Master - Slave question

[structuresguy]

We had another engineer (no longer with us) who modeled a 3 story steel moment frame building using the master slave command. He had it set to MASTER RIGID. When I took over the project after he left, I deleted the master slave command, and instead modeled flat plates to simulate the slab on metal deck floors. When I did this, everything starts failing.

I checked it again using a revised MASTER DIA XZ command, instead of RIGID command. It behaves basically exactly like my model with the plates.

Can someone explain what the RIGID command is doing please, and when you would want to use it?

It seems to be locking every node against any rotation, but allowing translation. Whereas my plates (and the DIA XZ master command) allow rotation at the nodes.

I am somewhat worried about the results, as many beams and columns are now failing, some by as much as 300%. They all worked when the master was set to RIGID.

Any insight is very much appreciated.

 

[Bill2001]

The concept of the "Master-Slave" facility is that the slave node will deflect as it’s master node. Precisely speaking, if the master sustains a rigid rotation of theta, the slave will undergo a displacement of the value of the translational displacement of the master plus R* sin (theta) – where R is the distance between the master and the slave. Thus, when you model a rigid link via master-slave it is important to know, which node will primarily be displaced due to the loads applied and which will follow the former due to rigidity in connection. The former should be specified as master and the nodes following it should be specified as slaves. Thus, a specification of master-slave would need an understanding of the deflection pattern of the model under the loads.
Hope this answers your query.
Bill

 

[RAYC1]

All members and plates that are embedded in a rigid master/slave will have zero forces and moments; so they will naturally pass the code checking.

Floor diaphragms modeled as SLAVE ZX will have embedded plates with zero inplane forces/stresses but will have bending moments and transverse shears. Floor members would have zero axial & My but would have nonzero Vy & Mz. So for those directions code checking has the possibility of failing.

For modal dynamics with a diagonal mass matrix (masses lumped at the nodes); the master node should be at the c.g. of the masses of the associated slave nodes.

 

[structuresguy]

Thanks all. This RIGID command is very dangerous. The Staad people need to make it clear when it should and should not be used. When I talked to their tech support, they said it should not be used for a floor diaphragm. but their manual says "for modeling rigid floor diaphragms". They need to get their act together. he said the RIGID command should only be used when you want no displacement/rotation between the master and slave nodes. For instance, maybe in a bracket off a column flange, or a corbel. Something that is rigidly attached to something else. But definitely not for floor diaphragms.

Anyway, the end result on this project is we are making the contractor weld in cover plates on about 30% of the buildings columns, and a few of the beams! This steel was already fabricated and erection had already just started, when we discovered the problem. So this one word is going to cost us big time.