# Shear force(x) at the tower base

**URL:** <https://forums.nlr.gov/t/shear-force-x-at-the-tower-base/245>\
**Category:** Computer-Aided Engineering Software Tools\
**Created:** [April 23, 2010, 1:37pm UTC](https://forums.nlr.gov/t/shear-force-x-at-the-tower-base/245 "2010-04-23T13:37:09Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Yinping.Yang](https://avatars.discourse-cdn.com/v4/letter/y/b487fb/32.png) [@Yinping.Yang](https://forums.nlr.gov/u/Yinping.Yang)\
**Post date:** [April 23, 2010, 1:37pm UTC](https://forums.nlr.gov/t/shear-force-x-at-the-tower-base/245/1 "2010-04-23T13:37:09Z")

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Hi,

I simulate the model under a linear increasing wind case. From the results comparison, I found the shear force in x axis at the tower base is different between FAST and Bladed. In contrast, the shear force in x axis at the tower top agrees well. Could someone give me some hints? I suspect maybe the tower leads to this problem? The yaw motion is disabled.

I do not understand what yields the moment about z axis at the tower top/base. The thrust on the rotor is not equally distributed, so there is a torque about z axis. Am I correct?

Thanks  
Best regards

Yinping Yang

 ![Power.jpg](https://europe1.discourse-cdn.com/nrel/original/1X/c51e0ca637e82bb71daecbb260b757fbf7603b6c.jpeg)  
 ![ShearTowrBase.jpg](https://europe1.discourse-cdn.com/nrel/original/1X/3dcd9b4acde7f4d41b1c029830cdc623354566e9.jpeg)  
 ![MomentTowerBase.jpg](https://europe1.discourse-cdn.com/nrel/original/1X/75ba2f342be58c01a223fea86f4c906fef0fa809.jpeg)

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**Author:** ![Yinping.Yang](https://avatars.discourse-cdn.com/v4/letter/y/b487fb/32.png) [@Yinping.Yang](https://forums.nlr.gov/u/Yinping.Yang)\
**Post date:** [April 23, 2010, 1:39pm UTC](https://forums.nlr.gov/t/shear-force-x-at-the-tower-base/245/2 "2010-04-23T13:39:15Z")

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The forces and moments at the tower top.

 ![ShearTowerTop.jpg](https://europe1.discourse-cdn.com/nrel/original/1X/12d48253a9b5613e6cbb42170e5a3932265bf931.jpeg)  
 ![MomentTowerTop.jpg](https://europe1.discourse-cdn.com/nrel/original/1X/b129deeb7629ca1d331ef27bbe939d6722a5c105.jpeg)

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**Author:** ![Jason.Jonkman](https://dub1.discourse-cdn.com/nrel/user_avatar/forums.nlr.gov/jason.jonkman/32/3086_2.png) [@Jason.Jonkman](https://forums.nlr.gov/u/Jason.Jonkman)\
**Post date:** [April 29, 2010, 1:56pm UTC](https://forums.nlr.gov/t/shear-force-x-at-the-tower-base/245/3 "2010-04-29T13:56:34Z")

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Dear Yinpang Yang,

The Bladed results show more variation than the FAST results, which suggests that your Bladed model has less tower damping than your FAST model does. I’m not sure why the mean values are different in some of the outputs.

A nonzero yaw moment can be caused by:  
\*Asymmetry in the aerodynamic rotor loads and response  
\*A Nonzero shaft tilt, which can cause a portion of the rotor torque to act about the yaw axis  
\*The drivetrain torque can induce a mean tower side-to-side displacement, which can cause the rotor thrust to induce a yaw moments.

Best regards,