prestressed column

huangyhg

prestressed column
normally the specificaitons for the design of concrete sections are stated such way that you get tensile side failures. p-delta in-member buckling once considered should be showing the same tensile side failures hence from the way the code imposes to treat the sections.
the axial loads of so slender colums are likely to be small, there being the moments, or flexure, what is going to dominate te behaviour at failure. hence a moderate increase in compression is unlikely to force the compression side fragile failure, and contrarily will retard the tensile side failure.
other way said, with some prestress, your section is to stay more integral and with more stiffnes along a bigger range of solicitations, what opposes to buckling (inertia factor in the euler equation).
however, if your prestress is excessive, buckling you won't help, because if geometrically central straightens the member, but you may get end squashing, hence fragile compression side failure; yet it is unlikely you go near such prestress.
so in my view and prior to some verification by calculation it is likely that a moderate level of prestress on a given member reinforced with some passive rebar will allow it take a somewhat higher load.
while agreeing with ishvaaag's reasoning, i can add that this might be an interesting topic for research and testing. but before such theories are tested and proven, we engineers cant take them to the field as the codes dont allow.
well, it is clear that the reasoning was in answer to the shown interest. obviously one must check the column as is (prestressed or not) in accord with the relevant applicable parts of the compulsory code.
oap,
prestressing would help a free-standing column subject to significant moments.  i doubt it would help much, if any, in a regular column.
the prestressing does not change the properties of the column section, thus, the buckling load would be the same than a regular concrete column of the same dimensions.
prestressing does not increase the capacity of the column to compressive loads.
prestressing will increase creeping in the column, and might introduce some additional stresses in the beams supported by that column.
the column you described seems to be very close to the upper limit for slenderness given by aci 318-89, para. 10.11.4.1.   maybe increasing slightly the column size it would comply with the code.
good luck!
aef
oap (visitor)18 apr 02 3:48
thanks all,
    i have further question as in case of slender column as i mention above, if i do the non-linear structural analysis and the result shown me that the magnitude of the bending moment don't have any different from linear analysis, let say 3% diff. can i use the bending moment from the non-linear analysis to design the column section without using the moment magnifier?  if i can, what are things which i will concern?
thanks again
oap
you can directly use the moment so obtained if the material and geometrical nonlinearities have been contemplated.
the material one it seems from your description you have, then the question is if you have the moment values from a p-delta analysis at the factored level consistent with the final (nonlinear) sectional properties in use.
visitor!!! (visitor)19 apr 02 0:52
you ask if you prestress the column would it help with the slenderness problem?
now if you prestress it,you are applying the prestress at every point on the column.so what is the effective length?  
->"0"("zero")
right?
hence slenderness will not at all come in the way as the effective length is reduced to "0".i suppose prestressing is a most ideal means of solving the slenderness problem unless proved uneconomical
visitor, i think moderate prestress may help moderately solicited moderately slender columns columns in that moment of inertia is bigger along the range of solicitations. however prestress itself is not the only solicitation, and when in combination with other solicitations -say a compression load applied at ends- buckling appears.
hence prestress does not solve the slenderness problem, is just something more to consider in the problem.
furthermore, in a extremely slender column or wall, the perfect centering of the load becomes critical, since any eccentricity will severy affect the initial stress status...this then may add to the buckling induced stresses to cause the failure before than if there was not prestress. in this case extremely tight tolerances would be required, and even then the lack of homogeneity of concrete could cause similar behaviour to that described. hence my clause in 1st paragraph "for moderately slender columns".
visitor!!! (visitor)19 apr 02 14:04
ishvaag,
what i am trying to point out(or thinking) is not the intensity of prestress or the quantum of slenderness.i am trying to say since the prestress is going to act on every point on the column , this almost nullifies the effective length of the column(effective length be reduced to "zero"),then how will slenderness come into effect?
the phenomena of buckling would mean more or less failure upon frank departure of initial shape. if you prestress a slender column well centered in the ends and with significant eccentricity at the center of the column, prestress itself upon growth of the prestress force to the required amount will make te column snap, buckling. by the way it is so that in prestressed beams gravity forces are counteracted.
now, for your perfecty centered prestress, prestress becomes uniform, hence prestress itself won't be making the member more prone to buckling that was prior to prestress.
yet the purpose of any structural member is to sustain loads, for columns mainly compressive loads. these will need be checked according to the science of construction to ascertain if they are safe, if contemplated presently according to what specified in the mandatory code. these checks one way or other need to address the modes of failure, as one colleague put here, squashing or buckling, failure in any case.
now imagine a hollow hss section prestressed perfectly centered. you can readily see that if you compress such column from both ends such member, or the same made part of some structure, it has the ability to buckle and in fact will if the distance between restraint points does not ensure strength enough for the force applied, or if you want, if one squashing or local mode of failure does not prevails.
respect your statement, what you can attest is not that the buckling length is zero, but that there's no trend in the perfectly central prestress to buckle out of perfect equality between the section's stresses.... something by the way also understood for any buckling, to later make the assumption of that in real structures some initial imperfection there is.
the buckling length needs yet be determined for the member in appropriate way (as long as we proceed in our customary memeber by   
you need to be careful about the concept of buckling under prestress application versus externally applied loads.
under the action of the prestressing force application, with internal tendons that are in close contact with the element being prestressed it is not possible to buckle the member whilst prestressing it. the reason is that, as long as the tendons and concrete are in close contact, they will "deflect" together, and any lateral movement of the concrete will be followed by a corresponding movement of the tendon, but since the tendon is in a field of applied tension this will counteract the tendency for the memebr to buckle. again, this is under the application of the prestress force and assuming no external loadings. if there is a significant space between the internal tendon and the concrete, such that lateral movement can occur before engaging the tendon, then buckling effects can arise.
as an analogy, if you take say an "s" shaped (or any generally curved, in single, reverse or mulitple curvature) concrete element under the effects on concentric internal prestress, during the prestressing force application the curved shapes will not tendon to straighten nor buckle. if the prestress was eccentric there would be a deflection of the shape (due to the pxe), but again, no tendency to buckle.
if the member is externally prestressed, then the above is not true.
to demostrate this concept to students, i have taken small rigid styrene foam blocks (say 1" in size and angled on opposite ends) with a concentric hole. assemble say 10 blocks to form a curved shape (c, s, other) and thread an elastic band in the center. use a match stick to grip one end (dead-fixed end) then pull on the other end (live end). the "prestress" provides uniform p/a and there is no tendency to straighten nor buckle because the internal tendon (under tension) will balance any column action effects. you can keep appplying tension to the elastic band and the block memeber will not produce and global opening of the joints etc and no buckling.
but, under the application of an external load to an already  prestressed element the member can buckle, as per other axially loaded structural   
ingenuity, it is only a matter of labeling and eccentricity and force to buckle or distroy a beam postensioned from the ends, precisely out ouf the straightening attempt imparted to the tendons, it is due to this that there are limits to the tensile stresses in the prestressing stage there. i came to name such example in order to indicate that prestress itself if eccentrical can itself be a contributor to buckling upon the imparted bending stresses, and that the likelihood of the eccentricity being significant to it increases with the   
ishvaaag,
i think the tensile limits imposed at tranfer of prestress are to control cracking and not much to do with buckling.
the prestress can only be a "contributor" to buckling in so far as eccentric prestress will laterally "deflect" the   
in a 100 mmm thick 100 m long member, the likelihood of some unwanted eccentricity becomes big, and only the fact of that prestress is limited in amount prevents this eccentricity causing the straightening effect caused by the prestress (or the bending action induced if in contact) to ruin the weak