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 ?

 

[IDS]

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/frame-analysis-with-excel/

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/

 

[rb1957]

doug, ex7 is pinned, ex8 is fixed

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

 

[jrbaus]

@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

 

[rb1957]

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 ?

 

[IDS]

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/

 

[canwesteng]

I get the same as IDS

 

[zaes1980]

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.

 

[zaes1980]

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