RECTANGULAR SECTION UNDER COMBINED BENDING & AXIAL LOAD

[StressMan2506]

Fellow stress engineers:

ESDU 01.06.03 provides a means for determining max. permissible bending moment in a rectangular section also subject to axial load. The moment is a shape factor x 0.2% proof stress x elastic section modulus. This method does not give me the moment capability I am seeking.

Bruhn C3 covers sections in pure bending and enables one to calculate both yield and ultimate moments. The ultimate capacity thus found is in excess of my ultimate moment, and my axial stresses are low, so the indication is that I have the capacity I need. However, I would like to be able to show the combined effect at ultimate. Does anyone know of a method (possibly a hybrid of ESDU & Bruhn) which does what I am seeking? I am tempted to take the ESDU 'K' (1.4) and put it into the Bruhn method in place of 1.5. Doing so would give me a 'good' number, but would it be meaningful?

Thanks in anticipation.

 

[rb1957]

i think you're confusing shape factor and form factor (i know i have ...).

shape factor for a rectangle is 1.5

form factor is (1+4k)/5 = 1.4

and Fbu = 1.4*Ftu

 

[StressMan2506]

Thanks, rb1957. So I don't put 1.4 into the Bruhn formulation. You have written Fbu = 1.4*Ftu whereas ESDU definitely uses 0.2% proof, i.e. Ft2; further comment would be appreciated.

 

[rb1957]

Fbu = 1.4*Ftu is equivalent to Bruhn's eqn (3) in sect C3.4

you're trying to predict stresses in the plastic region. Bruhn's method (Cozzone) uses a simple linear slope beyond yield, ESDU almost certainly is using something more complicated, presumably based on Ramberg/Osgood parameters.

perusing our company's manual i did find a curiously complicated interaction formula for plastic tension and bending ...

 

[StressMan2506]

Thanks again, rb1957.

Since my earlier posts, I have spotted that it is stated in Bruhn C3.13 that ultimate strength design under combined loading is treated in detail in a later chapter. I have not been able to find a likely section heading in the table of contents; does anyone know which chapter it is? I am presently searching the book to find it...

 

[rb1957]

i think you mean the interaction formulas in C4 (around C4.22)