence, lumping mass @ the centre of the vessel is not appropriate. Lumping of the vessel mass should be done @ the CG of table top.
We can imagine this vessel as an external attachement to the structure and not the structure (which is meant to resist impending forces / loads ). So, we need not account vessel / vessel CG while calculating time period of the structure.
However, vessel connections to the table top should be properly designed.
So, while preparing mathematical model, this system can be idealized as cantilever supporting forces (Inertial)@ free ends.
Hope this helps
Thanks & Regards
Ajit
On Tue, 12 Aug 2008 alltimepisal wrote :
| Quote: |
| Akshay, Whatever you are suggesting is absolutely right for the structure in which weight of vessel is less than 25% seismic weight of structure. But the structure i am considering is having different configuration. About structural configuration - It has 16m x 16m table top (Slab thickness - 3m) supported on 12 columns. Height of the table top is 8.3m It supports 13m dia vessel (height of vessel is 40m) which has 25000 kN (2500 ton) weight. But weight of the table top is 15000 kN only. Our interpretation - Due to this structural configuration and loading mode shapes will be governed by vessel only. This is very similar to vessel supported on rigid support / ground (Table top will be very rigid and will have very less time period) In this case mass will be concentrated on table top and vessel will be vibrating as cantilever fixed at table top. Hence lumping of mass at CG will not be justified. At the same time, if we lump the mass at the table top, then it will lead to wrong time period estimation. Hence we have found out time period of combined system and fixed the seismic force percentage and followed the procedure given in my query on 7th August. Query - Based on this, can you suggest me whether i should go with the seismic force given by vessel group or i should go with lumping the mass at CG / table top. (In the first case, again question arises about whether we should use their forces as it is or we should factor them by appropriate factor to take care of R) |
Engineer,
Strl. and Arch. Deptt.
Engineers India Limited,
New Delhi
call me at 99112 02590
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alltimepisal
Aug 14, 2008, 5:51:55 AM8/14/08
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Ajeet,
By lumping vessel mass, we will account for seismic shear due to vessel.
But, in this case we have to add seismic moment due to vessel to the seismic force.
Again, this moment will be based on calculations from vessel engineers. Whether we can accept moments given by vessel group directly or we need to factorise it based on R.
Let me know, your opinion about this.
Regards
Yogesh Pisal
09371217723
By lumping vessel mass, we will account for seismic shear due to vessel.
But, in this case we have to add seismic moment due to vessel to the seismic force.
Again, this moment will be based on calculations from vessel engineers. Whether we can accept moments given by vessel group directly or we need to factorise it based on R.
Let me know, your opinion about this.
Regards
Yogesh Pisal
09371217723
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adroshan
Aug 17, 2008, 1:37:44 PM8/17/08
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Hi Yogesh,
Though I am not an expert in the intricacies of coupled analysis, pl. consider the following aspects:
1. By using a R value of 2 for vessel design, the designers have assumed that the deflections will be about twice that from the linear analysis. In RCC design, if one assumes a R value more than 2, I am not sure whether it can be ensured that relative displacement between the supports will still be within the 2*linear analysis values (if it is greater than this, vessel supports can see more load).
2. Vessel designers also consider a dynamic amplification factor to take care of flexibility of supporting structure. Unless we have the details of the design this may not help.
3. Best way is to go for a coupled analysis. Also note that for the equipment, a model with a lumped mass assumption will be stiffer than the continuous mass assumption.
For a uniform cantilever , 1st natural freq (omega) is 3.52*sqrt( (EI)/(m.L^4) rad / sec
where as the idealisation with a mass in the middle, omega is
sqrt( (24EI)/(m.L^4)) (m is mass per unit length)
-Roshan
Though I am not an expert in the intricacies of coupled analysis, pl. consider the following aspects:
1. By using a R value of 2 for vessel design, the designers have assumed that the deflections will be about twice that from the linear analysis. In RCC design, if one assumes a R value more than 2, I am not sure whether it can be ensured that relative displacement between the supports will still be within the 2*linear analysis values (if it is greater than this, vessel supports can see more load).
2. Vessel designers also consider a dynamic amplification factor to take care of flexibility of supporting structure. Unless we have the details of the design this may not help.
3. Best way is to go for a coupled analysis. Also note that for the equipment, a model with a lumped mass assumption will be stiffer than the continuous mass assumption.
For a uniform cantilever , 1st natural freq (omega) is 3.52*sqrt( (EI)/(m.L^4) rad / sec
where as the idealisation with a mass in the middle, omega is
sqrt( (24EI)/(m.L^4)) (m is mass per unit length)
-Roshan
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