Structural Mechanics Bending Stresses

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JimCrown

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I am struggling to answer this question:

A timber beam having a rectangular cross section 240mm x 80mm is simply supported over a span of 4m. If the permissible bending stress is 5N/mm^2, determine the maximum allowable uniformly distributed load that the beam may carry, applied perpendicular to the x-x axis?

What I have done is:

Ixx = bd^3/12 = 80 x 240^3/12 = 92160000 mm^4 or 9216.0.0 x 10^3 mm^4

Then, using section modulus I done:

z = I/Y = 92160000/120 = 768000 mm^3

Now do not know what to do
 
JimCrown said:
I am struggling to answer this question:

A timber beam having a rectangular cross section 240mm x 80mm is simply supported over a span of 4m. If the permissible bending stress is 5N/mm^2, determine the maximum allowable uniformly distributed load that the beam may carry, applied perpendicular to the x-x axis?

What I have done is:

Ixx = bd^3/12 = 80 x 240^3/12 = 92160000 mm^4 or 9216.0.0 x 10^3 mm^4

Then, using section modulus I done:

z = I/Y = 92160000/120 = 768000 mm^3

Now do not know what to do
Click to expand...

Assume that the intensity of the allowable distributed load is w N/m.

Then calculate the maximum bending moment generated in the beam due to that distributed load.

Then use the "flexure formula" to calculate maximum flexural stress generated in the beam (as a function of [FONT=&quot]w[/FONT])

Now equate that stress to maximum allowable (5N/mm^2) and calculate [FONT=&quot]w[/FONT].
 
Subhotosh Khan said:
Assume that the intensity of the allowable distributed load is w N/m.

Then calculate the maximum bending moment generated in the beam due to that distributed load.

Then use the "flexure formula" to calculate maximum flexural stress generated in the beam (as a function of w)

Now equate that stress to maximum allowable (5N/mm^2) and calculate w.
Click to expand...

I do not understand what you mean by, 'w'

Is the assumption I make of the intensity of the allowable distributed load the stress of timber from a stress chart?
 
JimCrown said:
I do not understand what you mean by, 'w'

Is the assumption I make of the intensity of the allowable distributed load the stress of timber from a stress chart?
Click to expand...

Please can someone help as I am absolutely stumped
 
JimCrown said:
I do not understand what you mean by, 'w'

Is the assumption I make of the intensity of the allowable distributed load the stress of timber from a stress chart?
Click to expand...
Have you taken a formal "strength of materials" class? I f you did - what textbook did you follow?
 
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