[SEFI] MASTER SLAVE COMMAND IN STAAD
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ashutosh.jain
Aug 7, 2008, 12:47:03 AM8/7/08
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Dear All,
Recently while reviewing a RCC Framed Structure modeling in STAAD, designer has used master Slave command for nodes, giving master node near the centre of slab and all other as slave. He says this is to take into account for rigidity of the slab, and it reduces the amount of r/f in columns considerably.
My question is whether it is ok to use this command and what precautions we have to take while using this,
Secondly if slab has cutouts how can we use this command
Regards,
ASHUTOSH JAIN
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Recently while reviewing a RCC Framed Structure modeling in STAAD, designer has used master Slave command for nodes, giving master node near the centre of slab and all other as slave. He says this is to take into account for rigidity of the slab, and it reduces the amount of r/f in columns considerably.
My question is whether it is ok to use this command and what precautions we have to take while using this,
Secondly if slab has cutouts how can we use this command
Regards,
ASHUTOSH JAIN
Important notice:This e-mail and any attachment thereto contains corporate proprietary information. If you have received it by mistake, please notify us immediately by reply e-mail and delete this e-mail and its attachments from your system.
Thank You.
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sdec.in
Aug 7, 2008, 1:58:25 AM8/7/08
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Dear Ashutosh
When we are modelling a bare RCC structure in STAAD, the rigidity provided by the slab systems is substantial, and it helps in ensuring all parallel frames move parallel to each other, under horizontal loads. This is being ensured by modelling the slab within the 3D frame, generally or by using a master slave command. If it helps in reducing steel, and to what extent, I cant really comment, but the whole behaviour of the structure is better represented in the model.
regards
Sangeeta Wij
When we are modelling a bare RCC structure in STAAD, the rigidity provided by the slab systems is substantial, and it helps in ensuring all parallel frames move parallel to each other, under horizontal loads. This is being ensured by modelling the slab within the 3D frame, generally or by using a master slave command. If it helps in reducing steel, and to what extent, I cant really comment, but the whole behaviour of the structure is better represented in the model.
regards
Sangeeta Wij
| : |
faux_ph
Aug 11, 2008, 5:42:27 AM8/11/08
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Master-slave command basically means that all the nodes will beahve (usually translational directions) in the same manner to the master node. Master-slave command is used to simulate a rigid diaphragm behavior that is common to RC structures. For small openings, master-slave command is still acceptable. Be careful though in using the master-slave command, not all diaphragm are rigid and they may behave as semi-rigid or flexible.
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ykalamkar
Aug 11, 2008, 7:40:35 AM8/11/08
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Dear all
I have tried many times master slave command. However i found the change in r/f in columns also is not reduced to considerable level. I am not clear why it happens? it might be dependent on the geometry of the building. is it?
i have just now tried in a rectangular building in which all columns are of same size and equally placed. i found hardly any difference in total quantity of the steel.
Have any one tried it in such buildings?
Yogesh
I have tried many times master slave command. However i found the change in r/f in columns also is not reduced to considerable level. I am not clear why it happens? it might be dependent on the geometry of the building. is it?
i have just now tried in a rectangular building in which all columns are of same size and equally placed. i found hardly any difference in total quantity of the steel.
Have any one tried it in such buildings?
Yogesh
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ashutosh.jain
Aug 11, 2008, 8:49:20 AM8/11/08
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Dear Yogesh
try to give master node around the CG of slab , I think it should give better result.
Regards,
ASHUTOSH JAIN
try to give master node around the CG of slab , I think it should give better result.
Regards,
ASHUTOSH JAIN
From: ykalamkar [mailto:fo...@sefindia.org]
Sent: Monday, August 11, 2008 5:11 PM
To: gen...@sefindia.org
Subject: [SEFI] Re: MASTER SLAVE COMMAND IN STAAD
Dear all
I have tried many times master slave command. However i found the change in r/f in columns also is not reduced to considerable level. I am not clear why it happens? it might be dependent on the geometry of the building. is it?
i have just now tried in a rectangular building in which all columns are of same size and equally placed. i found hardly any difference in total quantity of the steel.
Have any one tried it in such buildings?
Yogesh
Important notice:This e-mail and any attachment thereto contains corporate proprietary information. If you have received it by mistake, please notify us immediately by reply e-mail and delete this e-mail and its attachments from your system.
Thank You.
Thank You.
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alltimepisal
Aug 13, 2008, 5:27:46 AM8/13/08
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Ashutosh,
- Master Slave command is provided to model in plane rigidity of slab.
- When RC slab is provided, it ties the columns and beams system rigidly
- This system will distribute the lateral forces in proportion to the stiffness of the columns. At the same time displacements of all points in plane of slab will be same.
- If we dont provide master slave command (i.e. if we did not model in plane stiffness of slabs) then force distribution will not be proper and this can cause force concentration in some of the columns. This may be the reason of overdesign of some of the columns.
- While giving master slave command, we need to create additional nodes at the CG of each floor. This node will be assigned as master node and other nodes will be assigned as slave nodes. Slaved direction should be in the plane of slab.
- When there is cutout in the plane of slab, then we have to judge whether slab is still providing enough stiffness to beam column framing or not. If it is providing enough stiffness, then only we can provide master slave command.
Other alternatives of the Master Slave command
Increasing in plane stiffness of beams - Here we need to specify higher value of Iy to the beams
Fictitious rigid links - Here we need to provide fictitious rigid links in the plane of slab. (These links should be modelled to join each and every node of the floor)
I think may help you.
- Master Slave command is provided to model in plane rigidity of slab.
- When RC slab is provided, it ties the columns and beams system rigidly
- This system will distribute the lateral forces in proportion to the stiffness of the columns. At the same time displacements of all points in plane of slab will be same.
- If we dont provide master slave command (i.e. if we did not model in plane stiffness of slabs) then force distribution will not be proper and this can cause force concentration in some of the columns. This may be the reason of overdesign of some of the columns.
- While giving master slave command, we need to create additional nodes at the CG of each floor. This node will be assigned as master node and other nodes will be assigned as slave nodes. Slaved direction should be in the plane of slab.
- When there is cutout in the plane of slab, then we have to judge whether slab is still providing enough stiffness to beam column framing or not. If it is providing enough stiffness, then only we can provide master slave command.
Other alternatives of the Master Slave command
Increasing in plane stiffness of beams - Here we need to specify higher value of Iy to the beams
Fictitious rigid links - Here we need to provide fictitious rigid links in the plane of slab. (These links should be modelled to join each and every node of the floor)
I think may help you.
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rahul.leslie
Aug 13, 2008, 12:41:11 PM8/13/08
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Do make sure the master-slave is active in the horizontal plane only. If you give "Rigid" instead of "FX FZ", it'll affect the support reactions too.
An alternative to Master-slave is to :-
1) model all slabs with plate elements - each slab (bounded by four beams) as a single rectangular element.
2) specify all those plates as 'plane stress' elements
3) giving material properties (E, poisson) same as that of Concrete, except density, which is put as zero, for the plates
4) give the thickness of slabs as thickness of the plates
5) self weight, wt. of finishes and live loads to be applied as 'Floor Load', just as usual practice
This will give, with enough accuracy, the diaphram action of slabs.
An alternative to Master-slave is to :-
1) model all slabs with plate elements - each slab (bounded by four beams) as a single rectangular element.
2) specify all those plates as 'plane stress' elements
3) giving material properties (E, poisson) same as that of Concrete, except density, which is put as zero, for the plates
4) give the thickness of slabs as thickness of the plates
5) self weight, wt. of finishes and live loads to be applied as 'Floor Load', just as usual practice
This will give, with enough accuracy, the diaphram action of slabs.
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ashutosh.jain
Aug 14, 2008, 12:09:11 AM8/14/08
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Dear Rahul,
Thank you,
Regards,
ASHUTOSH JAIN
- Is it ok to model slab as 4 noded plate, with nodes at four column/beam joints.
- What is difference between master slave as Rigid and master slave as FX FZ. Pls give some idea
Thank you,
Regards,
ASHUTOSH JAIN
From: rahul.leslie [mailto:fo...@sefindia.org]
Sent: Wednesday, August 13, 2008 10:11 PM
To: gen...@sefindia.org
Subject: [SEFI] Re: MASTER SLAVE COMMAND IN STAAD
Do make sure the master-slave is active in the horizontal plane only. If you give "Rigid" instead of "FX FZ", it'll affect the support reactions too.
An alternative to Master-slave is to :-
1) model all slabs with plate elements - each slab (bounded by four beams) as a single rectangular element.
2) specify all those plates as 'plane stress' elements
3) giving material properties (E, poisson) same as that of Concrete, except density, which is put as zero, for the plates
4) give the thickness of slabs as thickness of the plates
5) self weight, wt. of finishes and live loads to be applied as 'Floor Load', just as usual practice
This will give, with enough accuracy, the diaphram action of slabs.
An alternative to Master-slave is to :-
1) model all slabs with plate elements - each slab (bounded by four beams) as a single rectangular element.
2) specify all those plates as 'plane stress' elements
3) giving material properties (E, poisson) same as that of Concrete, except density, which is put as zero, for the plates
4) give the thickness of slabs as thickness of the plates
5) self weight, wt. of finishes and live loads to be applied as 'Floor Load', just as usual practice
This will give, with enough accuracy, the diaphram action of slabs.
Important notice:This e-mail and any attachment thereto contains corporate proprietary information. If you have received it by mistake, please notify us immediately by reply e-mail and delete this e-mail and its attachments from your system.
Thank You.
Thank You.
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alltimepisal
Aug 14, 2008, 12:37:38 AM8/14/08
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Rahul,
Modeling the slabs with plate elements are absolutely OK. But we have to take care of the meshing size for providing adequate stiffness to beams and columns. It will automatically take care of in plane stiffness of slab.
But there is no need of providing density equal to zero and providing floor load for the self weight of slab.
This will increase degrees of freedom drastically, hence the analysis time will be considerably more.
Modeling the slabs with plate elements are absolutely OK. But we have to take care of the meshing size for providing adequate stiffness to beams and columns. It will automatically take care of in plane stiffness of slab.
But there is no need of providing density equal to zero and providing floor load for the self weight of slab.
This will increase degrees of freedom drastically, hence the analysis time will be considerably more.
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alltimepisal
Aug 14, 2008, 5:39:34 AM8/14/08
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Ashutosh,
You can not model slab as 4 noded plates, with nodes at four column / beam joints.
Reason -
1. This will not satisfy compatibility conditions.
2. In this case load will be transferred at the 4 joints only.
3. As there is no node in the span of the beam, it is free to bend and this will not be the case in case of a slab.
Master - Slave
1. FX-FZ - In this case we are providing in plane stiffness only
2. Rigid - In this case in plane + out of plane stiffness will be provided
Basically slab will provide in plane stiffness only, hence you should not go for the rigid option
Regards,
Yogesh Pisal
You can not model slab as 4 noded plates, with nodes at four column / beam joints.
Reason -
1. This will not satisfy compatibility conditions.
2. In this case load will be transferred at the 4 joints only.
3. As there is no node in the span of the beam, it is free to bend and this will not be the case in case of a slab.
Master - Slave
1. FX-FZ - In this case we are providing in plane stiffness only
2. Rigid - In this case in plane + out of plane stiffness will be provided
Basically slab will provide in plane stiffness only, hence you should not go for the rigid option
Regards,
Yogesh Pisal
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rahul.leslie
Aug 14, 2008, 12:41:49 PM8/14/08
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No, no – I meant not to mesh the elements. See,
1) I want the slabs to take in plane forces only, and that too by its stiffness in connecting the beam column junctions only – so I’m not meshing the slab
2) I don’t want the plates to transfer loads to the structure, including self weight of the slabs – so I’m putting zero density, and using floor loads for all surface loads, and floor loads for self weight of slabs too.
3) I don’t want the bending stiffness of plates to contribute to the beam – so I’m not meshing the slab
4) I don’t want the bending stiffness of plates to contribute in any way to the structure (but only its in-plane stiffness) – so I’m specifying them as plane stress
Now I’ll try to respond to your views:
1) I want the slabs to take in plane forces only, and that too by its stiffness in connecting the beam column junctions only – so I’m not meshing the slab
2) I don’t want the plates to transfer loads to the structure, including self weight of the slabs – so I’m putting zero density, and using floor loads for all surface loads, and floor loads for self weight of slabs too.
3) I don’t want the bending stiffness of plates to contribute to the beam – so I’m not meshing the slab
4) I don’t want the bending stiffness of plates to contribute in any way to the structure (but only its in-plane stiffness) – so I’m specifying them as plane stress
Now I’ll try to respond to your views:
1. "This will not satisfy compatibility conditions".
Well, it was meant to be active in its in-plane stiffness only, wherein it does satisfy
2. "In this case load will be transferred at the 4 joints only".
That’s why I’m not putting any load on the slab, not even its own self weight
3. "As there is no node in the span of the beam, it is free to bend and this will not be the case in case of a slab".
Now, that’s how we always wanted he beam to behave, and that’s why we never modeled slabs in usual practice. Here there is a slab, but is made sure it doesn’t take part in bending (by making it ‘mesh free’ and plane stress). The slab is active only in its in-plane direction
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