equivalent frame method for analysis of continuous slabs

[nestyc]

Hi

I am trying to model a two way slab system on beams using the equivalent frame method since the direct design method won't work. I am using a simple plane frame program to analyse the equivalent frame, however, my analysis gives moments that are too high. The reason being, I do not know how to model the out of plane torsional members. That is the beams framing into the supports, which would cause the slab thickness to increase at that point some distance away from the column.

The CSA codes are of no help and simply inform that I should use PCA-Frame or ADOSS. I know the stiffnes of the members would be increased as we move along the axis towards the column based on the thickness of beams and the magnification factor specified in the code.


My problem is how do you model this? Do I add a new node and increase the member stiffness for that short distance into the column? What do I do? The method of looking up tables for carry over factors and Stiffness factors for the moment distribution method I find is too tedious. There must be an easier way without buying an elaborate program like STAADIII or PROKON. I can't invest in these at this point. Hopefully after 2 or 3 jobs this nature.

Also I'm located on the tiny island of Dominica where resource material is very scant.

Any help would be deeply appreciated.

Regrads,

Craig Nesty

 

[erkansamhal]

Dear Craig,

For equivalent frame slab-beam-column system, there are some descriptions and usefull tables in book "Reinforced Concrete Fundamentals" written by P.Moss FERGUSON. But, I don't know how to design in any computer-aided structural program.

Best Regard,
ERKAN
erksamhal@ixir.com

 

[Riz]

Craig,
Hi!
Your problem definition is not clear. You have tried to explain in depth analysis related to a 2 way spanning slab. However, what you have not described is whether you are proposing to analyse it as a 3d model( FE analysis ). The fact is that a 2d frame analysis by Hardy cross or any frame analysis software can sort out the forces. Torsional problems depend on the end conditions, out of balance forces etc. For doing a 2d frame analysis it is necessary for you to get your loads correct. This than can be carried out in the other direction as well. The BS8110 table suffice for the design u intend to carry out - if u r using this COP. It has torsional requirements as well.
Hope this helps
Riz