Abdul Malik Memon

WA,
1. Basic information regarding bucking analysis of structural steel elements is generally available in the books dealing with the stability of steel structures. For example, "Guide to Stability Design Criteria for Metal Structures', 6th edition by Ziemian, 2010.
2. Specifically, in relation to Eurocodes, following links may be helpful in providing the desired information:-
a. Design of Steel Structures: Eurocode 3
b. Designers’ Guide to Eurocode 3: Design of Steel Buildings
c. Buckling resistance of steel & composite bridge structures-SCI 2012
d. Guide on second order analysis-2009
HTH
Regards.

Salam,
I'm looking for books with steel structural behavior (especially buckling). If anyone could suggest me some of books according to Euro-code.
Many Thanks,
A.Malik

W/Salaam,
Free vibrations are vibrations of the structure (or any other thing) that result once the structure is excited and the vibrations are let to die down without additional interference. For example if you have a pendulum at rest, you pick the bob in your hand, move it on one side and let it go. Once you let go the bob, the pendulum vibrates around the initial rest point. The period of vibration is dependent on the length of pendulum (technically the parameter that defines its stiffness) and is independent of the mass of the bob. If you measure the time the pendulum would take even as it slows down to complete a cycle,  it is exactly the same. Amazing isn't it. This is an example of free vibrations.
Now, forced vibrations are different. In forced vibration, you have a situation where the load producing or causing vibration of structure (or any other thing)  is "continuously being applied" at a frequency. An example would be a reciporcating or centrifugal machine sitting on say concrete foundation. Operation of machine would put the foundation in vibration (machine load itself has a frequency which may or may not match with foundation natural frequency) and would force the foundation response under its frequency. The foundation will as a result vibrate based on its stiffness (like pendulum). Another example  considering the case of pendulum (so that you can relate) for forced vibrations would be if  you put a big fan infront of the pendulum and it vibrates under the wind of fan.
Summary: In free vibrations, you disturb a structure and let it vibrate without disturbing it further. In forced vibrations, the structure is continuously being disturbed by a force applied at regular interval. That is why they are called forced vibrations as we are forcing structural response under external cyclic load. Enjoy the attached forced vibrations video as well that shows breaking of a wine glass under sound. Sound waves are what are forcing the glass to vibrate and the frequency of the sound waves has been set so that its close to natural frequency of the glass causing it to resonate and break.

ACI Code has not specified any stain-range for under reinforced beams. They just say that the beam should fail in tension before failure of compression region. By that definition, the tensile strain in steel must have a value of more than 0.002 before the compression strain in concrete reaches 0.003 for under-reinforced section.

Salam,
What is the difference between the under reinforced and tension controlled section? Are they same with the exception of strain values of steel.?
Many Thanks 

WSalaam..
ASD is design based on actual non-factored loads considering reduced material strength capacties. For example, for 100 kN load on a steel beam with yield of 350 MPa, you will design it using 100 Kn load and a strength of 2/3*350 MPa= 232 MPa. The reduction in material strength would provide safety, calculated as ratio of actual strength/ (reduction factor * actual strength). This ratio is called Factor of Safety (FOS). FOS would vary for foundations, steel or concrete design etc.
ULS is based on factored loads (load factor as per the statistical probability of likely hood of occurance causing failure) with reduced material capacites. The reduction in material capacites is lower than what ASD has. Like for steel design, material reduction factor is 0.9 only which is a mere 10% reduction. ULS is improved and currently the way to go in most building codes worldwide. Some building codes even prohibit using ASD.
The following link contains an excellent reply by @EngrUzair about cost Compairson b/w ASD and LRFD. 
WSD is same as ASD.
Secant modulus represents elastic behavior between stress and strain values (as it is a straight line) whereas tangent modulus gives you relationship in any portion of stress strain curve (elastic, plastic etc). 
Thanks.

Doing masters back to back with bachelors is not a bad idea if you are doing research. If you are more like a design person, then you should better get a job, but if you want to stay academic, go for masters.

Asala,-o-alaikum,
I need an advice regarding postgraduate studies. By grace of Almighty I received an admission letter from Technical University Dresden, Germany in ACCESS  (Advanced Computational and Civil Engineering Structural Studies) program. 
And Inshallah will have admission letter from Swinburne university Australia as well.
As I graduated in 2015 and have not landed a proper structure job yet, so i need a suggestion that should i go with masters or gain some experience prior to it?
Thanks.
 

Pickup columns or planted columns are structural cols supported on transfer slabs or beams (which you are referring as pickup beams i think).
For seismic design you have to overdesign collector elements like transfer beams or slabs by omega factor.
Issues with pickup cols is loads in them depend on stiffness of slabs/beams supporting them. If supports are flexible they are literally just hanging threads from slabs above carrying little or no load at all, you cannot just manually transfer loads to them by distributary area. Another thing is to check deflection at upper and below slabs as well as punching on both slabs.
Make sure the transfer slab has sufficient shear strength.
Another issue might be consteuction sequence and long term deflections due to these columns.
Also make sure you check for irregularity clauses of seismic code.

Nothing specific. There is nothing wrong with having this many columns. These columns are more like confined masonry columns rather than structural columns.
Sanity checks. Your results should make sense and load path should be complete.
Pick-up beams?
 

First you must be clear about requirements (arch, mep etc). Have a clear understanding of dwgs, are the cols masonary, non load bearing? Have you prepared 2, 3 different struc schemes? If you think you can value engineer and reduce cols, check with client req if they too want it.
Then you make sure the scheme is working, deflections, load paths, foundations, framing, cost.
Reducing cols mean, bigger beam and slab spans and so more deflections. Have you checked that? Also more local load on foundations if cols are far, and more sway in wind/eq bcz less bending stiffness of vertical elements (if you are not increasing col/wall sizes to compensate that).
If above is okay, you can eliminate cols that are not required. As a struc engr you have to be proactive, pitchin and promote the "most efficient" scheme that is "simple" and "necessary".

In addition to above, in my previous practice we used to provide the same reinforcement for top and bottom. Whatever is the max of top and bottom, put that at top & bottom.