designing columns for concrete buildings

huangyhg

designing columns for concrete buildings
i'm designing a 15 story post tensioned building and i'm trying to design the columns in risa floor. i'm just wondering what the standard practice is for considering moments from the slab in the design of the column.
1) i want to design the columns as axial compression members only but i know the slab will put moment into the columns. is this a standard assumption that i am required to make and design for?
2) when i model the floor i connect all the columns with fixed-fixed beams so that the columns will suck in moment in the finite element analysis.
- will this put too much moment into the columns? i have a 3 span condition 13'-27'-13' so theres a lot of unbalanced moment from the dead load alone.
4) if what i'm doing is overkill for the moment, do you model the slab moment connection as spring, and how?
i have slab cantilevers too. does anyone have the magic bullet for designing these quickly. thanks!

stazz,
1) yes, the standard assumption is that you have to design the columns for all load effects on them. this includes vertical load effects and horizontal load effects, which should be significant in 1 15 storey concrete frame building, resulting in axial forces and moments on each column. you cannot design them as compression only!
2) i am not sure what fixed-fixed beams are. i hope you are only modelling the fixity that actually exists and can be generated by the frame!
you cannot deliberately over stiffen the connection to make the slab design work better. you should be analysing the frame as it will act in practice. some designers reduce column stiffness for the frame design (in asia they quite often ignore the columns completely in the slab/beam analysis) but that is dangerous as the column will attract moment and failures such as punching shear must be checked based on the real frame stiffnesses as redistribution is not possible. columns in this case still need to be designed for the loads and moments that they attract.
why do cantilevers make it difficult. i do not understand the need to design these quickly. you need to design them properly!
i hope you are doing this design under the supervision of an experienced concrete building designer!
what i think rapt is trying to say is "the consequences of overestimating or underestimating the actual stiffness's of structural   
don't forget to design the columns for lateral loads, even if the lateral loads are taken out by shear walls, you will still need to check the moments that are put into the columns from lateral displacement.
1) always consider the moment that is being transferred into the column from the slab. also because the moment capacity of the column is dependent on how much compressive stress the column is subject to, look at a maximum and minimum compressive load envelope. the greatest moment in the column may occur for a patterned or checkered live load case which will reduce the axial stress and may have a reduced capacity depending on where the load points lie on the interaction diagram.
2) are you referring to fixed-ended columns at the floor levels above and below the floor you are designing? or are you referring to fixed-end moments used in frame design by moment distribution/redistribution?
3) to be on the conservative side, when designing columns design them with full stiffness. i.e. icol=igross. some codes allow for the analysis to be conducted using 0.8*ic. this allows for some stress to be redistributed because of the loss in stiffness that occurs when a section cracks.
cantilevers can be designed quite easily, they are determinate structures, the moment cannot be redistributed. a spreadsheet can be set-up quite quickly to  design overhangs.
does risa analyse equivalent frames or meshes the slab and designs by fe?
when designing a concrete structure, in addition to gravity and lateral loads, you need to design for creep, shrinkage, temperature and, in your case, post-tensioning forces. the forces caused by restraining these strains is significant, particularly for the columns.
let me guess, the old we just need foundation plans to get started catch 22.
there is no easy button for multi-story concrete columns but i've found etabs does a nice job.  although i don't always agree with how it determines sway vs non-sway.  still a lengthy, input and multiple runs process to economize and back check.  
rowingengineer,
in a 15 storey building, under vertical loads, the columns at the lower floors will not crack as the axial compression effect will be far greater than the moment effect and the column will be in compression. why are you using .7ig if the columns cannot crack. you can only allow for reduced stiffness if there is going to be cracking.
the article you are talking about in aci is discussing stiffness for sway cases as i understand it. you cannot use this logic for vertical effects.
so in other words, i should get another career?
crapt,
i want to design them quickly because this is just a preliminary design for estimating purposes for a design-build project. i need to get column sizes to the architect so they can lay out their guest rooms. in risa the beams are fully fixed to the columns. i modeled the beams as the depth of the slab x the tributary width wide, then the program will calculate the correct ig of the beams which will produce the correct stiffness in the equivalent frame analysis. the building has a high aspect ratio so i figured that the critical direction to analyze the columns was in the short direction where the column spacing's are not the same, so i ran the floor slabs as one way so that the load went directly to the beams first and then to the columns. this is how the equivalent frames in the short direction are analyzed anyway so i figure that i can pull the end moments from the beams out of risa and use these for the post tension slab ultimate strength design.
rowengineer,
i'm using the full ig for the column stiffness. i think this is conservative for the column design because this pulls the maximum moment out of slab and into the column. but for the slab design i would think that you do the opposite -> use the weakest column (0.7ig) or maybe even no column stiffness so that all the moments stay in the slab so that there is adequate steel in the slab.
asixth,
i'm referring to fix ended beams. there is no fe plate in the risa slab, i'm hoping that these beams i'm modeling serve as column strips to collect the load and have a stiffness for the equivalent frame analysis. i have a spreadsheet now that i'm using for the pt slab design (since there are so many stress checks) and i'm pulling the end moments from these beam strips out of risa to design for. for the lateral loads i might just model an entire 15 story frame and push the frame 3" to the right (h/500) and see what moments develop. what load case would i use for this? 1.2+1.6(moments developed from displacement)?
spats,
to design for creep, you multiply the deflections by the factor in aci right? to design for temperature, we limit the length of the building? to design for shrinkage, we limit the length of the pours and use pour strips? but to account for the pt forces in the columns, aren't these good for column? don't they relieve the dead load moments that the equivalent frame calculates? because isn't the pt force effectively a negative load that counteracts the dead load?
ctseng,
if i prove that these gravity columns are non- sway, then do i have to analyze them for lateral loads?

quote:
if i prove that these gravity columns are non- sway, then do i have to analyze them for lateral loads?
yes, you still must include all applicable load combinations in the design of the columns.  the sway vs. non-sway criteria affects how you magnify the column moments.
now in a non-sway frame, moments from the lateral forces mightfff"> be low, but that depends upon the way the structure is braced, how combinations of alternating live loads and lateral loads interact, etc.  but your lateral forces will possibly produce some moments in the columns and they would simply be included in the m2 values of the non-sway derivation of the δns values in aci section 10.12.3.

stazz,
you keep referring to fixed ended beams. that is mixing us up. a fixed ended beam has full fixity, which you do not have. i assume your beams have a full moment connection to the column and the frame analysis sorts out where the moments go. if they were fixed ended, the moment would be wl^2/12 at each end and wl^2/24 at mid span and would be very unconservative.
agree with jae that you need to allow for sway moments no matter what the framing system. in a braced building there is still sway and some sway moments and axial forces go into the columns.
you have to look at all of the possible combinations of wind, earthquake, and vertical load effects that the your loading/design code requires.
for the sway case, it is usual to use reduced column and beam stiffnesses as the   
stazz,
creep, shrinkage, temerature & post-tensioning forces all represent volume changes in the structure. volume changes can significantly affect the stresses in your concrete frames... more so with taller buildings. temperature is more important in exposed structures such as a cip parking garage than it is in your instance. temerature differences between the upper and lower surface of a beam/slab are very important when analyzing a garage, as are changes in