Master-Slave Connection
Master-Slave Connection
(OP)
I am modeling a large flat 3D concrete slab using a square mesh, supported at regular intervals in both directions by piles. I've analyzed this model two ways and I can't make sense of the results. In the first analysis, I simply analyzed the slab using pinned connections, which, as expected, gave me large stress concentrations over the support node. To alleviate this, I specified each support node as a master, with the rigidly connected slave nodes being within the boundary of the pile's cross section. When I do this however, the results go completely out of wack. In fact, at the pinned supports (fixed in the vertical direction), the nodes undergo significant vertical displacement (impossible!). How can this be? Am I misinterpreting the use of the master-slave function for this application? Would the slaves not be considered rigidly connected? Is STAAD just screwing with me?
Appreciate any help. Thanks.
Appreciate any help. Thanks.





RE: Master-Slave Connection
I know what you mean about stress concentration at the pin supported node. I usually just use a little rational averaging of the highly concentrated stresses to come up with a reasonable number for use in designing the slab reinforcing.
Of course, you could average it out more by adding several nodes for each pile, say dividing the pile perimeter into 8 or 12 segments. That would spread the load out more, but I don't know if the end result would be much different.
RE: Master-Slave Connection
An advantage of this supposed method would be to reduce the design moments since the spans would be effectively decreased. In my model, 18" diameter supports are placed at 8' o.c., meaning that the slab's clear span would reduce from 8' to 6.5'. This would help in optimizing the rebar detailing if I could get the model to work.
Regardless, I guess I can't figure out why STAAD allows the master-slave command to superceed the support conditions...
RE: Master-Slave Connection
2) Master and/or slave joints may not be inclined supports.
Hope this helps you.
Bill
RE: Master-Slave Connection
3) Master & Slave directions must not connect to solid elements. Use plates instead.
Use SLAVE FY, if vertical is the only direction controlled; then fix FY & MX & MZ at the master. Use thicker and/or stiffer plates within the pile zones.
RE: Master-Slave Connection
In my experience, it is almmost always better to use these link members to tie nodes together rather than to use the master-slave.
It's been awhile since I've looked through it. But, in my opinion, the STAAD manual has always been somewhat irresponsible in how they present the Master-Slave node behavior. It's just not very often that this feature can be used without undesirable consequences.
RE: Master-Slave Connection
RE: Master-Slave Connection
So, using the method I describe, you could easily create a "rigid link" beam in your model.
And your point on the use of master-slave nodes, and the consequences of such use, is very appropriate. I have many times seen bad results from innapropraite use of this function. When I review the models that our junior engineers make, I always check to see if they are using it, and if so, are they using it correctly. I once got involved in a 5 story moment frame building that was modeled innapropriately using master-slave. The design was done by a senior level engineer at my company, who had just been laid off. I was helping one of our juniors engineers check something while checking shop drawings, as the staad model was giving odd results. As I dug into it, it turned out that all the moment frames were under-designed because of the master-slave command. This ended up being a major issue, as we had to redesign connections, add beam bottom chord bracing to the beams, and about 1/2 the steel was already in place when this happened.