# Calculating Shear on a Circular Section



## Bigwolf (Aug 20, 2009)

Just curious if anyone can help out with this.....

I've got a 5" diameter beam with a uniform load on it. NDS Equ 3.4-1 (pg 15) says fv = VQ/(Ib)

I've got V and I = pi*d^4/64

Here's the question----b is defined as "breadth (thickness) of rectangular bending member" on NDS pg 5 --&gt; yet this notation is specifically used in the Equ 3.4-1 ????

In this instance is "b" just going to be the diameter of the section or is there some other reference I am missing for "b" of a circular cross section.

Also, Q is defined as "statical moment of an area about the neutral axis" --&gt;my school days have long since escaped me--is there a formula for this for a circular cross section?

Thanks in adance! :reading:


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## MOOK (Aug 20, 2009)

Bigwolf said:


> Just curious if anyone can help out with this.....
> I've got a 5" diameter beam with a uniform load on it. NDS Equ 3.4-1 (pg 15) says fv = VQ/(Ib)
> 
> I've got V and I = pi*d^4/64
> ...


Shear stress incircular timber beams is the same as regular timber beams

fv = 1.5 v/bd

b is the width of the beam and d is the depth of the beam

The only difference is including the curvature factor Cc for bending stress in Glulam Beams


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## jta (Aug 26, 2009)

MOOK said:


> Shear stress incircular timber beams is the same as regular timber beamsfv = 1.5 v/bd
> 
> b is the width of the beam and d is the depth of the beam
> 
> The only difference is including the curvature factor Cc for bending stress in Glulam Beams


Actually this is not correct. Shear stress is a function of the section properties; therfore the distribution of shear stress will be different. For rectangtluar sections max shear works out to 1.5V/A for circular sections its 4/3(V/A). Check a any strength of materials text to see for yourself.


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## Bigwolf (Aug 28, 2009)

jta said:


> Actually this is not correct. Shear stress is a function of the section properties; therfore the distribution of shear stress will be different. For rectangtluar sections max shear works out to 1.5V/A for circular sections its 4/3(V/A). Check a any strength of materials text to see for yourself.



Thanks for the replys--I was on the same track as you, but was having a hard time finding my stregth of materials book. :beerchug:


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## MOOK (Aug 28, 2009)

I think I missunderstood your question.

I thought you mean circular laminated wooden beam with rectangle cross section.


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