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Been stumped with this question for way too long... its a beam with a thin-walled rectangular cross-section, and a shear force is acting at a distance from the shear center. I know my decomposition of that force into the torsional moment and shear-center acting shear force is correct, but for some reason the shear flows don't add up.... rather than causing a higher shear to the left of the cross-section like I'd expect from my diagram, they cause a higher shear on the right side of the cross-section, sort of the mirror image of what I would expect. I've tried finding my mistake looking at books, websites, notes but none of them address this topic directly, leaving me with no other choice. :(

Could anyone please tell me what I'm doing wrong?

Also sorry for the blurry image and messy diagrams... I'll draw a new one if it's useless.

enter image description here enter image description here

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Thank you for the edit, exploringnet! – AED Apr 4 '13 at 14:33
This isn't really homework though. :P It's just me not understanding the topic. – AED Apr 4 '13 at 15:42
Hi AED. Welcome to Phys.SE. If you haven't already done so, please take a minute to read the definition of when to use the homework tag, and the Phys.SE policy for homework-like problems. – Qmechanic Apr 4 '13 at 15:49
Will do, thanks! – AED Apr 4 '13 at 15:51
Voting to leave open - although it's civil engineering it's enough like a standard Newtonian statics problem to fit this site and there is as yet no civil engineering SE. I do recommend you should draw up your diagrams in detail and explain the notation (the little butterfly wings representing stress profiles) - I don't think it would be wonted to many physicists. – WetSavannaAnimal aka Rod Vance Nov 2 '13 at 6:02

This is fairly naive way of looking at the problem, but have you got your final shear force distribution the right way round? The way I look at it, the web closest to the point load will do more work than the web furthest from the point load because it is closer and therefore effectively stiffer. The shear lag in the flange effectively means that if the flanges are wide enough then no load will transfer into the web furthest from the point load and all the load will go in the web closest to the point load. This means that the shear force must be highest in the web closest to the point load and not the web furthest from the point load.

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Thank you! Well, I wasn't really solving the problem, I know how to do all the calculation. I was just trying to figure out how to draw shear flow diagrams by observation. And what you said sounds completely logical - that would explain the whole dilemma. So using superposition and each of their shear diagrams is the correct and most practical approach. It was wrong of me to assume that I could simply draw the complete shear flow diagram in one go, by assuming the shear is zero at the line of action of the force. – AED Apr 8 '13 at 16:11

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