Monorail Spur

[Tom]

Pre or post? I thought post.

Man, you Purdue folks are everywhere. (I work with a ton of Purdue engineers)

One of my structural engineering professors consulted on an accident at a parking garage which had been under construction.

The contractor torched off the end of one of the post-tensioning cables. The release of the tension in the cable shot it out of the garage and through the facade of an adjacent building. Fortunately no one was hurt.

OMG! Yikes!

Maybe I'm backwards, but I was pretty sure they were pre-tensioned. They run the cables through sleeves embedded in the foam core and affix the ends to steel plates on the ends of the beam, then make the pour.

When they were explaining it to us in Vegas, as they were making one, I got the impression that the cable was already in tension, and then they poured. Maybe I misunderstood, and it was just pulled "taught" and THEN pulled to "tension" after it cured. :veryconfu

 

[Phonedave]

OMG! Yikes!

Maybe I'm backwards, but I was pretty sure they were pre-tensioned. They run the cables through sleeves embedded in the foam core and affix the ends to steel plates on the ends of the beam, then make the pour.

When they were explaining it to us in Vegas, as they were making one, I got the impression that the cable was already in tension, and then they poured. Maybe I misunderstood, and it was just pulled "taught" and THEN pulled to "tension" after it cured. :veryconfu

As I said before Concrete is very poor in tensile loading. Prestressing serves to mitigate that weakness.

You stretch the cable and pour the concrete around it, then you release the cable. Now pre-tensioned cable is in effect compressing the concrete. 'Pulling' it together if you will. Now when you put a tensile load on that section of concrete (the bottom of a uniformly loaded beam will be in tension) the action of the cable pulling the concrete together (compressing it) and the action of the loading pulling the concrete apart (under tension) cancel each other out, so you can design the beam with a smaller cross section.

You can also tension the cable after you pour the concrete, but in that case you do need to have sleeves as you mentioned. Then they can either grout the sleeves or fasten the ends of the cable, but it has to be fastened to the ends of the beam. The end result is you want the stressed member applying a compressive load to the concrete.



-dave

 

[Master Yoda]

As I said before Concrete is very poor in tensile loading. Prestressing serves to mitigate that weakness.

You stretch the cable and pour the concrete around it, then you release the cable. Now pre-tensioned cable is in effect compressing the concrete. 'Pulling' it together if you will. Now when you put a tensile load on that section of concrete (the bottom of a uniformly loaded beam will be in tension) the action of the cable pulling the concrete together (compressing it) and the action of the loading pulling the concrete apart (under tension) cancel each other out, so you can design the beam with a smaller cross section.

You can also tension the cable after you pour the concrete, but in that case you do need to have sleeves as you mentioned. Then they can either grout the sleeves or fasten the ends of the cable, but it has to be fastened to the ends of the beam. The end result is you want the stressed member applying a compressive load to the concrete.



-dave

Post-tensioning is done but it is done during the curing process and not after it. The advantage to post tensioning is that it can be done more easily in the field. As you might of guessed pre tensioning is the stronger of the two methods but it is harder and more expensive to apply pre tensioning in structures such as slabs.

 

[Tom]

As I said before Concrete is very poor in tensile loading. Prestressing serves to mitigate that weakness.

You stretch the cable and pour the concrete around it, then you release the cable. Now pre-tensioned cable is in effect compressing the concrete. 'Pulling' it together if you will. Now when you put a tensile load on that section of concrete (the bottom of a uniformly loaded beam will be in tension) the action of the cable pulling the concrete together (compressing it) and the action of the loading pulling the concrete apart (under tension) cancel each other out, so you can design the beam with a smaller cross section.

You can also tension the cable after you pour the concrete, but in that case you do need to have sleeves as you mentioned. Then they can either grout the sleeves or fasten the ends of the cable, but it has to be fastened to the ends of the beam. The end result is you want the stressed member applying a compressive load to the concrete.



-dave

Post-tensioning is done but it is done during the curing process and not after it. The advantage to post tensioning is that it can be done more easily in the field. As you might of guessed pre tensioning is the stronger of the two methods but it is harder and more expensive to apply pre tensioning in structures such as slabs.

Right and right, of course.

I'm 99% sure the beams they were making in Vegas were PRE-tensioned.

 

[BalooChicago]

Am I reading this right?

http://www.concretetech.com/project reports/disneyworld.htm

It looks to me like the individual beams are prestressed, but then multiple units are post-tensioned together. Right?

 

[Tom]

Am I reading this right?

http://www.concretetech.com/project reports/disneyworld.htm

It looks to me like the individual beams are prestressed, but then multiple units are post-tensioned together. Right?

Interesting....it sure sounds like it.

 

[s8film40]

Am I reading this right?

http://www.concretetech.com/project reports/disneyworld.htm

It looks to me like the individual beams are prestressed, but then multiple units are post-tensioned together. Right?

My understanding is that six beams are post-tensioned together to form a complete section of six beams. You can see this if you look at the beam way there are six segments of continuos beamway and then at the end of each segment each beam has it's own pylon and are connected to the next section only by an expansion joint.

Here you can see where one segment ends and the other begins:

[/IMG]

 

[Master Yoda]

Am I reading this right?

http://www.concretetech.com/project reports/disneyworld.htm

It looks to me like the individual beams are prestressed, but then multiple units are post-tensioned together. Right?

Interesting....it sure sounds like it.

My understanding is that six beams are post-tensioned together to form a complete section of six beams. You can see this if you look at the beam way there are six segments of continuos beamway and then at the end of each segment each beam has it's own pylon and are connected to the next section only by an expansion joint.

I can not remember a time when I have enjoyed so much tension.

 

[Tom]

I can not remember a time when I have enjoyed so much tension.

*Rim Shot*

 

[BalooChicago]

Now I feel like doing some shear and bending moment diagrams.

One of my projects had post-tensioned limestone columns, the contractor over-stressed one of the columns and literally pulverized on of the drums.

 

[officeboy]

My understanding is that six beams are post-tensioned together to form a complete section of six beams. You can see this if you look at the beam way there are six segments of continuos beamway and then at the end of each segment each beam has it's own pylon and are connected to the next section only by an expansion joint.

Here you can see where one segment ends and the other begins:

[/IMG]

What is curious about this particular photo is the grid like pattern visible in the concrete to the right of the expansion joint. Where does this grid pattern come from if the horizontal structure is one pour?

 

[Tom]

What is curious about this particular photo is the grid like pattern visible in the concrete to the right of the expansion joint. Where does this grid pattern come from if the horizontal structure is one pour?

Those grid lines are just from the inside of the forms they used when pouring the beams.

I'm not exactly sure why there would have been any sort of grid pattern like that on the inside of the forms - they should have been smooth. But perhaps they were very old forms and had started to show the impression of their structural backing all the way through to their faces.

 

[Kamikaze]

Those grid lines are just from the inside of the forms they used when pouring the beams.

I'm not exactly sure why there would have been any sort of grid pattern like that on the inside of the forms - they should have been smooth. But perhaps they were very old forms and had started to show the impression of their structural backing all the way through to their faces.

Wooden forms have to have joints somewhere.

 

[Tom]

Wooden forms have to have joints somewhere.

Not usually every 6" on center.

 

[unkadug]

Perhaps the beam in question is a curved beam...perhaps they used some kind of form with flex joints to form curves where they were needed.

 

[Kamikaze]

Not usually every 6" on center.

Ah, I see where you're talking about now. I thought you meant the joint that runs vertically. The part you're talking about almost looks like they pressed cinder blocks into the side.

 

[Tom]

Ah, I see where you're talking about now. I thought you meant the joint that runs vertically. The part you're talking about almost looks like they pressed cinder blocks into the side.

Yeah, almost a waffle pattern.

Since that beam is coming out of the TTA, it may have a slightly upward curve to it, but probably not a horizontal curve.

 

[Timon]

Here some pictures of the Vegas beams being made and installed. I'm sure the Engineering types will love these.

 

[jt04]

Yeah, almost a waffle pattern.

Since that beam is coming out of the TTA, it may have a slightly upward curve to it, but probably not a horizontal curve.

Did someone mention waffles? :slurp: