Wind pressure shear force

[JamieHir]

Hi,

I meed some help on a very simple question.

For a pole -

After calculating wind pressure and multiplying it by the projected area to calculate the shear stress.

Where does this shear stress act? Is it at the centre of the pole?

Lets say im calculating the shear stress caused by wind pressure due to a sign on the pole. Is the shear stress located at the location of the sign?

Thus the moment is the centreline i.e. height from bottom of pole to centre of sign?

Or is the shear stresa enacted by the wind pressure at the bottom of the pole?

 

[Lomarandil]

The resultant wind force (wind pressure x area) will act at the center* of the projected area. You can simplify a complex shape into several regular shapes to simplify this.

*note: some codes, including ASCE 7, will actually have you apply the resultant force slightly offset from the center of area to account for various real-life conditions. I'm not sure about AS1170

Once the resultant force(s) have been calculated, you can apply those along the length of your pole to determine the shear and moment demands in the pole. For a cantilever pole, the largest shear and moments will occur at the base* of the pole. If your pole is straight, that will also be the location of the largest shear and flexural stresses.

*note that if the pole is embedded, the "base" of the pole (and locations of max shear/moment) may exist below ground.

 

[SWComposites]

The wind force is a distributed load as a function of area down the length of the pole. The shear load will then increase from the tip to the base of the pole, so it will be greatest at the bottom of the pole. (draw a fbd). But with a sign on the pole the shear at the base of the sign is probably close to the total shear at the bottom of the pole. If the pole tapers in diameter or thickness you will need to check multiple locations along the pole.

 

[JamieHir]

Thanks guys.

In the spreadsheet, wind pressure x projected area is labelled as shear force.

If this shear force acts at the base of the pole, how can your centreline be your lever arm (centre of attachment i.e. sign)

Aren't they in the same location?

How can you then do shear force x centreline to get the moment at the base?

 

[Luceid]

If your wind force is acting in the vertical direction - then yes there is no moment. However I'd be very surprised if that was the case.

The shear force is acting at the centroid of the surface it is being applied to, in this case the sign, not the pole. So there is a shear reaction at the base of the pole, but the demand is coming all the way from the centroid of the sign. That force causes a moment about the base of the pole.

 

[BAretired]

In the spreadsheet, wind pressure x projected area is labelled as shear force.

If this shear force acts at the base of the pole, how can your centreline be your lever arm (centre of attachment i.e. sign)

Aren't they in the same location?

How can you then do shear force x centreline to get the moment at the base?

The shear force doesn't "act at the base of the pole". It acts at the center of the sign, but shear is still present at the base of the pole. If the vertical distance from base to center of sign is 'h', the moment is Area*pressure*h. Shear is constant over the height 'h'. The pole is a vertical cantilever with a horizontal load applied near the top.

Turn this 90 degrees and you've got it.