Long span prestressed girder stability and lifting 1

[whydoikeepasking]

Hi, I've been reading up on Robert Mast's PCI article on long span girder stability during lifting and transportation. It is also printed in PCI Design manual chapter 8.10

He defines lateral moment, M_lat as top corner of flange to reach tentile rupture, and takes ratio of that to the moment caused by self weight at harping point relating it to small angle theory and calculates maximum angle of tilt when cracking occurs.

If I need to take wind load effect into account, the lateral moment caused by the wind should be subtracted giving lower cracking angle?

I'm dealing with tall (8ft) and long (190+ ft) girder, and lateral moment due to wind is larger than M_lat to cause rupture at top flange corner meaning even without consideration of girder sweep, girder is already cracked.

Also, I've seen that to size up temporary brace at midspan, 2% of compressive force in flange is used. If we are dealing with prestressed girder, shouldn't I account for compression and tension due to prestressing tendon and their eccentricity?


I just wanted to verify this.

Thank you very much in advance.

 

[ishvaaag]

I think you would need to account everything as you say, wind and standing state, so prestress when present. Any technical guide as by Mast of whom I always remember not to use less than 1/85 in plan as the width of the compressed head must be taken unto account.

Anyway, solution: you may attach some streuctural steel aditament to the compression head, to be left or removed after installation; or you can widen the prefab girder head.

 

[asixth]

Thanks for posting the paper, I haven't had a chance to read through the comments on lateral stability but it is a very important issue. With these type of structures we specify that the girder must be restrained laterally from overturning and that every fifth longitudinal bar in the deck slab reinforcement must be welded so the girders cannot overturn during construction. It was not long ago that some workmen were killed in my area from overturning of bridge girders during construction, it is not something that I would like to have on my mind.

 

[jorton]

hjin25,

I usually calculate the stress in the top flange due to prestressing and self weight (this changes with different support conditions), the stress in the top flange due to axial forces if the lifting cables aren't plumb (this includes a P-delta effect from the axial force acting on and creating additional sweep in the beam), and the stress in the top flange due to a lateral wind load. I use these stresses to find how much additional stress due to toppling can be applied before tensile rupture of the top flange. This reserve stress capacity is used to find an equivalent lateral moment that would cause the stress. This lateral moment divided by the moment in the beam at that point due to self weight gives you the tilt angle at which cracking occurs. This can be used to find the FS against cracking as shown in the PCI manual.

Some people still you the Laszlo method. It is usually a little less conservative.

 

[whydoikeepasking]

Thank you very much all for the input.
The trouble I had was that the top flange of the girder didn't have any reserve capacity due to wind load only even without any consideration of girder sweep. Therefore lateral moment was negative value which was meaningless.

I'll read up on Laszlo and see how much they differ.

Thanks you again.

 

[Mac75]

Hi i am looking for help on Post Tensioned Box Beam girder bridge assessment.I have modelled the bridge as 2D Grillage.
I have split the box beam into I section and C section at ends.I would like to know that how can you calculate stressess in the web & diaphragm opening in 2D Model.

 

[asixth]

Mac75

We will be more than willing to help but it would be best if you started a new thread for that question.

 

[afrancba]

Hi

Could you tell me more about "Laszlo method" (or some usable links)? I've never heard about it, or its name might be different here.

Thank you very much in advance.

 

[whydoikeepasking]

Here you go.

 

[afrancba]

Thanks a lot, i am reading it