Slope deflection method

[zaes1980]

For some time now i am involved mostly in design and manufacture of Pre-engineered buildings. I usually use a simple 2D frame analysis tool to get moments and then design the frame. Have been doing the same thing over and over again for last 12 years.

recently i decided to code(Python) my own tool to design a Pre-engineered building as to make things even faster. So i was choosing a method to analyze a Gable frame, so after ages browsed my University days memory and found that the "slope deflection method" was the easiest for me to analyze a 2D frame.

coming to the point, i found this very similar example on the internet, it is a lecture note of Purdue university:
Purdue university example

turn to Example 7. now the solution suggests that the moment BC - 9.68 and moment CB = 25.2. This very confusing, my finding through my 2d computer analysis(attached screenshot) is the other way around. Moment BC is around 25 and Moment CB is around 9.5

can someone kindly point out where i am wrong in understanding this example ?

 

[jrbaus]

Zaes1980,

I am not familiar enough with this method to comment on where the error lies, however I would like to commend your efforts with Python.

May I ask what libraries you used to produce the graphic output? That is quite impressive.

I have thought of doing something similar in Python, however I am tempted to leverage of the open source package Frame3DD to do the number crunching since it has modal analysis, buckling analysis and other nice features that I simply couldn't find enough time to code by myself.

If you haven't already done so, this site is worth a look:

http://frame3dd.sourceforge.net/

Regards Jake

 

[zaes1980]

With python :
Tkinter for the GUI input and output
numpy for solving equations and linear algebra.
matplotlip for graphing

i have been using a lot of frame analysis packages but i have started to feel much more comfortable with making my own scripts. it gives me flexibility and confidence. I run a small Pre-engineered manufacturing company so my work is limited to analysis and design of Gable frames. I am building python tools to do my job in minutes and provide clients with quotations within 24 hours.

I would say developing your own tools for repetitive assignments like retaining walls, 2D frames etc is effective, but if you are running a consultancy and there is a lot of variety in your work then a general analysis package becomes important. So i think it all comes down to nature of work. developing a python sheet for a one time project might not be a good idea. But if like me your dad to day work is limited to one type of structures then Python tools are good.

 

[jrbaus]

Thanks for the feedback. Agree Python tools are good for repetitive tasks. I have a few small tools put together with PyQt however always looking for better ways to solve certain problems. Your matplotlib output is very clean, gives me a some ideas to consider next time I need to show some graphical output....

I have always looked at Python or Matlab as good options where you have a dynamic problem that isn't easily solved in Excel. Problems such as multiple members, load cases and relational data can of course still be solved in Excel (and I do this routinely) but there are definitely better solutions to be had if you have time to persist with the learning curve of some of these other technologies.

Good luck - hope someone can help you with your original question.


Kind regards,
Jake

 

[zaes1980]

jake have you tried pygal ? its amazing and the plots are dynamic, created in svg format

http://pygal.org/chart_types/#idid3

 

[KootK]

The example problem is using a fixed base assumption where you appear to have assumed a pin base. I would expect the roof beam moments to be higher at the peak than the eve because the peak connection is effectively 100% fixed due to symmetry.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[rb1957]

example 8 is fixed, example 7 is pinned ... no?

have you tried an earlier (easierÉ) example ?

another day in paradise, or is paradise one day closer ?

 

[KootK]

You're quite right rb1957. I misread the fixity.

If OP has the right values at the wrong locations, I'm guessing that it'll be a pretty easy fix.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[RFreund]

sorry again off topic...
@zaes1980, @jrbaus - is there a good starting point for using Python in general for calculations. I create a lot of calcs with mathcad, excel, etc (don't really know VBA though) but I'd like to move to learning a language and creating something with its own interface, etc.

Thanks


EIT

http://www.HowToEngineer.com

 

[zaes1980]

i don't think i have wrong values. you can quickly model and review the example (example 7) the one with pinned connections

 

[rb1957]

how about solving the problem with another method ?

can you check your python with another (easier?) problem ?

another day in paradise, or is paradise one day closer ?

 

[zaes1980]

@rb, well eventually i would move to stiffness matrix, but i will have to open my university notes after 15 years But the question is how can online lecture of a prestigious university be wrong, and if it is wrong i want to write an email to the university to correct this as this will mislead a lot of students.

@Rfreund
well the answer is simple : versatility and easiness. programming languages offer a lot of packages to get the job done in seconds and in style . you can make your own GUI and get outputs you like the way you like. in two lines you can solve matrices, linear equations. with Mathcad and excel etc you are limited with the way you input and get output. Heck you can make your own GUI software, and i tell you its very easy to learn. Once you get the hang of it you will forget Mathcad and excel.

 

[zaes1980]

@RF
if you want to learn python the best option is to learn from

http://www.coursera.org.

Rice university offers a engaging course with weekly mini projects and quizes. You can learn it free but if you pay 50USD then you get a university certificate on successful completion.
it is two months long and it will teach you good. Don't buy courses Don't lurk round for bits and pieces of tutorials. This course will really teach you enough python to do anything an engineer would want to do.
however unfortunately i think there is no active session and the course will be offered in Feb 2015

https://www.coursera.org/course/interactivepython

https://www.coursera.org/course/pythonlearn

 

[KootK]

Everything that I said in my post above is wrong. Please disregard it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[rb1957]

i suspect that the example is correct and that your script isn't

that's why i suggested trying a different method, or trying your script with a simplier problem.

another day in paradise, or is paradise one day closer ?

 

[Archie264]

Everything that I said in my post above is wrong.

I've been there many times on this humbling forum.

 

[rb1957]

"it will teach you good" ... sigh

another day in paradise, or is paradise one day closer ?

 

[KootK]

Yeah. There are times when I wish that I could delete my own comments. Maybe red flag them as racist or something. I figure it's better to throw comments out there, even if I'm not 100% certain, than to sit on my hands and not participate. I'd say my batting average must be 0.800 or better.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[rb1957]

you can RF your own, and ask mgmt please delete to save me embarrassment, had a senior moment ... they'll understand

another day in paradise, or is paradise one day closer ?

 

[RFreund]

@zaes1980 - Thank you! I will check it out, I have been wanting to put some time in to learning it.

As for the problem at hand have you checked it against one of your other software packages? Is it indeed wrong? Could be that the equations are correct but they answers were put in the wrong place. I have not checked that though.

If you are looking to go over an FEM approach. There are a couple good references. One is by Doug (eng-tips = IDS)

http://newtonexcelbach.wordpress.com/2013/01/22/3dframe-3d-frame-analysis-for-excel/

Another place where it might be easier to see a general beam FEA solver is the SMath forums (like mathcad):

http://en.smath.info/forum/yaf_postst2447p5_SMath-For-Engineering-Calculations.aspx#post16421

http://smath.info/file/NESDC

EIT

http://www.HowToEngineer.com