Why section modulus is important 3

[iNitin]

Hi,
I would like know why section modulus is important while designing sheet metal component like cross member used in frame of commercial vehicle,why not consider all area

Thanks in advance

Nitin
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[rb1957]

is the cross member in bending or compression ? these are more affected by I than A.

if the cross member is in tension, then A is more relevent.

Quando Omni Flunkus Moritati

 

[Jboggs]

Because bending strength is affected more by section modulus than by area.

 

[iNitin]

Thanks Jboggs
i am little bit appreciate your answer,but can you explain me how it will affect with some suitable example

Nitin
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[JohnRBaker]

Just look at something as simple as a thin-walled square tube versus a square bar. It comes down to the fact that if both of these profiles had the same cross-sectional area, the 'tube' will have to have a much 'taller' profile and thus a much larger 'section modulus'.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[desertfox]

Hi

Here is a good example, get a 12" rule and hold it flat at each end in your hands, now see how easy it bends, turn the rule through ninety degrees so that the width of the rule is vertical, then try bending that in your hands, I am sure you will see the difference even though the rule hasn't changed in area.

 

[SnTMan]

http://en.wikipedia.org/wiki/Section_modulus

Regards,

Mike

 

[Jboggs]

Also, compare the bending strength of an I-beam against the strength of a solid round or square bar of the same total area. The difference will only be seen in bending, not in straight tension loading.

The difference comes in two terms: neutral axis and outer surface. In bending, the particles on one surface are in tension while those on the opposite surface are in compression. The particles near the center of the beam see near zero stress, thus "neutral axis". The strength of the beam is related to the distance from the neutral axis to the far outer surfaces of the beam where the material particles see the maximum tension and compression.

 

[Tmoose]

Are you concerned with stress or deflection in you design?

Stress= MC/I

http://www-classes.usc.edu/architecture/structures/Arch213A/213A-lectures/15-Flexure.pdf

Deflection is strongly related to I also.

http://ruina.tam.cornell.edu/Courses/ME4730/Rand4770Vibrations/BeamFormulas.pdf

 

[msquared48]

As the area changes shape, the section modulates.

Mike McCann
MMC Engineering

 

[rb1957]

stress (due to moment) = M/Z

stress (due to force) = F/A

as noted above, two sections can have the same area both different modulus (= I/c)

take a rectangle, A = bh, I = bh^3/12, Z = bh^2/6
1) a square b = B, h = B, Z = B^3/6
2) a rectangle, b = B/2, h = 2B (same area as 1), Z = 2B^3/6

Quando Omni Flunkus Moritati

 

[Tmoose]

Looks like iNitin logged in today, Feb 26, so he has likely seen all the replies to date

 

[AndyHarpist]

Its because "resistance to bending" is a function of "how much stuff" there is, and how "far out it is" from the mid-point of zero tension and compression.
And that`s best described by the modulus I.

Andy

 

[iNitin]

Thanks guys now i got it


Nitin
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