Temporary Top Strands in Prestressed I-Girders

[CalebA]

Do you all consider temporary top strands in your design? I found that they effect camber and prestress losses, but would this effect be small enough to ignore? Also if I have a matt of WWR in the top flange, do I need to include this in my design for tension forces or would I leave it out? I am using LEAP Concrete btw.

 

[BridgeSmith]

Maybe I'm misunderstanding your question, but you would typically only add the sacrificial strands if they are needed to control camber or tension stress in the top of the beam at release. If you can ignore them in the design and meet all the stress and camber limits, there's no reason to include them in the beam.

 

[rapt]

In what way are they "temporary"?

Are they somehow removed after a certain time?

 

[CalebA]

Well they are called support strands, so I suppose they would remain in the girder? I included a link to the NU Girder standard I'm referring to. Would I then need to include the WWR reinforcing in the top flnage as well as the support strands? Since I'm using LEAP and you can only define one type of strand I found the following recommendation from another thread:

"To model the top strands you have to use the 0.6" 270K LL and back-calculate the Pull % that is specified on the Beam stab under Strand Pattern->Debond/Pull%. Keep in mind that this pull % may be overwritten if you change anything about your strand pattern, but working with CONSPAN you should be used to having your patterns wiped out every time you change anything, shouldn't you? ".

https://www.modot.org/sites/default/files/documents/PSI_06_NU_WWR_1.pdf

 

[BridgeSmith]

In what way are they "temporary"?

Are they somehow removed after a certain time?

They are typically debonded through the center portion of the span, and cut at midspan after the girder is in place. They are usually referred to as "sacrificial strands".

 

[CalebA]

How do they cut them at midspan if they're encased in concrete?

 

[BridgeSmith]

How do they cut them at midspan if they're encased in concrete?

They leave pockets when they're cast, to be grouted later.

Were the temporary strands you referred to something different?

 

[CalebA]

Both of the DOT's in this area (same fabricator) call them support strands. I dug up some shop drawings and did not see any details for the pockets you mentioned. I believe these "support strands" are permanent.

 

[BridgeSmith]

I've never heard the term "support strands", but if they're permanent strands, whether straight or harped/draped, in the top or the bottom of the beam, then they're just strands to us. I thought you were talking about sacrificial strands in the top of the beam that reduce the tension in top top of the beam from the release of the strands (from the anchor blocks, once the concrete has cured sufficiently) until the beams are put in place, spanning between supports, and their selfweight reduces or reverses the tension in the top of the beam enough that they are not required anymore. They only do sacrificial strands when the compression in the top of the beam under full load would be too much with the strands adding to the compression.

Anyway, back to the the "temporary" top strands. If they aren't what I'm calling sacrificial strands, what are they?

As far as the WWR, if it's continuous (properly lapped) over the length of the top flange, it can be considered in the design to mitigate the tension in the top of the beam. Otherwise, it cannot be considered in the design. I suppose if the design (concrete stresses, etc.) works without including it in the model, you can ignore it.

 

[OSUCivlEng]

Well I learned something today. My prestressed girder designs use strands in the top of the beam, but they aren't temporary. I don't know why you would go to the trouble of cutting them later. Seems like if you have to go to that much trouble to make a design work then a different type of girder should be used.

 

[BridgeSmith]

I don't know why you would go to the trouble of cutting them later.

As I said, it allows the designer to reduce the tension in the top at release and reduce camber, without exceeding the maximum compression in the top under under full load. It's not done often, but sometimes on large bridge girders, it can allow the designer to use a smaller and/or shallower girder. It's really not much extra work. It just takes some debonding sleeve (which they typically use at the ends of some of the strands anyway), a chunk of foam, a pair of bolt cutters, and some grout (which they're typically using to grout keyways, etc. already).

 

[rapt]

If they are left in place, they will affect the calculations and should be included for both serviceability and ultimate strength.

 

[CalebA]

Okay thank you everyone for clearing that up, now I'm wondering if I'm back-calculating the pull percentage correctly. Say the default pull percentage for 0.6" 270K LL strands (43.94 kips max allowable force per strand) is 75% and the 3/8" support strands are tensioned to 8 kips. Would the pull percentage for the support strands be 75%*(8 kips/43.94 kips) = 13.7%?

 

[BridgeSmith]

Still not sure what the function of the support strands is. It looks as though they may be included to support the mild reinforcement, including the WWR.

I think the pull percentage would be whatever is required to get to the 8 kips (or the 2.02 kips for the outer strands), if you're following the MODOT approach.

 

[CalebA]

I believe they serve the same function as temporary/sacrificial strands, except they are left in place so it will negatively effect in-service bottom flange compression. My guess is that the fabricators around here just haven't adopted this practice for one reason or another. I found a good article that touches on both temporary and permanent strands in the top flange.

https://www.aspirebridge.com/magazine/2022Spring/CBT-TemporaryTopStrands.pdf

 

[BridgeSmith]

I believe they serve the same function as temporary/sacrificial strands, except they are left in place so it will negatively effect in-service bottom flange compression.

You mean it will negatively affect in-service top flange, right?

Apparently, what we call sacrificial strands are also called temporary.

 

[Lomarandil]

As an erection engineer, these are also useful for transport and lifting girders into place. Lots of girders we evaluate nominally have excess tension at the top flange (as mentioned) during those conditions, and somebody ends up owning the risk of the girders cracking before diaphragms and deck go in.

(Although it's only rarely a real world problem)

 

[triops1996]

You can ignore the top strands practically since 32 kips is likely far less than what you're pumping into the bottom of the beam ~1000 kips.

You can also account for it if you wish to be more accurate. Just make sure in your model the collective centroid of prestressing and prestressing force are equivalent to what it should be by hand calculations.

 

[bugbus]

In the past we have added temporary strands in ungrouted ducts at the top of girders for transportation/lifting/handling, which are then either completely removed once the girder is in place, or alternatively just de-stressed, left in place and grouted. This was done to avoid potentially excessive tension / cracking at the top flange due to dynamic effects.