I created a model (frame) with a tapered section (h is various through the length of the section). When i compare the self-weight of the structure with the manual calculation i get a difference about 15-20%. Robot offers for example 6841kg for the whole structure and my hand calculation gives 5903. As for me then its quite big difference for the same geometry. Any ideas or assumptions?
Solved! Go to Solution.
Here is the Robot file and dwg sketch of the frame. Some assumtion is made about the drawing (its not 100% the same like in Robot, but very close to it) but i dont think that it can be the reason.
Actually i took into account the webs which are continued in frame upper joint (works as a stiffeners) + some plates in middle connection point and it all gives 254,8kg.
Sum: 4709,2+939,2=5648,4kg vs 6841 in Robot or 5648,4+254,8=5903,2kg vs 6841kg in Robot.
PS I also mentioned that in Robot the density of steel is 7701kg/m3, if i take this fact into account then difference will grow even more.
What is th difference? Angle of column? The thickness of flanges/webs is the same, width is the same, length is the same. Some "inaccuracy" between Robot and drawing can be in the upper joint, but its influence should be small. By the way what do you mean in flange calculation "0,55-0,15=0,4"? Even now i dont understand where i did a mistake..
PS Probably u can give some advice how to model such structure? I had a simple sketch (shape) of the frame and estimators were asked from us approx. weight of such frame. Which weight is more accurate: mine (calculated manually) or Robot ones? My logic was: design as accurate as possible frame in Robot and get the base dimentions (web and flange thickness, width etc), then to calculate the weight by hand. As Robot also had a possibility to calculate the wight of the structure i decided to compere 2 calculations. I wonder that i get such difference...
What is th difference? Angle of column?
Geometry - real vs simplified=robot
By the way what do you mean in flange calculation "0,55-0,15=0,4"?
Robot selfweight of tapered structures is generally OK - in Your example there are some differences between AutoCad drawing and Robot model, so results must be different.
Assume that AutoCad drawing is correct, then:
1. Total length of flanges (measured in AutoCad DWG) is 60,00m so flanges weight is = 60,00m * 0,40m * 0,025m * 7850kG/m3 = 4710kG
2. Total area of webs (measured in AutoCad DWG) is 24,06m2, so webs weight is = 12,06m2 * 0,01m * 7850kG/m3 = 1889kG
Total weight of structure measured in AutoCad drawing is = 4710kG + 1889kG = 6599kG
Attached is corrected Robot model (there were some differences in web height, between DWG and Robot model) - total weight of structure in corrected Robot model is 6588kG so difference is neglectible (less than 0,2%).
Your hand calculations took into consideration 2 rafters and 1 column - should be 2 rafters and 2 columns .
in my hand calc i forgot to multipile by 2 web area, that was my mistake. The result now is comperable with your hand calculation 6588 (you get 6599). In my initial Robot model i didnt find any difference in web height... So "updated" manual result is now compereble with Robot ones, 6588 vs 6840. Still there are 3-4% difference. To remove this last disagreement i need to design columns with an angle which corresponds to dwg scheme, then i get also closer results. Thank you, problem solved.