Sect.2 q.2 - cantilever I-section

[studentlife]

hello all

followin my quest to teach myself all about beams etc iv come across this it seems straight forward well, first bit does anyway :)

is it basicaly looking at the cross section of the I-section and stating the distance from the base to the middle??

forthe second part do you jus write the formula for the moment where the I-section fixes to the wall using the first "F"

and the third has baffled me, i have no idea where to use the compression data to find the force "F"

 

[FredGarvin]

is it basicaly looking at the cross section of the I-section and stating the distance from the base to the middle??

I am assuming you mean part A here. The centroid is going to be the center of mass of the cross section of the beam. If the beam is completely symmetric then what you said is correct. However, in your case, it is not symmetric about the horizontal axis. This means that the centroid is going to move towards the side with more mass. So the centroid will be displaced slightly lower than the center of the section. Do you know how to do the calculations?

forthe second part do you jus write the formula for the moment where the I-section fixes to the wall using the first "F"

You need to read up on your statics to find out what the 2nd moment of the area is. It is a numerical description of the distribution of mass about a reference axis and helps determine how resistant to bending a beam will be.

http://en.wikipedia.org/wiki/Second_moment_of_area
http://www2.umist.ac.uk/construction/intranet/teaching/ul222/exp/2maex.htm

and the third has baffled me, i have no idea where to use the compression data to find the force "F"

The top of the beam is going to be in compression and the bottom of the beam will be in tension. Even though he shouldn't have posted answers like that, take a look at Lax's post. Make sure you understand how the summation of forces comes out and how the distance to the centroid comes into play.