Transfer Beam - Column Moments

[slickdeals]

Folks,
When you design post-tensioned concrete transfer beams, do you account for the reduction in column moment due to the equivalent upward loading from the tendons? Typically, the practice is to stress the tendons at the centroid of the beam or T-section, but sometimes it may be advantageous to not stress at the neutral axis and induce an eccentricity to help the column moments.

What is the common practice when you design such transfer beams?

 

[rowingengineer]

slickdeals,
Talking from an Australian point of view we use bonded partial post-tension, and normally the PT is not in the centroid of the member. We vary the PT above and below the centroid to take advantage of the P.E effects. However I don’t see this as affecting the column moments.


An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[paddingtongreen]

Typically, the beam would be post tensioned before any load was added above or else it would have to be shored into position, leading to a very complicated situation on how much weight to place on it before tensioning the strands and removing the shores. The beam will hog when the cables are tightened and released to the beam if the cables are below the centroid. Generally the cables would be below the centroid for gravity loading, so as to compress the fibers that will be in tension after the load is applied.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

The position of the anchorage in transfer beams is chosen based on the transfer requirements, not on the columns. Transfer beams are stressed sequentially as the load comes onto them. There may be some effect on columns, but this is not a primary consideration.

 

[rapt]

slickdeals,

Even if the anchorage is at the centroid, there will still be a prestress moment in the columns. For ultimate strength design, this prestress moment (which is equal to the secondary or Parasitic prestress moment) should be included in column design. The method of its inclusion will depend on the design code. Some codes will use a different load factor, possible .8 if the PT moment is reverse sigh to the applied load moments (similar to the logic if LL is of opposite sigh to the DL).

I have never seen anyone actually adjust the tendon height to modify the column moment deliberately. Nornammy the tendon profile is determined to provide the best profile for the beam design and the locations of the applied loads. I do not believe in playing games with transfer beams or their supports!

 

[ron9876]

Also don't believe in messing around with transfer beams. You haven't lived until you have seen a cracked transfer beam.

When you run the pt beam if you model the columns in the run you will find out the reactions. Normally the moment of inertia of the column is much less than the beam so it doesn't attract much moment.

 

[slickdeals]

I guess I did not ask the question correctly and maybe you misunderstood that I was trying to fool nature.

My question was pertaining to the dead load moment that gets alleviated by post-tensioning. In your analysis models, a transfer beam probably gets modeled as a reinforced concrete beam of given cross section. But in reality the moments are less because some of the dead load is balanced by the PT. PTI has equivalent load diagrams in the Appendix for PT.

How many of you actually take the time and effort to model an upward load in the beam to account for the post-tensioning effects? SAP2000 has a built-in tendon module, but not many other programs.

 

[csd72]

slickdeals,

It is definately a terminology thing, column moment usually refers to the moment in the column not in the beam. I am sure you meant 'moment from the column reaction'.

Of all the PT beams you ever design, transfer beams are the ones that you should consider the load cases in the most detail. You should always consider all load cases, I say consider not design as experience will allow you to dictate certain load cases as not critical. If you do not have the experience to make this call then you should run numbers on all the cases until you do have sufficient experience.

 

[BAretired]

slickdeals,

I have not done post tensioning in a long time, so I may be a bit rusty on the theory. Forces exerted by post tensioned anchorages and the uplift of harped cables can and should be considered as external loads.

BA

 

[rapt]

Slickdeals,

Understood you perfectly the first time.

I always model the prestress forces properly, upward line loads, downward line loads, force couples from section changes, concentrated moments for anchorage eccentricity.

Why you would use SAP for this I do not know.

But I still would not play with the anchorage height from the centroid to try to minimise column moments from the prestress as you have suggested.

I also would not rely on the vertical load reactions for the transfer beam from a SAP 3D model as this model does not take into account construction sequence and its effect on load transfers through the structure. A SAP (or other) model assumes the whole building is constructed while propped and then all of the props for the structure are released. This is not what happens in practice. The full 3D model allows for a lot of support settlement effects on reactions, dispersing them through the frame, that are not real because of this. I would look also at reactions based on contributory areas.

The only loading from prestress that goes into your columns is the Secondary or Parasitic Prestress moment. There will also be a small change in your column reactions from secondary/Parasitic effects, either increasing or decreasing them.

 

[coolidiot]

Slickdeals,

I would Lower the Anchorages below the CG if i have to adjust the Column moments to a minor value. But I would suggest you to reduce the Column Fixidity or design the Beam as Pinned.

Any Comments?

 

[rapt]

coolidiot,

What are you going to do with the moments that are attracted by the columns? They do not disappear just because you make that analysis assumption!