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Slope deflection method

Slope deflection method

Slope deflection method

(OP)
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 ?

RE: Slope deflection method

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

RE: Slope deflection method

(OP)
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.


RE: Slope deflection method

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

RE: Slope deflection method

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.

RE: Slope deflection method

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 ?

RE: Slope deflection method

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.

RE: Slope deflection method

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
www.HowToEngineer.com

RE: Slope deflection method

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

RE: Slope deflection method

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 ?

RE: Slope deflection method

(OP)
@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.

RE: Slope deflection method

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

RE: Slope deflection method

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.

RE: Slope deflection method

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 ?

RE: Slope deflection method

Quote:

Everything that I said in my post above is wrong.

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

RE: Slope deflection method

"it will teach you good" ... sigh

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

RE: Slope deflection method

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.

RE: Slope deflection method

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 ?

RE: Slope deflection method

@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/2013/01/22/3d...

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-...
http://smath.info/file/NESDC

EIT
www.HowToEngineer.com

RE: Slope deflection method

zaes1980 - I get the same answers as you if modelling the frame as pinned at the base, with a UDL, but the example is fixed at the base, and has the loads applied as point loads at the purlins. Modelling it this way I get

Base joints: 16.3 k.ft
Haunches: -22.1 k.ft
Crown: -13.1 k.ft

So it looks like the results shown in the example are wrong.

I used an Excel based frame analysis program that you can download from:
http://newtonexcelbach.wordpress.com/2014/01/22/fr...

The current version is VBA based, with the option to link to compiled solvers using the Alglib library, but I'm working on a Python version, which I will be publishing soon (still with an Excel front end, but most of the work will be done by Python code, using Numpy, Scipy and PySparse).

I'm not into Matplotlib yet, but I'd be interested to see your code, if you choose to make it available.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Slope deflection method

doug, ex7 is pinned, ex8 is fixed

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

RE: Slope deflection method

@Zaes1980: pygal looks nice. I will keep that one in mind to for next time and might see how well it plays with pyqt.

@RFreund: I gained a lot of good insight into python programming during my university days where I took an introductory course. I did get started with Python before I had any formal training though, and it is probably one of the easiest languages to get started in. The best advice I can give is try and complete a project that adds real value to your work - you will learn a lot more and see more of the languages strengths. There are many generic tutorials out there to get you going, but solving problems unique to your situation will teach you a lot since it will force you to find your own path to the end result (most of my early projects are good example of how NOT to write code). Feel free to shoot me a PM anytime if you need a few hints on getting started and I will share all my bad habits with you :)

Regards Jake

RE: Slope deflection method

FWIW, using a constant UDL the (FEM)bc and cd is wL^2/12 = 22.5 k.ft

"simplify" the load to the purlins, you get 3000:6000:6000:3000 (so much simpler) and i get their 20 k.ft

their ex8 is like doug`s results, but reversed ... humm, maybe something smells in denmark ?

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

RE: Slope deflection method

Thanks rb1957.

For pinned supports I get:

Haunch: -25.3 k.ft
Crown: -6.5 k.ft

So still very different from the answers in the text.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Slope deflection method

I get the same as IDS

RE: Slope deflection method

(OP)
all in all here the screenshot of the program being written

the GUI is pretty much ready for my script.(attached screenshot) it is interactive, it will take the values from the input panel (loading to be added) and as the values change in the input panel the diagram will change accordingly. there is also a Zoom in and Zoom out option. The blue screen is left empty for results etc. The idea is to perform all checks as per AISC guide 25, off coarse after performing analysis and by simply changing values a Pre-engineered should be design within minutes.

it would be great if some one could collaborate with me. write down the python class to analyze the frame. this would be great teamwork if three or four engineers start collaborating in projects like this. i am all up for any programing of the design tools. I can pretty much do anything with python, its the structural analysis calculations that slowing me down, during my 15 years of practice i hardly ever performed manual calculations.


RE: Slope deflection method

(OP)
So i think it is safe to say that the example it incorrect. The university people should be notified.

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