Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
(OP)
Hi everyone!
I am trying to develop a spreadsheet that calculates reaction forces for a simple braced frame using direct stiffness method. I did the regular process of
Calculation of stiffness matrix in local coordinate
Calculation of stiffness matrix in global coordinate
Calculation of global stiffness matrix
Calculation of reaction forced from known values
My problem is that my spreadsheet calculated right reaction forces for unbraced frames. It does not calculate right reaction forces for braced frame though. Is Direct Stiffness Method limited to unbraced frames only? I didn't think so but I feel like I have reached a road block can't find out why. :( Please help.I can upload the spreadsheet if you all want me to.
I am trying to develop a spreadsheet that calculates reaction forces for a simple braced frame using direct stiffness method. I did the regular process of
Calculation of stiffness matrix in local coordinate
Calculation of stiffness matrix in global coordinate
Calculation of global stiffness matrix
Calculation of reaction forced from known values
My problem is that my spreadsheet calculated right reaction forces for unbraced frames. It does not calculate right reaction forces for braced frame though. Is Direct Stiffness Method limited to unbraced frames only? I didn't think so but I feel like I have reached a road block can't find out why. :( Please help.I can upload the spreadsheet if you all want me to.






RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
Do your reactions satisfy equilibrium?
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
you must get smarter than the software you're using.
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
@BigInch, my brace is a full vertical cross brace i.e. it goes from the top of one column to the base of the other column. My matrix does consider the presence of brace as well and as far as I know it should be correct as well.
I have attached that spreadsheet with this post for you all to look at. That you so much for your views. Please let me know your thoughts.
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
Yes, by equilibrium, I mean the sum of the loads in any direction should equal 0 in the end. Yes, loads are attracted to stiffness, therefore reactions will be greater where the members have more stiffness. However, you should be able to come really close to the reactions in concentric brace frames buy using statics.
Sorry, I don't really want to go through your spreadsheet as it has been 10+ years since I have had to do this type of work (when I was at uni) and it would take me a while to decipher.
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
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RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
JAE, I am not a student. I am trying to develop this spreadsheet so that anybody can figure out the approximate weight of steel needed to support a specified load without having to use a structural analysis software.
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
In your matrices, it appears that you're only accounting for the flexural stiffness of all the members, but your brace (especially with the shape chosen) is going to carry axial forces - you need to account for the axial stiffness of the brace to get the forces correct in your members. Please note that I only spent 5 minutes looking at your spreadsheet, so if I missed where it's accounted for, I apologize.
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
bookowski, our support structure condiciton/configuration does not change much. I am doing it in excel so that any one without prior structural experience can get an approximate steel weight values.
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
http://www.cedengineering.com/courseoutline.asp?ci...
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
While I understand that you have two supports and 4 unknowns, I think any engineer would eliminate the horizontal component of the support w/o the brace attachment making the system statically determinant at that point no matter what the connection was at the top of the column. How else did they design building w/o computers 100 years ago.
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
SteelPE, I did not understand you. When you say elimination of the horizontal component of the support, which support are you referring to? Are you talking about the top beam sitting on top of the two columns? Please elaborate a little bit further.
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
http://newtonexcelbach.wordpress.com/2014/01/22/fr...
The link is a series of blog posts on frame analysis in Excel, starting with simple on-sheet solutions, through to 2D and 3D program using VBA or compiled solvers that will deal with structures of just about any complexity.
The stiffness method should certainly cope with a braced frame, so there must be an error in your formulation somewhere. Unfortunately I won't have much time to look at it today, but if I do find anything I'll let you know.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
Yikes!
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RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Direct Stiffness Method works for Simple Unbraced Frame, but does not for Braced Frame
I really appreciate you all helping me out on this project.