engineers, i need your help with mechanics of materials

[xl2ockl3ottomx]

whats up, i know there are some engineering students and engineers on here and i need some help. these two problems have been driving me crazy and i am just flat out stuck.

first one:
the question is: two cyclindrical rods, one of steel and the other brass, are joined at C and restrained by rigid supports at A and E. For the loading shown and knowing the E-steel=200 GPa and E-brass= 105GPa, determine the reactions at A and E and the deflection of C.

the answers from the back of the book are:
Ra= 63.8kN left, Re=37.2kN left
C- 46.3μm right

second question:
the question is: a 4ft concrete post is reinforced by four steel bars, each 3/4 in in diameter. knowing E-steel=29x10^6psi, α-steel = 6.5x10^-6 / degree F and E-concrete= 3.6x10^6 psi, α-concrete= 5.5x10^-6/ degree F, determine the normal stresses induced in the steel and concrete by a termperature rise of 80 degrees F.

the concrete post is 8 x 8 inch and 4 ft tall. the steel bars are 3/4" in diameter and run the height of the post.

the answers from the back of the book are:
steel: -1.888x10^3 psi
concrete: 53.6 psi



thanks guys.
marc d

 

[thomas91169]

bump cause goddamn, engineering is crazy shit!

 

[Coiled03]

Dude - I can do it, but not here at work. It's really not that hard. Finding the reactions is more of a statics problem. Then, once you know the loading, the deflection is your MOM problem based on the material properties.

 

[20stang02]

Ill try and help ya out when I get home...not sure if I have any of my stuff from that class

 

[speedofsound]

the deflection is your MOM

I am an engineer and know what that is, but I snickered just a little. :beer:

 

[olgreydog7]

The first one will need some buckling equations I think. I can't remember the qualities or equations of the top of my head, but it may be of sufficient diameter that it doesn't matter anyway. It may be a straight compression problem. From looking at the answers, it doens't look like the moment matters, since they don't give the weight of the material, or really enough info to use the density to determine weight. So, you should just need the E from the steel and brass in order to ind how much of the energy is absorbing in each one. This will also send you down the road to finding the deflection of C. Remember equal and opposite reactions, then solve based on deflection.

On the second one, what you need to do is find out how much the materials are growing based on temp. Assume that the stress is zero at the begining. Again, equal and opposite reactions, so the concrete is pushing on the steel and the steel is pushing on the concrete. The difference in stress is due to difference in area. As long as your sign convention is correct, you may get a + rather than a -, but if they are opposite for steel and concrete you are ok. Now, the steel will dflect more for temp than the concrete, so take that length, and the area of teh steel and figure out the force that the steel is pushing and you'll get the stress. Do the same thing for the concrete, but you'll find it's pushing against the steel. I think the steel will come out of teh concrete too, so that shoudl relax some of the pressure on the steel.

Fun problem man, good luck.