huangyhg

load balancing eo value
hi,
i am a recent civil engineering graduate and am trying to design a post-tesioned beam, for the first time, for a parking structure.
because most of the beams (17.5ft apart) will be only for one span (30 ft), i have elected to use the "load balancing" method. my slab is 8" and the beam drops 12" deeper (so i am assuming i can call it a 20" beam) and is 32" wide.
my question is: when calculating the e value, do i include both the post tension reinforcement and the regular reinforcement (to calculate the location of the netural axis)?
also, in getting the design moments i used the moment distribution method for the two span beams. one span was 45ft and the other was 20ft. from my understanding it is not appropriate to include the columns and do a frame analysis. can anyone tell me why?
thanks
aci code 18.2.6 and the commentary seems to indicate that the section properties can be based on the total area of the concrete.
my 2nd edition of t.y. lin's "design of prestressed concrete structures" works out examples comparing use of the transformed & gross sections and notes that the results are close enuff to not bother with the transformed section properties.
thanks oldpapermaker,
just take make sure that i'm understanding you correctly, i can take the neutral axis at 1/2 the depth of the beam?
that's what i thought, because it's just balancing the dead load. also, i've never seen post-tension construction, but if the effects are immediate, couldn't you just jack the tendons till the beam was level? - i'm assuming no.
thesinghman,
i think that you should get a copy of a text book about prestressed concrete and review the approach and examples in that text.
you didn't note if the floor slab is monolithic with the beam section. unless you are designing a precast, prestressed beam the slab and beam are usually monolithic. if that is the case, then one has a t-section, not a rectangular section. the aci code makes recommendations about the effective flange width to consider in computing the section properties of the t-beam.
in my first response i didn't address your question about the columns in your analysis of the shear & bending moments of the structure. but, if the columns and beams are cast so that you have a rigid frame then you should do your analysis based on that configuration. the concrete & reinforcing (post tensioned and normal rebar) should then be sized for those shear & flexural requirements.
oldpapermaker,
thanks for the suggestion. actually, i read a post-tensioning text book but they were pretty vague. it was almost like one big advertisement for companies that specialize in pt.
the floor slab would be monolithic with the beam section (which i found out is why when doing to moment distribution that you assume in to be pins/rollers more than fixed - cause the steel is the only thing holding it together). however, with the beam not dropping a significant amount, i don't think it's appropriate to use any effective flange width. i ran some quick calculations and it didn't look like it behaved like a t-section.
thanks also for the reply regarding the moment distribution; a lot is gained if the columns and slab can be poured together. but given the geometry and location of the site, i don't think it would be practical to attempt it.
thanks again for such a quick reply - this site is great!
thesinghman,
a good text book on pt design will tell you how to do it all. not a lot of the stuff from the pti.
if the slab and beam are monolithic, then you use a t section. i am not sure what calculation you are doing to disprove this. the deflection of the member will not tell you whether or not to include the t section.
if the slab/beam and columns are poured and connected with a moment connection before the forms are stripped and the loading is applied then you include the columns in the analysis. the slab and columns do not need to be poured at the same time. in fact this practice is frowned upon in many areas. there just needs to be a moment connection.
there is no real need to balance a speciific portion of the load. load balancing can be used to calculate a number of tendons and tendon profile. this is then used to calculate the prestress actions and resulting bending moments and shears and the secondary prestress moments. from this the ultimate strength and service stresses and deflections can be checked. the design will then tell you if the tendons are adequate by themselves or extra reinforcement is required. the deflections will tell you if you have enough prestressing (as long a cracking and long term effects are allowed for in the calculations properly).

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