Deflection of the post tensioned beam

[Aet20]

Have question regarding deflection of PTB results under the alternate loaded pattern, by logical thinking the deflection shall be the highest when the cantilever span is loaded as compared with both spans loaded and when the middle span loaded. I have modeled a few beams and the results turned out to be similar to where the cantilever span deflection is the highest under the both span loaded case instead of when it is loaded alone. Please enlighten me if there is other factor to look at.

 

[centondollar]

The loading leading to maximum deflection depends on the relative position of the right roller support. There is no general rule or factor that determines the most critical loadcase - you have to decide the place of the roller and run all the load cases.

For the beam in question, atleast ULS, SLS and stability (uplift at the left support) need to be checked. The required combinations depend on the code you are designing to; I count at least 3 for ULS (DL all spans + LL left span; DL all spans + LL right span; DL all spans + LL all spans), 3 for SLS (same as for ULS, but with different factors), and 1 for stability (1.5*LL right span + DL*(facorable_factor*leftspan + unfavorable_factor*rightspan)).

 

[KootK]

For gravity direction load, I would agree that the relative, downwards deflection at the tip of the cantilever should be at a maximum for the case of live load being present only on the cantilever. Whether or not that's the worst case deflection that you'll see anywhere on the beam is, as centondollar mentioned, a function of the back span. You might even get negative deflection at the cantilever tip.

 

[jayrod12]

The negative tip deflection is a real thing. It's especially true for wood structures I have found. Everyone is trying to span wood further and then they add a short(ish) cantilever to the end. The governing deflection case for the cantilever is often upwards deflection under backspan loading only.

 

[dik]

with balconies, it helps with drainage.

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-Dik