Enquiry: Bending not along principle axis?

[Su Solberg]

Hi, everyone.

I was told that when a beam is short, L<=5H, the equation of "Stres=Moment*/2nd moment of initeria" is not valid (how).
(where H = deapth of beam ; L is the location of F acting on that beam)

Is that ture and is that relate to principle axis?

Also if it is ture, why people use the following kind of calculation for Bolt Connection in steel design.


Thanks alots of your advise in advance.

 

[Su Solberg]

Sorry, attachment missing.
The following is the method of calculation in Steel Design.

Assuming principle axis is unknown. find the principle axis by try and error in the equation of Sum of Torque by Tension +bearing = moment encounter in the connection group.