Prestressing plant girder bed design

[Prestressed Guy]

I am working on the design of a new prestressed girder casting bed and end anchorage abutments. My plan is to just design them as gravity mass abutments with heavy section wide-flange end posts in cast sockets. The slab/grade beam between the abutments would resist the sliding force from prestressing and hold down forces from harping. I am limited to about 6' depth due to ground water levels so will need to make the mass abutments wide and long. This bed will have a casting length of 200' - 250' long so I am planning to have some post tensioning in the grade beam to eliminate cracking and to help in gathering dead load for the harping hold-downs. Any thoughts and/or suggestions?

 

[Prestressed Guy]

Bump.
No prestress engineers out there?

 

[HTURKAK]

The GWL 6ft depth is discouraging.. What is the total pretension load ? what is the soil conditions?

I am thinking inverted U shaped bed with heavy nibs and slab.. The ends will be solid blok .

If the soil conditions favorable , you may consider passive thrust of the soil , both ends could be abutment buried in the ground.

 

[Prestressed Guy]

The ground is pretty good sandy gravel. The problem with the ground water is the river that flows by the site.
The prestressing is very high with a max of 44 bulb strand plus harped strand and a max height of over 8'. I do not see any real advantage to a "U" shaped bed. I need a center rail for harping hold down and I am thinking more of a rectangular section with slab extensions for the form opening tracks.

 

[HTURKAK]

The prestressing is very high with a max of 44 bulb strand plus harped strand and a max height of over 8'.

The number of strands implies the total prestressing load could be in the range of 1000 tons..My personnel opinion is to consider two options ;

i) Consider ,gravity mass abutments with shear nib at far side , with heavy section UC end posts and with diagonal strut which will be cast together with abutment , and inverted U shaped bed , ( the bed shall resist to axial load due to prestressing force )

ii) Consider ,Driven battered piles ( could be UC heavy sections) under the abutments.

I do not think that it is reasonable to rely on passive thrust of the soil, which will be mobilized after 50-60 mm soil displacement.

 

[Prestressed Guy]

For this application I would never consider passive soil for prestressing load. the amount of horizontal deflection would make stressing calcs impossible.
HTURKAK, I do not getting a mental picture of "shear nib", can you clarify? ("i) Consider ,gravity mass abutments with shear nib at far side".)
I prefer to not use any heavy driven piles due to possible disruption of nearby existing infrastructure so will be using gravity mass abutments.
One thing that I have been thinking about is using reinforced concrete for the end posts instead of heavy steel sections. The last bed I designed end-posts for used W36x393 and this one has more strand and taller sections.
The minimum clear casting bed length will be 220' so the total bed length will be 250'-300' long. The plant would prefer to cast the bed with all mild reinforcement but I am worried about shrinkage cracking and am thinking about some post tensioning to control cracking.

 

[HTURKAK]

Dear Haydenwse (Structural);

The available info. , the casting length 250'-300' long , the soil is is pretty good sandy gravel, the gravity abutments will be considered, the prestressed beam ht more than 8'. ..and you do not consider abutment with battered driven steel pile .

I prepared the following sketch with preliminary dimensions, assuming the total prestressing load around 1000 tons and with eccentricity 0.6 m.


I do not know the ambient temperature , weather cond.s and set up will be outdoor or not . A nominal post tensioning could be helpful.
The bed could be casted the with mild reinforcement , but with construction joints and with staggered casting .

 

[Prestressed Guy]

HTURKAK that is very similar to what I am thinking of except that I did not include the nib and the bed was rectangular section the same width as the end posts which in this case is about 5.5' rather than the "U" shape. But I can be swayed with a convincing argument.
What is your reasoning for the nib. I can see that it will move the CG out farther but will make the excavation more difficult and add to the detailing of the reinforcement. Given the very high harping force along the centerline of the bed, I do not see any advantage to the "U" shape. I plan to use a continuous ST section inverted with holes punched in the upturned stem at 6" o.c. for the harping anchor and That will be anchored to the bed with cast in anchor bolts.

 

[HTURKAK]

My approach was The bed and the abutments will resist to axial load and OT moment with combined reaction. I have prepared that sketch assuming the abutment will resist 60% of OT moment with counter weight action and the bed will resist to remaining with gravity and bending , the bed will need a certain inertia (i presumed 1.0 m depth and the width of bed 3.0m slab thk. 0.3 m ) and proposed As= 500 mm2 rebar at beginning ( of course will reduce at center ). The set up could be modelled with beam on elastic foundation..after analysis, the reasonable approach is to observe that all of the winkler springs are at compression .




The reasoning for the massive nib is, to mobilize base friction and shifting the C.G of abutment to resist OT moment due to prestressing load .

P.S. If i were the designer , i would prefer steel end posts with diagonal struts rather than RC abutment ...steel option is more flexible . and if you provide more refined data ( total prestressing load (for straight and harping strands ..), the grid of the loads on vertical axis , you may get better responses..