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## Robot Structural Analysis

Distinguished Contributor
Posts: 132
Registered: ‎11-16-2011

# Self weight of tapered profile

371 Views, 9 Replies
02-11-2014 02:29 AM

Hi,

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?

Product Support
Posts: 5,482
Registered: ‎04-26-2010

# Re: Self weight of tapered profile

02-11-2014 02:34 AM in reply to: Jummybear

Rafal Gaweda
Distinguished Contributor
Posts: 132
Registered: ‎11-16-2011

# Re: Self weight of tapered profile

02-11-2014 04:07 AM in reply to: Rafal.Gaweda

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.

Manual calculation:

(14,015m+15,98m)*2pcs*0,4m*0,025m*7850kg/m3=4709,2kg (flanges)

(1,94m2+1,044m2+8,98m2)*2pcs*0,010m*7850kg/m3=939,2kg (wall)

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.

Product Support
Posts: 5,482
Registered: ‎04-26-2010

# Re: Self weight of tapered profile

02-11-2014 06:56 AM in reply to: Jummybear

You should calcaulate model  \ sections as they are in Robot not as drawn in ACAD

Example:

Rafal Gaweda
Distinguished Contributor
Posts: 132
Registered: ‎11-16-2011

# Re: Self weight of tapered profile

02-12-2014 12:40 AM in reply to: Rafal.Gaweda

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...

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Posts: 5,482
Registered: ‎04-26-2010

# Re: Self weight of tapered profile

02-12-2014 01:00 AM in reply to: Jummybear

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"?

Rafal Gaweda
Active Contributor
Posts: 43
Registered: ‎09-24-2008

# Re: Self weight of tapered profile

02-12-2014 03:17 AM in reply to: Jummybear

Hi,

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%).

BTW

Your hand calculations took into consideration 2 rafters and 1 column - should be 2 rafters and 2 columns .

Jerzy

Distinguished Contributor
Posts: 132
Registered: ‎11-16-2011

# Re: Self weight of tapered profile

02-12-2014 04:58 AM in reply to: legar

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.

Active Contributor
Posts: 43
Registered: ‎09-24-2008

# Re: Self weight of tapered profile

02-12-2014 11:59 PM in reply to: Jummybear

ilja.jevglevski wrote:

"In my initial Robot model i didnt find any difference in web height..."

I can't agree, see below differences: